Boeing 737 Flight Control Back-up Systems
The backup and standby systems are separated for the different flight controls. The aileron and the elevator can be used in manual reversion flight, and the rudder is provided with a separate standby system. The high lift devices have an electrical and hydraulical backup system while the spoilers are not provided with any backup system.
Primary Flight Controls Back-up

The backup and standby systems for the primary flight controls are grouped in three categories:

  • Ailerons
  • Elevators
  • Rudder

If a hydraulic failure occurs (manual reversion), the pilots can mechanically move the ailerons with the control wheels. The PCU does no longer work with hydraulic power, and this causes significantly more friction. The overall force the pilots have to excite will be higher, because of cable friction and aerodynamic loads. Especially at high speeds, the load on the ailerons will be high, and thus will the steering be heavier. If the aileron system jams, the first officer can use the control wheel for rolling the aircraft, via the spoiler mixer. In this condition, only the spoilers are used for rolling the aircraft.

In the event of an hydraulic failure, the consequences for the elevators are the same as with the ailerons. If one control column jams, applying sufficient force will physically separate the control columns and so the transfer mechanism will break. The control column that moves freely can be used to control the elevators. The force required to control the elevator, is higher than in manual reversion flight. The maximum travel of the elevator is significantly reduced. The stabilizer trim can still be used to reduce the force required to control the elevators.

If hydraulic system A and/or B fails, the rudder can be controlled by the standby hydraulic sys-tem through a separate PCU. It is not possible to control the rudder if only one of the hydraulic systems fail.

Secondary Flight Controls Back-up

The backup and standby systems for the secondary flight controls are grouped in two catego-ries:
- Spoilers
- High lift devices

If the automated spoiler extension system fails, the speed brake lever must be manually moved at landing or rejected takeoff procedures. There is no backup system available for the spoilers. The spoilers are partly controlled by hydraulic system A and partly by hydraulic system B, which is a safety measure in case one of two systems fail.

If the trailing edge flaps are deployed asymmetrical, which is sensed by the FSEU, hydraulic pressure is automatically removed from the flap drive unit. This prevents damage on the surfaces. If hydraulic system B fails, the trailing edge flaps can be electrically extended and retracted by the alternate flaps switch. The alternate flaps switch controls an electrical motor that performs the extension. The extension can take up to two minutes. The leading edge devices can be controlled by the standby hydraulic system, in an event of a system B failure. The leading edge devices move simultaneously with the trailing edge flaps in case of a failure. The leading edge devices can not be retracted by the standby hydraulic system.

 
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