How does autopilots using barometric pressure maintain altitude, when these can vary wildly in non-calm situations? I would expect at least gps added to the mix (although with russia blocking skies randomly that may not be the smartest option these days). Or somehow keeping inclination (dont know the proper name of instrument here).
It uses the pressure altitude relative to the sea level pressure you configure in the plane (the altimeter in inHg / QNH). Above transition altitude (in the US usually 18000) you switch to standard pressure (29.92 inHg), so for those higher flight levels we ignore pressure differences. But for altitudes below this, yes you constantly have to change the altimeter in the plane or else your altitude / flight level chance / VNAV descent / climb will be incorrect. Of course you can always get the pressure by tuning into a ATIS / AWOS etc radio station for a nearby airport if you aren't getting the latest pressure from the ATC enroute.
Just to add for completeness, for ICAO/the rest of the world:
Transition altitude (ascent) and transition level (descent) is typically defined per FIR (flight information region), but can be airport-dependent. It is listed in the relevant airport charts. 7000 is fairly common, but it varies. Altimeter setting is measured in hPa/millibars (with the standard being 1013.25, which corresponds to 29.92 inHg).
To expand on verst's answer, the altimeter setting is defined based on your location, to roughly match meteorological conditions. When flying IFR or otherwise directed by ATC, you will be given altimeter settings. They will also be repeated during arrival and landing. In addition, they are given using an automated weather service called ATIS. ATIS broadcasts are given an incremental letter, that way you can verify you have the latest update. When checking in with a controller you will give the letter, and they will tell you if you're outdated.
The most important thing is that everyone in an area reference the same altitude/altimeter setting for traffic avoidance. Whether or not that is off from the actual altitude by a few hundred feet or not is secondary, since ground avoidance typically uses a lot bigger safety margins. Airliners and military planes have radar altimeters to measure height above ground when flying in IMC (low vis), while smaller planes typically fly VFR (based on visual rather than instruments).
> Airliners and military planes have radar altimeters to measure height above ground when flying in IMC (low vis),
But the RA are used primarily in the later phases of the approach. The barometric altimeter setting remains vitally important throughout most IFR ops in all categories of aircraft. (Radar altimeters are a key component of the ground proximity warning system GPWS in transport aircraft.)
> while smaller planes typically fly VFR (based on visual rather than instruments).
General aviation aircraft are quite frequently well-equipped with instrumentation for IFR ops. I’ve flown approaches down to published minima in single engine aircraft and have (legally) departed in low IMC conditions that grounded Part 121 flights. (Not necessarily best judgement…but here to tell the tale that I’ve become wiser in older age…)
> But the RA are used primarily in the later phases of the approach.
Indeed, the context of my sentence was using it for ground avoidance.
> General aviation aircraft are quite frequently well-equipped with instrumentation for IFR ops.
Yes, but most GA flights are VFR flights. Again, the context was ground avoidance and my point was that since they are usually used for VFR, more advanced means of ground avoidance are not generally required.
