An aircraft had to make an emergency landing after one engine caught fire and the other stalled due to a fuel additive error, a report has found.
The plane took off from Gatwick Airport on 26 February after an engineer using an internet calculator added 37 times too much of the chemical.
The engineer had not carried out the task before, the Air Accidents Investigation Branch (AAIB) said.
The Titan Airlines plane returned to the airport safely.
The Airbus A321 was being moved from Gatwick Airport to Stansted Airport at the time. There were seven crew members on board, but no passengers.
The amount of Kathon - which is used to prevent contamination by microbes - to be added to the plane's fuel was about 1.6kg, but after using an internet calculator the engineer added 60kg, the AAIB reported.
"[The engineer] did not recognise that he was using an excessive quantity."
'Banging and surging'
The task had not been designated as "critical", which meant no additional checks were carried out to ensure it was done properly, the AAIB said in its interim report.
The watchdog was told that at about 500ft (152m) one of the plane's engines started "banging and surging", shortly after it took off at about 00:10 GMT.
The flight commander said the plane was "yawing and fishtailing all over the place", before flames were seen coming from the engine.
The commander issued a Mayday call before seeing the other engine's controls "begin to fluctuate". A "stall" message was displayed in the cockpit three times, he said.
The plane landed at Gatwick at 00:20, the AAIB said.
A full report will be published in due course, the investigators said.