Jump to content

Archived

This topic is now archived and is closed to further replies.

Pilot53

FBW Trim Reference Speed

Recommended Posts

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?


Mike Iacovetta

Windows 10 x64 | Cooler Master H500p Mesh | Intel i9-10900k @5.2GHZ EVGA 360 AIO | Gigabyte Z490 Aorus Ultra | 4x8GB DDR4-4000 C18 B-Die | GTX1080Ti H50 AIO | Seasonic PX-750 Focus Platinum | Samsung Odyssey HMD

 

Share this post


Link to post
Share on other sites

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.


Dan Downs KCRP

Share this post


Link to post
Share on other sites

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?


Mike Iacovetta

Windows 10 x64 | Cooler Master H500p Mesh | Intel i9-10900k @5.2GHZ EVGA 360 AIO | Gigabyte Z490 Aorus Ultra | 4x8GB DDR4-4000 C18 B-Die | GTX1080Ti H50 AIO | Seasonic PX-750 Focus Platinum | Samsung Odyssey HMD

 

Share this post


Link to post
Share on other sites

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.


Dan Downs KCRP

Share this post


Link to post
Share on other sites

Ok great, thanks for clearing that up.


Mike Iacovetta

Windows 10 x64 | Cooler Master H500p Mesh | Intel i9-10900k @5.2GHZ EVGA 360 AIO | Gigabyte Z490 Aorus Ultra | 4x8GB DDR4-4000 C18 B-Die | GTX1080Ti H50 AIO | Seasonic PX-750 Focus Platinum | Samsung Odyssey HMD

 

Share this post


Link to post
Share on other sites

  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    22%
    $5,585.00 of $25,000.00 Donate Now
×
×
  • Create New...