I read some pilots manual which indicates that the AT retard mode will be active at 30feet AGL to retract the throttle to idle, but the flare mode starts from 50 feet AGL? Should the throttle to be retracted to idle before flare? If not what is auto throttle doing during the flare maneuver, try to maintain the speed?
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2 Answers
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The auto-throttle system of a typical airliner (like e.g. the Boeing 737) will maintain the approach speed (usually $ v_\mathrm{ref} + 5 \, \mathrm{kt} $, depends on the wind) during final approach. It should not reduce thrust before starting the flare because the speed could decrease too much, which could in the worst case result in a stall.
The Boeing 737 FCOM (v2 4.20.19 Automatic Flight - System Description, see also this answer for a full description of the autoland) says:
The A/P flare maneuver starts at approximately 50 feet RA and is completed at touchdown. [...]
The A/T begins retarding thrust at approximately 27 feet RA so as to reach idle at touchdown. A/T FMA annunciates RETARD.
So during the flare the thrust is decreased such that the thrust levers are closed at touchdown. This (together with the increase in pitch) will start to reduce airspeed. Ideally, the aircraft then touches down at or about reference speed $ v_\mathrm{ref} $. The thrust reduction should be smooth because the thrust-pitch-coupling of engines mounted under the wing will cause a pitch down moment, which needs to be counteracted with control inputs.
answered Jul 4, 2019 at 16:05
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$\begingroup$ Thanks for the clarifications, so ideally autoland flare at 50 feet with airspeed Vref+5, touch down at Vref, does that mean(ideally) I always land with same pitch attitude at touch down? $\endgroup$– VvVCommented Jul 4, 2019 at 17:07
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$\begingroup$ Not necessarily, the pitch attitude will depend on the weight of the aircraft and the configuration (737 typically lands with flaps 30 or 40) and to some lesser extent atmospheric conditions (air pressure, temperature). $\endgroup$ Commented Jul 4, 2019 at 20:34
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Airbus is very similar.
During the approach A/THR is in SPEED mode, targeting the pre-determined approach speed, usually VLS+5 kt. During the flare, if autopilot is engaged in LAND mode A/THR will automatically reduce thrust to idle. Excerpt from FCOM:
RETARD MODE
The RETARD mode is only available during automatic landing (AP engaged in LAND mode). At approximately 40 feet RA, the RETARD mode engages and remains engaged after touchdown. The A/THR commands IDLE thrust during the flare, and the FMA and engine warning display “IDLE”. If the autopilot is disengaged during the flare before touchdown, the SPEED mode replaces the RETARD mode, and the pilot has to manually reduce thrust.
Note: In an automatic landing, the system generates a “RETARD” callout at 10 ft radioaltitude (RA), which prompts the pilot to move the thrust levers to IDLE in order to confirm thrust reduction. In manual landing conditions, the system generates this callout at 20 ft RA, as a reminder.
Don't be confused about "remains engaged after touchdown" phrase above. In practice pulling thrust levers back to idle at 10 ft as suggested by RETARD auto callout will disconnect the auto thrust just before touchdown.
