AltairLabs:
Howdy Gerg!
Hello!
AltairLabs:
Yes, I did mention Flight Levels, you even quoted me on it:
Please note I said I was going to nit pick - meaning being needlessly, overly critical. I was simply adding to and slightly clarifying the use of 29.92 on the altimeter at >= FL18. I did not mean to offend. It was more of an, "if anyone cares", level of detail. And even then I could have been less ambiguous in what I stated.
AltairLabs:
But I passed over it briefly, since this thread by @Fabio Varesano seems to deal with ground altitude rather than flying. @Fabio Varesano biggest error source by far is failing to use the baro setting, surprised nobody else caught it.
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@Fabio Varesano can indeed get accurate terrain elevation from a barometer, but you do have to accurately account for the local pressure.
I thought I had and addressed it. You can get an accurate altitude, but only so long as weather remains constant. In most places, this simply doesn't happen for very long, especially in the summer months. Even the heating and cooling of the day will change the reading. If you have an error of 2' when you're 1000' in the air, no one cares. But if you are 4' off the ground, that's a 50% error. And that's exactly why when above FL18, everyone uses 29.92 because you're not likely to run into the ground at that altitude and everything else flying close enough to hit you should have the same error +- 50'. And so while your true altitude may be off by hundreds of feet, you're still so high its still not a noteworthy concern.
In this case, chances are his craft will be spending the majority of its time close to the ground. In fact, much closer than most private aircraft which spends most of its time < FL18 (average something like 6k-8k MSL), whereby pilots do constantly, manually, correct for barometric changes. There's a very good reason for that - weather happens and is rarely static for long.
So yes, absolutely, you can get accurate altitude, but only reliably for short periods of time. The further out you go from your baseline reference, the greater the probability of induced error and greater the error is likely to be. Accordingly, from the aircraft's perspective, it has no way of knowing if the induced error is because of a change in altitude or a change in weather. Now you can attempt to correct some with a temp/humidity sensor, but even that is only going to lessen to probable rate of decay over time rather than mitigate the error entirely. This may allow for enough compensation over a large enough window to not worry about things, unless you know some type of pressure system is moving through.
Furthermore, on days where fronts are moving through the area, even at a slow pace, the change over a span of fifteen minutes to an hour can still be significant, especially if beginning at mid morning whereby rapid heating is also likely to soon follow.
Long story short, unless you're willing to move forward knowing you have a very long list of caveats and an error which is very likely to grow over time, such a method for an aircraft is far from ideal without using a reference station as I previously outlined.
I don't recall reading anything about flight durations, but IMOHO, if they are around or over fifteen minutes or so, I wouldn't trust it without at least temp/humidity compensation. And beyond that, I wouldn't trust it for but maybe another fifteen or so; or at least not without a close eye on current weather and fixed altitude barometric readings so that you know with what error you're actually flying.