Boeing Crash Clues Spark Questions Over Why Plane Cleared to Fly

(Bloomberg) -- Investigators are slowly piecing together clues of how Indonesia’s worst air disaster in two decades transpired, raising questions over how a near brand-new Boeing Co. jet that had recurring instrument failures was cleared for its ill-fated flight.

The Lion Air Max 8 plane’s angle-of-attack sensor, which helps aircraft maintain the correct pitch to stay airborne, was replaced the day before the Oct. 29 crash after erroneous readings on a previous trip, the Indonesia National Transportation Safety Committee said Wednesday. Faulty airspeed readings plagued the jet on its last four flights before it plunged into the Java Sea with 189 people aboard.

Boeing Crash Clues Spark Questions Over Why Plane Cleared to Fly

The revelations spurred Boeing to alert operators of the 737 Max aircraft worldwide that the airflow sensor can provide false readings in certain circumstances. Misleading data from that device could trick the plane into pointing its nose down.

That warning and the investigation team’s statement suggest the pilots on JT610 may have been battling with the aircraft as its computers commanded a dive. In addition, the faulty cockpit data over multiple flights leading up to the accident and the replacement of a wildly misleading sensor have raised questions about maintenance, oversight and the plane’s suitability for service.

“The aircraft, with that recurring problem, should not have been released to fly,” Neil Hansford, chairman of Australian consultancy firm Strategic Aviation Solutions who’s worked with airlines worldwide for more than 30 years, said by phone Thursday. “It should have been grounded.”

The almost brand-new plane, with just 800 hours of flight time, was cleared for the Oct. 29 flight after maintenance overnight, Lion Air has said.

On a previous flight from Bali to Jakarta, the same jet’s angle-of-attack sensor feeding the captain’s displays registered a 20-degree difference from the device on the copilot’s side of the cockpit, the committee said. The malfunction can cause the computers to erroneously detect a mid-flight stall in airflow, triggering a dive to regain speed to keep flying.

Moments into the aircraft’s final flight, the pilots asked to return to Jakarta but never turned back, according to Indonesia’s safety commission and flight-tracking data. Shortly afterward, JT610 plunged into the water, nosing downward so suddenly that it may have hit speeds of 600 miles an hour.

Boeing Crash Clues Spark Questions Over Why Plane Cleared to Fly

Investigators will want to examine the pilots’ actions, how flight crews were trained and whether maintenance that was performed was adequate, said Roger Cox, a former U.S. National Transportation Safety Board investigator.

“I would definitely be looking at the man-machine interface and how pilots respond,” said Cox, a former airline pilot who flew earlier versions of the 737 and specialized at the NTSB in cockpit actions.

Adding to the airline’s problems, a Boeing 737-900ER operated by PT Lion Mentari Airlines clipped a pole late Wednesday while taxiing at Bengkulu airport in southern Sumatra, shearing the tip of its left wing. Authorities grounded the aircraft for investigation.

The Boeing directive didn’t call for operators to carry out new inspections or take other action. It stressed that pilots should follow procedures in the flight manual when encountering erroneous data. Boeing has delivered 219 Max planes -- the latest and most advanced 737 jets -- since the models made their commercial debut last year with a Lion Air subsidiary.

American aviation regulators followed by issuing an emergency order Wednesday requiring that airlines follow Boeing’s instructions and add information to pilot manuals showing how to diagnose the problem and respond.

Carriers will have three days to update their manuals under the order, issued by the Federal Aviation Administration. The FAA said the problem “could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain.”

India’s aviation regulator said in a statement Thursday that it has ensured that all Indian operators are aware of the FAA directive and have taken appropriate corrective action.

Boeing, which is headquartered in Chicago, said it is cooperating fully and providing technical assistance as the investigation continues. United Technologies Corp. supplies the angle-of-attack sensors and indicator for the 737 Max, according to Airframers.net. The company didn’t respond to requests for comment. Honeywell International Inc. provides the air data inertial reference unit.

While modern jetliners primarily operate on autopilot, the computerized system can disengage when airspeed indicators malfunction, forcing pilots to manually fly the aircraft. If there is an “uncommanded nose-down stabilizer trim” on the Max, pilots can counteract it by pushing a switch on their control yoke. But the plane’s computers will resume trying to dive as soon as they release the switch, the Boeing bulletin said.

©2018 Bloomberg L.P.