Primary controls

Primary Flight Controls

The primary controls are the three main control surfaces of an aircraft that rotate it about its three axes.

The three axes

1. Ailerons

Function

Move opposite on left and right wings.

• Left up, right down → more lift right, less lift left → roll left.
• Yoke action: yoke wheel left → roll left.

Position

• At the trailing edge of the outer wing — as far outboard as possible for maximum moment arm.
• Typical length: 25-40 % of half-span.

Secondary effect

• Adverse yaw: see separate lesson "Combating Adverse Yaw".

2. Elevator

Function

Symmetrically moveable on the horizontal stabilizer.

• Pull yoke back → elevator up → down-lift at stab → tail drops → nose rises (pitch-up).
• Push yoke forward → elevator down → pitch-down.

Position

• Conventional tail: top rear of stab.
• T-tail: elevator at top of vertical fin (e.g. PA-28 vs DA-40).
• All-moving stabilator: whole stab rotates → more effective but more sensitive.

Secondary effect

• Speed change: pitch-up → α rises → drag rises → speed drops.
• In turn: pitch-up amplifies bank (auto-drift).

3. Rudder

Function

Vertical control surface at the tail.

• Push left pedal → rudder left → side force right at tail → yaw left.
• Pilot operates with feet (rudder pedals).

Position

• On the vertical fin as moveable trailing piece.

Secondary effect

• Roll effect via dihedral (yaw left → sideslip → roll left via dihedral).
• Adverse yaw of ailerons can be compensated by rudder.

Coordinated flight

Coordinated flight uses all three controls at once — the ball in the slip-skid indicator centred.

Example left turn:

1. Yoke left (aileron) → roll start.
2. Left pedal (rudder) — compensates adverse yaw, keeps nose in heading.
3. Yoke slightly back (elevator) — maintains altitude despite bank.

Yoke / Joystick / Stick

Versions:

• Yoke: two-handed, in most GA trainers (C172, PA-28).
• Joystick (side stick): one-handed, in high-performance and newer aircraft (DA-40 centre, Cirrus side, Airbus airliners).
• Stick (centre): in aerobatic and glider aircraft.

Control effectiveness

• Scales with velocity squared (dynamic pressure).
• At low speed: controls "mushy", less effect.
• At high speed: controls "tight" but more sensitive.
• Maximum at Vne: control-load limit (Va concept, see lesson "Va").

Control travel

POH-specific:

• Aileron: typically ±20-30° per side.
• Elevator: typically ±20-25°.
• Rudder: typically ±20-30°.

CS-23 requirements

CS-23 Subpart B requires for every angle-of-attack range:

• Sufficient control effectiveness (demonstrated in certification testing).
• No reversal tendencies (controls must not act backwards at extremes).
• Force levels within accepted ranges.

Cable / push-rod / fly-by-wire

• Cable: standard in most GA aircraft.
• Push-rod: some modern designs for more direct action.
• Fly-by-wire: in airliners and newer fighters — electronic signals replace mechanical linkage.