Understanding Unusual Flap Asymmetry During Approach

Flap asymmetry is one of the more challenging malfunctions a pilot can face during the critical approach phase. It occurs when the flaps on one wing extend or retract at a different rate or to a different degree than those on the opposite wing. This asymmetry creates a difference in lift and drag between the two wings, inducing roll and yaw forces that must be actively managed. While modern aircraft have redundant systems and safety mechanisms to prevent asymmetry, mechanical failures, hydraulic leaks, or electrical faults can still lead to this condition. Recognizing and responding to unusual flap asymmetry correctly is essential for maintaining control and ensuring a safe outcome.

Recognizing Flap Asymmetry

Early recognition is the first step in managing any in-flight abnormality. Pilots must be alert to several cues that may indicate flap asymmetry has developed during approach:

  • Control force changes: A sudden increase in roll or yaw control inputs is required to keep the aircraft straight and level.
  • Aircraft attitude anomalies: The wings may appear not level, or the aircraft may require a persistent bank angle to track the glideslope.
  • Instrument indications: Flap position indicators on the overhead panel or EICAS (Engine Indicating and Crew Alerting System) may show mismatched values between left and right flaps.
  • Unusual sounds or vibrations: A whining or grinding noise from one wing, or a change in airframe vibration, can signal a flap system problem.
  • Autopilot warnings: If engaged, the autopilot may struggle to maintain the selected mode, potentially disconnecting or generating bank/roll limit alerts.

Because approach is a high-workload, low-altitude phase, pilots must be trained to recognize these cues quickly without being distracted from primary flight tasks. The FAA Advisory Circulars provide guidance on recognizing flight control malfunctions, though specific aircraft flight manuals (AFMs) remain the primary reference.

Causes of Flap Asymmetry

Understanding the root causes helps pilots anticipate and diagnose the issue. Common causes include:

  • Mechanical jam or binding: Debris, corrosion, or physical obstruction can prevent one flap from moving freely.
  • Hydraulic system failure: Loss of hydraulic pressure to one side or a leak in the system serving one wing.
  • Electrical or actuator failure: A malfunction in the electric motor, gearbox, or position sensor serving one flap.
  • Pilot error: Inadvertent selection of different flap positions for left and right sides (rare, but possible on aircraft with independent flap controls).

In turbine-powered aircraft, the flap system is often driven by a torque tube or linear actuator, and asymmetry protection systems (such as torque limiters or mechanical locking mechanisms) are designed to stop further movement when a preset difference is detected. When these protections fail or are overwhelmed, asymmetry develops.

Immediate Actions When Flap Asymmetry Is Detected

Once flap asymmetry is suspected or confirmed, prompt and measured actions are necessary. The following steps are based on industry best practices and typical AFM checklists. Always consult your specific aircraft’s QRH (Quick Reference Handbook) or AFM for your type.

  1. Maintain aircraft control. The first priority is to fly the airplane. Use rudder and aileron inputs to counteract roll and yaw. Avoid abrupt control movements, which could exacerbate the asymmetry or cause a stall.
  2. Set and hold a safe approach speed. If the flaps are asymmetric, the aircraft’s stall speed may increase, and the normal approach speed may no longer be adequate. Increase the target speed by 10–20 knots (or as recommended for flap-up or asymmetric landing) to provide a margin above the stall. Refer to the AFM or QRH for specific speeds.
  3. Cross-check flap indications. Verify that the asymmetry is real and not an instrument or indicator error. Look at both primary and standby flap position indicators. If the aircraft has a flap asymmetry warning (visual and aural), acknowledge it but do not allow the warning to distract from flying.
  4. Brief the situation. Announce the asymmetry to the other pilot (if CRM is applicable) and assign tasks. The flying pilot (PF) continues to control and fly the approach, while the pilot monitoring (PM) runs the checklist and communicates with ATC if needed. Never rush the checklist; safety is paramount.
  5. Consider flap position. In many aircraft, the asymmetry is less critical if the flaps are already near the landing setting (e.g., full flaps) rather than at a mid-range setting. Some QRH procedures will direct you to leave the flaps as they are, while others may advise retracting flaps to a symmetrical position above the current asymmetric position. Do not move flaps without a specific checklist instruction, as changing flap selection could worsen the asymmetry.
  6. Advise ATC. Inform air traffic control of the situation. Request vectors for an extended final approach or a straight-in approach as needed. If available, request runway length and landing distance data. ATC can also provide priority handling and clear the way for an emergency landing.

The Role of Crew Resource Management (CRM)

Effective communication and task sharing are vital when dealing with flap asymmetry. The PF should focus entirely on flight path and energy management, while the PM handles checklists, ATC communications, and monitoring. Both pilots should challenge and verify each other’s actions, especially when performing non-normal checklists. For example, if the PM reads a step to “select flaps to UP,” the PF should verify that the aircraft is at a safe speed and altitude before executing. In high-stress scenarios, cross-checking prevents errors that could lead to loss of control.

Handling Techniques for Asymmetric Flap Configurations

Once the aircraft is under control and the immediate actions are taken, the pilot must adjust flying technique to compensate for the asymmetry. The exact technique varies by aircraft type, but general aerodynamic principles apply.

Using Rudder and Ailerons

Flap asymmetry causes both roll and yaw moments. The extended flap generates additional lift and drag on that wing, producing a roll toward the wing with less flap. At the same time, the increased drag on the low-flap side causes it to yaw toward the high-drag side. To counter this:

  • Apply aileron into the roll direction (typically toward the wing with less flap) to level the wings.
  • Apply rudder to counteract yaw and keep the nose aligned with the runway. In many cases, opposite rudder is needed (i.e., if the right flap is extended more, the aircraft yaws left, requiring right rudder).

It is crucial to use coordinated inputs and avoid overcontrolling. Aileron inputs should be smooth and deliberate; excessive aileron may cause adverse yaw or exceed control surface limits. The rudder is the most effective tool for maintaining directional control at low speeds, so pilots should be comfortable using it actively during the approach.

Speed Management

Speed is a critical factor. Asymmetric flap configurations increase drag and may raise the stall speed, especially if the flaps are extended on only one side. The recommended approach speed is usually calculated as VREF (reference landing speed) plus an additive for the asymmetric condition. For many transport-category aircraft, the QRH will specify something like “VREF + 20 knots” or a minimum speed. The extra speed provides energy to maneuver and reduces the angle of attack, decreasing the roll tendency. However, do not exceed the maximum flap extension speed (VFE) for the extended flap side. If the asymmetry is severe, it may be safer to retract flaps completely and perform a no-flap landing, which eliminates asymmetry forces (though it increases approach speed and landing distance).

Managing Landing Gear and Flaps

Some AFM procedures allow landing gear extension to help stabilize the aircraft. Gear extension adds drag and may reduce the asymmetric roll moment by acting as a stabilizing force. In other cases, the checklist may recommend delaying gear extension until a go-around decision is made. The timing of flap or gear changes must be carefully coordinated. Avoid making multiple configuration changes simultaneously, as this can overwhelm the PF and lead to an unstable approach.

Special Considerations for Different Flap Positions

  • Flaps retracted (asymmetric in mid-extension): If the asymmetry occurs while flaps are extending from zero to takeoff or approach setting, the aircraft may be in a high-drag, low-lift configuration on one side. Consider stopping the extension and landing with no flaps if possible.
  • Flaps at landing setting (asymmetric at full or near full): The asymmetry may be more manageable because both flaps are close to their final positions and the aircraft is already in a high-lift configuration. The main risk is increased landing distance and roll control. Some aircraft can land safely with full flaps asymmetric if the pilot uses sufficient aileron and rudder trim.
  • Flaps retracting (asymmetric on go-around): If a go-around is attempted and asymmetry occurs, the pilot must prioritize positive rate of climb and retract flaps only if consistent with the asymmetry procedure. Do not select flap UP if it will worsen asymmetry.

Always follow your aircraft manufacturer’s approved procedures. The EASA aircraft operations guidance also emphasizes adherence to AFM limitations and crew coordination during unusual-flight-handling events.

Decision to Go-Around or Continue

Not all flap asymmetry situations force a go-around, but the pilot must make a go/no-go decision early enough to maintain a stable approach. The stabilized approach criteria should be evaluated:

  • Airplane configuration: Is the aircraft properly configured for landing (flaps, gear) and within safe speed limits?
  • Control margins: Are full aileron and rudder deflections required to maintain wings level and directional control? If inputs are near the stops, the approach is not stable.
  • Energy state: Is the approach speed too high or too low? Can you maintain the required descent profile?
  • Crew comfort: If the PF is fighting the aircraft and unable to scan instruments or perform a normal landing, a go-around is prudent.

When to Go-Around

A go-around should be initiated if:

  • The aircraft cannot be made stable by 1,000 feet AGL (500 feet AGL in VMC for some operators).
  • The asymmetry worsens or cannot be managed.
  • The QRH procedure instructs a go-around for the specific condition.
  • Any stall warning (stick shaker, margin less than 1.3 VS with flaps asymmetric) is triggered.

Executing a go-around with flap asymmetry requires careful technique. Smoothly apply go-around power, maintain directional control with rudder, and retract flaps only in accordance with the QRH. Some aircraft procedures call for retracting flaps to an intermediate setting (e.g., half) or leaving them as is until a safe altitude and speed are reached. The go-around itself may cause a further asymmetry if the flap system attempts to retract asymmetrically. Therefore, the PF must be ready to overcome strong yaw/roll forces during the transition to a climb.

When to Continue

If the aircraft is stable, control forces are manageable, and the approach can be flown to a normal touchdown point, continuing may be safer than risking a go-around in a critical configuration. Factors to consider include:

  • Runway length: A longer runway provides more margin for asymmetric landing.
  • Weather: In crosswinds, asymmetry may increase the demand on roll and rudder capabilities.
  • Landing distance: Add at least 25% to the calculated landing distance for asymmetric configurations (use the AFM landing distance for flap-up if no asymmetric data exists).

Ultimately, the captain or pilot-in-command makes the decision based on the specific situation. There is no shame in executing a go-around; it is often the safest option. As the FAA Airplane Flying Handbook states, “A go-around is not a failure—it is a normal part of pilot judgment.”

Post-Approach Procedures: After Landing or Missed Approach

Once the aircraft is safely on the ground and parked, the crew must ensure the flap system is properly inspected and documented before the next flight. For a go-around scenario, the same inspection steps apply after landing.

On-Ground Inspection

  1. Visual check of flaps and tracks: Inspect the flap panels, tracks, and actuators for visible damage, binding, misalignment, or hydraulic leaks.
  2. Functional test: With hydraulic power applied (if safe), cycle the flaps slowly and observe symmetry using ground personnel or cameras. Do not cycle flaps if there is a mechanical jam or risk of further damage.
  3. Check control circuits: Verify the flap control system electrical feedback and sensors for any faults.
  4. Refer to MEL/CDL: If the aircraft is to be dispatched, consult the Minimum Equipment List or Configuration Deviation List for dispatch with inoperative flap components. Many flap asymmetry conditions require the aircraft to be grounded until repairs are made.

Reporting and Documentation

  • Pilot report (PIREP): Submit a pilot report detailing the asymmetry, the phase of flight, corrective actions taken, and any symptoms observed.
  • Maintenance log entry: Document the event in the aircraft flight log or maintenance record, including aircraft registration, date, time, and nature of the malfunction.
  • Report to authorities: If the asymmetry was severe, or if any damage occurred, file an Aviation Safety Report (ASRS in the US, or equivalent in other jurisdictions). Reporting helps improve design and procedures.

Safety management systems (SMS) should capture such events to analyze trends and prevent recurrences. Crews are encouraged to debrief the event with safety officers to share lessons learned.

Prevention and Training

The best way to handle unusual flap asymmetry is to prevent it from happening in the first place. Pre-flight inspections should include a careful check of flap actuators, tracks, and hydraulic lines. During walk-around, look for any debris, damage, or abnormal fluid leaks near the flaps. Additionally, pilots should be familiar with the aircraft’s asymmetry protection system—knowing how it functions helps troubleshoot when it doesn’t activate.

Simulator training is invaluable. Recurrent training should include scenarios involving flap asymmetry at various phases, including approach and go-around. The crew should practice using the QRH, applying correct control inputs, and making the go-or-no-go decision. AOPA training resources note that simulator practice builds muscle memory and confidence, reducing the shock of a real event.

In addition, operators should ensure that their standard operating procedures (SOPs) cover flap asymmetry thoroughly. Periodic audits of flap system maintenance and test intervals can catch potential failures before they occur.

Conclusion

Unusual flap asymmetry during approach is a serious but manageable emergency. By recognizing the condition early, executing immediate control actions, applying appropriate handling techniques, and making a sound decision to continue or go-around, pilots can safely conclude an otherwise challenging flight. The key is to stay calm, fly the airplane first, and use the approved checklist. Post-flight actions—inspection, reporting, and debriefing—close the loop and enhance safety for future operations. With proper training and procedural discipline, the risks associated with flap asymmetry can be significantly mitigated, allowing pilots to maintain control and land safely.