Boeing has sent instructions to airlines using its 737 Max jet about how pilots should react to erroneous readings from sensors, an issue investigators believe was a key factor in the Lion Air crash in Indonesia, in which all 189 people on board were killed.
Aviation experts said the initial findings indicate that the the pilot and first officer of flight JT610 did not know at what speed they were flying, similar to the problem experienced on Air France flight 447, which plunged into the Atlantic sea in 2009.
Boeing said it had issued an operations manual bulletin, advising pilots on how to deal with “erroneous input from an angle of attack (AOA) sensor”.
The AOA sensor sends out information about the angle at which the aircraft is flying, which indicates to the captain and first officer whether the plane may be at risk of stalling.
An AOA giving out erroneous data can lead to incorrect speed readings, potentially causing confusion among the flight crew and a rapid loss of altitude.
An AOA had been changed by mechanics on the ground in Bali the day before the crash, when similar problems occurred, Indonesia’s National Transportation Safety Committee said.
Aviation safety expert David Gleave said this might lead to the captain and first officer seeing different speed readings and reacting poorly, particularly if they were inexperienced..
“Quickly after take-off, you engage autopilot and the autopilot wouldn’t know which speed is correct, so it says ‘Over to you chaps, this is what you get paid for’,” Gleave said.
“The correct reaction is to work out which set of instruments is generating good info and which is generating bad info. The person with the good info displayed then flies the aeroplane manually.
“The first thing you’d need to do is relax and not overreact. Alarms may be going off and the first thing to do is do nothing. The aeroplane was OK beforehand and will be OK for a little while until you figure things out.”
He said there were several checks that flight crew could perform that would help identify which airspeed reading was correct. But he warned a fatal accident might ensue due to a “startled reaction” from the pilot, or if flight crew were not sufficiently well trained to perform the required checks.
Lion Jet’s Boeing 737 Max8 crashed into the Java Sea just 13 minutes after take-off, indicating pilots would have limited time to identify the issue and react correctly.
On Monday Indonesian accident investigators said an airspeed indicator on the crashed jet was damaged for its last four flights, but US authorities responded cautiously to suggestions of fleet-wide checks.
While Boeing has issued a bulletin explaining how flight crew should react to erroneous readings, the power to ground aircraft and insist upon further checks rests with regulators such as the Federal Aviation Authority in the US and the Civil Aviation Authority in the UK.
Boeing said: “The investigation into Lion Air flight 610 is ongoing and Boeing continues to cooperate fully and provide technical assistance at the request and under the direction of government authorities investigating the accident.”
The Boeing 737 Max is the US aircraft manufacture’s fastest-selling plane of all time, garnering about 4,700 orders from 100 customers in the first few years of its life.
It is an upgrade to the 737, the most popular plane ever built, with added range that allows airlines to use it for long haul flights, unlike its predecessor.
John Strickland, director of independent transport consultancy JLS Consulting, said: “The 737 used to be a short-haul aircraft but is now long haul capable and Norwegian, for example, is using it for that.
“It has proven very popular with a number of airlines who previously wouldn’t have taken a risk on some long-haul markets due to the larger capacity and poorer economics of previous generation aircraft.”
Airlines using the 737 Max include Norwegian, Tui, the Singapore Airlines offshoot SilkAir, Garuda Indonesia and Canada’s WestJet.