FBW Trim Reference Speed

[Pilot53 544]

On the 777 what other than the trim causes the fbw reference speed to change?  How does this differ with and without the autopilot or the flight director engaged?

[downscc 1,327]

See FCOM 9.20.10.  The trim reference speed is changed with the trim switches, which do not move the stabilizer directly rather they make input to the Primary Flight Computers (PFC).  Note that the AFCS also affects trim while in A/P, for example on takeoff the trim speed will be the appropriate V2 or V2+10 airspeed.  The trim reference speed is that speed which will be maintained without input to the control column. There is a good brief discussion in the FCOM.

[Pilot53 544]

Ok Thanks, the fcom also doesnt seem to mention anything other than the a/p or trim switches effecting the reference speed.  So I take it things like the flight director alone or the mcp speed window have no effect on the reference speed?  Typically when I fly an approach I use the autopilot with vnav or flch mode until I reach around 5000 feet at 240 kts.  If i then disengage the a/p and manually set  an mcp speed of lets say 190 kts with flaps 5 will the reference speed remain 240 knots?

[downscc 1,327]

Mike, don't complicate the FBW reference speed. It's really very simple. It only applies when you are not using the autopilot and it is that speed that the aircraft is trimmed to fly.  It may descend or climb depending on power but it will tend to hold the reference speed.  The only way to change the reference speed is with trim. By definition, it is the speed that the aircraft is trimmed to fly.

 

In your example, you are both cancelling A/P and changing A/T... since the power required to sustain 190 kias is less than 240 kias (the speed last trimmed to by A/P) the aircraft will want to pitch down  to accelerate which A/T then reduces thrust.... I suggest you use speed intervention with A/P, then cancel A/P once speed is at your approach target.

[Pilot53 544]

Ok great, thanks for clearing that up.