pmdg 747 400 loc and app not coming on

veshagoo · Trainee Joined: Feb 02, 2007 Posts: 72

hi...when i fly anywhere and have the fmc all set up for landing, my loc and app button does not light up. is this a software problem or is my fmc not setup correctly? just before i am ready to punch the loc button i get a ...vref not selected...any help is apprec..rgds ken

99jolegg · Posted: Thu Sep 11, 2008 6:01 pm Post subject:

You don't have the FMC / MCP setup correctly. You need to select Vref and the subsequent flap setting in the FMC. Without seeing the panel / FMC it's hard to say much more than that Confused

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veshagoo · Trainee Joined: Feb 02, 2007 Posts: 72

hi..thanks for your reply, i usually get a ...vref not selected, i dont fully understand what that means..would it explain it in the fmc section of the pmdg manual?..thanks again for your help..rgds ken

99jolegg · Posted: Thu Sep 11, 2008 7:43 pm Post subject:

Yes, it should do.

Vref is simply a multiple of dirty stall speed (Vso) that denotes threshold speed. Add 5 to this number and you have (roughly) Vapp...the approach speed.

Simply put, Vref is the speed you should aim for at touchdown. Vref is dependent upon how dirty the aircraft is (as in how much drag from flaps / landing gear) which means you'll find different Vref speeds for different flap settings. It's up to you to choose the flap setting / Vref combination in the FMC. This speed can then be set on the MCP (autopilot) to have the autothrust maintain it. I have an idea that this info is on the INIT page once you start the approach...could be wrong though.
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