Originally Posted by
FlightDetent
Yes, error is not a good naming.
For recording and display purposes, different from the source ADS-B transmission, FR24 nullifies some of the altitude data.
When an aircraft is geometrically below the 1013.25 isobaric surface, the FR24 presentation would need to show negative values, since ADS-B transmits altitude referenced to standard pressure which actually is a negative numeber in that case.
To avoid confusion for the common inernaut, all negative reports are truncted to 0 for FRIf FL < 0 then, for display their records.
When converting the ADS-B FL as reported FR24 to ALT, some of that data are simply not there.
The cutoff layer is
FL < 0, which is the same as
ALT < (QNH - 1013.25)x27
With the modest 1018 hPa at local station in the discussed case, where 0 is dispalyed by FR24 the ALT was (anything) less than 125 ft. (XPDR codes level in 25 ft increments).
Good explanation of an often (as we have seen) misunderstood FR24 idiosyncrasy.