Originally Posted by
First_Principal
This a worthwhile and pertinent discussion to have.
To answer your question simply; yes such data
may add to understanding of incidents and accidents in some scenarios.
However I think some caution should be exercised, particularly for those using the the interpreted graphical displays often shown up here on PPRuNe.
Why? - a while ago
Pilot DAR enquired about an accident in which some ADSBExchange data was presented showing, as I recall, a very significant rate of descent. At least with ADSBExchange the underlaying code (tar1090 IIRC) is open-source and available for inspection. Having had a, admittedly brief, look at that code it suggested to me that the raw data may be interpreted to some extent before it's displayed to the end user. While I have no doubt the intent would be honourable and designed to improve utility, when it comes to important accident investigation I suggest that only the most base raw data should be used. It may be that some interpretation would still be necessary, but at least that would (should) be known and under the control of the investigators. In such cases it's my further view that were any data interpretation carried out that detail should included in the report so it is available for scrutiny by all.
In the matter of FR24 and other similar systems I claim no real knowledge, although I'm aware of the name and the purpose of the site. If, however, it's a proprietary system and the methodology/code is not not available for inspection then I would be even more cautious about
relying upon information from these sources. It could still be useful to assist in understanding, but without full knowledge of the path the data takes from the transmitter to your screen I'd have to consider it not necessarily reliable
per se.
I'd like to think that investigators would be aware of such possible issues, and be able to obtain raw unadulterated data, but at the present that would seem nothing more than conjecture? Thus I reiterate full disclosure a la the open-source philosophy would be advisable, and allow readers of results to be assured (and be able to check if they felt so inclined) how such results come to pass.
Some very good points made.
The (professional) investigation report that I linked to makes a distinction between the data captured by the ANSP and that provided by FlightRadar24 (the latter presumably similar to the data it publishes for public users).
To be fair to FR24, they make no attempt to run sanity checks on the data that they host - doing that is down to the end user.
One fundamental limitation of FR24 ADS-B data is that they fabricate transmissions that include both position and velocity data - that isn't how ADS-B works, and the inevitable consequence is that you can't be sure whether a supplied timestamp relates to the position or the velocity transmission from the aircraft. The Norwegian reports states that "Many [velocity and track] datapoints [in the FR24 data] were deleted because of great deviations".
A further limitation (regardless of the data source) is the Vertical Rate (i.e. RoC/RoD) ADS-B data item. Clearly it can't be an instantaneous value as it's normally baro-derived, so it's often at odds with the altitude timeseries in the data.
Lastly, the Norwegian report references Mode S EHS (Enhanced Surveillance) data. FR24 can't provide that data, so the ANSP was the only source for that, which include lots of useful parameters including IAS, TAS, roll angle (particularly relevant for the investigation in question).
So, in summary - is FR24 data useful in understanding what might have happened in an accident/incident ? Answer: Possibly, but only up to a point, and only if one appreciates the caveats and limitations associated with it.