Understanding High-Lift Devices: Flaps and Slats

The wings of a transport category aircraft are a marvel of compromise. Engineers design them to be highly efficient for the cruise phase, where the aircraft spends most of its airborne time. However, a wing optimized for Mach 0.78 cruise performs poorly at the low speeds required for takeoff and landing. This is where high-lift devices—flaps and slats—become indispensable.

Flaps and slats are movable surfaces that temporarily alter the wing’s aerodynamic profile. Slats deploy from the leading edge of the wing. Their primary function is to manage the airflow over the top of the wing, delaying airflow separation and the resulting stall at high angles of attack. Flaps extend from the trailing edge, increasing the wing’s camber (curvature) and surface area. Together, they allow an aircraft to generate significantly more lift at a given speed, or fly at a much slower speed without stalling.

This operational flexibility comes at a cost: drag. The precise management of the trade-off between increased lift and induced drag dictates how pilots configure the aircraft during takeoff, climb, cruise, descent, and landing.

The Aerodynamics of Lift Augmentation and Drag Penalties

To understand how pilots choose specific configurations, one must first grasp the aerodynamic principles at play. The total lift generated by a wing is governed by the equation:

L = CL × ½ ρ V² S

Where CL is the coefficient of lift, ρ is air density, V is velocity, and S is wing area. Flaps and slats primarily increase the maximum possible coefficient of lift (CL_max) and the effective wing area (S).

  • Increased Camber: Deflecting flaps downward increases the camber of the wing. The airflow traveling over the curved upper surface accelerates more sharply, creating a stronger pressure differential between the top and bottom of the wing. This directly increases lift.
  • Increased Surface Area: Fowler flaps (common on large aircraft) extend backward as they deflect, physically enlarging the wing’s surface area. This provides an additional lift boost, as the lift formula directly includes area (S).
  • Delayed Stall: Slats create a small duct between the leading edge and the wing. High-energy air from the lower surface is accelerated through this slot and blown over the upper surface. This re-energizes the boundary layer, allowing the wing to reach a higher angle of attack before the airflow separates (stalls).

While these changes dramatically increase lift, they also increase drag. Form drag increases due to the larger frontal area of the deployed devices. More significantly, the modification of the lift distribution across the wing increases induced drag. The pilot must respect this penalty; attempting to accelerate or climb aggressively with full flaps can lead to performance limitations.

Types of Flaps and Their Specific Characteristics

Different flap designs offer varying ratios of lift-to-drag increase.

  • Plain Flaps: The simplest type. The trailing edge hinges downward. They are effective at increasing camber but produce a relatively high amount of drag for the lift generated. Common on light general aviation aircraft.
  • Slotted Flaps: When deployed, a gap opens between the flap and the main wing. This slot allows high-energy air from the lower surface to flow upward, delaying separation on the flap itself. Multi-slotted flaps (double and triple slotted) are common on large transport aircraft like the Boeing 737 and Airbus A320, allowing for very high CL_max values.
  • Fowler Flaps: These extend aft on tracks before deflecting downward. The extension increases the wing area significantly, providing exceptional lift increases. The drag increase is initially smaller than other flap types, making them excellent for takeoff. Full extension provides the high drag needed for landing.
  • Krueger Flaps: Found on the leading edge of some aircraft (like the Boeing 737), these are hinged panels that fold out and downward from the bottom of the leading edge. They increase camber and delay the stall, performing a similar function to slats. Check out this Boeing Aero article on high-lift systems for a deeper dive into the mechanics of Krueger flaps and slats.

Configuration Management Across Flight Phases

The decision of which flap and slat setting to use is not arbitrary. Pilots follow strict operating manuals and standard operating procedures (SOPs) that dictate the specific configuration for each phase of flight. The goal is to utilize the exact amount of lift required to mitigate the risks of low-speed flight while minimizing the drag penalty.

Pre-Flight and Taxi

Prior to departure, the flight crew performs a pre-flight inspection of the wings, visually confirming the integrity of the flaps and slats. During taxi, the devices remain fully retracted (Flaps 0/Flaps UP) to prevent damage from debris and to avoid unnecessary drag. Some aircraft use a small flap setting (e.g., Flaps 1) during taxi to improve control authority in crosswinds, but this is type-specific.

Takeoff and Initial Climb

Takeoff is the phase where the high-lift devices provide their most critical performance benefit. The aircraft must accelerate to a safe lift-off speed (V_LOF) within the confines of the runway length. A clean wing would require a prohibitively high takeoff speed.

  • Takeoff Flap Setting: This is a carefully calculated compromise. A higher flap setting (e.g., Flaps 15 or 20) generates more lift, allowing a lower lift-off speed and shorter ground roll. However, it also increases drag, which reduces the acceleration capability and degrades the climb gradient, particularly if an engine fails (OEI).
  • Optimal Balance: Most transport aircraft use a lower takeoff flap setting (e.g., Flaps 5 in the Boeing 737, Flaps 1+F in the Airbus A320). This provides enough lift for a reasonable takeoff speed without creating so much drag that the aircraft cannot climb out safely on a single engine.
  • Initial Climb: After lift-off, the aircraft retracts the landing gear and then initiates the “Flap Retraction Schedule.” The pilot will begin cleaning up the wing as the aircraft accelerates. Retracting flaps too early or too quickly can cause the aircraft to settle back towards the runway. The standard procedure is to retract flaps incrementally at specific speeds (e.g., at V_F+15 for acceleration to V_S).

Cruise and High-Speed Flight

Once the aircraft reaches the transition altitude and accelerates to cruise speed, the wing is fully cleaned up. Flaps 0 and Slats Retracted. In this configuration, the wing is optimized for lift-to-drag ratio (L/D max). Any deployment of high-lift devices at Mach 0.78 would be structurally dangerous (due to aerodynamic loads) and aerodynamically disastrous, causing a massive increase in drag and a potential shock-stall condition. The SKYbrary reference on high-lift devices provides excellent background on the risks of deploying these devices at high speeds.

During the climb to cruise, pilots must manage the “Flap Retraction Speed” (V_F). The flap and slat mechanisms are not designed for high-speed flight. Exceeding V_F can cause structural deformation or failure. Similarly, during descent and holding, there is a specific “Maximum Flap Extended Speed” (V_FE).

Descent, Holding, and Approach

As the aircraft descends for landing, the wing must once again become a low-speed, high-lift device. The process begins with the selection of a small flap setting to increase drag and steepen the descent path without gaining excessive speed.

  • Holding: Some operators use a small flap setting (e.g., Flaps 1 or 1+F) during holding patterns to lower the stall speed and provide better stability at the holding speed, while generating less drag than the landing configuration.
  • Approach Sequencing: As the aircraft decelerates on the approach, pilots extend flaps and slats in a specific sequence. For example, an A320 will go from CONF 1 to CONF 2, then CONF 3, and finally CONF FULL. Each step generates more lift (lowering the stall speed and the approach speed, V_REF) while generating significantly more drag.
  • Speed Management: The pilot must manage the aircraft’s energy state perfectly. Extending full flaps at a high speed can overstress the structure or cause a significant pitch change. Retracting them at a low speed can cause the aircraft to drop. The FAA's Pilot's Handbook of Aeronautical Knowledge covers these aerodynamic transitions in depth, detailing the relationship between load factor, speed, and angle of attack when configuring the aircraft.

Landing and Go-Around

For landing, the aircraft is placed in the full landing configuration (Flaps FULL or Flaps 30/40). This provides the lowest stall speed, allowing the aircraft to fly the final approach at the lowest possible speed while still maintaining a safe margin (typically 1.3 times the stall speed, or V_REF).

  • Full Flaps: The high drag created by full flaps is also beneficial. It allows the pilot to make a steeper, more stabilized approach without gaining speed. It also aids in decelerating after touchdown.
  • Go-Around: If a landing is rejected and a go-around is initiated, the pilot calls for “Max Power” and “Flaps 15” (or the go-around setting). The goal is to reduce drag instantly. The pilot will retract the flaps to the go-around setting immediately. If the pilot attempted to retract flaps fully at low altitude and low speed, the aircraft would lose lift and sink. The go-around setting is a compromise setting that provides sufficient lift for the climb-out while removing the massive drag of the full landing configuration.

Non-Normal Operations and Failure Management

The critical nature of flaps and slats requires robust systems and procedures for failures. A malfunctioning high-lift system can be a major emergency.

  • Asymmetric Flaps/Slats: If a mechanical failure prevents one side from deploying or retracting, the aircraft will experience a severe roll imbalance. The pilot must immediately stop the movement of the devices. If an asymmetric condition is detected, the flight computer usually blocks further movement. The crew must then use the Alternate or Emergency Extension system, if available, or lock the system and fly the approach with a higher approach speed (V_REF + a correction factor) to account for the roll control limitations. An excellent resource for understanding these drill-down procedures is the Airbus Flight Operations and Training documentation.
  • Flap Load Relief: Some aircraft have a system that automatically retracts flaps one setting if the aircraft exceeds the speed limit for that setting. This is a protective feature designed to prevent structural damage.
  • Slat Disagreement: This is a serious fault. If the slats are not in the commanded position, the aircraft may have a significantly different stall speed than expected. Procedures often dictate a higher approach speed and a specific landing distance calculation.

Integrating Configuration Management into Flight Safety

The impact of flap and slat configurations extends far beyond simple lift and drag. It directly influences the aircraft’s stall margin, engine performance, structural loads, and handling qualities. A pilot must always consider the “why” behind a specific setting.

Takeoff Configuration Warnings: Most modern aircraft are equipped with a “Takeoff Configuration Warning” which sounds if the aircraft is configured incorrectly for takeoff (e.g., slats not set, speed brake deployed). This is a direct result of the critical aerodynamic role these devices play. Attempting a takeoff without the correct high-lift configuration would require a much longer runway and higher speed, potentially far exceeding the aircraft’s performance capabilities and the runway’s physical limits.

Fuel Economy: In the cruise phase, the retraction of these devices is what allows the aircraft to achieve its designed fuel efficiency. An airline losing a flap fairing or having a slat that will not fully retract would be forced to divert for maintenance or fly at a lower, less fuel-efficient altitude. The cost of such an aerodynamic penalty can be tens of thousands of dollars in fuel per flight.

Conclusion: The Art of Aerodynamic Compromise

The flap and slat system is a perfect example of the compromises inherent in aircraft design. A wing cannot be simultaneously optimized for Mach 0.78 cruise and 130-knot approach speeds without mechanical assistance. The pilot’s role as a system manager is to select the precise configuration that balances the demands of lift and drag for each phase of flight.

From the low-speed high-lift demands of takeoff to the drag-free efficiency of cruise, and back to the high-lift, high-drag requirements of landing, the proper use of flaps and slats is a fundamental skill of professional flight crews. Understanding the aerodynamic principles behind these devices empowers pilots to make better decisions, handle failures effectively, and operate the aircraft safely and efficiently throughout the entire flight envelope.