Boeing has admitted that it knew about a problem with its 737 Max jets a year before the aircraft was involved in two fatal accidents, but took no action.
The firm said it had inadvertently made an alarm feature optional instead of standard, but insisted that this did not jeopardise flight safety.
All 737 Max planes were grounded in March after an Ethiopian Airlines flight crashed, killing 157 people.
Five months earlier, 189 people were killed in a Lion Air crash.
The worldwide fleet of 737 Max planes totalled 387 aircraft at the time of the grounding.