Why some aircraft don't show up on flight trackers

Open any flight tracking app, point it at the sky above a busy airport, and you will find aircraft that it can name but cannot route. It knows the type. It knows the registration. Ask it where the aircraft is going and it has nothing.

This surprises people, because it seems like the hard part — finding an aeroplane in the sky — has already been solved. In fact the route is the hard part, and the reason is worth understanding.

Aircraft don't broadcast where they're going

Almost all modern flight tracking is built on ADS-B: Automatic Dependent Surveillance–Broadcast. Aircraft work out their own position from satellite navigation and transmit it, roughly twice a second, unencrypted, for anyone with a receiver.

An ADS-B message carries the aircraft's identity, its position, its altitude, its speed, its heading, and its rate of climb or descent. What it does not carry, at all, is a route. There is no origin field and no destination field. The system was designed so that air traffic control can see where aircraft are, not to publish their itineraries.

What an aircraft does broadcast is a callsign — something like BAW123. For scheduled commercial flights, that callsign maps to a published schedule, and a separate database can turn it into "London Heathrow to Doha". Every route you have ever seen on a flight tracker came from that second lookup, not from the aircraft.

Which is why the gaps exist

Once you know the route is a lookup rather than a broadcast, the missing aircraft make sense. A route is only available if somebody has published a schedule under that callsign.

• Private and business aviation. No published schedule. A business jet flies where its passengers ask, often filed only hours ahead.
• Training flights. A light aircraft doing circuits has no destination in any meaningful sense — it is going to land where it took off, several times.
• Ferry and positioning flights. Moving an empty aircraft between airports is not a scheduled service.
• Military. Frequently absent from public feeds entirely, by arrangement.
• Charters and one-offs. May never appear in a schedule database.

In practice, expect a route for somewhere between 60% and 80% of the aircraft you can see. Near a general aviation field the figure is far worse; near a major hub at peak hours, better.

The other gap: aircraft on the ground

There is a second category that confuses people in the opposite direction. Point a tracker at a large airport and you will find dozens of aircraft that are not flying at all. Parked aircraft keep transmitting. So do aircraft under tow, and aircraft taxiing.

They are real, and they are correctly reported, but they are not spottable — you cannot look up and see an aeroplane that is sitting on a stand two miles away behind a terminal building. Any tool that shows you "aircraft nearby" without distinguishing these will bury the handful of aircraft actually in your sky under a pile of stationary ones.

What honest handling looks like

When we built Where To, this shaped two decisions.

Aircraft that are parked are filtered out; aircraft that are moving on the ground are kept, because you genuinely can see one taxiing from a terminal window or a perimeter fence.

Aircraft with no known route are still shown, greyed out, and you can tap one to see everything that is known about it. The alternative — quietly hiding them — is worse. If somebody is standing outside looking at an aeroplane that the app insists is not there, they will not conclude "that must be a private flight". They will conclude the app is broken.

A gap in the data is not a failure. Pretending it isn't there is.