737 Flight Crew Operations Manual
Flight Management, Navigation -
Flight Management System
Operation
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
D6-27370-866-EGP 11.31.23
(SB Changes YK981 - YQ456)
Altitude intervention cannot be used to decrease cruise altitude. Setting a lower
altitude then pushing the altitude intervention switch causes the FMC to enter an
early descent in the selected descent mode.
YW273, YW274
(SB Changes YK981 - YQ456)
The Altitude Intervention function will only apply to the active flight plan.
The altitude intervention software will enabled a climb or descent to a new cruise
altitude to be initiated from the Mode Control Panel.
The altitude intervention option will allow the crew to initiate a cruise descent
using the altitude intervention feature on the Mode Control Panel when the
airplane is not in close proximity to the top of descent. A distance of 50 NM will
be used as a determinant for this operation. If the airplane is greater than 50 NM
from T/D. The pilot may dial the MCP altitude down and press the ALT INTV
button on the MCP. FMC response is similar to use of altitude intervention for a
cruise climb initiation. FMC ALT is set to the new cruise altitude and cruise
descent is initiated using existing guidance techniques for cruise descent. A CRZ
DES can be initiated using altitude intervention when greater than 50 NM from
top of descent provided that the MCP ALT is not set below a descent altitude
constraint. For this condition of more than 50 nm to T/D, and the MCP altitude
dialed below a descent constraint, the result of pressing the ALT INTV button will
be to transition from CRZ to EARLY DES and to honor the constraint. Subsequent
presses of the ALT INTV button may be used to delete the constraint.
This feature enables a cruise descent to sequence into a path descent to eliminate
a condition which can occur when the pilot initiates a cruise descent close to the
original top of descent, less than 50 nm to top of descent. The change monitors an
extension of the descent path during a cruise descent, in a manner similar to an
early descent operation, to capture the extension of the path if it is intersected.
Transition from cruise descent to descent occurs at the time of path capture in a
manner similar to a transition from early descent to descent.
Cruise
At cruise altitude, the FMC sets cruise speed at the default or pilot entered speed
until reaching the top–of–descent (T/D) point. Alternate cruise speed options are:
• long range (LRC)
• flight crew entered speed.
Cruise thrust is set as required to maintain level flight at the target speed, with the
autothrottle engaged. The FMC uses maximum range cruise speed if cost index is
set to zero.
Fuel and ETA predictions are based on a constant altitude cruise unless a step
climb altitude is entered.
September 15, 2016
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