primary flight controls

These consist of the flight control surfaces and the engine power management (unless you are flying a glider that is!).

control activation

In light aircraft the control surfaces mentioned above are moved by the power of the pilot's muscles. Each control surface is connected directly to the control column or rudder pedals with a series of cables and pulleys or rods. In such a control system the control column can move the control surface, but the control surface can also move the column. This is called a reversible control.

In large aircraft the pilot requires assistance in moving the controls. Assistance may be provided by electrical motors or hydraulic jacks. When such a system is employed the controls become irreversible.

simple controls



A simple basic control system as operated by a pilot

A simple mechanical cable operated system as you will find on aircraft such as the Cessna C152. The cables in some aircraft are replaced by rods. The control column can be moved by raising and lowering the elevator

boosted control system

This is a simplified boost system. When the pilot moves the column, tension in the cable or rod also opens a valve letting pressure from a hydraulic or pneumatic pump expand a slave cylinder that assists moving the control. The control column can be moved by raising and lowering the elevator but a considerably more force is required.

fly-by-wire system

There is no mechanical connection between the control column and the flight surface in a 'fly-by-wire' system. There is a sensor on the control column which transmits the column's position to an actuator. The actuator then moves the control surface to a position which matches the column's deflection. This system is not reversible. (The control column will not be moved by moving the control surface). The system is much lighter than the boosted system and is used on all large aircraft nowadays.

Tab Control Systems

In the early days of large aircraft many designers avoided the need to provide boosted controls, as described above, by using tab activated controls. The DC-9 for example uses tab actuated controls. In a tab controlled system the pilot moves only a small actuating tab on the larger control surface. The force generated by the tab then moves the main control. This is of course the same way trim tabs work. Therefore, you can think of this system as being like trim tabs if they were connected to the control wheel instead of a separate control wheel. Note that in a tab controlled system there is no direct connection between the control column and the control surface.

servo and anti-servo tabs

Another way of changing the amount of force the pilot must apply to the control column is through servo and anti-servo tabs. In this system the control column is directly connected to the control surface (just like a C-172) but a tab is geared to the movement of the control surface so that it either assists the movement of the control, or counters the movement of the control. Thus, the controls can be made to feel heavier or lighter than they would otherwise.

servo tab

anti servo tab

Control Horns 

On an aircraft with reversible controls the pilot must apply a force to the control column sufficient to keep the the control surface deflected in the air stream. To assist the pilot the aircraft design will usually provide a control horn, such as the one shown to the right. The horn is simply an extension to the control surface which projects ahead of the hinge. The air striking the horn assists the pilot to deflect the control surface. Horns are usually provided on elevators and rudders. Ailerons usually do not require horns.


Mass Balance

When a control is deflected a low pressure area forms on the cambered side. This tends to pull the control back into alignment with the wing, stabilizer or fin as the case may be. However, the control surface has mass and therefore momentum. If the centre of gravity of the control surface is behind the hinge, the control tends to overshoot the point of alignment. The result is a tendency for the control to flutter. Flutter could become sufficiently severe that the aircraft could break up in flight.

To solve the above problem the control must be balanced, so that its centre of gravity is in line with the hinge.

click here to see flutter movie

The exact distribution of weight on a control surface is very important. For this reason, when a control surface is repainted, repaired or component parts replaced, it is essential to check for proper balance and have it rebalanced if necessary. To do this, the control surface is removed, placed in a jig and the position of the centre of gravity checked against the manufacturer's specifications. Without any airflow over the control surface, it must balance about its specified C.G. This is known as static balance. For example, the aileron of the Bonanza is designed for a static nose heavy balance of 0.2 inch pounds. The C.G. of the aileron is forward of the hinge centreline causing the control surface to be nose heavy.

Bob weights

Bob weights are sometimes known as counter weights. Their purpose is to change the amount of control force required to deflect the control column under different g-loadings. Normally the amount of force the pilot must apply to the control column, assuming reversible controls, varies with airspeed only. However, by installing a bob weight the aeronautical engineer can make it more difficult to pull on the control column as g-force increases. The purpose of the bob weight is to reduce the likely hood the pilot will overstress the aircraft.


Differential Ailerons

Differential ailerons are designed so that the up-going aileron rises a greater angle than the down going aileron.

When we use ailerons we want the ship to roll only on its longitudinal axis. The problem is that to raise a wing the aileron increases lift on that wing with the resultant increase in drag. At the same time there usually is a decreased lift on the opposite wing with a decrease in drag.

The descending wing has less drag and moves forward while the rising wing has more drag and moves backwards. This produces a tendency to yaw (turn) in the wrong direction or into the rising wing and away from the intended turn direction. This usually results in a nose high slip with the fuselage side presented to the relative wind with high drag. This is called "adverse yaw" and is fine if you need to lose altitude with lateral fuselage drag as in a landing approach, but bad for beginning a coordinated turn with the fuselage parallel to the relative wind.

To compensate for this problem and make flying easier, aircraft are usually designed with one or a combination of a number of methods to decrease adverse yaw.

The common fix for adverse yaw is to mechanically produce differential aileron movement so that there is more up travel than down. In other planes the aileron is hinged towards the top of the wing/aileron joint so that a portion of the leading edge of the aileron sticks down into the slipstream creating drag when the wing is descending to balance the resultant drag from the rising wing.

Frieze Ailerons

Frieze ailerons are designed so that when the aileron is deflected upward a lip extend down into the air stream. As a result the up-going aileron produces more drag than the down going aileron. This helps provide some of the force required to start the aircraft yawing in the desired direction of the turn. Usually this is enough force to provide the required turn moment but not enough to overcome aileron drag. Therefore, the pilot will still need to use the rudder to coordinate the turn when large aileron deflections are employed.

engine management

On simple aircraft on which student pilots usually begin, there are two controls. The throttle, which is always black in colour, and the fuel mixture control, which is always red. Vernier 'push pull' controls such as in the illustration below appear to be popular with Cessna. More power is applied if the vernier is pushed in. Fuel is shut off to the engine if the mixture control is pulled out. As simple aircraft have fixed pitch propellers, increase in throttle will increase the RPM of the propeller. For more information about propellers, see the propeller section.

typical Cessna 152 engine management controls

Other aircraft are fitted with quadrant lever controls, (Piper for example). Power is applied by pushing the lever FORWARD. The fuel supple to the engine is cut off by pulling the lever back.

typical quadrant engine management controls


This is operated by the red control. At close to sea level altitudes, the mixture shown be maintained in the fully rich position. This should always also be the case when power setting are used that are more than 75% available, the the fuel contributes to the cooling of the engine. Once cruising altitude is reached, the mixture can be leaned. This results in considerable fuel savings. As the mixture is leaned, the exhaust gas temperature increases. The gauge is marked with a red line, and leaning should never exceed this. If an exhaust gas temperature gauge is not fitted the mixture can be adjusted by observation. As the mixture is leaned, there will be a small increase in engine RPM, or power in the case of a complex aircraft. Continued leaning will result in a decrease in RPM. The mixture should then be enriched until it is just on the rich side of maximum RPM.

At airfields of high altitude, the mixture must be leaned to produce full power on the ground, prior to take-off.

carburettor heat

While carburettor heat is not a primary instrument, it is useful to include it in this chapter. Incredibly many aircraft engines are still fitted with carburettors, despite the fact that fuel injection has been generally available since half way through the last century. These archaic devices have a nasty habit of freezing up and preventing the fuel mixture from entering the engine, which begins to run a tad rough and then stops, sometimes with fatal results. This will happen when operating close to the dew point, and also when the engine is running under reduced power, such as during a descent. It is up to the pilot to periodically operate the carburettor (carb) heat control, which is usually to the right of the throttle and mixture controls. Carb heat control diverts hot air into the carburettor from a sleeve around the exhaust system and which will melt the offending ice. The engine must ingest the resulting water, so sometimes it will run rougher than before for a short period. Carburettors can suffer from a variety of different icing and this will be looked at later (see aircraft technical 'induction system').  The carb. heat should always be applied during descent. The power to the engine is reduced when carb heat is applied so it t is good practice to remove it just before touchdown (say 100 feet) so that full power is instantly available should a go around become necessary.


Aircraft which are fitted with a constant speed propeller have a third system which manages the RPM of the propeller. This is always blue in colour. The throttle in this case just increases available power  and RPM is controlled by the propeller control. There is an order in which these controls should be used to avoid a possible overspeed of propeller.

increase RPM

Blue first Black second

decrease RPM

Black first Blue second.

Students almost always begin flying in simple aircraft. Further training is required to operate complex types such as the Piper Arrow or Mooney series.