Learning hub · Aircraft General Knowledge · Lesson 2

Airframe, flight controls, flaps and trim

Your hands and feet move three control surfaces and a few levers, and the whole aeroplane answers. Knowing what each surface does, and what it does as a side effect, makes the first lessons much easier.

About 12 minutes, plus the quiz. Part of Aircraft General Knowledge. It builds on Lift, angle of attack and the stall.

1. The airframe

The airframe is everything except the engine and the systems: the fuselage, the wings, the tail (the empennage) and the landing gear.

In a modern light aeroplane the fuselage is usually semi-monocoque: a thin skin stiffened by frames and stringers, so the skin carries part of the load. Older designs use a welded tube truss covered in fabric.
The wing’s main strength is in one or more spars running along the span. Ribs give the wing its shape.
Light aeroplanes are mostly aluminium alloy. Composite materials (glass or carbon fibre) are common in newer types.

The structure is designed for a maximum load factor, a multiple of normal gravity. You will meet it again in drag, stability and turning flight.

2. The primary controls

Three surfaces move the aeroplane about its three axes.

Surface	Moved by	Movement	Axis
Ailerons	Control column or stick, left and right	Roll	Longitudinal
Elevator	Control column or stick, back and forward	Pitch	Lateral
Rudder	Rudder pedals	Yaw	Vertical

The ailerons move in opposite directions: roll to the right and the right aileron goes up while the left goes down. Pulling back raises the elevator trailing edge, the tail is pushed down and the nose rises.

Each control works through the airflow over it, so it feels lighter and less effective at low speed and firmer at high speed. Use the controls together and with the horizon in view, not one at a time.

3. Secondary effects of controls

Every primary control causes a second, unwanted movement. Learn them, because you will be correcting them from your first lesson.

Ailerons cause yaw. The rising wing makes more drag and pulls the nose the wrong way. This is adverse yaw, covered in the next lesson on turning flight.
Rudder causes roll. Yawing to the left makes the right wing travel faster and the left slower, so the aeroplane also rolls to the left. A sustained yaw makes a turn.
Power affects pitch and yaw. A change of power usually changes the pitching moment and the slipstream effect on the tail, and changes the amount of yaw. The size is different for each type.

4. Flaps and trim

Flaps are secondary controls on the trailing edge of the wing. Lowering them increases the camber of the wing, so it makes more lift at a lower speed and the stalling speed is lower. They also add drag, which lets you descend more steeply without gaining speed, and they give a better view over the nose on approach. The pitching effect when you lower them differs by type, so learn your own aeroplane’s behaviour.

Flaps must not be extended above the maximum flap-extended speed (Vfe), because the structure of the flap and its mounting is only designed for lower airspeed. Reduce speed first, then select flap, using the stages in the flight manual.

Trim removes the control force you would otherwise hold. A small tab on the elevator moves the opposite way to the elevator itself, so the airflow holds the elevator in the position you choose. The aeroplane then flies at a speed that matches the trim setting. Fly the attitude with the control column first, then trim away the force. Trim does not move the aeroplane, it takes away your effort.

Some aeroplanes have an all-moving tailplane (a stabilator) with an anti-servo tab, which moves in the same direction as the trailing edge of the tailplane. It makes the control feel heavier and helps stability, and on many such types the same tab is also used for trim. Some elevators have mass balance weights ahead of the hinge to prevent flutter.

5. Locks and pre-flight control checks

A control lock (gust lock) clamps the controls on the ground so wind cannot slam the surfaces against their stops. It is a red-flagged pin, a clamp or a pedal lock. Remove it during the external check and stow it where you will see it. Starting with a lock in place is a classic cause of accidents.

Pre-flight, you check the controls twice: externally, looking at the hinges, the pins, the split pins and the surface for damage, and from the seat. In the cockpit move each control to its full travel and check that it is free, that it has the correct sense (the right aileron goes up for right stick, the elevator goes down for forward stick) and that the flaps and trim work in both directions. Do it after any maintenance too, as a wrong sense is a known maintenance error.

Training material only. The flight manual and your school’s checklist define the check for your aeroplane and the flap and speed limits. If a control is stiff, loose, or moves the wrong way, do not fly.

6. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

You move the control column to the right and slightly back. Describe what every moving surface does, and name the two axes about which the aeroplane moves.
Show the answer
1. Right movement: the right aileron goes up and the left aileron goes down. The aeroplane rolls to the right about the longitudinal axis.
2. Back movement: the elevator trailing edge goes up, the tail goes down and the nose goes up. This is pitch about the lateral axis.
3. The rudder does not move, but you may need it to deal with adverse yaw.
In an invented aeroplane the maximum flap-extended speed is 85 kt. You are on the downwind leg at 100 kt and about to select the first stage of flap. What do you do and why?
Show the answer
1. 100 kt is above 85 kt, so selecting flap now would exceed Vfe and could damage the flap or its mounting.
2. Reduce power and let the speed fall to below the Vfe, or to the speed the flight manual gives for the stage you want.
3. Then select the flap, in the stages the manual allows, and trim for the new speed.
You trimmed for 80 kt in level flight. You reduce power and hold the nose level while the speed falls to 65 kt. Which way do you feel the stick force? What do you do about it?
Show the answer
1. The aeroplane is trimmed for 80 kt, so at a lower speed the nose wants to drop back to that speed.
2. To hold the nose level at 65 kt you need to pull back, so you feel a steady pull force.
3. Trim nose-up until the force is gone. The trim now matches the lower speed.

7. Watch

A walk-through of the main control surfaces of an aeroplane and what each one does.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

A short explanation of trim and why you trim after you set the attitude. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB.

8. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

Next: Fuel, oil, electrics and the propeller. Back to the learning hub.

For training and situational awareness only. Not certified for navigation. Always use official charts, the AIP, NOTAMs and a proper weather briefing for real flights.