What happens if the motor that trims the elevator of an airplane fails and there are no trim wheels in the cockpit (like in the Boeing 747). Is there a backup system or hidden trim wheels?
Yes, there is a backup system available when there is no trim wheel. On the Boeing 747, the stabilizer trim has two control modules, normal and alternate:
The stabilizer trim system provides pitch trim by varying the angle of incidence of the horizontal stabilizer. Normal and alternate electrical channels control two stabilizer trim control modules. Each control module hydraulically powers a trim actuator. Actuator outputs mechanically sum to drive the stabilizer. Trim rate is reduced at high airspeeds.
(Boeing 747-400 FCOM 9.20.4 - Flight Controls - System Description)
While the normal trim is controlled via the switches on the yoke or via the autopilot, the alternate trim is controlled via two switches located on the center pedestal:
1 Stabilizer (STAB) Trim Cut Out Switches
ON - supplies hydraulic power for stabilizer trim.
- supplies hydraulic power for the stabilizer trim
- shuts off related system hydraulic power if unscheduled trim detected
CUTOUT - shuts off related hydraulic power to stabilizer trim.
2 Alternate (ALTN) Stabilizer Trim Switches
Push (both switches) - trims stabilizer in desired direction using alternate control channel.
(Boeing 747-400 FCOM 9.10.2 - Flight Controls - Controls and Indicators)
All other Boeing aircraft without a trim wheel work similar.
On modern Airbus aircraft, manual pitch trim is usually not required because the fly-by-wire system takes care of that. These aircraft do not have trim switches on the sidestick. There is however always an alternate trim possibility. On the earlier models (up to A340), this is done manually via the trim wheels. Starting with the A380, the trim wheels were removed. There are now two pitch trim switches on the center pedestal for alternate control:
Controls the position of the Trimmable Horizontal Stabilizer.
It moves the THS at the following speed:
- In clean configuration : 0.15 °/s
- With slats/flaps extended : 0.25 °/s
The PITCH TRIM sw is composed of two switches, which are spring loaded to neutral. In order to move the pitch trim, both switches must be pressed simultaneously. On ground, the PITCH TRIM sw has priority over the automatic takeoff trim setting. The PITCH TRIM sw is inhibited in flight, except in direct law. (The auto trim function is lost in direct law. USE MAN PITCH TRIM is displayed on the PFD).
(Airbus A380 FCOM 27 - Flight Controls - Pitch Trim Sw)
Some aircraft have no pitch trim at all. The result in most cases is a continuous pressure on the controls. Most airliners have three kinds of pitch trim control. An automated system allowing an autopilot to adjust trim by activating powered control, a switch on the yoke controlling the same thing and the trim wheels, adjusting the trim by means of physical force. But that's not all. Any controllable influence changing the pitch can be regarded and used as a pitch control system.
One of the best examples of that is the flight of United Airlines Flight 232 which ended in Sioux City in 1989.
This airplane had lost all hydraulics, making it virtually uncontrollable.
Eventually the pilots managed to crash land the airplane in Sioux city, controlling both pitch, yaw and roll by means of thrust input only. Many passengers and crew survived that crash. A truly phenomenal achievement.