The three aircraft axes

An aircraft moves about three axes intersecting at the centre of gravity (CG):

Axis	Position	Rotation
Longitudinal axis	nose-tail	Roll (bank) — primarily by the ailerons
Lateral axis	wingtip to wingtip	Pitch — by the elevator
Vertical axis	perpendicular to earth	Yaw — by the rudder

Primary controls

The three primary controls are ailerons, elevator, and rudder:

Surface	Axis	Motion
Ailerons	longitudinal	Roll (bank)
Elevator	lateral	Pitch
Rudder	vertical	Yaw

Primary and secondary control effects

Each control has a primary effect (intended) and may produce a secondary effect on other axes:

Rudder input

Primary effect on right rudder: yaw to the right — the nose turns right.
Secondary effect: roll to the right — through the yaw rotation the left wing flies faster, generates more lift, and the aircraft rolls into the turn (right).

Aileron input

Primary effect: bank in the input direction.
Secondary effect: adverse yaw — yaw in the opposite direction (from differential induced drag).

Elevator input

Yoke back (pull): tail surface produces an increased downward force → the tail is pushed down → the nose rises (pitch up).
Yoke forward (push): the nose drops, speed increases and sink rate increases — the aircraft accelerates in descent.

Secondary controls

Purpose: improve the aircraft's performance characteristics AND relieve the pilot of continuous control forces (through trimming).

Element	Purpose
Flaps	Increase CLmax (more lift at lower speed), increase drag (steeper approach)
Trim systems	Relieve the pilot of continuous control pressure — via tab, spring, or whole stabiliser (e.g. PA-28)
Spoilers / speed brakes	Reduce lift / increase drag for faster sink; also after landing; rare on PPL trainers
Leading-edge devices (slats)	Delay stall by energising the boundary layer; rare on PPL trainers

Flap types

Type	Mechanism	Lift gain
Plain	trailing edge hinges down	low
Split	lower half hinges — high drag	medium
Slotted	gap between wing and flap energises upper surface	high
Fowler	extends chord (more area) AND hinges; slotted	very high — best CLmax/drag trade

Tabs — special cases

Trim tab (classic)

A small auxiliary surface that the pilot adjusts from the cockpit to hold the control in a specific position — no continuous pressure needed.

Balance tab

A balance tab is an auxiliary surface mechanically coupled to the primary control and moves in the opposite direction to it:

Pilot trims nose up (yoke back) → elevator moves up → the balance tab moves down (opposite).
Result: the aerodynamic force on the balance tab acts in the same sense as the pilot input → the control force the pilot must apply decreases.

→ Balance tabs are an elegant mechanical solution to reduce control forces without electric servos.

Ground adjustable trim tab

A ground adjustable trim tab is a non-movable metal tab on a control surface that can only be adjusted on the ground with a tool (often on the rudder or aileron):

Not adjustable in flight — the pilot cannot operate it from the cockpit.
On the ground, maintenance personnel bend it into a specific position to optimise the in-flight characteristics of the aircraft (e.g. eliminate continuous rudder pressure in normal cruise).
Typical on light aircraft (C152, PA-28) as a cost-effective alternative to cockpit-adjustable trim.

Control balance

Mass balance — weight forward of the hinge line, prevents flutter (aerodynamic oscillation).
Aerodynamic balance — horn (forward of hinge), offset hinge, or balance tab — reduces the control force required from the pilot.

Control transmission — typical by weight class

Transmission of control inputs from the cockpit to the control surfaces varies by aircraft size:

Aircraft mass	Control transmission
Up to ~2 t (PPL trainers, light aircraft)	Mechanical — via control cables or push-pull rods. Direct mechanical link from yoke/pedal to control surface.
2-15 t	Usually still mechanical, sometimes with hydraulic assistance (servo tabs, boosted systems).
Over 15 t	Hydraulic (servo) or fly-by-wire (electronic).

→ Standard for PPL trainers (Cessna 172, PA-28, DA40, up to 2 t): mechanical cable or push-pull rod transmission.

Control lock / rudder lock when parking

When an aircraft is parked outside (not in a hangar), the control column / rudder is locked ("rudder lock" or "control lock" engaged) to prevent wind from displacing and damaging the control surfaces:

A mechanical lock (plastic or metal bracket) on the yoke or stick locks the elevator and ailerons.
Some aircraft have a control-wheel lock pin on the yoke itself.
The pilot removes the lock before flight as part of pre-flight inspection — a forgotten lock is a known accident cause (pilot attempts take-off, elevator blocked → cannot lift off or land safely).