737NG FMC 10.8 VNAV path

[lospilotos]

Hi!

Sometimes during the approach the FMC calculated vertical path just drops a couple of hundred feet, putting the aircraft high compared to the profile. It seems to be happening around the entered Transition Level in the Decent Forecast page. This happens even if the landing QNH is entered as well in the mentioned page.

Last time my colleague on the flight deck said it was due the the fact that we had switched from STD to QNH well above the TL, but that does not make sense to me. Isnīt the path calculated from TOD to EOD irrespective of altimeter setting, using the forecast QNH from Decent Forecast page. Switching between STD/QNH on the PFD doesnīt affect the path. Where does the FMC get itīs pressure information from and isnīt it always in relation to STD?

I of course always compare height vs. distance so I donīt need pointers like "donīt trust VNAV", I just want to understand the systems.

Thanks!
/L.

[BOAC]

I would guess that the 'puter is working on your altitude reading being 1013(.2) UNTIL TL and then QNH below, so it is technically getting 'wrong' information above TL? When it gets to TL it will look at the numbers and say 'doh' or whatever that is in 'puterspeak. Without a sophisticated algorithm to 'notice' that you do not have 1013 set or to 'compare' what you have set with the FMC QNH entry (which I don't imagine anyone bothered with), it has no way of knowing that what you have set is not 1013 or that 1013 is not the QNH.

I cannot think of any other reason. I was never sharp enough to notice, or more probably just doing my 3 x tables at the time/looking out of the window.

[Denti]

The FMC uses the pressure setting of the side with the MAster light on (if you don't have it on i suspect it takes the left sides setting). And the rest from my observation is exactly what BOAC describes. It calculates its path with standard until the entered TL, then the entered QNH value (if you used the descend forecast page at all, standard else).

[de facto]

Dear Boac,

say 'doh' or whatever that is in 'puterspeak

Whats that 'puterspeak'?
Thanks,

[BOAC]

I trust you are familiar with the slang word 'puter'? If not 'oogle will help you. 'Speak' is slang for a form of language used by certain groups, as in 'geek-speak'. Again, 'oogle will help.

[de facto]

Finger troubles there young man?
Doh ,yes from the simpsons,a classic.
Geek,yes but never was considered as one,always prefered running after girls instead of doing my maths and physics..

I aint chinese so cant help you there...ill google via VPN for the word puter