High lift devices are among the most critical aerodynamic tools on modern aircraft, directly influencing performance during the most demanding phases of flight: takeoff and landing. Their role becomes even more pronounced in emergency scenarios such as aborted takeoffs and forced landings, where the margin between a safe outcome and a catastrophe narrows dramatically. This article examines the engineering, operation, and strategic use of high lift devices—flaps, slats, slots, and leading-edge constructs—in emergency contexts, with a focus on how they enhance control, reduce stall speeds, and enable pilots to manage high-risk situations effectively.

Understanding High Lift Devices: Types and Functions

High lift devices are movable surfaces or fixed aerodynamic features that increase the maximum lift coefficient of a wing, allowing an aircraft to fly at lower speeds without stalling. Their design is a trade-off between lift augmentation and drag penalty, optimized for temporary use during takeoff, approach, and landing.

Flaps

Flaps are hinged surfaces on the trailing edge of the wing. When extended, they increase camber and sometimes wing area, boosting lift. Common types include plain, split, slotted, and Fowler flaps. The Fowler flap, widely used on commercial jets, slides rearward and downward, enlarging both chord and camber for maximum lift gain. In an emergency abort, flaps can be deployed to a lower setting (e.g., flaps 5 or 10) to increase lift for a go-around or to reduce stopping distance by enabling a slower, more controlled deceleration.

Slats and Leading-Edge Devices

Slats are movable panels on the leading edge of the wing that extend forward to create a slot. This slot channels high-energy air from below the wing over the upper surface, delaying boundary-layer separation and stall. Fixed slots perform a similar function but are non-moving. Krueger flaps, used on some Boeing aircraft, are hinged panels that extend from the lower leading edge. In emergency situations, slats are typically extended simultaneously with flaps (slats/flaps schedule), providing the necessary low-speed lift without inducing an abrupt stall characteristic.

Slotted Wings

A slotted wing incorporates fixed or movable slots to improve lift at high angles of attack. The slot accelerates the airflow over the wing’s top surface, raising the stall angle. This is particularly beneficial in emergency maneuvering where pilots may need to pull high angles of attack to avoid obstacles. The extra margin before aerodynamic stall is a safety buffer.

Aerodynamic Principles: Lift, Drag, and Stall Margins

The fundamental equation for lift is L = ½ ρ V² S CL. High lift devices directly increase the lift coefficient CL by altering the wing geometry. Flaps add camber, slats modify air flow, and both increase the wing area S. The result is a lower stall speed, which is critical during emergencies when the aircraft must remain controllable at low velocities.

However, increased lift comes with an increase in drag. The drag penalty is acceptable because high lift devices are used only during phases where low speed is mandatory. In a rejected takeoff (RTO), deploying flaps reduces the ground roll distance by allowing a higher average deceleration—the aircraft can brake harder and the aerodynamic drag adds to the stopping force. In a forced landing scenario, the reduced stall speed allows the pilot to fly a slower, steeper, and more precise approach over obstacles, minimizing the landing distance required.

Effect on Stall Speed

Stall speed varies proportionally to the square root of the wing loading divided by the maximum lift coefficient. Doubling CL on a typical transport wing can reduce stall speed by about 30%. For example, a clean wing might stall at 130 knots, while a fully configured wing with flaps and slats could stall at 90 knots. That 40-knot margin is often the difference between clearing a treeline and colliding with it. In emergency aborts, pilots can rotate at lower speeds, reducing the risk of overrunning the runway.

Emergency Abort Scenarios: Maximizing Control from the Ground Up

An aborted takeoff, or rejected takeoff (RTO), is one of the most time-critical emergencies. The decision to abort may stem from engine failure, bird strike, tire failure, or a warning indication. High lift devices are integral to the procedure.

Pre-Takeoff Configuration

For takeoff, flaps are typically set to a designated “takeoff flaps” position—typically 5°, 10°, or 15° depending on aircraft type. This setting shortens ground roll and improves climb capability. In the event of an abort after V1 (decision speed), the pilots initiate the RTO sequence, which includes deploying speed brakes, applying maximum wheel brakes, and ensuring the high lift devices remain in their takeoff setting. Deliberately leaving flaps and slats extended during an RTO provides additional aerodynamic drag (helping deceleration) and higher lift for rotation if needed, though at high speed the drag may be significant. The standard procedure is to keep the configuration unchanged until the aircraft stops.

Aborts at High Speed

Above V1 but still below VR (rotation speed), an abort is still possible but requires careful management. High lift devices already deployed keep the wing in a high-drag, high-lift state. The aircraft’s deceleration is aided by the induced drag from flaps. The FAA’s Airplane Flying Handbook emphasizes that the configuration should not be changed during an RTO unless directed by the aircraft’s manual, because retracting flaps can alter the pitch attitude and potentially reduce tire-to-ground friction. For example, if flaps are inadvertently retracted, lift decreases, causing the nosewheel to press harder on the runway and possibly overloading the nose gear.

Engine Failure After V1

If a critical engine fails after V1, the takeoff must continue. Here, high lift devices enable a climb with one engine inoperative. The takeoff flap setting provides enough lift to maintain a positive rate of climb on the remaining engine(s), often at a reduced speed (V2). The pilot will not retract flaps until the aircraft reaches an obstacle clearance altitude, ensuring that the climb gradient remains sufficient. In emergencies with multiple failures, the ability to retract flaps in stages allows the pilot to trade lift for speed or climb performance as needed.

Emergency Landing Scenarios: Slowing Down to Stay Alive

Forced landings due to engine failure, fuel exhaustion, weather, or system malfunctions require the pilot to maximize gliding distance and then achieve a controlled touchdown on a limited surface. High lift devices are the primary tool for managing kinetic energy and trajectory.

Engine-Out Glide and Configuration

Upon total engine failure, the first action is to establish a best glide speed (typically much higher than stall speed with flaps up). At this stage, high lift devices should remain retracted to minimize drag and maximize range. As the pilot selects a landing site and begins the approach, flaps and slats are deployed in stages to reduce speed and steepen the descent path. The goal is to arrive over the landing threshold with enough energy to safely flare, but not so much that the aircraft floats past a short field.

The landing distance required increases significantly with approach speed. The Aircraft Owners and Pilots Association (AOPA) notes that a 10% increase in approach speed can extend landing distance by 20%. By using full flaps, the pilot reduces the approach speed, thereby shrinking the landing footprint. On rough or unprepared surfaces, the slower touchdown reduces impact loads and helps maintain directional control.

Obstacle Clearance

In confined landing areas—fields, highways, small airstrips—obstacles such as trees, fences, or buildings may loom at the approach end. High lift devices allow the pilot to fly a steeper glide path without increasing speed, because the wing can operate at a higher angle of attack before stalling. Slats and slots are especially valuable here; they improve the maximum lift coefficient by about 40% compared to a clean wing, enabling a descent angle of 6–8 degrees rather than the standard 3 degrees. This “flak approach” may be the only way to clear a 50-foot obstacle and stop before a barrier.

Partial Power and Asymmetric Control

In an emergency where one engine is feathered and the other is at reduced thrust, high lift devices help manage asymmetric lift. Extending flaps on both wings (symmetrically) increases overall lift, but careful use of speed brake and rudder trims is essential. In some light twins, the manufacturer recommends using only takeoff flap setting for single-engine landings to maximize control authority. Leading-edge slats can improve aileron effectiveness at low speeds, countering the roll from the live engine.

Operational and Maintenance Considerations

The reliability of high lift devices is directly linked to the probability of a successful emergency outcome. Modern aircraft use redundant actuation systems—hydraulic, electric, or mechanical—to ensure deployment even after multiple failures. For example, the Boeing 787 uses electro-hydrostatic actuators that can operate independently. In the event of total hydraulic failure, some aircraft have manual reversion or emergency systems that allow pilots to extend flaps using an alternate mechanism (e.g., backup electric motor or hand crank).

Indications and Automation

Pilots rely on flap position indicators, slat asymmetry detection, and flight control computers that monitor the health of the system. In many aircraft, the auto-flap or autoslat function automatically deploys leading-edge devices at high angles of attack during a go-around or when approaching stall. In an emergency, these automations can ease pilot workload but must be understood so as not to misinterpret a system degradation.

Failure Modes and Risks

If a high lift device fails to deploy symmetrically, the aircraft experiences a roll moment and increased stall speed on the failed side. Procedures for asymmetric flap or slat failures involve cross-checking indicators and limiting bank angles. Some aircraft allow “flaps up” landings, which require higher approach speeds and longer distances—critical knowledge for anytime an emergency threatens normal system function. Regular maintenance checks, including lubrication, actuator cycling, and load testing, are mandated by aviation authorities to uphold the system’s integrity.

Pilot Training and Emergency Procedures

Simulator training exposes pilots to engine failures, aborts, and forced landings, requiring proper use of high lift devices. Emergency checklists (e.g., "Engine Failure After Takeoff," "Rejected Takeoff," "Forced Landing") explicitly state flap and slat settings. For instance, the Airbus A320’s “ENGINE FAILURE AFTER V1” procedure directs "THRUST LEVERS … TOGA, SRS, FLAPS RETAINED." This ensures the high lift configuration remains until a safe altitude. In smaller general aviation aircraft, the pilot must manually extend flaps before touchdown, often in stages, to avoid a sudden increase in drag that could cause an unwanted descent.

Key training points include:

  • Recognizing the impact of flap settings on stall speed and stopping distance.
  • Practicing rejected takeoffs with various flap configurations.
  • Executing emergency landings with and without high lift devices (simulating failure).
  • Understanding the aerodynamic effects of asymmetric flap/slat deployment.
  • Using external references such as FAA Advisory Circulars and aircraft-specific manuals.

An excellent resource for U.S. pilots is the FAA Regulatory & Guidance Library, which provides access to the Airplane Flying Handbook (FAA-H-8083-3C) and the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B), both containing detailed sections on high lift devices and emergency operations.

Real-World Examples and Case Studies

Several incident reports highlight the critical role of high lift devices. In 2009, an Airbus A320 experienced a dual engine failure after a bird strike and executed a successful forced landing in the Hudson River. The pilots used flaps to configure the aircraft for the ditching, achieving a low speed and a stable attitude. Similarly, in 1989, a DC-10 suffered a catastrophic engine failure and loss of hydraulic systems; the crew used differential thrust and limited flap extension to control the aircraft, eventually landing safely. The NTSB report noted that the proper use of the leading-edge slats and trailing-edge flaps was instrumental in maintaining aerodynamic control.

For a deeper dive, the NASA Technical Report Server offers papers on high lift system design and performance under extreme conditions, which can inform pilots and engineers alike.

Conclusion

High lift devices are far more than comfort features for slow flight—they are indispensable assets for emergency management. In abort scenarios, they reduce stopping distances and enable safe continued climb with reduced power. In forced landings, they lower stall speeds, steepen approach paths, and allow pilots to reach and stop within confined spaces. The interaction between aerodynamic design, system redundancy, maintenance discipline, and pilot skill ensures that these devices perform when needed most. Understanding and respecting the role of flaps, slats, and slots in emergency operations is a hallmark of professional aviation practice. As aircraft technology evolves, the principles remain: high lift devices convert aerodynamic potential into margin—the margin that saves lives.

Additional Reading: For those seeking a deeper technical understanding, Boeing’s Aero Magazine has published articles on flap system design and failure scenarios, and the AOPA’s Safety Center provides free courses on emergency procedures.