FlightAware Discussions

Missing VFR Flight


#1

This morning (Jan 10, 2018) I flew my ADS-B equipped aircraft VFR from KAPF to KPGD. This afternoon, I flew the same aircraft back to KAPF.

FlightAware shows the afternoon flight, but not the flight this morning. Any idea why?


#2

What’s the N-number?


#3

The aircraft identifier is N541MN


#4

We did have full ADS-B coverage for the morning flight but it looks like it never quite met the criteria (altitude/groundspeed) to get turned into a proper flight on the website. The afternoon flight was a bit higher & faster so did make it in. We didn’t get a reliable in-the-air indication in the ADS-B data itself (I’d guess no weight-on-wheels sensor?) and in that case we fall back to some fairly conservative checks on groundspeed/altitude. Nice clean ADS-B data otherwise though.


#5

My altitude northbound was 2,500 ft. Southbound was 3,500.

My aircraft (N541MN) is a motorglider, so my cruising airspeed is ~110 knots (~60 knots while climbing). This morning, there was a 15-20 Knot headwind, so my groundspeed was significantly lower than normal.

I got an FAA ADS-B Performance Report for the flight, and it shows that my ADS-B OUT system was working properly and was indicating that I was airborne, not on the ground (my Dynon ADS-B OUT system automatically switches Air/Ground mode as required).

I am confused why FlightAware would have additional filters that would block an otherwise valid ADS-B flight. Given the ground stations that FlightAware has in SW Florida, I am sure that my flight data was being received for almost the entire duration of the flights in both directions.

I love the FlightAware ADS-B network, and have two ADS-B receivers setup myself (one in Minneapolis, and one in Naples, FL), so I am a BIG fan. Just want to make sure that all this great data is fully visible and not getting lost.


#6

Yep, there was no problem with your transponder and we had ADS-B coverage including ground coverage the whole way.

The air/ground issue is an unfortunate combination of problems. There’s a field in ADS-B messages which carries air/ground status, but it has some ambiguous values. Here’s what the spec says:

Unfortunately your transponder looks like it falls into case (a) and is transmitting CA=6 (“either airborne or on the ground”) which means we can’t unambiguously decide that it’s in the air based only on the ADS-B message’s CA field in that case.

There is also a distinction in ADS-B between “surface position messages” and “airborne position messages” (essentially, surface position messages are more precise but omit altitude). In your case, your transponder does appear to send surface position messages when on the ground, and airborne position messages when in the air. Unfortunately there are lots of other transponders in the wild that do not do this and continue to send airborne position messages with CA=6 even while on the ground. The extra checks on groundspeed/altitude are to deal with those transponders and you’re unfortunately getting caught in the crossfire.

(a caveat here: I’m not looking at the raw messages sent by your aircraft, just inferring what probably happened from the summarized data that we collect)

I’ll look at whether we can tweak the thresholds but there is a trade-off, we don’t want to create false positives, it’s a delicate balance.

edit: it’s also worth noting that if we have a flightplan from another source (e.g. you filed IFR and we got that from the FAA) then we’re generally much happier to assign positions to that existing flight with fewer checks.


#7

It would be great if you could get this fixed. Note that gliders can fly at quite low speeds (Air speeds <40 knots). If you are in mountain waves, the ground speed could actually be 0.

The FAA ADS Performance Report shows that the flight was airborne, so I’m not sure where the discrepancy is between that and what you are seeing.