What Are Flaps?

Flaps are high-lift devices mounted on the trailing edge of an aircraft’s wing. By extending downward and rearward, they increase both the camber (curvature) and the surface area of the wing. This modification raises the coefficient of lift (CL) at a given angle of attack, enabling the aircraft to generate sufficient lift at lower airspeeds. Without flaps, commercial airliners would require much longer runways and dangerously high approach speeds. The aerodynamic principles behind flaps—chiefly the enhancement of lift and controlled increase in drag—make them indispensable for safe takeoff and landing operations.

Flaps also influence the wing’s stall characteristics. By delaying airflow separation over the upper surface, properly deployed flaps allow the wing to maintain attached flow at lower speeds. This effect is particularly critical during the final approach and landing flare, where precise speed control and stability are paramount.

Types of Flap Systems

Commercial airliners employ several flap configurations, each offering a specific balance of lift increase, drag penalty, mechanical complexity, and weight. The choice depends on the aircraft’s design mission, operational speeds, and certification requirements.

Plain Flaps

Plain flaps are the simplest design: a hinged portion of the trailing edge that rotates downward. When deployed, they increase wing camber and lift, but the increase is modest compared to more advanced types. Plain flaps are lightweight and mechanically straightforward, making them suitable for light aircraft and some regional turboprops. However, their limited lift enhancement and high drag at large deflection angles restrict their use on larger commercial jets.

Split Flaps

Split flaps consist of a panel on the lower surface of the wing that deflects downward while the upper surface remains smooth. This creates a high-pressure region below and a low-pressure region above, generating additional lift. The abrupt change in airflow behind the flap produces significant drag, which can be useful for steep approaches. Split flaps were common on early jet airliners like the Boeing 707 and are still found on some business jets, but they are less efficient than slotted designs for modern high-performance aircraft.

Fowler Flaps

Fowler flaps are among the most effective high-lift devices used on commercial airliners. They extend rearward on tracks before rotating downward, simultaneously increasing wing area and camber. The rearward motion expands the total lifting surface, while the downward deflection boosts camber. This combination yields large lift increments with relatively low drag penalties. Fowler flaps are standard on many narrow-body and wide-body aircraft, including the Boeing 737 and Airbus A320 families. Their complex mechanism requires robust actuators and precise rigging but rewards operators with superior takeoff and landing performance.

Slotted Flaps

Slotted flaps incorporate a gap—or slot—between the flap and the fixed wing. When deployed, high-energy air from the lower surface is directed through the slot and over the flap’s upper surface. This energizes the boundary layer, delays flow separation, and allows higher lift coefficients before stall. Single-slotted flaps are common on smaller jets; double- and triple-slotted designs are used on large aircraft like the Boeing 747 to achieve exceptional lift at low speeds. Each slot adds complexity but can increase maximum lift by 40–60% compared to a plain flap.

Fowler-Slat Flaps (Combined Systems)

Many modern airliners combine Fowler flaps on the trailing edge with leading-edge slats or Krueger flaps. These integrated systems provide the highest lift coefficients available, enabling steep approach angles and short-field operations. For example, the Boeing 787 uses advanced trailing-edge flaps alongside variable-camber leading edges. Such combinations are carefully scheduled to optimize lift, drag, and pitching moments throughout the takeoff, climb, approach, and landing phases. The synergy between leading- and trailing-edge devices is a hallmark of modern aerodynamic design.

How Flap Systems Work

Flap actuation on commercial airliners is typically powered by hydraulic or electric systems, with manual backup in some older designs. Fly-by-wire aircraft use electronic signals from the flight control computers to command flap positions, ensuring precise scheduling and redundancy. The flap control lever in the cockpit allows the flight crew to select specific settings—commonly labeled 1, 2, 3, 5, 10, 15, 20, 25, 30, or 40 degrees, depending on the aircraft type. Each setting corresponds to a particular flap angle and, if applicable, slat extension.

Flap Settings and Flight Phases

During takeoff, flaps are typically extended to a moderate setting (e.g., 5–15°) to increase lift and reduce takeoff distance while keeping drag low enough for acceleration and climb. After liftoff, the crew retracts flaps in a gradual sequence to avoid abrupt changes in lift and drag while maintaining positive climb performance. In the cruise phase, flaps are fully retracted to minimize drag and maximize fuel efficiency.

On approach, flaps are extended incrementally as the aircraft slows. The landing setting (often 30–40°) provides maximum lift and drag, allowing a steep descent path and low touchdown speed. Some aircraft, such as the Boeing 777, use a “flap load relief” system that automatically retracts flaps slightly if airspeed exceeds a safe limit, protecting the structure from excessive loads.

Leading-Edge Devices: Slats and Krueger Flaps

Most modern airliners complement trailing-edge flaps with leading-edge high-lift devices. Slats are movable surfaces that extend forward from the wing’s leading edge, creating a slot that channels airflow over the wing and delays stall. Krueger flaps are hinged panels that deploy from the lower leading edge, increasing camber. The coordination between leading-edge and trailing-edge devices is critical; asymmetric or mis-timed deployment can cause handling difficulties or structural damage. On fly-by-wire aircraft, the flight control computers manage this coordination automatically.

Actuation and Redundancy

Flap systems are designed with multiple redundant power sources and control paths. Typical architectures include two independent hydraulic systems supplying power to flap actuators, with an electric backup that can operate the flaps at a reduced rate. In case of total hydraulic failure, a standby electric motor or manual crank (in older aircraft) can extend or retract the flaps. Modern aircraft like the Airbus A350 use all-electric flap actuation, eliminating hydraulic lines and simplifying maintenance. These redundancies ensure that even with multiple failures, the crew can still manage the flap configuration safely.

Importance of Flap Systems in Aviation Safety

Properly functioning flaps are critical for safe operations. A flap asymmetry or jam can lead to severe roll moments, stall at unexpected speeds, or structural overload. For this reason, flap systems are subject to rigorous certification testing and scheduled maintenance.

Failure Modes and Monitoring

Flap asymmetry sensors detect when the flap position differs between left and right wings beyond a small tolerance. If an asymmetry is detected, the system automatically locks the flaps in place and alerts the crew. Similarly, flap position sensors feed data to the flight data recorder and maintenance systems. Pilots train extensively on flap failures, including scenarios where flaps are stuck in a single setting, requiring alternate procedures for landing with higher speeds and longer rollouts.

Maintenance and Inspection

Each aircraft undergoes periodic inspections of the flap tracks, rollers, actuators, and control rods. Lubrication, wear checks, and functional tests are performed per the manufacturer’s maintenance planning document. Non-destructive testing (e.g., eddy current, ultrasonic) is used to detect cracks in flap structures. Airlines also monitor flap usage data to predict component life and schedule replacements proactively. The reliability of modern flap systems is high, but the consequences of failure demand a culture of meticulous care.

Conclusion

Flap systems exemplify the sophisticated engineering behind commercial aviation. By transforming the wing’s shape on demand, they enable airlines to operate safely over a wide range of speeds and conditions—from short runways at high-altitude airports to long-haul cruises. Understanding the types, mechanisms, and safety implications of flaps helps students and aviation professionals appreciate the delicate balance between aerodynamics, structures, and systems design. As aircraft evolve toward more electric architectures and higher efficiency, flap systems will continue to be refined, but their fundamental role in providing safe, low-speed flight will remain unchanged for decades to come.

For further reading, consult the Boeing Aero magazine on high-lift systems, the FAA advisory circulars on flap system design, and textbooks like Aerospaceweb’s explanation of slotted flaps.