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I was watching Flight Radar 24, positioned near my house, to see what planes I could spot, and noticed a Cessna 550's transponder just dropped off @ 5,325 ft in a high-speed descent!

Is there something I should do about it? Or do I assume somebody in ATC has seen it?

Is there anything anyone should do in similar circumstances?

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This is almost a dupe of this question –  Pondlife Jan 7 at 15:55
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@Pondlife Same fish, different pond -- I think this one can stand on its own as a reminder to people that they don't need to call emergency services if they see a flight disappear from FlightAware. –  voretaq7 Jan 7 at 17:56
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@voretaq7 Yes, I agree; I find it's often interesting/useful to link similar-but-different questions. The different perspectives and details are often good to know. –  Pondlife Jan 7 at 18:15
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In addition to the answers below, have a look at flightradar24.com/faq - "I was following an airplane when it seemed to have disappeared, why?" –  Paul Cager Jan 7 at 23:20

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up vote 32 down vote accepted

Really there's nothing you should do in cases like these.

Flightradar24, FlightAware, and similar services should not be used for flight safety purposes, and most of them specifically state so in their terms of service (such as sections 12 and 14 of Flightradar24's terms and conditions). Their sources may go down for whatever reason -- that does not mean that the plane has crashed.

These services can only track what ATC controllers can (for the most part), so it's more likely that if there is a real issue, that they'd notice without the general public making a fuss.


In this specific case the flight was being tracked using MLAT (designated by the radar type T-MLAT, which is shown in your animated image file). MLAT flight track accuracy relies on the accuracy and number of the receivers in the area, and according to Flightradar24's website, MLAT generally only works above 10000 feet:

MLAT

In some regions with coverage from several FR24-receivers we also calculate positions of aircraft with the help of Multilateration (MLAT), by using a method known as Time Difference of Arrival (TDOA). By measuring the difference in time to receive the signal from aircraft with an older ModeS-transponder, it's possible to calculate the position of these aircraft. Four FR24-receivers or more, receiving signals from the same aircraft, are needed to make MLAT work. That means that MLAT coverage can only be achieved above about 10000-20000 feet as the probability that signal can be received by four or more receivers increases with increased altitude.

MLAT coverage is today limited to some parts of Europe and North America, but expanding fast.

So as the aircraft descends below 10,000 feet there is an increasing chance that you will lose it on Flightradar24's tracking system.

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I would add that loss of RADAR contact does not mean that they are not still in contact with ATC –  p1l0t Feb 18 at 1:13
    
one more thing: those websites/systems are always delayed by quite a lot, and have a low sample rate, so by the time you see something drop away people will already be on it if there's anything wrong. –  jwenting Feb 19 at 12:54

Nothing you should do.

Flight Radar 24 does not have coverage down to ground level in all areas. ATC has better surveillance and will be aware of the aircraft. Probably it's just in a fast descent, the Citation II / Bravo is quite a nice toy.

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