Ethiopian Airlines Group suspended a plane crew pending an investigation after they lost communication with air-traffic control in Addis Ababa before later landing safely.

Flight ET343 was en route to the Ethiopian capital from Khartoum in neighboring Sudan on Monday, Africa’s largest airline said in a statement. Appropriate action will be taken against the aviators based on the outcome of the investigation.

The pilots had both fallen asleep, according to the Aviation Herald, and were only woken by a warning siren triggered when the autopilot disconnected. They were then able to return the plane to its correct flightpath. 

Ethiopian didn’t immediately respond to a request for comment on the report.