Changing QNH while maintaining altitude

[N1G 88]

Hi,Maybe I am missing something but I was not far into my flight and decided to check QNH at local position. The change resulted in a few hundred feet difference. When I change it, of course I get hit by the controller to maintain my altitude. My cruise altitude for this GA flight was 6000. How can I avoid the slap from ATC and still maintain the correct altitude. RegardsBob

[Ray Proudfoot 8,947]

Hi Bob,If I understand you correctly your assigned cruise altitude was an altitude and not a flight level. Perhaps you could give me your departure and arrival airports to confirm this.As RC instructs you to climb to a new altitude it should issue the current QNH. Always make sure you adjust your altimeter to the pressure advised by RC. If it changes RC will advise you.I don't understand why RC wouldn't pick up the new QNH and not advise you but more info on your flight details might help. If all else fails and you can easily reproduce the problem then a log to JD will clear things up.

[jd 57]

Hi Bob,If I understand you correctly your assigned cruise altitude was an altitude and not a flight level. Perhaps you could give me your departure and arrival airports to confirm this.As RC instructs you to climb to a new altitude it should issue the current QNH. Always make sure you adjust your altimeter to the pressure advised by RC. If it changes RC will advise you.I don't understand why RC wouldn't pick up the new QNH and not advise you but more info on your flight details might help. If all else fails and you can easily reproduce the problem then a log to JD will clear things up.

and a further note, don't use the "B" key. it does not work everywhere. only in the USA.keep this in mind - if your assigned altitude is in feet, put local pressure into the altimeter.if your assigned altitude is a FL, enter the standard pressure (29.92) into the altimeterjd

[N1G 88]

Hi guys,Well I dont remember if it was FL or just altitude. My cruise altitude was 6000. I had gone of frequency to check weather and did receive a new pressure which of course I adjusted.So when you say put local pressure, do you mean "maintain that pressure" regardless of what another ATIS report might state enroute ? Bob

and a further note, don't use the "B" key. it does not work everywhere. only in the USA.keep this in mind - if your assigned altitude is in feet, put local pressure into the altimeter.if your assigned altitude is a FL, enter the standard pressure (29.92) into the altimeterjd

[Ray Proudfoot 8,947]

Hi Bob,I'm guessing you haven't flown the tutorials as those will teach you all about transition altitudes and transition levels. This knowledge is fundamental in operating your aircraft correctly so RC doesn't bawl you out.An altitude of 6000ft and a flight level of 60 will only be the same when QNH is 29.92 / 1013.2. If it is any different your aircraft could be hundreds of feet higher or lower than the altitude RC expects. That's why it's important you understand the difference.Sorry if that sounds patronising but once you know the difference you won't have this problem again.To answer your question you should only change pressure when advised by RC, not by listening in to a local weather report.

[N1G 88]

Thanks again Ray for your explanation and I am not thinned skinned so go ahead and holler at me!!!Bob

Hi Bob,I'm guessing you haven't flown the tutorials as those will teach you all about transition altitudes and transition levels. This knowledge is fundamental in operating your aircraft correctly so RC doesn't bawl you out.An altitude of 6000ft and a flight level of 60 will only be the same when QNH is 29.92 / 1013.2. If it is any different your aircraft could be hundreds of feet higher or lower than the altitude RC expects. That's why it's important you understand the difference.Sorry if that sounds patronising but once you know the difference you won't have this problem again.To answer your question you should only change pressure when advised by RC, not by listening in to a local weather report.