The honest answer: usually nothing happened
An aircraft vanishing from a flight tracker is one of the most common things people notice, and it is almost never a sign of trouble. Tracking is a picture assembled from radio signals picked up by receivers on the ground, and there are a dozen ordinary reasons a signal stops arriving. Almost all of them are about the ground, not the aeroplane.
It is worth being blunt about this, because a disappearing aircraft is regularly mistaken for an emergency. If something genuinely goes wrong, the aircraft usually keeps transmitting throughout. Silence is far more often a receiver problem.
1. It flew over the horizon
This is the big one. 1090 MHz is line of sight — the signal does not follow the curve of the Earth. Once the aircraft drops below your receiver's horizon, the transmission simply does not reach it.
The geometry is unforgiving and easy to estimate. Range in nautical miles is roughly 1.23 × √(altitude in feet), plus the same term for your antenna height.
| Aircraft altitude | Theoretical maximum range |
|---|---|
| 1,000 ft | ~39 nm |
| 10,000 ft | ~123 nm |
| 30,000 ft | ~213 nm |
| 40,000 ft | ~246 nm |
Those are ceilings, not promises. They assume a clear path and no obstructions. What they explain is the single most common disappearance of all: an aircraft descending towards an airport a long way from you drops off not because anything changed on board, but because it went below your horizon.
2. Something solid got in the way
A hill, a tower block, or the house next door will block 1090 MHz completely. Most receivers have a permanent blind sector they never notice until they plot a coverage map and find a wedge missing in one direction.
The effect is strongest at low elevation angles, which is precisely where distant aircraft are. An obstruction that costs you nothing overhead can remove sixty miles of range on one bearing.
3. The aircraft is not broadcasting a position at all
Not every transponder sends position. Older airliners, a lot of business aviation and most general aviation carry Mode S, which replies with an identity and altitude but no location. Those aircraft can only be placed by multilateration, which needs four receivers to hear the same reply at once.
When an aircraft crosses out of the area where four stations overlap, MLAT stops dead. There is no graceful fade — the position is either solvable or it is not. This is why some aircraft disappear at a hard, repeatable boundary while others fade gradually.
4. It is on the ground
Aircraft on the ground are surrounded by terminal buildings, hangars and other aircraft, all of which block the signal. Many transponders also reduce output power when the aircraft senses it is on the ground, and some stop transmitting position entirely once weight is on wheels.
So an arriving flight that vanishes seconds after touchdown has usually just landed normally. It is one of the most reliable disappearances there is.
5. Deliberate privacy
Some aircraft are hidden on purpose, and this is a legitimate, documented arrangement rather than anything sinister.
- LADD — Limiting Aircraft Data Displayed. An owner can ask that their aircraft not be shown by participating flight trackers. The aircraft still broadcasts; the display is filtered.
- PIA — Privacy ICAO Address. The aircraft is assigned a temporary 24-bit address unconnected to its registration, so it appears but cannot easily be tied to an owner.
- Military and state aircraft, which may not transmit position at all, or may transmit only intermittently.
Different trackers honour different filters, which is why an aircraft can be visible on one site and absent from another at the same moment. That is a policy difference between the two services, not a difference in what the receivers heard.
6. Nobody is listening there
Coverage is built entirely from volunteers' receivers. Over oceans, deserts, mountains and sparsely populated regions there is often nobody within range at all. A transatlantic flight typically drops off the map west of Ireland and reappears near Newfoundland; it was broadcasting the whole way, and for most of it nothing was there to hear.
Satellite-based ADS-B reception now fills much of that gap for commercial services, but it is not something a ground-based community network can replicate.
7. Interference and a saturated receiver
1090 MHz sits in a busy neighbourhood. Strong nearby transmitters — broadcast FM, pagers, mobile masts — can overload the front end of an inexpensive receiver, and the symptom is a general loss of weak, distant aircraft while strong local ones stay solid. A filter usually fixes it, and the improvement can be startling.
Rain and cloud, incidentally, are not the problem people expect. At 1090 MHz atmospheric attenuation is small; a storm will not remove an aircraft from your screen.
8. The transponder really was switched off
It does happen. Transponders are occasionally selected to standby by mistake, and they can fail. Even then, an aircraft in controlled airspace is still on primary radar, still in contact with controllers and still being separated from other traffic. What you have lost is one broadcast, not the aircraft.
When something you were watching disappears, run down the list in order of likelihood: was it descending or low? was it heading away from you? was it near a known blind sector? had it just landed? was it an MLAT target? Only after all of those is anything unusual worth considering — and even then, the aircraft is fine and it is your reception that changed.