Little more than six months after Boeing's 737 Max was cleared to fly again by US regulators, the aircraft finds itself under intense scrutiny once again.
The discovery of a potential electrical problem last month led to the renewed grounding of more than 100 aeroplanes, belonging to 24 airlines around the world.
Deliveries of many more new aircraft have been suspended. Boeing and the US regulator, the Federal Aviation Administration say they are working closely to address the issue.
But the affair has given new energy to critics who claim the 737 Max was allowed back into service prematurely - and that issues which could have contributed to two fatal crashes have not been fully analysed or addressed.
Those critics include a high profile whistle-blower, Ed Pierson, who has already sought to link allegedly poor production standards at the 737 factory with electrical defects on the crashed planes, which he claims may have been implicated in both accidents.