UK investigators believe that a Turkish Airlines Boeing 737-800 pilot misinterpreted information on a departure plate, resulting in the aircraft's levelling off at extremely low altitude after taking off from London Stansted airport.
It flew for about 11km (6nm) at a height of just 500ft (150m) above the ground during the incident on 16 October last year.
The jet was operating a service to Istanbul and had departed runway 05 with instructions to follow the standard instrument departure pattern Dover Five Sierra.
Dover Five Sierra involves a climb straight ahead to 850ft - equivalent to about 500ft above Stansted ground level - then a right turn to a heading of 209e_SDgr, with the aircraft maintaining a 5.5% climb gradient before levelling off at 5,000ft.
But the Turkish 737 instead levelled off within seconds of departing. London area air traffic control contacted the crew and the pilot reported the jet's altitude as 900ft above sea level, below the minimum safe altitude of 1,800ft.
Controllers ordered the aircraft to climb immediately to 5,000ft and the aircraft proceeded to Istanbul without further incident.
In a bulletin on the event the UK Air Accidents Investigation Branch says the crew had misunderstood the notes on the Stansted standard instrument departure plate, apparently confusing the straight-ahead climb instruction with the initial level-off altitude.
Before take-off the co-pilot had entered 900ft in the altitude pre-selector. Although the captain had thought the figure unusual he did not seek clarification from air traffic control.
"After the incident the commander realised that he and the co-pilot had not registered the exact meaning of the 'initial climb' note on the SID plate and thought this might have been due to a language issue," says the AAIB.
It says that the occurrence, the first of its kind on record in the UK, was too rare to warrant a safety recommendation. But the AAIB also says the crew was fortunate that visibility was good and the surrounding terrain was relatively flat.
"Had [the aircraft] been in instrument meteorological conditions and operating from an airport where terrain was more prevalent," it says, "this incident could have quickly become more serious."
Source: Flight International
