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Students

Congratulations on learning to fly. Whether for private flying or commercial career, you have embarked on a path of continuous learning and enjoyment, interspersed with moments of reflection on your choice! But that is all part of the experience of flying. May your flying activities last a lifetime.

AOPA would like to be with you all the way. The first step is to join AOPA as a Student member. All we ask for is a £20 administration fee, which is refundable in full against your first full Pilot (or Instructor) membership. For that you get up to two years membership with full member privileges and benefits. Your Student membership expires after two years or when you gain your Pilot licence, whichever comes first.

Without AOPA representing the interests of General Aviation you would find your freedom to fly restricted by:

  • Harder and more expensive to achieve licences
  • Fewer privileges with your licence
  • Less airspace accessible to you as a VFR Pilot
  • Fewer airfields to use
  • Higher duty on AVGAS
  • Higher navigation charges
  • Higher maintenance costs
  • Increased medical requirements

Supporting AOPA provides us with the funding we need to cover the cost of protecting your interests against over regulation and anti-aviation voices.
You also get personal advice in the event of you having an issue related to your training and flying.

AOPA UK, part of the worlds largest association representing General Aviation (GA), is the only truly independent GA organisation. We are only answerable to our members.

Once you have your Pilot Licence and get a few hours in your log book as Pilot in Command, free of your Instructor and the Local area or Circuits, you are at your most vulnerable and the choices you now make will determine how long you keep flying.

We should tell you now that the majority of Private Pilots give up flying within 5 years of gaining their licence. If it is not your objective to just gain a pilot licence to prove you can do it, then move on to something else, you may well need a plan to keep from losing interest in flying.

We know that the chances of getting past the 5 year drop out period are enhanced by:

  • engaging in a Flying Club's social side
  • buddying up with other Pilots and plan shared flights
  • extending your flying range and experience
  • joining an aircraft ownership group
  • making use of schemes such as the AOPA Wings Scheme to set goals
  • making use of a mentor if you need confidence building
  • training for new ratings
  • undertaking differences training

Right, lets get straight into it, there’s a lot to cover. Is everyone sitting comfortably? Then I’ll begin…

AIR LAW

Why is it Air Law has the reputation for being the boring or uninteresting subject? It might surprise you to find out it in fact contains very important and practical facts, and instructions and rules to help keep you and others around you safe. It is the highway code of the air and has to be respected and appreciated. During the course I teach at AOPA, you learn the rules for avoiding collisions, rights of way, airport and runway markings, aerodrome traffic zones and flights in their vicinity.

Learn about visual and instrument flight rules and the weather conditions that determine them. Learn about controlled airspace and the services you can get from air traffic control units.

OPERATIONAL PROCEDURES

This is one of the newer exams that was part of Air Law. Operational Procedures in commercial aviation is really about how companies are going to operate, the company, minima and standard operating procedures they negotiate with the CAA.

PPL holders and training organisations don’t operate like this, but we follow ICAO recommendations. They are based on ICAO Annex 6, and part II is relevant to General Aviation operations.

Look up the Annex and have a read. It’s not for everyone but interesting if you have a legal mind. Apparently. You do need to study the  practical and useful information contained within it. It deals with safety equipment and its use, wake turbulence separation, wind-shear, what to do if you have an accident, search and rescue, and other general information about the procedures involved in operating an aircraft safely.

HUMAN PERFORMANCE AND LIMITATIONS

A bit of biology, a bit of physiology and a smattering of psychology. This is really interesting stuff. You learn how your eyes and ears work and how they can be tricked or give incorrect information to the brain.

During the AOPA lectures we cover what it means to become a competent and proficient pilot. Most accidents are caused by a lack of good judgement, and analysis of accidents involving human factors shows that there is hardly ever a single cause. Apart from passing the exam, learning about Human Factors is learning about yourself and your body and how it can be affected by a myriad of environmental factors, such as hurtling through the air at great speed  thousands of feet up in a metal tube with an engine stuck to the front.

NAVIGATION

The shortest distance between two points is a straight line. But how do you draw a straight line on a sphere? Or how can you put a curved line on a flat piece of paper?

This topic will teach you about course and headings,true and magnetic tracks, variation and deviation, longitude and latitude, and that the Earth is in fact not a sphere but an oblate spheroid, a useless fact but it has come up in University Challenge.

This is probably the most technical and practical of the exams and requires a bit of maths and mental arithmetic. We will help you learn the easiest way to grasp these concepts. To get a historic perspective on how we navigate and for a really interesting tale of intrigue and the establishment of the Royal Observatory at Greenwich, I highly recommend the book Longitude by Dava Sabel and a visit to the Greenwich Observatory.

The course covers General Navigation and Radio Navigation. It also covers the use of the navigation computer, the famous whizz-wheel, that I hope you will learn to love. 

FLIGHT PLANNING AND PERFORMANCE

This subject is about how to plan a safe journey. It is about how much weight you can carry, how much fuel you should carry and how much runway you need. It explains how long will it take to get to your destination and how to re-plan on route if necessary.

It is also about reading, understanding and then using the information from charts to plan a safe route. Some is intuitive; the consequence of an overloaded aircraft is obvious, but what about an aircraft with a forward or aft C of G? How will this affect the take off and landing performance, stall speeds, fuel consumption, glide speed and so on? Learn about the aircraft and the system you use to operate it and your decisions will come from an intellectual depth of knowledge. That can only be a good thing where safety is paramount.

METEOROLOGY

The atmosphere is an envelope of gas held down by gravity within which a constant battle for equilibrium is staged by areas of higher and lower pressures, higher and lower temperatures, and higher and lower densities. Add water into the mix and it is torn between its solid, liquid and gaseous states, radiating or absorbing huge amounts of energy as it changes.

These global and energetic currents of moving air and billions of tonnes of water generate static build-ups of such potential that they can instantly ‘boil’ a tree and cause it to explode.

You need to learn about meteorology, about the mixture of gases that move horizontally and vertically, and the implications of trying to fly a machine through it. The weather poses the greatest safety challenge to your flying. You will have to make decisions based on observations you make and those predicted by professionals. You will get it wrong, they will get it wrong. Deciding not to go somewhere because of the weather and getting it wrong is the safest scenario.

Being armed with theoretical and practical knowledge of aviation meteorology will lead to your being a better all-round pilot. Learn to interpret the actual and forecasted weather charts and observations. Learn to read into the weather what you see around you and the conditions that they might pose to an aircraft in flight.

Bore everyone else to tears with tales of good decision making and by repeating the mantra that it is ‘better to be down here wishing you were up there than up there wishing you were down here’.

PRINCIPLE OF FLIGHT

This subject is about what makes a plane fly and the forces around it. A decent knowledge of physics is useful, though not essential, in order to really understand the dynamics of moving an aircraft through the air at different velocities. The effects of different shaped aerofoils on lift and drag, the effects of acceleration and angle of attack, stalling, stability and so on.

Our course at AOPA covers all the topics, starting with a short introduction to Newton and the three laws. These seem quite simple and intuitive, and they are up to a point. It is the equations and principles that can be derived from them that make them so profound. They help us describe many things, from how an aircraft flies to plotting a satellite’s orbit. We will stick to how aircraft fly and what keeps them flying.

AIRCRAFT GENERAL KNOWLEDGE

The Aircraft General Knowledge topic covers all of the components of a light piston driven aircraft. Understanding how an engine works and the limitations imposed on the airframe is an essential part to becoming a good all-round pilot.

You learn how engines and instruments work, how to cope if something goes wrong – like a pitot blockage – how the altimeter and VSI work on static pressure but the airspeed indicator measures dynamic. Make sense of all those dials! They are your aircraft’s ‘life support’.

COMMUNICATIONS

This is the last topic we cover during the AOPA course. It is not demanding intellectually and hopefully you will have picked up a lot from listening and using the radio during flying lessons.

Again it is a very practical exam, and it is all stuff you need to know, like the difference between your Rogers and Wilcos. Who and what you are. Where you are from and where you are going. Your position and altitude. Your request. QDMs and QDRs. Urgencies and Emergencies.

My philosophy for teaching these topics at this level is that for the most part you are not doing this as the day job. Therefore it is important to enjoy the learning as much as the flying. I am starting the course at AOPA in October. Everyone who has done the course and taken the exams have passed.

See here for times and dates of the AOPA Ground School Course in London.

Adam Winter

If you have any questions regarding any flying issues Adam can advise. Email him: adam.winter@aopa.co.uk

Altimeters can do the job quite well, although they can only tell you the difference in height between two points, namely the one you set on the subscale and where you are. The altimeter does not know where the top of a hill, the ground or the sea is. That is up to you.

You need know how high to fly to avoid an obstacle, and you need to know how high you are above the sea in order to avoid it. A clear understanding of the altimeter settings is essential for this.

‘Q’ codes were originally created by the British Government in the early 1900s and were a list of abbreviations licensed by the Postmaster General for the use on ships over the radio. They were either a question or an answer (advice). It was a way of abbreviating the Morse Code. The list is extensive, even the original had 45 codes, and is still widely used by amateur radio enthusiasts.

QNH and QFE are both altimeter settings. When QNH is set on the subscale, the altimeter will tell you how high you are above sea level (your altitude), and when QFE is set, it will tell you your height above the ground.

It might be an obvious thing to say at this stage, but sea level remains constant as far as aviators are concerned. Ground level does not remain constant.

QNH, when set on your altimeter, will give your height above sea level. So if you are on the ground at an aerodrome and you set the QNH (the pressure at sea level) the altimeter will read the elevation. Your height above sea level is called ‘altitude’, so QNH set on the subscale gives your altitude.

If you are at Elstree your altitude at ground level is about 330 feet, at Biggin Hill it is about 600 feet, and Southend about 60 feet.

Remember the QNH is a measurement or calculation of air pressure at sea level. Pressure changes constantly, whereas the height of an airfield doesn’t, so the difference between QHN and QFE is constant.

At Elstree the QFE is always 11hPa less than the QNH, Biggin Hill will always be 20hPa less, and Southend 2hPa (see the diagram below).

When you set the QFE on the ground at any airfield your altimeter should read 0ft. Most circuits are then flown as a height, usually 1000 feet above ground level (AGL).

DIVIDED REGIONS

When you are flying cross country and you are in the vicinity of, or below, the controlled airspace of a large airport like Heathrow or Birmingham, you would use the QNH for that airfield.

The UK is also divided into several “altimeter setting regions”, and if you are not flying in the vicinity of a large aerodrome you could use one of the regional pressure settings. It is defined as the lowest forecast setting for that hour. They can be obtained from any ATCSU (Air Traffic Control Service Unit).

UNDER PRESSURE

Finally there are ‘Pressure Altitude’ and ‘Flight Levels’.

I doubt as a PPL you will use these often but you should know about them.

Pressure Altitude is simply the altitude the altimeter gives you when you set 1013hPa on it. If you have a transponder in your aircraft with a digital readout, you might have noticed that the height given on the display is a pressure
altitude (transponders have no barometric components).

Sometimes flying around Elstree on a day with particularly low pressure, our
transponder reads 2700 feet, even though we are 2300 feet above sea level (on the QNH). But the London TMA is at 2500 feet! Don’t worry, the local radar stations know we are not infringing and it is because of the low pressure. 

Flight Levels are generally used when you are high enough that a large drop in sea level pressure would not result in the aircraft descending enough to put the plane in danger from obstacles or aircraft flying on the QNH.

When you fly a Flight Level on the pressure setting 1013hPa, you call your altitude ‘Flight Level’. Flight Levels are stated in increments of 500ft; you knock off the last two zeros and call it ‘Flight Level’. So reading 5000 feet on your altimeter with 1013 set on it becomes ‘Flight Level Five Zero’, or FL50.

Imagine all the planes flying across the Atlantic having to change altimeter settings as pressure increases or decreases. Even airline pilots aren’t paid enough for that, so they use 1013hPa. If the pressure on route drops by 20 hPa, and the aircraft descends 600 feet, as long as all the other planes flying that route have the same pressure setting, there would be no problem that the actual altitude is changing as there are no obstacles.

Things become slightly more complex when looking at the altitude at which you can change from using an actual pressure setting (QNH) and 1013. This is known as the ‘Transition Altitude’. Have a look at the diagram. As long as the
actual QNH is above 1013hPa,then flying on the 1013hPa pressure setting as a Flight Level is safe. When the pressure is above 1013hPa, there will be no conflict with aircraft on QNH below the transition altitude. If however the sea level pressure is below 1013hPa, then if you were to fly at the first Flight Level above the transition altitude you might be in conflict with an aircraft using altitude.

There are lots of books and apps that deal with altimetry, and as I mentioned earlier, it is a really important subject to understand.

For example, it you are doing a standard overhead join to an airfield below a TMA (eg Elstree), and have the wrong altimeter setting, you could, at 2200 feet indicated (on QFE) be in the London TMA, which starts at 2500 feet altitude.

Make sure this is a topic you understand both mathematically and practically. Your instructor will help you with the practical side of altimetry.

Adam Winter

Adam is a commercial pilot with more than 30 years' experience. His career has included bush flying in Africa and island hopping in the West indies, as well as a stint in the airlines. He is also a qualified physics teachers with three years classroom experience, that aged him 10 years, so he is back doing what he loves, teaching PPL.

"...I fly aircraft with tricycle undercarriage and have no tailwheel experience to talk about, except to say that when I tried landing one a few years ago it was the last time I heard the words “I have control” with urgency coming from the seat next to me (thanks Tony). The following therefore doesn’t hold for taildraggers, which have a different centre of gravity and therefore a mind of their own!"

For students it can be a challenge to keep an aircraft in a straight line coming in on final. There is the speed and rate of descent to control – a fine balance between nose attitude and power to maintain a constant angle. And the aircraft has to be flown down the centre line. Hard enough when the wind is from straight ahead, but then add a crosswind into the equation and the capacity bucket fills up very quickly!

Let us assume you have mastered the normal approach landing. You can trim the aircraft for the speed you want, and select a power setting that gives you a sensible approach angle or rate of descent.

Now we put in a crosswind. In order to approach the runway down the centre line, you have to point the nose into wind. If the wind is from the left, in order to fly down the centre line you have to point the nose to the left. So if it is an approach to runway 26, your heading will be about 250 degrees.

For you in the cockpit and to an observer on the ground the aircraft appears to be approaching the runway slightly side-on. Some people call this crabbing, for obvious reasons. The aircraft is at this stage in perfectly balanced flight, the ‘ball’ is central and the ‘crab angle’ is caused by drift. In a perfect world this angle would remain constant all the way down the approach, but the wind velocity does change, so small changes to the approach angle will be required.

Now the pilot is flying the aircraft down the centre line and is laying off for drift. When teaching this I like to make it a fairly long final. It can be useful (especially for inexperienced pilots) to try out, or rehearse, the rudder and aileron inputs they will need to straighten up before landing.

So the aircraft is heading 250 degrees and tracking down runway 26 (260 degrees). The maximum angle will be about 10 degrees; much more than that and you are probably going beyond the aircraft’s crosswind limit (see calculating crosswinds).

At 300-500 feet, apply some right rudder and point the nose down the centre line. To stop the aircraft banking when applying right rudder, use left aileron. The aircraft is now no longer in balanced flight, but it is flying down the centre line. A look at the turn-and-slip indicator will show this as the ball will be off slightly to the left.

I let students experiment with rudder and aileron on final, but don’t recommend they fly the whole approach like this for a couple of reasons: first, this is not balanced flight and in the sideslip you are creating extra drag which will make speed control quite difficult. Second, although it is a good way to ‘rehearse’ which controls you will be crossing when you need to straighten the aircraft, the wind gradient changes down the approach, backing and slowing down (in theory), so the input will be different. This experimenting on final can also be useful as it can be quite daunting approaching at an angle, and I have had countless instances of students correcting for drift using the wrong inputs simply because they have convinced themselves (nerves?) that they will get it wrong.

Pilot Skill

As you approach the ground and prepare to ‘hold off’, keep the nose pointing into wind and the aircraft tracking down the centre line. Round out, and now the magic. Hold off (a fine balancing act), while straightening up with rudder. Use opposite aileron into wind to stop the left wing rising (with the right rudder input) which should also stop the aircraft drifting to the right of the runway. I won’t write much about this as it just needs to be practiced. Over and over.

It is not easy but once you have mastered this, it can be very satisfying. When you have touched down, you still need to control the aircraft. With PA28s the rudder also controls the nose wheel steering, so after touch down you should maintain back pressure to allow for aerodynamic steering from the rudder.

If you touch down and release the back pressure, you put weight on the nose wheel and the right rudder you were using to straighten up becomes a right turn. This is accompanied by a short burst of screeching from the nosewheel tyre, so left input is needed, which gives left rudder, left nose wheel steering plus left turn from the weather cocking effect – with more screeching of the tyres, which is about to happen again because you now need to compensate to the right again. This can happen five or six times! So, keep the back pressure and the weight off the nose wheel, and have the advantage of a bit of aerodynamic braking from the tailplane. Breath deeply…

Calculating Crosswind Component

This is a nice and easy thing to do on your whizz wheel. In this example I have used a wind from the north at 40 knots. (Please don’t fly if the wind is from the north at 40 knots). I have put the wind direction in at the top and marked the 40 knots down with a dot (Picture 1). The arrow shows that if you are on a northerly runway, there is a 40 knot headwind component and 0 knot crosswind. Picture 2 shows what happens to the 40 knot wind if the runway is 40 degrees off (i.e. runway 04). We now have a 30 knot headwind component and a 25 knot crosswind. Picture 3 shows that if you took off on runway 09 (to the east), there would be a 40 knot crosswind component and zero knots of headwind. Finally, much has been written about crosswind limits and debates around when or not one should fly. I will only add that you need to have a personal limit that lies within your personal ability, confidence and experience. If you are about to fly and are not sure, grab an instructor and fly some circuits. Happy flying, and happy crosswind landings. And keep applying the back pressure after landing!

Adam Winter

Adam is a commercial pilot with more than 30 years' experience. His career has included bush flying in Africa and island hopping in the West indies, as well as a stint in the airlines. He is also a qualified physics teachers with three years classroom experience, that aged him 10 years, so he is back doing what he loves, teaching PPL.

Apart from in an emergency, there is never going to be a decent excuse for infringing controlled airspace. There is only going to be a reason it has happened.

The airspace around South East England can seem rather daunting with myriads of restrictions; Control Zones, TMAs, danger areas, military zones, temporary restrictions such as the Red Arrows, memorial flights, Airshows (found in the NOTAMs) and so on. Whilst it might look congested on a 1:500 000 map, there is actually an enormous volume of uncontrolled airspace to manoeuvre in and many ways to get through.

As a general aviation pilot, there are foolproof ways to either avoid controlled airspace or go through it.

Navigation is the Key

The key to avoiding infringing is good navigation, and the key to that is simply knowing where you are, and where you are headed. Knowing where you are is a function of keeping up with the aircraft, and knowing where you are going is all about keeping ahead of it. Then fly straight and level.

Let us imagine a short route. There is a minimum safe amount of preparation for the route you should be doing. The first thing is draw a line on a 1:500 000 map. The line avoids or goes through or around controlled airspace, and note the upper limits of the airspace for the route. Now look at the route and note any distinguishing features along the it. Circle them and note the EET (estimated en-route time) to those features. Good features are towns, cities, roads, airfields, railways, forests and so forth. I tend not to use rivers, as they can be hard to spot, or dried out. So a typical navigation leg on my map will consist of a black line, with small circles along it around features, with a number representing the minutes it will take to get there. I don’t believe in clogging up the map with fan lines and correction angles, not for a PPL-holding pilot, or student navigator.

Now, in order to follow that line, you have either calculated the headings using your amazing and wonderful CRP circular slide rule, or you have printed the headings from some demonic website (apps are useful tools – I always check and compare the weather and winds with the met office directly when using them). Either way you have a reasonable calculation and assessment of winds and drift angles, so you are well prepared.

Flying the Route

You should be able to fly that route using time and heading as you have done the minimum but essential preparation. Don’t forget this is visual navigation. Stay ahead of the aircraft by literally looking ahead and spotting the features you circled (and other aircraft). If you see the outline of a town that you estimate passing in four minutes, you are four minutes ahead, and can think about that forest you might be looking for after twelve minutes. Of course life is not always this simple, but by making these easy preparations you will have more capacity to make corrections when that first town is off to the right, or work out why it took five minutes longer than expected. You are giving yourself more of a chance of navigating successfully, and less of a chance of going into controlled airspace.

Try always to prepare to navigate this way primarily, and then you can add layers of assistance on top, such as satnav or radio aids to help you confirm you are following your line.

There are still other tools at your disposal to avoid controlled airspace. There are some excellent ATSOCAS (Air Traffic Service Outside Controlled Air Space) such as LARS (Lower Airspace Radar Service). I couldn’t resist that. These services have been provided for us in order to help keep us out of controlled airspace and to keep controlled airspace safe. A lot of people use Farnborough LARS around the South East.

Their best product for the PPL navigator is the ‘Basic Service’. When you get that from them, they offer you a transponder code then monitor your progress as best they can on radar. If they are not too busy they will tell you about traffic if it is going to pass close by, but it is your job really to look out. If you do happen wander too close to controlled airspace and they see it, they will warn you and steer you away if they spot it in time. If you infringe, alarm bells go off for them, and they will steer you out straight away. They will be very calm and professional and will let you get on with the rest of your flight. You will probably be asked to call them on your return and will be asked to fill in a form which details the infringement.

The reason they do this is to establish why it occurred, how it might be avoided in the future, and if you need any further training before flying again. With my infringement a few years ago (I’ll tell you later), and a recent one by a student of mine, they have been very reasonable and forgiving because were talking to them. Getting a basic service is very basic.

Here is a typical call format:

“Farnborough G-AOPA for basic service”
“G-AOPA Farnborough pass your message”
“G-AOPA is a PA28 from Elstree to Wellesbourne, South of Bovingdon 2000 feet, request basic service”
“G-PA squawk 5024 London QNH 1005”
“G-PA squawk 5024 QNH 1005."

And that is about it. It is the same basic RT format used for most situations and you should be fluent with it. It is nice to get it all out in one short professional-sounding spurt of RT, but it is OK if you forget something or get it wrong. Often students say something completely random, or forget to state something. It doesn’t matter, the controller will ask. It isn’t just Farnborough that offer this service. Most large airports offer it such as Luton or Brize Radar.

As an instructor I use this service often, both to help me out and to get my students used to using it. I often hear controllers steering aircraft away from or out of controlled airspace. Those are the cases that don’t usually cause commercial aviation any problems because although the infringements occur, they are quickly diverted. The problems occur when the infringing aircraft aren’t talking to ATC.

When you are using a service, you have to listen out and respond to the controller if he calls. I am very familiar with our local training area and know visually where all the controlled airspace is. Sometimes ATC are very busy and I don’t really need a basic service so I simply select the frequency and monitor it. I can let the LARS provider know I am listening in by squawking what is called a ‘conspicuity code’. You can use these codes for cross country routes as well.

Even the Best Falter 

Unfortunately, just like avoiding parking tickets in central London, avoiding controlled airspace is something that can trip up even the most experienced of us. About six years ago I was flying as safety pilot with a friend who could still fly but no longer held a medical.

We were at the holding point for runway 26 at Elstree, mid August. The blackberry bushes a few feet off the wing tips were heavy with huge berries and I was imagining a juicy warm rumble dripping with thick cool fresh cream. I was also vaguely aware of my friend doing the pre take-off checks and I saw him adjust the altimeter, set the DI, fuel pump, flaps and so on; only to be woken from my berry reveries as he was given the discretion to line up. We took off and climbed and turned towards the north-east. The top of uncontrolled airspace around Elstree is altitude 2500 ft. At 2400 ft I warned my friend and he levelled off and started to descend at just under 2450 ft. But looking out the local area, things didn’t look right; we looked too high. Checking the QNH I’d written down earlier and the altimeter confirmed my suspicion and we had been flying on QFE. So 2450 ft height was in fact over 2750 altitude. We descended very rapidly and I contacted Farnborough straight away for a basic service and admitted the error.

We continued the flight without incident and contacted Farnborough by phone on our return. I filled in the online form and I stated that I had been distracted at the holding point and hadn’t noticed the wrong altimeter pressure setting. I stated that it would never happen again and it hasn’t. I remember the incident every time I set the altimeter now.

Finally, don’t forget you can go through controlled airspace. Luton seem more than happy to let you fly over their thresholds. You just have to know where you are, sound competent on the radio, use the correct phraseology and ask. It is a good experience.

If there is a jet on final, ATC usually ask you to orbit north or south of the runway until you see it. When you see it and report to them, they ask you to fly over the 26 threshold (if wind is westerly - 08 if easterly) and pass behind the jet. They also tell the jet pilot about you (look up ‘D’ controlled airspace).

It's a good way to build confidence and is great fun!

Adam Winter

If you have any issues regarding any flying, Adam can advise. Email him via: adam.winter@aopa.co.uk

Being pilots can give us the sense that as we are in control of our craft; we ‘understand’ its behavior, and the physics behind what we are doing. But understanding physics and having a good ‘feel’ or instinct for how an aircraft behaves are two very different sides of the coin; you can be an excellent pilot with little or no intellectual knowledge of the physics behind what you are doing.

I can’t address all of the above, so will look at Newton’s Three Laws of Motion. We will also look at inertia and momentum, and how they apply to aircraft and their contents.

There is an awful lot of physics going on when we fly. Thankfully, with Newtonian mechanics, there are only three measurements we are concerned with. They are distance, mass and time.

Distance is a measure of how far apart two things are, mass is a measure of how much ‘stuff ’ an object contains, and time describes the order and duration of events. All other measurements (e.g. force, momentum, weight, acceleration) are a derivation of these.

For example, weight is a measure of an object’s mass undergoing acceleration due to gravity, which in turn is a measurement of an increase in speed or direction change over a period of time. Speed is a measure of change in distance over time, and so on.

We should also be aware of the difference between a scalar and a vector. A scalar is a measurement with magnitude only. Distance, mass and time are all scalars. A vector has both magnitude and direction. So if we give distance a direction, it becomes a displacement, or giving speed a direction makes it a velocity.

Force is also a vector, with both magnitude and direction. I’m not particularly worried about a punch from Mike Tyson – unless it is directed at my face!

Newton’s First Law

The First Law states that a body at rest will remain at rest and a body in motion will continue in a straight line (uniform motion) unless acted on by an outside force. This is often referred to as the ‘Law of Inertia’. There is no unit or formula to describe it as such. Inertia is something an object has, and we can’t change it. It can be thought of as a term describing how difficult or easy it is to get something moving (I’m not referring to ‘Inertial Moments’ that apply to longitudinal stability, these are considerations designers and engineers take into account, not us pilots).

So when you are pushing an aircraft back, the force you have to apply to get it moving is due to what we think of as its inertia. Other considerations such as if it is on tarmac or long grass and did you take the brakes off will also affect how inert it ‘feels’. This is only partly true; we are not completely correct because if we were looking at a purely Newtonian inertial system (i.e. no friction and a fixed reference in space), it would start moving no matter what force we apply, but the acceleration would be different.

On the ground, friction and gravity become the law’s ‘outside forces’ that are acting on the airplane, the ‘body’ initially at rest. The airplane’s inertia is just the mass, it is a scalar, and we can’t change it (unless we add passengers or fuel of course!) The heavier the aircraft, the more inert it is, but it will be harder to start moving because we have to overcome friction which opposes the force you are applying.

Newton’s Second Law

The Second Law states that if you apply a force to an object, it will accelerate at a rate directly proportional to the force and inversely proportional to its mass.

The formula describes this as well, but is usually written in the form F=m.a, or force equals mass times acceleration. The SI unit for force is thus kgms-², usually referred to as a newton. It is the force required to accelerate a mass of one kilogram at a rate of one metre per second every second. So after one second the object is moving at 1m/s, after two seconds, 2m/s, after 15 seconds 15m/s, and so on.

When considering accelerating an aircraft to take-off velocity we do calculations that obey the second law. How much force is required to accelerate this mass to 70kts (36m/s)? What distance will it cover over the ground and through the air? How much ground will it cover before it reaches 50 feet above that obstacle?

You use the graphs of motion from the aircraft’s technical manual to calculate the take-off-run or distance. These are graphs obeying Newton’s second law with a few ‘real world’ variables thrown in, such as temperature, pressure altitude and wind. Then you add factors for runway surface friction and slope.

When an aircraft is moving, it can be useful to think of its momentum, especially when it comes to stopping.

Momentum is the product of mass and movement, of applying a force to an object for a period of time. Momentum can change with a change in speed or direction (velocity) as it is a vector. At the end of the trip, we think of the aircraft in terms of its momentum, because it is a moving mass (mass x velocity = momentum) and a force has to be applied for a period of time to stop it. This can be expressed as:

Mass x Velocity = Force x Time

This shows that the momentum of an aircraft can be viewed in two ways, in two different reference frames, and this can cause confusion. First there is the product of its mass and velocity through the air. This is the speed the aircraft is flying according to the air speed indicator, and its direction, so is relevant for calculating stresses in a turn, for example.

From this perspective, when turning in still air or a stable wind, the aircraft’s momentum is changing only due to the its change in direction, assuming its air speed remains constant. There is no numerical change to its momentum, although it is in fact accelerating, as illustrated in the diagram below.

During a turn in windy conditions, momentum change due to the wind is only by reference to the ground. One time to consider this momentum is when looking at the angle you are coming in on final approach. Your velocity, and thus momentum, can be considered to have components along two axes. One is the horizontal velocity which relates to the ground speed of the aircraft; the braking system will cope with that (which is actually an energy exchange–a whole other story).

"Understanding physics and having a good ‘feel’ or instinct for how an aircraft behaves are two very different sides of the coin."

The second is the vertical momentum (by measurement of vertical speed) that has to be overcome aerodynamically; if your vertical momentum is too great you will come down hard. You overcome some of this vertical momentum by rounding out (flaring), making your descent angle less steep and bringing your flight path parallel with the runway, before holding off. In this case, the momentum we are considering is with respect to the reference frame provided by the ground.

Wind shear also causes a change in momentum (this is just one way of looking at this situation), because it is a sudden change in velocity of the air surrounding the aircraft. If you are flying into a 40 knot headwind on final at 75 knots IAS, and the wind suddenly shears to zero, the aircraft can’t instantly accelerate, so it will stall as the airspeed decreases to only 35 knots.

Thankfully most changes in wind velocity are less abrupt.

Back to Mike Tyson. We can say that relative to the fist, his glove has zero momentum, but that is not very consoling when it hits you. It is important to tackle a problem from the correct perspective.

Newton’s Third Law

The Third Law is the law of action and negative reaction (“To every action there is an equal and opposite reaction”). In other words, it states that all forces act in pairs, and the two forces are equal in magnitude and opposite in direction.

A simple example is to imagine lying on the ground and a person whose mass is say 75kg sits on your upraised hands. They are exerting a weight of about 750 newtons down on you, (the acceleration due to gravity being around 10ms-2, and then using F=m.a). In order to hold the person up (and assuming the acceleration due to gravity is exactly 10ms-2), you have to be pushing up with a force of 750 Newtons.

Exert 751 Newtons or more and the person goes up, less than 750 newtons and they will go down. When you sit in an aircraft you exert a force of 750 Newtons on the seat, and to support your weight the seat pushes back with the same force. At 60 degrees of bank you are pulling 2g (this means the force you feel from the seat is twice as much as the force due to gravity, which hasn’t changed one iota). But – in your frame of reference – you are still sitting upright in your seat, and in the aircraft ‘down’ is still towards the floor.

Look outside, however, and down is very acute, while at the same time you feel as if you are being pushed outwards. Physics tells us that you are in fact accelerating towards the centre of a circle that the aircraft is flying around, and that in your trajectory, your actual velocity is at any instant straight ahead, see diagram:

Your body isn’t pushing down on the seat. Rather, the seat is acting to pull you in to the centre of the circle. Imagine being a child swung around by your arms. In a similar way, you are being pulled into the centre of the circle that your trajectory is taking you around. There is no force pushing you outwards, just one pulling you in.

It’s the same in an aircraft. The force that causes you to feel this push in the seat is called ‘centripetal’ and is caused by centripetal acceleration. This is an acceleration caused not by a change in speed but a change in direction, which therefore means the velocity is changing constantly even while its magnitude (i.e. speed) is constant.

At face value, Newton’s Three Laws of Motion are not hard to understand. However, they do describe steady state systems (that don’t really exist in our universe, like ideal gasses, as they are just ‘idealised’ models to help us to understand, and to analyse).

To quote Walter Lewin, professor of physics at MIT, “Can the Laws be proven? No. Do we believe in them? Yes. Why do we believe in them? Because all measurement, all experiments are consistent with the Three Laws.”

They describe a basis from which many more complex calculations can be made, and this is what makes them the essential starting point. I have stuck to a few simple applications of these laws. In aviation we know that systems are never simple or steady. There are forces lurking in the bushes that can cause chaos.

They are applicable to all the situations we encounter and, as we know, these situations lead to endless discussions and arguments.

Adam Winter

Adam is a commercial pilot with more than 30 years' experience. His career has included bush flying in Africa and island hopping in the West indies, as well as a stint in the airlines. He is also a qualified physics teachers with three years classroom experience, that aged him 10 years, so he is back doing what he loves, teaching PPL. Adam is also a qualified secondary physics teacher.

If you get into a car with the intention of driving to Scotland, a quick glance at a road map will tell you that generally you will be heading north. It doesn’t really matter, the roads are fixed tracks and the friction between the road and your tyres will keep you on track (literally). The wind won’t be drifting you off in the wrong direction.

Also it doesn’t matter if you are referring to true north or magnetic north; you follow the road and so your track is predetermined.

However, anything that is airborne travels through a moving medium, and travelling in a straight line on a predetermined course becomes a bit more awkward. We need to calculate how much off course we will be blown, and correct for it. We also need to work out how long it will take us to get there.

So let’s go back to the map and draw a line. We have drawn an imaginary line that we would like to follow (i.e. our desired track).

Now we have to measure its direction with a protractor and take a measurement. The lines we are measuring against on the map are the lines of longitude and latitude that all line up to the north/south rotational poles. These are ‘True Tracks’. The rotational poles are known as True North and True South.

Effect of Wind

Now is a good time to work out what the wind is going to do to our track, in order to determine a True Heading. The winds you get from the Met Office are also measured in degrees true. So if the wind is blowing from the west, causing easterly drift, you are going to have to turn slightly to the west.

There are several ways of calculating the drift angle. By far the easiest way is to draw a scale diagram. Let’s say the wind is from 270 degrees at 20 knots.

If a journey is 100 nautical miles heading north (true), and our aircraft’s true airspeed is 100 knots.

We draw a line pointing to the top of the page (north)and make it 100 units long:

Then, from the top of the lines draw another line to the right (from the west) that is 20 units in length. You have drawn two vectors (a vector is a line with both magnitude and direction).

Draw another line from the bottom of your track line to the end of the wind line, and that is the track you will take if you don’t correct for wind.

You need to measure the angle at the bottom, and that is your drift, and you correct for that drift in the opposite direction.

Looking at the diagram, however, there is a slight problem: measuring the angle this way doesn’t take account of the fact that once you point the aircraft into wind, you slow down. So now we need some trigonometry.

This is easy for a right-angle triangle, but the wind is rarely exactly 90 degrees to your track, and then the maths becomes more complex.

There is an easier way of doing a scale diagram. The tool is the much loved (by me anyway) circular slide rule, or “Whizz Wheel”.

This immediately tells me my drift correction - 12 degrees to port - and the small headwind component. All in one!!

Now we know that to go north on this particular day we have to make our true heading 348 degrees and our ground speed will be 98 knots.

The next problem is that the aircraft compass is not aligned to True North, as it aligns itself with Magnetic North, and they are not in the same place (see diagram).

The angular difference between pointing to the true North Pole and the Magnetic Pole is ‘Variation’, and we apply the Variation to the True Heading to get the Magnetic Heading.

And then we get in to the aircraft and turn the electrics on and all the electromagnetic waves they generate make the compass itself deviate. So we need to make a correction to our compass heading to get a correct Magnetic Heading. This is Deviation.

If you have a PPL, the above should be old hat. You did it for your general flight test, and your navigation skills should have improved. However, with the advent of Sky Demon and other navigation programs, as well as GPS navigation, you might feel a bit rusty or reliant on the technology.

Another part of your PPL training that may have eroded is the use of traditional navigation aids like VORs and NDBs. I teach PPL Navigation at AOPA in Victoria and we have a great simulator for teaching PPL radio navigation or refreshing VOR/NDB tracking. It is also very useful if you have an IMC rating and want to practice radio navigation and approaches, especially if you are coming up for renewal.

Don’t forget the pleasure of navigating using a stopwatch, compass and a line on your map!

Adam Winter

Adam is a commercial pilot with more than 30 years' experience. His career has included bush flying in Africa and island hopping in the West indies, as well as a stint in the airlines. He is also a qualified physics teachers with three years classroom experience, that aged him 10 years, so he is back doing what he loves, teaching PPL.

Why is an interesting and grossly underused word in flight training! Why for example do people do so many strange things when flying aeroplanes?

There you are on final approach, power set, flaps deployed and trundling down the glide path nice and stable, when at about 50ft above the ground you chop the power. Go stand next to your favourite GA runway and watch.

See if you can spot the wobble when the power comes off, the nose tries to drop, and the trim goes to pot. Why do we do this? Please somebody explain to me the advantages of destabilising a perfectly good approach!

How about a gentle reduction of power during the initial flare thus giving better elevator control, and a civilised harmonious return to what Neil and Buzz described as ‘the good earth’.

Here are a few more ‘whys’! Why are there nine examinations to pass to obtain a PPL? Most of the information a student has to force into his or her brain is pretty irrelevant. I have never used the 1 in 60 rule in my life and please somebody tell me – where is the signals square at Heathrow? I once had an Aldis lamp conversation with the old tower at London Airport but we were just showing off as the radio was working perfectly well!

Staying with examinations – why on earth is there a requirement to pass them all in six sittings? What’s that all about and how is this relevant to leisure flying?

Another question is why are we so short of flying instructors and why do they move on so quickly? Could it be that nobody can afford to live on an average instructor’s wages? I recently came across a flying school paying the national minimum wage to its part time instructors on a zero hour’s contract. I suppose you could live with this if work was available for 10 hours day, there is lots of lovely daylight all year round, and the weather never strays from that portrayed in English Heritage brochures.

It would also help if all the students bother to turn up.

So it really is not that surprising that when the airlines' seductive call is heard, the response is amorous.

Why do pilots fly so close to controlled airspace? I am getting pretty bored with providing the training required by the CAA to expunge the sin of an airspace incursion. Keep at least two miles and 500ft vertically away from these blasted zones. Remember your transponder only has to be plus or minus 100ft accurate, therefore bombing along just below controlled airspace can easily trigger an alert leading to a letter dropping on the door mat!

Another quandary with asking "why?" can be when you get a reasonable answer. However this can be very refreshing! Years ago you could come and go in and out of the country as a pilot by just waving your flying licence at customs. The Commercial licence actually stated that the holder had the right of entry into the UK.

One day, all of a sudden, our passports were demanded at what was usually the end of a heavy day. This was very annoying so one day I tackled the border guards. To my horror my hostility was met with a perfectly good explanation. The UK had become a flying training centre of excellence and considerable numbers of overseas citizens had gained UK licences. Therefore this system was no longer viable.

Mind you there were consequences of having to report to customs as one evening we arrived late and duly trudged off to their office. We were greeted by scuffling noises from behind some lockers, followed shortly by silence and then the appearance of a young couple of somewhat embarrassed border officers, who had been indulging in what my mother would have described as ‘courting’.

So what is the point of asking why? Well if you are under training you are probably overwhelmed by all the rules, regulations, and training information. By asking why a little more often you will be able to make your own mind up about the efficiency of your training. Does your instructor understand the subject sufficiently to explain why you are doing something? Is your flight school giving you the very best it can? Are you doing something just to appease officials in EASA? Asking why will put you in command of your training and develop the self-reliance to be the captain of your aircraft.

Meanwhile back at the Training Committee we have been trying to answer some of other people’s why’s.

Why does the Wings scheme not cover people undertaking Tiger Moth type flying or indeed give instructors an improvement route?

Why are pilots still coming to grief using flap 40 in Cessna 150s? It must be a training issue as the go around at this flap setting has been known as all-but impossible for decades.

What is the industry doing about the flying instructor supply? My club has been using European instructors to supplement supply for years, but due to Brexit uncertainty, and the falling pound, that source has dried up.

I have spoken with many retired airline pilots who started their careers instructing with the aim of getting them back into the training industry. Unfortunately very few are prepared to go back to basics and teach again. I think the cost of renewing their instructor privileges outweigh any desire to pass on their invaluable knowledge. Shame.

We have electric aeroplanes on the horizon so what are the regulations going to be about them, and of course there are drones to avoid. Or should it be they must avoid us? Sadly there are far more drone owners than active pilots so guess where the votes are.

Two last thoughts. If you are flying in the hot weather or are popping over to the continent please be careful with pressure altitude. Your engine may well be struggling in the heat and the wing won’t work that well.

The second is my AOPA email account is now up and running, Hurrah! So if you have any training issues or comments please drop me a line. AOPA is here to help!

Ian Marshall
ian.marshall@aopa.co.uk

A few years ago a colleague of mine, a very experienced instructor/examiner asked me to fly with a friend of his with whom he shared a Cessna 210. His friend had been making awful landings, either coming down hard or touching down and bouncing. It was worse in a crosswind where he was making no rudder corrections to speak of. My colleague asked me to fly with him. I let him do the first circuit and landing. On runway 26 there was a slight crosswind from the right. We touched down to the left of the centre line, bounced and went around. On the second approach I asked for control on final at about 250 feet. Straight away I realised the aircraft was not trimmed. I trimmed it and handed control back to him (which took all of 5 seconds). There followed a textbook landing, followed by two or three more, now with good trimming, and the problem was solved. Because the aircraft was not trimmed, he was concentrating too much on keeping the correct trajectory, having to use large inputs and didn’t have any capacity remaining for the finer touches. Once the airplane was trimmed however, the inputs he needed were smaller, lighter to the touch and didn’t require as much concentration.

All light aircraft I can think of have at least an elevator trim tab of some sort, and that is what I want to look at in this issue. It might be a servo tab on the elevator of a Cessna, or an anti-servo on the stabilator of a PA28. Whatever type of tab it is, it does the same thing. It sets the aircraft on a certain course and by staying on that course allows for a less tiring and probably more enjoyable flight.

I can drive a student mad with incessant “are you trimmed?” questioning. Every time they change power setting or configuration, they learn that I am about to ask “are you trimmed?” I then take control and if the nose goes up or down, I ask “then why’s the nose gone up? or down?”. I’m not actually that annoying and it is done with humour, but it gets the point across.

If you are at an early stage in your PPL, you should have started practicing trimming the aircraft. I start showing the use of trim on the very first lesson, if only to introduce the student to the idea of setting the aircraft on a stable course without having a constant input. I like to put the plane out of trim and challenge them to maintain a stable nose attitude for a while. Then let them try the same thing with a well trimmed plane. Recognising small changes in nose attitude can also be a challenge, but once these can be recognised, putting the nose back where it was is also a good exercise to start right from the beginning, as well as being fun.

The first thing that has to be done before attempting to trim the plane is to understand that you are trimming for a stable configuration. It can be straight and level, climbing or descending, base leg or final. When I say stable I mean that the aircraft is neither accelerating or decelerating while being trimmed. Being trimmed means that you can take your hands away from the control column and on a stable day the nose will stay where it is for a long time. On a turbulent day the trend is that the nose stays put.

Trimming for straight and level consists of making sure you have a stable nose attitude; if you are levelling off from a climb, choose your nose attitude, allow the airplane to accelerate then set the power you want. Flying with a light touch (try three fingers), release the control column and watch the nose attitude. If the nose goes up (which it will in this situation), put it back where it was as soon as you notice the change. Don’t look vacantly out the window while the nose attitude changes to the point the aircraft slows down and climbs. Trim the aircraft so that when you release the control column again, there is no change in nose attitude. In this example the nose went up when the control column was released. In order to maintain straight and level a forward pressure was needed. Forward trim of the wheel is needed until the nose stays where it is on its own. The same principle applies to trimming for any configuration of power or flap. Once you have the power setting required, a stable nose attitude and speed, then you can trim.

Let’s say you are going to start a climb at 80kts from straight and level at 95kts. In this instance you should apply full power first, then choose a sensible nose attitude for the climb, hold it and then trim. Until you are more experienced it is a good idea to do this without looking at the ASI. Once you feel trimmed, if you then look at the speed and find you are doing 85kts, simply raise the nose a touch more and re-trim. Or if you are too slow, lower the nose a touch and re-trim. In this way you should find the correct trim within two attempts.

A common error is trimming too soon or too late which leads to chasing the trim. Let me illustrate this by exaggerating what happens. Going back to levelling at 2,000 feet from a climb. The first thing you do is to lower the nose for the straight and level attitude, then almost immediately reduce the power and trim. You feel the control column and you think “Job done!” But you didn’t give the plane enough time to accelerate, so what is happening now is the plane continues accelerating and the RPM will also increase. You have trimmed at about 85-90kts, and that is the speed the plane wants to now fly at. So the nose attitude starts to rise, and you only notice this after maybe 100/200 feet. So you have to descend. During the descent you accelerate to say 105kts. You get to your 2,000ft, raise the nose, power seems fine and you trim. Job done! But now plane is going too fast, and as you trimmed at around 100kts, that is the speed it wants to do, so down goes the nose. It might be another 100/200 feet of descent before you notice it (don’t worry you WILL do this a couple of times until it ‘clicks’). Dohhhhh! So you need to climb again; up goes the nose, she slows down (unless you add a bit of power), you get to the correct height, nose to straight and level, power okay, trim. But the plane is still accelerating so... Okay, I have tortured you enough. I would not let a student get into this never-ending loop. It is interesting how it can happen to PPL holders after not flying with an instructor for a while. It is one of the ‘habits’ people tend towards and I have watched it happening for a good 10 to 15 minutes before pointing it out.

Another erroneous use of the trim wheel is to use it not for relieving control pressure but for applying it.

Trim should only be used following a change of pitch, power or configuration (gear or flap changes).

So learn to trim properly, understand what you are doing and why, and keep mindful of what you are doing, especially once you are out there on your own. Not trimming is like having a car that pulls to the left or right the whole time trimming is like a good cruise control. It relieves control column pressure and allows you to fly with a light touch. In turbulence a well trimmed plane means it is easier to notice updrafts and downdrafts. Trimming in turbulence is a matter of practice, and you will get there as long as you understand and can trim correctly on a smooth day.

Finally a word on electric trim. Personally I don’t like it. I have flown medium sized twins with all the trimmings (ha-ha), including electric trim and found I felt a lot better disconnecting it and using the trim wheel. If you do like the electric trimmer and use it regularly, make sure you know how to pull the circuit breaker in a hurry. Also, try to use it only for large adjustments in trim, and then use the wheel for finer changes. This is because unless you use the electric trim a lot and are very used to it, you really don’t know how fast it is trimming for you, and it is easy to over-trim, and end up chasing it again.

Adam Winter

Adam is a commercial pilot with more than 30 years' experience. His career has included bush flying in Africa and island hopping in the West indies, as well as a stint in the airlines. He is also a qualified physics teachers with three years classroom experience, that aged him 10 years, so he is back doing what he loves, teaching PPL.

Force Mass, weight, moment, pivot, torque, moment arm. These terms are all used in the quest for CoG. So let’s make sure we understand what they mean
and how they come together to allow us to calculate whether we will take-off.

To start with some definitions. Mass, (S.I. unit is the kilogram) is a measure of how much ‘stuff’ something has in it. A one kg mass has one kg of molecules in it. If you take it to the moon it still measures one kg, and on the sun, and anywhere else.

A Force is a push, or pull, or twist on an object that will cause it to change its motion. For weight and balance calculations we are concerned with the force
of gravity, which causes an object to accelerate downwards, at say 10 metres per second every second. The S.I. unit for force is the ‘newton’, and the definition
of a newton is the force required to accelerate a mass of one kg by one metre per second every second.

The Weight of an object is the result of gravity acting on it, and can be defined as the mass times the acceleration due to gravity.

Since weight is a force, its unit is also the newton. We often mix the terms mass and weight in aviation, but it really doesn’t matter, as long as the calculations are meant for Earth. On the moon your weight would be 1/6th of your Earth weight.

A Moment is another force, but this time it is a force applied then felt at a distance, and it causes a turning movement, a Torque. Imagine holding a plastic bag weighing one kilogram out horizontally on your 1 metre long arm. The bag is exerting a 10 newton (1kg x 10 metre/s/s acceleration) force downwards one metre away from your shoulder.

The torque force you are feeling in you shoulder is the ‘moment’, the result of the force exerted at a distance. The unit for moment is a newton metre, or pound inch or pound feet*.

The ‘moment arm’ is the distance between the shoulder and the bag. Now move the bag up to your elbow. The obvious result is less torque felt at your shoulder because now the moment arm is 0.5 metre away. The moment force felt at your shoulder is now 10 newtons x 0.5 metres, so 5 newton metres. Much less torque than holding it at arm’s length. 

CENTRE OF GRAVITY

The centre of gravity (CofG) of an object is the point where the total weight of a body can be thought to be concentrated. For an aircraft it is also the centre point
through which pitch, yaw and roll will occur.

When we do a CofG calculation, we are doing it because the manufacturers and engineers of the aircraft have guaranteed that it will perform well as long as the CofG is within the stated parameters. The forward and aft CofG limits affect the longitudinal stability of an aircraft.

As the CofG moves forward it becomes harder to raise the nose, which can cause serious problems at low landing speed if you can’t raise the nose enough, and can’t slow down.

With the CofG too far aft, the increased elevator sensitivity in pitch up can lead to an inadvertent stall. This is just the tip of the iceberg for problems caused by being out of the suggested range.

So now the terminology is defined and hopefully understood, it should be easy to grasp the weight and balance calculation.

WEIGHT AND BALANCE

Look at the basic weight and balance form:

The columns are for Weight in pounds (lbs), the Arm in inches (meaning the length of the moment arm), and Moment, (which is that force felt at the shoulder)
calculated by multiplying the weight and arm together.

You might notice that the arms, the position from which the distance is measured, are all aft of the datum. This is a  way of making the maths easier and less prone
to error.

By choosing an arbitrary point somewhere at the front of the aircraft, all the calculated moments will be clockwise, so they will all be added. If the datum was the CofG itself the calculation would be harder and easier to make an error as you would be adding all moments rear of the CofG and subtracting all the anticlockwise moments forward of the CofG.

On the Weight and Balance form, the first row is for the Basic Empty Weight.

Rows two and three are calculated for the pilot and passengers.

You can see the front seats are 80.5in aft and the rear passengers 118.1in aft.

The fuel tanks’ datum is between the two rows of seats and the baggage is quite far back.

To do the full calculation you have to fill in the weights and multiply them by the arms (as has been done for the pilot and passengers). When that is done, add all the weights together then check it does not exceed the maximum take off weight.

There is no need to add all the arms, but do add all the moments together. Leave that number in your calculator, because you then divide the total moment by the total weight.

You have just calculated the new position of the CofG.

You then take that weight and CofG position and plot it on the CofG Range graph from the aircraft manual (example far left).

This article is not intended to teach you how to do a weight and balance, but rather familiarise you with the terms and show you how it is done. If you have learnt from it, great.

Adam Winter

Adam is a commercial pilot with more than 30 years' experience. His career has included bush flying in Africa and island hopping in the West indies, as well as a stint in the airlines. He is also a qualified physics teachers with three years classroom experience, that aged him 10 years, so he is back doing what he loves, teaching PPL.

*the unit newton metre is not the same as the newton metre (joule) that refers to work done. A joule is force times distance where the force vector is in the same direction of movement of an object, whereas in moments the newton metre is a torque, not a straight line.

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