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

> Secondary control operation, flap load-relief and asymmetry protection logic, and trim architecture including runaway safeguards.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-05 — Flight Controls
- **Topic code:** 021-05-02
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/flight-controls/aeroplane-secondary-flight-controls/

---

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.

---

Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.