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 said the Distributed Denial of Service (DDoS) attack had hit "the availability of our services" but not compromised 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.
⚠️ For the third time in two days Flightradar24 is under attack. Our engineers are working to mitigate the attack as quickly as possible and we hope to be back tracking flights soon. We appreciate your patience and apologize for the inconvenience. Updates to follow in thread.— Flightradar24 (@flightradar24) September 28, 2020
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, and distress signal codes, among an array of other advanced features, some of which are locked behind a subscription payment.
Such tools make the website, and its competitors, a valuable tool for plane-watchers and for journalists covering accidents and other airline incidents.