Automatic Flight -System Description 737 Flight Crew Operations Manual
Command Speed Limiting
The AFS provides speed, pitch and thrust commands to avoid exceeding the following limit speeds:
.
Vmo/Mmo
.
wing flap placards
.
landing gear placard
.
minimum speed. The commanded speed can be equal to, but does not exceed a limit speed.
Speeds greater than Vmo/Mmo cannot be selected from the MCP. Speeds can be selected which exceed flap and gear placards or are less than minimum speed.
Minimum speed is based on angle of attack and is approximately 1.3 Vs for the current flap configuration. It is sensed by the angle of attack vanes, one on either side of the forward fuselage.
If a speed greater than a placard speed, or less than minimum speed is selected, the AFS allows acceleration or deceleration to slightly short of the limit, then commands the limit speed. The overspeed or underspeed limiting symbol appears in the MCP IAS/Mach display when the commanded speed cannot be reached.
Either pitch or thrust, whichever is engaged in a speed mode, attempts to hold the limit speed. The commanded limit speed and MCP speed condition symbol, remain until another speed is selected which does not exceed the limit. A speed 15 knots greater than the minimum speed must be selected to remove the underspeed limiting symbol.
Reversion Modes
During some flight situations, speed control by the AFDS or A/T alone could be insufficient to prevent exceeding a limit speed. If this occurs, AFDS or A/T modes automatically revert to a more effective combination. The reversion modes are:
.
placard limit reversion
.
minimum airspeed reversion. Mode reversion occurs slightly before reaching the limit speed. Both the AFDS
and A/T have reversion modes which activate according to the condition causing the reversion.
Copyright . The Boeing Company. See title page for details.
Automatic Flight -System Description 737 Flight Crew Operations Manual
Placard Limit Reversion
When one of the placard limit reversions (gear, flap or Vmo/Mmo) is reached, the overspeed limiting symbol appears in the MCP IAS/Mach display and the following occurs:
.
if not in AFDS or A/T speed control and the A/T is armed, the A/T reverts
to SPEED and controls speed to the placard limit
.
if in AFDS or A/T speed control, no reversion is necessary. The AFDS or
A/T, whichever is controlling speed, holds speed slightly below the
placard limit
.
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
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.
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.
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