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For small aircraft flying below 2,500 ft. AGL, is it appropriate to compare the landed ADS-B altitude (pressure calibrated AMSL) to the elevation of the runway in order to compute a reference offset for flight altitudes?

For example, if an aircraft's ADS-B reports a landed altitude of 700 ft. and the known runway elevation is 500 ft. above sea level, is it appropriate to apply that -200 ft. offset to reported flight altitudes? Thus a reported ADS-B altitude of 1,500 ft. would be approximately 1,300 ft. for that aircraft/flight?

This question is regarding aircraft flying in the general vicinity of the airport and with relatively short flight durations (less than 20 minutes).

Edit: For clarity, this question is related to data from flight tracking websites such as FlightAware.

bhamjman
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    ASE answers, and other resources, linked in comments under this ASE question may be relevant: https://aviation.stackexchange.com/q/93379/34686 – quiet flyer Jun 02 '22 at 01:14

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For an after-the-fact analysis, for example looking at flight data recorded by an EFB or displayed on a flight-tracking website, what you describe sounds very reasonable.

For use during a flight this is absolutely unacceptable; even for a flight conducted under Day VFR conditions, an aircraft must be equipped with a pressure-sensitive altimeter. In fact this pressure altimeter is what provides the transponder (ADS-B) with its altitude information in the first place (unless the parameter you are looking at is explicitly labeled "GPS altitude").
In an emergency situation where your altimeter becomes inoperative but the pitot-static system as a whole is still reliable, you might be able to justify the use of ADS-B altitude information to provide situational awareness until you land, which should be as soon as practicable—but the mental math required to correct the ADS-B altitude means it should be considered an emergency procedure only.

randomhead
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  • My guess is that OP wants to look at data from FlightAware (or similar) and try to apply a correction to the observed altitudes. – Jim Jun 02 '22 at 02:30
  • @Jim is correct that my question is related to sites like FlightAware (I've edited for clarity) but the additional information is helpful nonetheless. Thank you.

    So does this mean that the aircraft's altimeter should be calibrated for local pressure which would then make ADS-B out more accurate?

     – bhamjman Jun 02 '22 at 02:57
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    @bhamjman: See the question (and associated resources) linked by QF for more information. The upshot is: the altimeter in the aircraft is indeed calibrated as you say, but the altitude encoder which transmits information to ATC is never calibrated. Because the ATC radar scope knows the local altimeter setting, it automatically applies the correction on the ground. This ensures that ATC sees accurate information even if the pilot mis-calibrates their altimeter. It has the side effect that systems which do not calibrate the ADS-B information, like flight tracking sites, are inaccurate. – randomhead Jun 02 '22 at 03:07
  • Thanks @randomhead. So if I'm understanding correctly, the pressure altimeter can be calibrated for local pressure, and then the altitude value is fed to the ADS-B transponder for broadcast. And I'm guessing this explains why multiple planes can all broadcast different altitudes (100-200 ft. variances) when landed? QF's link has also been very helpful. – bhamjman Jun 02 '22 at 03:33
  • Not quite @bhamjman. The altimeter is a device which shows the pilot what altitude they are at, and the pilot can adjust this device to account for the local pressure. But the altitude sent to ATC, via the transponder, does not account for local pressure. It assumes that the local pressure is equal to the standard pressure and sends that information. The ATC systems on the ground then take that number and adjust for the actual pressure, just as the pilot adjusts the altimeter in the aircraft. It is possible that ATC and the pilot could use different adjustment factors. – randomhead Jun 02 '22 at 03:42
  • Finally, what would you say is the reason for ADS-B altitude variances on the ground from multiple planes. In my example of a 500 ft. elevation runway, I'm trying to understand what contributes to ADS-B landed altitudes of 500ft, 600ft, and 700ft from 3 different planes. Trying to understand why the ADS-B altitude data if off so much from one plane to the next. I appreciate your time. – bhamjman Jun 02 '22 at 03:51
  • I would suspect it is variation in the physical equipment installed in the aircraft but I am not sure. Perhaps it is also a function of where the target "dropped off," provided the tracking site does not have coverage literally down to the ground. Both are merely guesses. That does sound like a question which deserves a post of its own! – randomhead Jun 02 '22 at 03:53