Aircraft Secondary Flight Controls: Flaps, Slats, and Trim Systems

021-05-02Pablo Asensio Martínez2026-03-272 min

Secondary flight controls include high-lift devices (flaps and slats) and trim systems. These devices allow the aircraft to change its aerodynamic configuration for different phases of flight, such as takeoff, landing, and efficient cruising.

Flaps and Slats

Flaps and slats are used to increase lift and drag, allowing flight at lower speeds.

  • Types:
    • Light aircraft often use plain or camber flaps.
    • Large transport aircraft typically use slotted Fowler flaps.
  • Control & Operation:
    • A cockpit selector signals control computers, which drive a hydro-mechanical unit or hydraulic motor. This motor drives a transmission system to move the surfaces.
    • Emergency Operation: If the main hydraulic system fails, emergency extension is often achieved via an electric motor driving the same gear train, or an alternate hydraulic system. Pneumatic systems are not used for flaps as they lack precise control.

Protection Systems

  • Flap Load Relief (Load Limiter):

    • Purpose: Protects flaps from structural damage due to excessive air loads.
    • Function: If airspeed exceeds the maximum flap extended speed ($V_{FE}$) for a specific setting, the system automatically retracts the flaps to a lower or intermediate setting. Once speed decreases, they may extend again.
    • Mechanism: Some systems use "blowback" (airflow pushing flaps up), while others use hydraulic pressure to physically move them.

  • Asymmetry Protection:

    • Risk: Asymmetric flap deployment (one side extending more than the other) creates a dangerous rolling moment due to uneven lift and drag.
    • Function: Sensors compare flap positions on both wings with great accuracy. If an asymmetry is detected, the system stops the flaps from moving immediately and alerts the pilots.
    • Important: It does NOT attempt to automatically correct the asymmetry (e.g., by extending/retracting one side) as this could aggravate the situation if a surface is jammed.

Trim Systems

Trim systems reduce pilot workload by neutralizing control forces.

  • Trimmable Horizontal Stabilizer (THS):

    • A fully moving horizontal tail surface usually moved by a hydraulically actuated jack (screw jack).
    • It is controlled by trim switches, not directly by the control column.

  • Trim Runaway Protection:

    • Definition: Uncommanded movement of the trim, which can lead to loss of control.
    • Safety Feature: Most electrical trim systems require two toggle switches to be operated simultaneously. One switch releases the trim brake, and the other powers the motor. This prevents runaway if a single switch malfunctions or shorts.