Pilots Warned Lion Air of Fault on Doomed Jet Night Before Disaster

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The Lion Air jet that crashed into the Java Sea off Indonesia earlier this week, with the presumed loss of 189 lives, had experienced problems with the sensors used to calculate altitude and speed on its previous flight, an issue that could help explain why the plane dove into the water.

Pilots on the nearly new Boeing 737 Max reported the issue after flying from Denpasar to Jakarta the night before Monday’s accident, Lion Air spokesman Danang Mandala Prihantoro said on Wednesday.

The instruments were checked by maintenance workers overnight and the plane was cleared to fly, Prihantoro said.

While it will be days or weeks before definitive information emerges in the crash, which occurred shortly after takeoff, discrepancies in speed and altitude readings can cause confusion on the cockpit and have led to accidents in the past, including the 2009 crash of an Air France plane in the Atlantic Ocean.

Erroneous sensors could be an explanation for the flight track data, said John Cox, president of Safety Operating Systems and a former airline pilot.

But Cox and others cautioned that it is too soon to say what happened on the Lion Air flight and some of the flight data – such as speeds that weren’t extreme and none of the highly abrupt manoeuvres that preceded the Air France jet’s loss of control – may suggest some other cause.

Indonesia ordered Lion Air to fire its technical director and started an audit of the carrier’s maintenance facility after the disaster. The airline also dismissed the engineer who cleared the ill-fated flight JT610 even after the pilots had reported the problems a day before the crash.

The removals were ordered on the recommendations from the National Transport Safety Board, the Transport Ministry said in a statement in Jakarta on Wednesday.

Indonesia’s National Search and Rescue Agency hopes to find the plane’s crash-proof flight recorders and the main wreckage of Flight JT610 on Wednesday night, said M Syaugi, head of the agency.

Search teams have picked up sound signals from the so-called pingers attached to the recorders, which activate when they come in contact with water to make them easier to locate, Syaugi said.

Search and rescue teams are confident they have located jet’s fuselage, National Military Chief Hadi Tjahjanto said.

“We have located the area where we strongly believe the plane’s black box is located. The large parts of the aircraft should be nearby,” Tjahjanto said.

Even with modern GPS tracking, planes need to calculate their precise speed through the air. To determine airspeed – which can vary substantially compared to the speed over the ground due to winds – aircraft rely on Pitot tubes which measure the air rushing into them.

By comparing that pressure against the ambient air pressure – which is obtained by what are known as static ports – aircraft can determine airspeed.

If either of the pressure sensors is blocked, it can cause erroneous readings. In the case of the Air France flight, investigators concluded that a high-altitude ice storm clogged the Pitot tubes.

Modern jetliners are equipped with three separate airspeed sensor systems as backups. If one goes bad, pilots are trained to check the other readings and disregard the one that’s incorrect.

Lion Air and investigators haven’t provided details about the issue on the previous flight Sunday night. Data provided by flight tracking company FlightRadar24 showed that the jet took off and reached an altitude of 1,692 metres, then dropped to 1,410 metres, an unusual altitude loss at a time when aircraft normally climb steadily.

The plane then resumed its climb, but never climbed above 8,500 metres. Jetliners almost never fly below 9,100 metres because cruising at the lower altitudes is less fuel efficient. However, planes with partially malfunctioning altitude sensors aren’t allowed above 8,500 metres.

“Every aircraft that we have will go through transit, preflight and post-flight checks,” said Lion Air’s Prihantoro. “We are conducting inspection and maintenance if needed on every aircraft.”

United Technologies Corp is a supplier of the systems to the 737 Max, according to Airframer.com, a website that tracks suppliers of aircraft components. A company representative said she wasn’t immediately able to confirm whether its equipment was on the Lion Air plane.