I wanted to know a simple thing: if you ask different free flight tracking APIs about the same patch of sky, do you get the same planes? So I called three of them, none needing an API key, over a small box around New York. I did it five times in a row at about 06:48 UTC on 1 November 2026.
They did not agree.
What came back
- OpenSky returned 7 to 8 aircraft.
- adsb.fi returned 18 to 19.
- adsb.lol returned 29.
Across the three, 31 different aircraft were seen. All three answered in under half a second (median 0.32 to 0.45 seconds), so speed is not what separates them. Coverage is.
I do not know why OpenSky returned so few. Each service uses its own receivers and rules, and OpenSky's docs describe anonymous access as limited, but I did not test the cause. It was also the middle of the night in New York, so the sky was quiet. Treat the numbers as one snapshot from one internet connection, not a ranking.
What none of them tell you
None of the responses I got contained an origin or destination. These APIs give you what an aircraft broadcasts about itself: position, altitude, speed and call sign. If you want departures and arrivals, you need a flight status API with a key. Aviationstack, for example, has a free plan of 100 requests a month.
The part most people skip: the licence
"No key" does not mean "no rules". I read the terms of each one.
- OpenSky's terms of use say the data is licensed for non-profit research and education. Any use by a for-profit or commercial entity needs a written licence from OpenSky.
- adsb.fi's page says its open data is for personal, non-commercial use only, with a credit link back to it.
- adsb.lol's docs say its API is licensed under ODbL 1.0, an open database licence with attribution and share-alike conditions. I read a summary of ODbL, not the full text, so check it yourself.
- airplanes.live refused my first call with a 403 and a message asking me to email them with a description of my project. I did not contact them. Secondhand sources describe it as non-commercial, but its guide page returned a 404 when I looked, so that is unconfirmed.
So a free API that answers instantly can still be one you are not allowed to build a business on.
If I were starting today
- For a hobby map, I would use adsb.lol with a small radius and cache the results.
- For a school or university project, OpenSky is designed for that. Credit the network.
- For a product, I would not build on a non-commercial API. I would read the ODbL conditions properly and plan to move to a paid or licensed source before launch.
What I did not verify
- I did not run any API that needs a key. Aviationstack's numbers come from its pricing page.
- I did not contact airplanes.live.
- I tested one area at one quiet hour, so counts will differ at busy times and elsewhere.
- This is not legal advice.
Which flight data API are you using, and did you check its licence? I would like to know if anyone has had OpenSky grant them a commercial licence.
I wrote up the full comparison, with the code I ran and the paid options, on the APIHiver blog: https://apihiver.com/blog/free-flight-data-api
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