Popular real-time flight-tracking website Flightradar24 was hit by a cyber-attack that knocked out access to its services for hours.
The attack is the third the company has suffered in two days, it said.
Early attempts to restore the site failed, with “significant instability due to the sustained attacks”, it said.
It did not say exactly what kind of attack it had experienced, but said it hit “the availability of our services” but not user data.
The site appeared to be improving on Tuesday, with intermittent loading errors.
The website allows users to track planes – both commercial passenger flights and private ones – in mid-air, following flight paths live.
Flightradar24 says it has about two million users and tracks 180,000 flights every day. It includes aircraft manufacturers – such as Airbus and Boeing – among its customers.
Its tools can identify a plane’s model, flight number,