The problem came to light when the latest version of the software was loaded onto an actual aircraft, according to one of the people. While it has been tested on planes in flight, most of the software reviews have occurred in a special simulator used by engineers on the ground.
Airlines have already built months of delay into their schedules to resume flying the plane, so it's possible the software work won't require additional changes. Southwest Airlines Co., American Airlines Group Inc. and United Airlines Holdings Inc. have said they won't fly the plane again until June.
"Boeing has made us aware of the issue but it's too early to provide any indication regarding potential impact to timing" of the plane's return to service, said Brandy King, a Southwest spokeswoman.
Carriers have said they'll need to adopt new pilot training and to work on planes to prepare them for service once the grounding is lifted by the FAA.
Boeing announced on Jan. 7 that it will recommend pilots undergo additional simulator training on the Max, a reversal of their long-held view that crews qualified on other 737 models only needed computer-based instruction. That action makes it more likely the FAA and other nations will require the additional training.
The crash of a Lion Air 737 Max on Oct. 29, 2018, and an Ethiopian Airlines plane on March 10 both occurred after a system known as Maneuvering Characteristics Augmentation System repeatedly pushed the planes into dives. Pilots in both cases were able to temporarily maintain control, but eventually the jets entered steep dives and crashed.
Boeing has been working for more than a year on fixing software to ensure that MCAS is safe. The process has been bumpy at times as new glitches arose and tension flared with regulators.
During the process of assessing the plane last year, Boeing discovered another issue and had to redesign its flight-control computers. The reworking of that software has been one of the reasons that the repairs have taken so long.
A Boeing audit of the aircraft at the end of last year also discovered that wiring on the plane was potentially vulnerable to short-circuits that could trigger flight-control problems. That will require moving some wiring in the plane.
(With assistance by Justin Bachman.)
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