There are comments on PPRUNE that FlightRadar24 data is not reliable in that sort of terrain? Everybody here seems to treat FR24 data like it's gospel but I wonder?
I actually think those spikes are erroneous data, or out of order data.
I think this is a good topic. I often see people mention that flight tracker data isn't always reliable, but what exactly can be unreliable about it? After all, those values are directly coming on onboard instruments. It's not like some they're some estimated values from an outside third-party measurement.
Here's my thoughts about what is/isn't reliable about ADS-B / flight tracker data. Feel free to correct or add more:
Timestamp - Assuming the timestamp on the ADS-B message comes from the transmitter, this should always be 100% accurate (Unless the time on the transmitter was off. But even then, the relative time would be accurate, so you would not get out-of-order data.)
Lat/Long - This comes from the onboard GPS. This could be slightly off as GPS isn't always perfectly accurate. This could explain some of the "backtracking" that you see in data, especially the granular data. But I think it is a result of GPS and not out-of-order data.
Altitude - This one is often misused because ADS-B uses standard Pressure Altitude in ft (like Flight Level). So it's not that it's wrong, but often people treat it as True Altitude or AGL. Otherwise the Pressure Altitude should be mostly accurate (in 25 ft increments). And even if misused, at least the Relative Altitude would be accurate.
Speed - is the horizontal ground speed in kts. I'm not sure if it comes from GPS or IRS (inertial reference system).
VSpeed - is the vertical speed in fpm. I believe this comes from IRS.
Track - degrees. I assume this would be accurate.
ADS-B receiver - In mountainous terrain, the receiver may not be able to pick up the ADS-B signals. So it may be missing some. Or in this case, I believe the last ~7s of ADS-B signals were missing because it dipped below the ridge (the numbers check out. The last ADS-B was 1570 adj altitude. The ridge was 1500 ft.) But again, you will be able to tell in the data if signals were dropped by gaps in the timeline.
Flight tracking websites - By default, they may only save/use the ADS-B data like every 30 seconds or 1 minute (otherwise too much data for them). This can lead to misleading conclusions because they interpolate or smooth the graph with very sparse data points. That's fine for normal, smooth flights. But when something goes wrong, the flight tracking sites may not show it because the data points are too sparse. Thankfully, FR24 will usually publish the granular data for major incidents. That gives the ADS-B data about every 2s.