737 Flight Crew Operations Manual
Automatic Flight -
System Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
4.20.26 D6-27370-866-EGP
YW273, YW274
• for VMO/MMO only, if the A/T is engaged in a speed mode and the
thrust levers are at idle, the AFDS, if in a V/S mode or CWS P, will
automatically engage to LVL CHG mode.
• if the A/T is not available, no reversion response to gear or flap placard
speeds is available. The AFDS reverts to speed control for Vmo/Mmo
speed limiting.
Minimum Speed Reversion
YK981 - YQ456
The AFDS and A/T do not control to a speed which is less than minimum speed
for the current flap configuration. This speed is approximately 1.3 Vs. Minimum
speed, FMC speed, or selected speed, whichever is higher, becomes the AFS
commanded speed. If actual speed becomes equal to or slightly less than the
minimum speed, the underspeed limiting symbol appears in the MCP IAS/Mach
Display, and if operating in the V/S mode, the AFDS reverts to LVL CHG. The
AFDS will also revert to LVL CHG from VNAV PTH, except when flying a level
segment. There is no reversion from Vertical Speed Modes (VNAV or MCP) to
level change mode when MIN speed is reached and the flaps are greater than 12.5.
YW273, YW274
The AFDS and A/T do not control to a speed which is less than minimum speed
for the current flap configuration. This speed is approximately 1.3 Vs. Minimum
speed, FMC speed, or selected speed, whichever is higher, becomes the AFS
commanded speed. If actual speed becomes equal to or slightly less than the
minimum speed, the underspeed limiting symbol appears in the MCP IAS/Mach
Display, and if operating in the V/S mode or CWS P, the AFDS reverts to LVL
CHG. The AFDS will also revert to LVL CHG from VNAV PTH, except when
flying a level segment. There is no reversion from Vertical Speed Modes (VNAV
or MCP) to level change mode when MIN speed is reached and the flaps are
greater than 12.5.
The AFS commands a speed 5 knots greater than minimum speed. Reaching a
speed 5 knots greater than minimum speed reactivates normal MCP speed
selection control. The AFDS commands nose down pitch to increase airspeed if
the thrust levers are not advanced. When actual speed becomes 5 knots greater
than minimum speed, the underspeed limiting symbol disappears.
The A/P disengages and the F/D command bars retract when in a LVL CHG climb
with a command speed equal to minimum speed and a minimum rate of climb
cannot be maintained without decelerating.
Minimum speed reversion is not available when the A/T is OFF and the AFDS is
in ALT HOLD or after G/S capture. Minimum speed reversion is also not
available when in VNAV PTH and flying a level segment.
September 15, 2016
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