737 Flight Crew Operations Manual
Automatic Flight -
System Description
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D6-27370-866-EGP 4.20.25
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.
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 the AFDS is engaged but not in speed or CWS mode, and the A/T is
armed but not in speed control, the A/T reverts to SPEED and controls
speed to slightly below the placard limit
• if the AFDS or A/T is in speed control, speed is maintained slightly
below the placard limit
YK981 - YQ456
• 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, will automatically
engage to LVL CHG mode.
September 15, 2016
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