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GBO
8th Jul 2019, 10:17
A B777 is cruising at M0.83/FL360 with the autopilot engaged in SPD/LNAV/VNAV PTH.

The programmed ECON DES SPD is M0.80

If ATC has NOT issued a descent clearance, after passing Top of Descent, VNAV PTH changes to VNAV ALT, but what speed will the aircraft fly?

Does it reduce to M0.80 or maintain M0.83?

EGGW
8th Jul 2019, 10:52
Always stays the same as cruise speed/mach, well at least in my last 13 years on the 777. Descent Path has to be active for it to reduce to that mach/speed.

EGGW

BuzzBox
9th Jul 2019, 02:47
Always stays the same as cruise speed/mach, well at least in my last 13 years on the 777. Descent Path has to be active for it to reduce to that mach/speed.

The Honeywell Boeing 777 FMS Pilot Guide states (my emphasis):

The FMS DESCENT flight phase begins when the aircraft departs the cruise altitude at the top-of-descent or begins the deceleration segment prior to reaching the top-of-descent. The DESCENT phase extends to the last constraint in the descent (end-of-descent point).

The descent path is calculated starting at the end-of-descent and going backwards to the cruise altitude. The descent vertical path is calculated to satisfy decelerations, configuration changes, altitude and airspeed constraints, forecast winds, preselected descent speeds, and other constraining factors.

The FMS creates a deceleration segment at the top-of-descent point when the cruise speed is greater than the descent speed. If necessary, an acceleration segment is also created to meet speed constraints during the descent.

In this case, where the cruise speed is M0.83 and the descent speed is M0.80, the FMS should create a deceleration segment and the speed should reduce to M0.80, even if the aircraft doesn't descend.