Robertfm wrote in Wed Sep 18, 2019 4:28 pm:I've noticed several planes including the Piper PA28 and the Lioneau actually don't have either hPa (hectopascal) and inHg (inches of mercury on them. I have a chart that converts to +- feet and this can be set when plane is on the ground but how can you do it when flying and pressure changes.
You should never have to convert to or from feet; if you do, then you're doing it wrong. Altimeter setting in any reasonably modern aircraft is going to be either in hPa or inHg, or there will be a switch selecting either. The PA-28 definitely has a QNH knob, though it may be hard to read in FG due to limited screen and texture resolutions. There may be a tooltip though that tells you the current setting when you hover over the knob.
So here's what you would normally do:
On the ground, you get your ATIS information on the radio. This will give you a QNH, either in hPa, or in inHg. If it's the wrong unit, you grab a conversion table from your glareshield (or wherever you have it) and convert it. Then you dial that number into the altimeter's QNH setting. If the ATIS says "QNH 1020", then that's what you dial into the altimeter. You may then want to double check the altitude displayed on the altimeter against the known airfield elevation; it should be within a few tens of feet. E.g., if you know the airfield is at 150 ft above sea level, and you have dialed in your QNH, then the altimeter should read something between 140 and 160 feet or so. You can also do it the other way around: turn the QNH knob until the altimeter matches the airfield elevation, then verify that the QNH setting is the same as what the ATIS said.
Once airborne, you don't need to do anything until one of the following happens:
a) You climb above transition altitude (18,000 ft in the continental US, often considerably lower elsewhere, e.g. 5000 ft in many parts of Europe); at this point, you switch to "standard" QNH (1013 / 29.92), and you leave it there until you descend below transition level again.
b) You cross into an airspace for which a different ATIS is available, or there is a significant weather change. In this case, you would get the new relevant ATIS information, and dial the new QNH in (again converting between hPa and inHg as needed).
c) You descend below transition level. This happens a good while after planning your descent, so you have plenty of time beforehand to get the ATIS for your destination, do conversions as necessary, and write down what you need to enter when you cross the transition level.
d) ATC informs you of a changed QNH for your current position. The most up-to-date QNH is often part of your approach clearance, but should rarely come as a surprise at this point - including it in the clearance is more of a sanity check than anything.
Again, you should never need to convert between QNH (whether hPa or inHg) and feet, there simply isn't a use case.