Altimeter settings/AutoPilots.

[novicef]

I am studying for my ATPL exams and was wondering what would happen when there are large differences between QNH and 1013 with an autopilot engaged.

For example if the QNH was say 993 or say 1028 and I was at Transition Level of 60 and then cleared to an altitude of 5000 feet. How would the Auto Pilot react when I changed the datum setting and I selected LVCH.

Is there any sequence of resetting the altimeter and selecting LVCH to prevent the Auto Pilot from behaving in an un toward manner.

[towser]

Initiate the climb or descent first and then change pressure setting. on some aircraft if you change the pressure setting first the autopilot will attempt to regain its previous level/altitude.

[AerocatS2A]

First of all, there will be more than 1000' between the lowest available flight level and the highest available altitude if you have a low QNH. For example, in Australia, with a transition altitude of 10,000', FL110 is not available if the QNH is below 1013, FL120 is not available with a QNH below 980. This is to ensure that there is always adequate vertical separation between aircraft at flight levels and aircraft on QNH.

In your scenario with a QNH of 993 there's only 400' between FL060 and A050, a more likely scenario is if you were at FL070 and given A050, then you've got 1400' to do what towser suggested. Set and arm 5000', start your descent, call "transition", change to 993, call the alert for 5000' (if that's in your SOPs), and then monitor the altitude capture.

In the aircraft I fly (Dash 8 ), if you changed to the QNH first, the autopilot will climb to maintain the indicated altitude (given a low QNH). The problem with this is that you are reducing vertical separation with opposite direction traffic that should be 1000' above you. And you will directly conflict with any VFR traffic that would normally only be 500' above you.

[Beakor]

B757/767 will adjust to new datum in ALT HOLD or level in VNAV PATH if the subscale is changed. However, our SOPs (and most others I believe) say: Select new level or alt on MCP, then change subscale, then initiate climb or descent. Any other order, if you were to get distracted after iniating climb or descent and forget to reset the subscale then you're going to get a level bust. The transitory climb or descent as you reset the subscale is not noticeable as it takes at least 30secs for the jet to start climbing or descending and by then you've initiated the required mode.

[FullWings]

However, our SOPs (and most others I believe) say: Select new level or alt on MCP, then change subscale, then initiate climb or descent.
I would agree with that. On a practical note, it is useful to brief for unusually high/low QNH and to work out what effect it will have before you get to press the button! If you keep the rate of climb/descent down while close to transition, you might avoid an inadvertent "bust" as well. In the last few weeks we have had QNHs from 970-ish to 1045+ in the London FIR...