Originally Posted by
bigduke6
Since VNAV is prohibited (B744) when using QFE, I have used it on take-off from Moscow several times, and in Bishkek years ago when they still used QFE.
Anybody know why LNAV and VNAV is prohibited on the 744 when using QFE?
Answer discovered later...
Limitation on LNAV/VNAV with QFE is that any conditional altitudes in the FMS are referenced to QNH, not QFE. Secondary (Minor) consideration is airplane without GPS and doing DME updates will get slant angle/range wrong using QFE. In reality, if the departure doesn't have conditional waypoints, and the plane has GPS, VNAV will work without any issues for the very short time you are below the transition altitude.
FMC does not know altimeter is on QFE, so a conditional alt is with a condition, like after reaching 4000' turn left, where you can't manually code the point in yourself. Something like 5000B at a specific point you could actually could make work, as you could adjust the altitude and change it 4000B, (assuming 1000' field elevation using QFE)
For LNAV, the NAV accuracy for non-GPS planes is degraded by the slant range error.
In reality, most planes have GPS, or the slant range is not going to make a big enough error to really make a difference in LNAV unless you really need a low RNP.