French investigators have reiterated that the only strategy to limit injury risk from turbulence is for passengers to keep seat-belts fastened while seated.
Investigation authority BEA states that turbulence forecasting is not precise – able to provide only probabilities – and detection of clear-air turbulence is “not possible” with current on-board technology.
BEA gave its verdict in its inquiry into a serious turbulence incident involving a Transavia Boeing 737-800 (F-GZHM) operating between Lyon and Tel Aviv on 13 February last year.
The captain had expected light to moderate turbulence around 1h 30min after departure. While in the cruise over Montenegro at 37,000ft – about 86min after take-off – the crew was duly informed by air traffic control that the aircraft was entering a region in which turbulence had been reported between 38,000-40,000ft.
BEA says the captain advised the cabin crew chief of turbulence in “two or three minutes”, and subsequently illuminated the seat-belt light. The crew reduced airspeed from M0.79 to M0.77.
One flight attendant made an announcement to passengers while another began checking seat-belts were fastened. The onset of the turbulence occurred about 1min after the seat-belt light was turned on.
BEA says the aircraft experienced moderate turbulence for about 10s, with the air current changing from 35kt tailwind to a 11kt headwind, while the lateral component from the left dropped from 77kt to 50kt. Vertical load factor fluctuated between 0.52g and 1.7g.
Airspeed increased sharply, says the inquiry, and an overspeed alarm was triggered. The first officer, who was flying, deployed speedbrakes just before the turbulence intensified for 6s, with vertical loads changing from negative 0.7g, to 1.71g, then back to negative 0.32g.
The aircraft rolled up to 37° left at a rate of up to 38°/s.
BEA says the captain took control of the aircraft from the first officer and rolled the aircraft level, before commencing a descent to 35,000ft.
Cabin crew had not had time to prepare for the sudden turbulence and, while two members were able to find seats, two others were twice thrown towards the ceiling.
“Unbuckled passengers, seated towards the rear of the cabin, were also thrown upwards and struck the luggage compartments,” the inquiry says.
BEA says the incident injured two flight attendants and eight passengers. But the crew, having assessed the aircraft and the condition of those on board, opted to proceed to the destination.
The crew had been unaware of forecasts of strong turbulence on the route, because they did not have the latest information to hand, and were not familiar with colour coding on a weather application.
But BEA points out that turbulence forecasting remains an inexact field, and that – in the event of unforeseen turbulence – crews should use an emergency instruction to issue a warning to cabin crew and order an immediate fastening of seat-belts.
It notes that the “very short interval” between the pilots’ initial advisory to cabin crew and the onset of turbulence meant there was insufficient time to conduct a full cabin check.
There is an “intrinsic limitation” to procedures relating to unforeseen turbulence, it states, and the best defence against injury is for passengers to keep seat-belts fastened during flight.