VNAV Selection Limitation B744
 

"If leveling off within 2000 feet after changing altimeter setting from QNE to
QNH, or QNH to QNE, do not use VNAV to execute the level-off if QNH is less
than 29.70 in/1006 hPa. After the level-off is complete, VNAV may be
re-engaged."

Anyone know the results if this is not followed? Overshoot the level off?

Thanks

Correct, depending on rate of decent/climb, you may pass through the desired altitude by a few hundred feet and then return to it.

In a decent the easy solution is just to "transition" early and set the QHN prior to the 2000 limitation. So set QNH when cleared to an altitude below TL yet still prior to the 2000' limitation gives the autopilot plenty of time to catch the large down drop in altitude from changing from STD/29.92/1013 to QNH.

But........it says not to do this while in VNAV. Is there any reason that an overshoot of altitude is less likely when in FLCH. In fact, it might be more likely when descending as FLCH has a tendency to be with throttles closed and therefore, a higher rate of descent.

So why not allowed in VNAV but allowed in V/S and FLCH?

Must be something in the FMS/autopilot algorithms that handle the change in QNH...