This article seeks to provide general guidance on flight control malfunctions. Nothing herein should be taken as a substitute for aircraft-specific guidance in the Quick Reference Handbook (QRH) or other relevant manuals. This discussion is aimed primarily at crews of transport category jets.

Definition

Flight control malfunctions can include a wide variety of problems ranging from hydraulic failure, control surface jamming, control surface hardovers, component failure, trim tab failure, trim tab runaway, structural damage, fly-by-wire system failure, and others. QRH procedures and related flight manual guidance provide the best remedies for most of these problems. However, no manual can cover all possible contingencies, and strong systems and aerodynamics knowledge can help pilots handle situations for which there are no full and specific instructions.

Flight Control Loss Due to Hydraulic Failure

Modern aircraft are designed so that loss of one hydraulic system will not cause complete loss of pitch, roll, or yaw control. However, failure of multiple hydraulic systems could cause loss of control in at least one axis. Depending on system design, defences could include manual control reversion, or use of electrically operated trim tabs for at least partial control. QRH and flight manuals should provide guidance in these cases.

Effects of Airspeed on Flight Control Malfunctions

Flight controls become more effective as airspeed increases. In normal flight, less control deflection will achieve the same pitch, yaw, or roll as airspeed increases. Conversely, more deflection is required as the aircraft slows down. This concept can become a factor as a crew diagnoses a flight control problem. For example, if a rudder becomes jammed at a given position, its effect on yaw will decrease as airspeed decreases. The aircraft may become easier to handle at lower speeds. On the other hand, if aileron control becomes limited, and circumstances require landing with a crosswind, whatever aileron control remains available will be more effective at higher speeds. Therefore, if runway length permits, pilots may choose to approach and land at a higher than normal speed. 

In the case of failed high-lift devices such as flaps and slats, emergency procedures will call for landing at higher speeds due to the higher stall speeds encountered during no-flap/no-slat landings.

Disconnect of Jammed Flight Controls

Many transport aircraft have dual-rigged flight controls, with separate rigging for the pilot's and copilot's sides. If a control cable or other component on one side jams, the two sides can be disconnected so that the unjammed side can be used to control the aircraft. The disconnect is normally conducted by pulling an aileron or elevator disconnect handle in accordance with QRH procedures. This particular emergency procedure is not usually a memory item. However, pilots should familiarize themselves with the procedure so they can perform it efficiently, especially if the control jam takes place at low altitude.

Runaway Trim

Most flight control trim systems in transport aircraft have functions that limit the extent of a runaway elevator, aileron, or rudder trim. This usually involves a cutout switch that deactivates trim after it runs continuously for a few seconds. However, in rare cases, the automatic cutout can fail. QRH procedures for this emergency typically involve pressing a manual trim cutout button. Then, if the system uses dual-channel trim motors, the motors can be reactivated one at a time to identify the failed motor/channel. Trim can be reset with the functioning channel. As with the jammed flight control procedure discussed above, pilots should be familiar with the runaway trim procedure so they can perform it efficiently if this problem takes place at low altitude.

Use of Bank Angle to Mitigate Runaway Pitch Trim

If a pitch trim tab runs away to a full pitch-up deflection, it may become difficult to keep the nose down and prevent a stall. However, by placing the aircraft into a steep bank, this reduces the vertical component of lift, making it easier to hold down the nose enough to keep the aircraft above stall speed. Once established at a level altitude, it may be possible to add flaps, slow the aircraft gradually by reducing power, and begin taking out some of the bank. With full flaps, the aircraft might be slowed enough to reduce the effectiveness of the runaway trim tab enough to return to a wings-level attitude.

In some airline training programs, this maneuver is taught as a technique. As simulator time permits, crews should consider practicing the maneuver.

Use of Thrust to Aid in Flight Control

If flight controls malfunction, pilots can use thrust to help control the aircraft. In a typical transport jet with engines mounted under the wings, adding thrust will increase the aircraft's pitch, while reducing thrust decreases pitch. Asymmetric thrust can be used to help with roll and yaw. For example, reducing power on the right engine while adding power on the left will result in roll and yaw toward the right. 

Fly-By-Wire Malfunctions

In some cases, a malfunction of an aircraft's fly-by-wire or flight augmentation system can induce flight control malfunction. If undesired control responses take place during automated flight, or during manual flight assisted by flight augmentation systems, consider deactivating flight augmentation. For example, certain QRH procedures for the Embraer 170/190 aircraft will call for switching flight controls from "normal" mode to "direct" mode. This deactivates higher-level functions such as yaw damper and turn coordination, elevator thrust compensation, and configuration change compensation with the horizontal stabilizer. In general terms, if a flight control module or flight augmentation computer is suspected of unwanted inputs, turn it off.

Flight Control Check

In some circumstances involving flight control malfunction and/or structural failure, crews may choose to perform a flight control check. This involves gradually slowing the aircraft toward final approach speed, adding flaps and lowering landing gear as speed permits, until the aircraft is configured for landing. At various points during that process, pilots should assess roll, pitch, and yaw authority. Crews should perform this check at high enough altitude to recover from loss of control. If control becomes marginal at a given speed, then a speed somewhat higher than that becomes the minimum approach speed. In no case should the aircraft be slowed until full control deflection is required to maintain flight.

Runway Selection for Landing

Select the longest available runway with the least crosswind. Even if the crew has performed a flight control check, aircraft response could become unpredictable near touchdown. Also consider the availability of airport crash/fire/rescue services.

Approach and Landing

 Most aircraft manuals call for a long, flat final approach when landing with compromised flight controls. If circumstances require landing with a higher than normal approach speed, fly the aircraft onto the runway with minimal flare. If the aircraft balloons due to excessive flare, control response for a recovery and/or a go-around could be marginal.