lospilotos
4th Feb 2012, 09:09
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.
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.