Aerospace manufacturer Boeing knew a cockpit safety alert was not working as intended in its 737 Max plane but did not disclose this to airlines or regulators.

The company said it knew about the problem with the aircraft before a fatal crash in Indonesia last October – although it is not clear if the catastrophe could have been avoided had the alert worked as intended.

The feature was designed to warn pilots when a key sensor might be providing incorrect information about the pitch of the plane’s nose, but within months of the plane’s debut in 2017 engineers realised that the sensor warning light only worked when airlines also bought a separate, optional feature.

The sensors malfunctioned during an October flight in Indonesia and another in March in Ethiopia, causing software on the plane to push the nose down.

Pilots were unable to regain control of either plane and both crashed, killing 346 people.

Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures Show all 9 1 /9 Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures This picture taken on March 11, 2019, shows debris of the crashed airplane of Ethiopia Airlines, near Bishoftu, a town some 60 kilometres southeast of Addis Ababa, Ethiopia. - An Ethiopian Airlines Boeing 737 crashed on March 10 morning en route from Addis Ababa to Nairobi with 149 passengers and eight crew believed to be on board, Ethiopian Airlines said. (Photo by Michael TEWELDE / AFP)MICHAEL TEWELDE/AFP/Getty Images AFP/Getty Images Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures Family members mourn the victims at the crash site of the Ethiopian Airlines operated Boeing 737 MAX aircraft, at Hama Quntushele village in the Oromia region, on March 13, 2019. - A Nairobi-bound Ethiopian Airlines Boeing crashed minutes after takeoff from Addis Ababa on March 10, 2019, killing all eight crew and 149 passengers on board, including tourists, business travellers, and "at least a dozen" UN staff. Families of the victims were taken to the remote site on March 13, 2019, where the plane smashed into a field with 157 passengers and crew from 35 countries, leaving a deep black crater and tiny scraps of debris. (Photo by TONY KARUMBA / AFP)TONY KARUMBA/AFP/Getty Images AFP/Getty Images Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures A page of a Boeing flight crew operations manual is seen at the scene of the Ethiopian Airlines Flight ET 302 plane crash, near the town of Bishoftu, near Addis Ababa, Ethiopia March 12, 2019. REUTERS/Baz Ratner REUTERS Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures epa07434278 Rescue workers search the site for pieces of the wreckage of an Ethiopia Airlines Boeing 737 Max 8 aircraft near Bishoftu, Ethiopia, 13 March 2019. Ethiopian Airlines flight ET 302 carrying 149 passengers and 8 crew was en route to Nairobi, Kenya, when it crashed on 10 March 2019 by yet undetermined reason. All passengers and crew aboard died in the crash. The Boeing 737 Max 8 aircraft has come under scrutiny after similar deadly crashes in Ethiopia and Indonesia within a few months. Several countries have banned the plane type from their airspace and many airlines have grounded their 737 Max 8 planes for safety concerns after the Ethiopian Airlines plane crashed minutes after take-off on 10 March. EPA/STRINGER EPA Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures A grounded Boeing 737 MAX 8 passenger plane of the Norwegian low-cost airline Norwegian is parked at the tarmac at Vantaa airport in Vantaa near Helsinki, Finland on March 13, 2019. - A number of countries have banned Boeing's 737 MAX 8 medium-haul workhorse jet from their airspace in response to the Ethiopian Airlines crash that killed all 157 people on board. (Photo by Heikki Saukkomaa / Lehtikuva / AFP) / Finland OUTHEIKKI SAUKKOMAA/AFP/Getty Images AFP/Getty Images Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures Rescue workers search the site for pieces of the wreckage of an Ethiopia Airlines Boeing 737 MAX 8 aircraft near Bishoftu, Ethiopia, 13 March 2019. Ethiopian Airlines flight ET 302 carrying 149 passengers and 8 crew was en route to Nairobi, Kenya, when it crashed on 10 March 2019 by yet undetermined reason. All passengers and crew aboard died in the crash. The Boeing 737 Max 8 aircraft has come under scrutiny after similar deadly crashes in Ethiopia and Indonesia within a few months. Several countries have banned the plane type from their airspace and many airlines have grounded their 737 Max 8 planes for safety concerns after the Ethiopian Airlines plane crashed minutes after take-off on 10 March. EPA/STRINGER EPA Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures A heap of debris from the wreckage of an Ethiopia Airlines Boeing 737 MAX 8 aircraft are piled at the crash site near Bishoftu, Ethiopia, 13 March 2019. Ethiopian Airlines flight ET 302 carrying 149 passengers and 8 crew was en route to Nairobi, Kenya, when it crashed on 10 March 2019 by yet undetermined reason. All passengers and crew aboard died in the crash. The Boeing 737 Max 8 aircraft has come under scrutiny after similar deadly crashes in Ethiopia and Indonesia within a few months. Several countries have banned the plane type from their airspace and many airlines have grounded their 737 Max 8 planes for safety concerns after the Ethiopian Airlines plane crashed minutes after take-off on 10 March. EPA/STR EPA Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures A crew working with an investigative team to clear the site after the Sunday crash of the Ethiopian Airlines operated Boeing 737 MAX aircraft, carry debris at Hama Quntushele village in the Oromia region, on March 13, 2019. - A Nairobi-bound Ethiopian Airlines Boeing crashed minutes after takeoff from Addis Ababa on March 10, killing all eight crew and 149 passengers on board, including tourists, business travellers, and "at least a dozen" UN staff. Families of the victims were taken to the remote site on March 13, 2019, where the plane smashed into a field with 157 passengers and crew from 35 countries, leaving a deep black crater and tiny scraps of debris. (Photo by TONY KARUMBA / AFP)TONY KARUMBA/AFP/Getty Images AFP/Getty Images Boeing 737 MAX grounding: Ethiopia Airlines Flight 302 in pictures FILE PHOTO: An American Airlines Boeing 737 MAX 8, on a flight from Miami to New York City, comes in for landing at LaGuardia Airport in New York, U.S., March 12, 2019. REUTERS/Shannon Stapleton/File Photo REUTERS

It is not clear whether having the warning light would have prevented either the Lion Air crash or the 10 March Ethiopian Airlines crash near Addis Ababa.

Boeing’s disclosure on Sunday, however, raised fresh questions about the company’s candour with regulators and airline customers.

Boeing said again that the plane was safe to fly without the sensor alert, called an angle-of-attack disagree light.

Other gauges tell pilots enough about the plane’s speed, altitude, engine performance and other factors to fly safely, the company said.

A spokesperson for the American Federal Aviation Administration (FAA) said the agency was notified of the non-working warning light in November, after the Lion Air crash on 29 October.

He said FAA experts determined that the non-working cockpit indicator presented a low risk.

“However, Boeing’s timely or earlier communication with [airlines] would have helped to reduce or eliminate possible confusion,” the spokesperson said in a statement.

The indicator was supposed to tell pilots when sensors that measure the pitch of the plane’s nose appear to be in conflict, a sign that the sensor information is unreliable.

Boeing told airlines that the warning light was standard equipment on all Max jets.

Video shows flames coming from plane engine as Boeing 737 catches fire in Russia

Boeing engineers quickly learned, however, that the warning light only worked if airlines also bought an optional gauge that told pilots how the plane’s nose was aimed in relation to the onrushing air.

The company decided to fix the problem by disconnecting the alert from the optional indicators at the next planned update of cockpit display software, as in-house experts decided that the non-working light did not affect safety.

Nearly 400 Max jets were grounded at airlines worldwide in mid-March after the Ethiopia crash.