Every certificated aircraft relies on a set of primary flight controls—ailerons, elevators, and rudder—to govern pitch, roll, and yaw. But mastering flight in the real world, especially during takeoff and landing, demands more than these three surfaces can deliver alone. That is where secondary flight controls step in. Flaps, slats, and spoilers each manipulate the aerodynamic behavior of the wing in specific, purposeful ways, allowing aircraft to fly slowly and safely during approach while still cruising efficiently at altitude. For the Aviation Maintenance Technician (AMT) working on airframe systems, a thorough understanding of how these devices work, how they are constructed, and how they are inspected is not just exam knowledge—it is foundational to airworthiness.
This article covers all three major categories of secondary flight controls as addressed in FAA guidance, explaining the aerodynamic principles behind each, the mechanical and hydraulic systems that drive them, and the maintenance considerations every AMT must know.
Flaps: Increasing Lift and Drag on Demand
Flaps are movable surfaces attached to the trailing edge of the wing (and in some designs, the leading edge). Their primary function is to increase the camber and, in many designs, the chord and surface area of the wing, which raises the coefficient of lift at a given angle of attack. This allows the aircraft to maintain controlled flight at speeds well below the normal cruise airspeed—critical during approach and landing. At the same time, most flap configurations also increase drag, which helps the aircraft descend steeply without building excess airspeed.
Types of Flaps
- Plain flap: The simplest design. The trailing edge of the wing hinges downward, increasing camber. It adds both lift and drag, though it is less aerodynamically efficient than other designs.
- Split flap: Only the lower surface of the trailing edge deflects downward. This configuration produces lift similar to or slightly greater than a plain flap at a given deflection, but generates significantly more drag, giving it a much higher drag-to-lift ratio—making it useful where steep approaches are needed.
- Slotted flap: A gap (slot) between the flap and the main wing allows high-energy air from the lower wing surface to energize the boundary layer on top of the flap, delaying separation and significantly improving lift. This is the most common design on modern light aircraft and many transport-category aircraft.
- Fowler flap: Moves rearward on tracks before rotating downward, simultaneously increasing wing chord (surface area) and camber. This produces a large lift increase with relatively modest drag at partial deflection, making Fowler flaps ideal for transport-category aircraft where gentle takeoff configurations are needed. At full deflection, drag increases substantially to aid landing.
- Double-slotted and triple-slotted flaps: Found on large transport aircraft, these combine multiple Fowler-style panels with multiple slots, achieving very high lift coefficients for low-speed operation on heavy airframes.
Flap Actuation Systems
On light aircraft, flaps are commonly actuated by a simple mechanical linkage operated by a cockpit lever, or by an electric motor driving a jackscrew or torque tube. Transport-category aircraft use hydraulic actuators driven by the aircraft's main hydraulic system, with mechanical or electrical position-sensing feedback to ensure symmetrical deployment. Asymmetric flap extension—one wing's flaps extending while the other's do not—creates a dangerous rolling moment. For this reason, most modern systems include asymmetry detection circuits that halt flap movement and alert the crew if the two sides diverge beyond a set limit.
The AMT must verify that flap tracks and rollers are lubricated and free of corrosion, that jackscrew threads show no signs of wear or contamination, that torque tube connections are secure, and that limit switches are properly calibrated to stop flap travel at the correct deflection angles specified in the manufacturer's maintenance manual.
Slats: Energizing the Leading Edge
While flaps act on the trailing edge, slats address lift augmentation at the leading edge of the wing. A slat is a small, curved airfoil section that deploys forward and downward from the wing's leading edge, opening a slot between itself and the main wing surface. This slot directs high-energy airflow from beneath the wing up and over the leading edge, re-energizing the boundary layer and allowing the wing to reach a much higher angle of attack before stalling.
The practical effect is a significant reduction in stall speed. Because the wing can now generate sufficient lift at a higher angle of attack, the aircraft can fly more slowly—valuable during approach, especially for heavily loaded aircraft. Slats are often paired with large Fowler flaps on transport aircraft: together they provide the very high lift coefficients needed to keep approach speeds manageable even when the aircraft is near maximum landing weight.
Fixed vs. Automatic vs. Powered Slats
- Fixed slats: Permanently extended from the leading edge. No actuation system required, but a small aerodynamic penalty exists at cruise. Common on some light sport and training designs.
- Automatic (aerodynamic) slats: Held flush against the wing by aerodynamic pressure at normal speeds. As angle of attack increases and leading-edge pressure changes, the slat is pushed forward automatically—no pilot input needed. The system is passive and mechanically simple.
- Powered slats: Driven by hydraulic or electric actuators and controlled by the flight crew, often interconnected with the flap system so both deploy together through a single lever. This is the standard on commercial transport aircraft.
Maintenance inspection of slats focuses on the condition of the slat tracks and carriages, the integrity of actuation rods or hydraulic lines, and freedom of movement through the full range of travel. Seals and gap geometry must be verified to ensure the slot produces the intended aerodynamic effect; a deformed or misaligned slat can produce asymmetric stall characteristics that compromise safety.
Spoilers: Dumping Lift and Controlling Roll
Spoilers are panels on the upper wing surface that, when raised, disrupt (spoil) the smooth airflow over the wing, rapidly reducing lift and simultaneously increasing drag. Their versatility makes them useful in several distinct operational modes.
Roles of Spoilers
- Ground spoilers (speed brakes on ground): On landing rollout, all spoiler panels extend fully to destroy lift, transfer aircraft weight onto the wheels, and dramatically improve braking effectiveness. Because aerodynamic lift is eliminated, the tires grip the runway surface with the full weight of the aircraft. Ground spoilers are automatically armed and deploy on touchdown (confirmed by weight-on-wheels sensors), though the AMT must verify that automatic deployment logic and squat switch circuits function correctly.
- Flight spoilers (in-flight speed brakes): Selected by the pilot in flight to increase drag and steepen descent without increasing airspeed. Useful during rapid descents from cruise altitude or when the aircraft must lose altitude quickly on an instrument approach. Flight spoilers typically extend symmetrically on both wings to avoid inducing roll.
- Roll control spoilers: On many large aircraft, spoilers augment or even replace ailerons for roll control, especially at high speeds. At these speeds, large aileron deflections can twist an aeroelastic wing enough to reduce or reverse the intended rolling moment (aileron reversal); using spoilers for roll control at high speed helps avoid inducing this wing twist in the first place, rather than correcting it after the fact. The spoiler on the down-going wing rises to reduce lift on that side, dropping the wing and initiating the turn. This is controlled through the same aileron input from the pilot, with a mixing unit proportioning spoiler and aileron deflection depending on airspeed and configuration.
Spoiler Actuation and Maintenance
Spoilers on transport aircraft are actuated by hydraulic actuators, with each panel often powered by a separate hydraulic system to provide redundancy. The AMT must check that spoiler panels seat flush against the wing upper surface when retracted—any gap or protrusion causes parasitic drag and can affect handling. Hinge fittings must be inspected for cracks and corrosion, and actuator attachment points checked for security. The weight-on-wheels (squat switch) system that governs automatic ground spoiler deployment must be rigorously tested during scheduled maintenance; a failed squat switch that causes inadvertent in-flight ground spoiler deployment has been a factor in serious accidents.
Why Secondary Controls Matter to the AMT
Secondary flight controls directly affect the aircraft's stall speed, landing distance, and roll authority—all airworthiness-critical characteristics. A flap that binds, a slat that deploys asymmetrically, or a spoiler that fails to extend on landing can each lead to a loss-of-control accident. The AMT's responsibility is to ensure these systems operate within the tolerances specified in the manufacturer's maintenance manual and the limitations published in the Type Certificate Data Sheet (TCDS).
After any maintenance on secondary flight controls, a functional check is required: surfaces must be cycled through their full range of travel, rigging must be verified against approved data, and limit switches and asymmetry detectors must be tested. The work must be documented in the maintenance record as required by 14 CFR Part 43.
Key Numbers and Rules
- Flap deflection angles are specific to each aircraft model and are published in the AFM/POH and TCDS; the AMT must rig to these exact values.
- Asymmetry protection systems must detect and halt flap travel before the difference between left and right panels exceeds the manufacturer's stated limit (varies by aircraft type).
- Ground spoilers must be confirmed armed (weight-on-wheels squat switch tested) before return to service after any landing gear or spoiler system maintenance.
- All secondary flight control work is subject to 14 CFR Part 43 maintenance recording requirements, and major alterations require FAA Form 337 or equivalent approved data.
- Fowler flap tracks must be inspected for wear per manufacturer intervals; excessive play can cause the flap to misalign and change the aerodynamic gap geometry critically.
Common Test Traps
- Confusing lift vs. drag effects: Partial flap deflection primarily increases lift with modest drag; full deflection dramatically increases drag. AMT examinees must know that the drag penalty at full flap is intentional and required for steep approach angles.
- Slat vs. flap function: Slats reduce stall speed by allowing a higher angle of attack; they do not primarily increase cruise lift. Flaps increase camber and area to generate more lift at the same angle of attack. These are different mechanisms and different benefits.
- Spoiler symmetry: Asymmetric spoiler deployment (one wing up, one wing not) causes roll. Examiners may present scenarios where a stuck or failed spoiler panel produces unexpected rolling. Understand which mode (roll control vs. speed brake) uses symmetric vs. asymmetric actuation.
- Ground spoiler arming: Students sometimes assume spoilers deploy automatically without pre-arming. In practice, the crew must arm the system (or the system auto-arms on final), and the squat switch triggers deployment. An un-armed system will not deploy, significantly increasing landing distance.
- Maintenance authority: Secondary flight control rigging is a maintenance task requiring an appropriately rated AMT or repairman; it is not among the specific items pilots are authorized to perform as preventive maintenance under 14 CFR Part 43, Appendix A, paragraph (c).