Copyright . The Boeing Company. See title page for details.
11.31.24 September 29, 2005
Flight Management, Navigation -Flight Management System Operation
737 Flight Crew Operations Manual
YD652 - YD655 On non-idle descent segments, the pitch mode will switch to VNAV PTH if not already in VNAV PTH. In VNAV PTH, thrust controls speed.
When a navigation data base vertical angle leg is flown (GP x.xx on RTE LEGS page), VNAV switches to VNAV PTH if not already in VNAV PTH.
In approach phase during speed intervention, the pitch mode remains in VNAV PTH after speed intervention is exited. The FMC shall remain in the current vertical mode regardless of IAS MACH selector changes.
When speed intervention is exited the descent mode will switch back to path mode.
Descent
VNAV can perform a descent in either of two modes – path descent or speed descent. During a path descent, the FMC uses idle thrust and pitch control to maintain a vertical path, similar to a glideslope in three dimensions. During a speed descent, the FMC uses idle thrust and pitch control to maintain a target descent speed, similar to a level change descent.
Top Of Descent (T/D)
The point where the cruise phase changes to the descent phase is the top of descent. The T/D point is shown on the map as a green open circle with the label T/D. T/D is calculated from an end of descent (E/D) point.
Intermediate T/D points show on the map as green open circles with the label T/D–XXXXX (altitude). Intermediate T/D points exist when path segments between altitude restricted waypoints produce a level path segment. The intermediate T/D point shows where the descent will resume.
End of Descent (E/D)
The FMC calculates a descent path based on airspeed restrictions, altitude restrictions and the end of descent (E/D) point. The E/D point is shown on the map as a green open circle with the label E/D. The E/D is the last of the following which is not preceded by a lateral discontinuity:
.
the runway threshold for approaches with a runway waypoint on the RTE
LEGS page, or
.
the missed approach point for approaches not showing a runway waypoint
on the RTE LEGS page, or
YD652 - YD655
.
the last descent waypoint, or
.
the lowest “at” altitude restriction if no arrival procedure is entered.
Entering an instrument arrival procedure provides an E/D point.
Copyright . The Boeing Company. See title page for details.
Flight Management, Navigation -Flight Management System
Operation
737 Flight Crew Operations Manual
If there is no E/D point, FMC predictions assume a computed profile to 1000 feet above the destination field elevation, at a position which will vary according to selection of arrival procedures. The FMC will provide a slowdown profile for approach. VNAV path descent is not available if there is no E/D point.
VNAV Descent and Approach Path
The descent path starts at the calculated top of descent (T/D) point and includes waypoint altitude restrictions. The path is based on:
.
idle thrust . descent wind speed decreasing
.
speedbrakes retracted with decreasing altitude
. applicable target speed.
YA613 - YD651, YK101 - YK104 After the first “at” or “at or below” restriction, the path angle is level until intercepting the idle thrust descent path to the next altitude constrained waypoint.
YD652 - YD655 After the first “at” or “at or below” restriction, the path angle is constant between waypoints.
Note: When passing top of descent following high speed cruise operation (within
approximately 6 knots of Vmo/Mmo, cost index of 100 or higher), VNAV
may revert to LVL CHG to prevent overspeed. Reduce airspeed to the
VNAV target descent speed prior to reengaging VNAV.
Normally, the target speed is economy speed above the airspeed restriction altitude and 240 knots below that altitude, until deceleration for approach. VNAV will not permit descent below the airspeed restriction altitude until the airspeed is at or below the restricted value plus ten knots. The start and end of the airport speed restriction deceleration segment is shown on the map as a green open circles with no labels.
The descent path assumes deceleration to reach the final approach fix (FAF), or the glideslope intercept point at VREF 40+20 knots.
Target speeds are changed by entries on the DESCENT page. Entries made on the LEGS page are “at or below” and may limit the target speed. Wind and thrust assumptions are changed on the DES FORECASTS page.
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