The US Federal Aviation Administration has issued an emergency airworthiness directive for the Boeing 737 MAX 8 and MAX 9 after one crashed in Indonesia last month killing all 189 people on board.

The directive focuses on how to handle erroneous data from a sensor that investigators believe malfunctioned on the Lion Air jet that plunged into the Java Sea shortly after taking off from Jakarta on October 29.

FAA directives are usually followed by other airline regulators internationally.

The FAA said erroneous data from the “angle of attack” sensor, which helps prevent the plane from stalling and diving, could cause flight crew to have difficulty controlling the plane and lead to “excessive nose-down attitude, significant altitude loss, and possible impact with the terrain”.

The directive instructs airlines to make specific changes to flight manual procedures for responding to the problem.

Indonesian investigators on Wednesday said the sensor was replaced on the Lion Air plane the day before its fatal flight and may have compounded other problems with the aircraft.

The two-month-old Boeing 737 MAX 8 crashed into the sea 13 minutes after take-off from Jakarta. Both that flight and its October 28 trip from Bali to Jakarta showed erratic speed and altitude shortly after take-off.

Indonesia’s National Transportation Safety Committee earlier this week announced the plane had a malfunctioning airspeed indicator on its last four flights, based on analysis of the flight data recorder.

‘Is this fatal?’

Chairman Soerjanto Tjahjono said the airspeed indicator and sensor problems were related.

Lion Air’s first two attempts to address the airspeed problem didn’t work, and for the jet’s second-to-last flight the “angle of attack” sensors were replaced, Tjahjono said.

On that October 28 flight, the pilot’s and co-pilot’s sensors disagreed by about 20 degrees. The plane went into a sudden dive but the pilots were able to recover control and decided to fly on to Jakarta at a lower-than-normal altitude.

On the fatal flight, the plane hit the water at very high speed after it had been cleared to return to the airport minutes after becoming airborne.

“The point is that after the AOA [sensor] is replaced, the problem is not solved but the problem might even increase. Is this fatal? NTSC wants to explore this,” Tjahjono said.

Airline safety experts said pilots are trained to handle a plane safely even if those crucial sensors fail. Backup systems are generally in place as well.

Search extended

Investigators are expected to focus on how a single sensor’s failure resulted in a faulty command that didn’t take into account information from a second sensor, said John Cox, CEO of Safety Operating Systems.

“We don’t know what the crew knew and didn’t know yet,” Cox said. “We will.”

Indonesia’s search and rescue agency has extended the search until Sunday.

Body parts are still being recovered and divers continue to hunt for the cockpit voice recorder.

Indonesia’s transportation safety committee said it had agreed with Boeing on procedures that the plane manufacturer should distribute globally on how flight crews can deal with the sensor problems.

The flight procedure recommendations to Boeing were based on how the flight crew responded to problems on the Bali-to-Jakarta flight, said investigator Nurcahyo Utomo.

Lion Air is one of Indonesia’s youngest and largest airlines, flying to dozens of destinations at home and internationally.