A dizzy and nauseous captain and his co-pilot were forced to wear oxygen masks as they landed a plane carrying 139 passengers at Heathrow Airport.
A strong smell in the cockpit and cabin caused caused crew and some passengers to suffer eye and throat irritation.
But a report by the Air Accidents Investigation Branch found "no explanation" for the odour.
The incident, described as "serious" by the AAIB, happened as the Lufthansa Airbus approached London from Frankfurt on October 21 last year.
The plane was brought down safely after the captain and co-pilot put on the masks and requested priority landing clearance.
Once the engines were shut down, the situation in the cabin improved.
Six crew members were taken to a local hospital and released several hours later.
Despite taking blood tests, no medical reasons for the symptoms were found.
The report raised concern with "a growing number of cases in which there has been a similar lack of conclusive evidence as to the cause(s) of aircraft cabin air quality issues".
According to the UK Civil Aviation Authority, "fume events" like these happen on approximately 0.05% of all commercial passenger and cargo flights.