How should the autopilot or pilot handle asymmetric yaw, as with one failed engine on a multi-engine turboprop whose engines are not on the centerline? Should one apply roll control or rudder control?
The details will depend on the exact aircraft type, so I will answer for the Bombardier Dash-8 Q400 as an example.
The autopilot does not have rudder authority on the Dash-8 (source: FCOM Autoflight chapter):
The Autopilot (AP) couples the FD commands to the flight control surfaces using pitch and roll servos for automatic control of the aircraft flight path.
If not disengaged, the autopilot would try to maintain heading with roll only.
The yaw damper would not be enough to overcome the yaw asymmetry either:
12.3.9 Yaw Damper
The Yaw Damper provides through control of the rudder:
- Damping of the aircraft Dutch Roll mode.
- Turn coordination.
The Yaw Damper authority is limited to a maximum of ± 4.5 degrees of rudder by mechanical stops. Yaw Damper commands are limited in software as a function of airspeed.
The correct procedure in case of an engine failure would be to hold rudder towards the live engine and then trim the rudder whenever time permits: (image source: FCOM Flight Controls chapter)
- RUDDER TRIM INDICATOR
- indicates trimmed rudder position
- RUDDER TRIM KNOB (rotary action spring loaded to neutral)
- trims rudder in desired direction
- first graduation trims slow, second graduation trims fast
The target for the rudder trim should be coordinated flight, i.e. no sideslip indicated on the primary flight display (PFD):
(image source: FCOM Indicating and Recording Systems chapter)
- SLIP/SKID INDICATOR (white)
- the Slip/Skid Indicator shows the lateral acceleration of the aeroplane and is a trapezoid symbol. It turns with the roll pointer
- the maximum deflection indication shows a 0.14 g lateral acceleration. This is equivalent to one and a half the thickness of the upper part of the slip/skid symbol (trapezoid shape)
- the slip skid Indicator is shown to the left when the aeroplane is slipping to the right and to the right when the aeroplane is slipping to the left
- the indicator goes out of view when the aeroplane roll is more than 60 degrees or the related AHRU attitude has malfunctioned