The manufacturer said it had now flown the Max for more than 360 hours on 207 test flights.
It is modifying the Manoeuvring Characteristics Augmentation System (MCAS), which counteracts the risk of the aircraft stalling by tilting the aircraft’s nose downwards.
The Max has an issue with this, as the new engines fitted to it are heavier and fixed further forward on the wings.
Boeing faces victim compensation claims totalling millions of dollars following the Lion Air and Ethiopian Airlines disasters that led to the aircraft being banned from airspace around the world. It will also face billions in compensation from airlines forced to lease replacements.
In a statement, Boeing said it was in talks with the US Federal Aviation Administration detailing “how pilots interact with the airplane controls and displays in different flight scenarios”.
“Once the requests are addressed, Boeing will work with the FAA to schedule its certification test flight and submit final certification documentation,” it said.
Tui Airways, which has 15 of the aircraft type with another eight on order, said it will need to know by the end of May whether it can schedule the 737 Max into the summer peak or wait until the winter season for its return.
An early return would cost Tui €200 million in lost revenue, but the aircraft’s absence until November would add another €100 million to the bill.
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