Changes to controversial control systems linked to two fatal crashes of Boeing 737 Max 8s in the last six months have been issued by Boeing.

But it remains uncertain when the aircraft, grounded worldwide this month, will be allowed to resume sevice.

Investigators have not yet determined the cause of the accidents.

Boeing will install as a standard a warning system, which was previously an optional safety feature, as part of the upgrade.

Neither of the 737 Max 8s, operated by Indonesian carrier Lion Air and Ethiopian Airlines, that were involved in the fatal crashes, carried the alert systems, designed to warn pilots when sensors produce contradictory readings.

Boeing said in future airlines would no longer be charged extra for that safety system to be installed.

The US manufacturer said it was continuing to work with the US Federal Aviation Administration and other regulatory agencies on the certification of the software update.

“The software was put through hundreds of hours of analysis, laboratory testing, verification in a simulator and two test flights, including an in-flight certification test with FAA representatives on board as observers,” Boeing said

The updates reduce the crew’s workload in “non-normal flight situations” and prevent erroneous data from causing the activation of theManoeuvring Characteristics Augmentation System (MCAS), designed to keep the aircraft from stalling.

In a media briefing, Boeing said that the upgrade to MCAS was not an admission that the system had caused the crashes.

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