The list of items Boeing could be forced to fix before US safety officials let the grounded 737 Max airliner fly again has grown to include a problem with electrical wiring used for the plane's controls.
Separately on Monday, the financial damage to Boeing came into sharper focus with American Airlines announcing a compensation deal for thousands of flights canceled because Max jets were grounded most of the year.
American did not say how much it will receive but said it will give $US30 million ($A43 million) to employees as profit-sharing.
American estimated in October the Max grounding would cost it $US540 million in pre-tax income for 2019.
Aeromexico said on Monday (local time) it had also settled with Boeing but declined to release details. The airline has six Max jets.
The wiring issue came to light after the Federal Aviation Administration asked Boeing to audit key systems on the Max after two crashes in which new software and faulty sensors were implicated.
During that review of changes Boeing is making to the plane, it discovered bundles of electrical wiring were too close together and raised the potential for a short circuit that could cause pilots to lose control.
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"We identified this wiring-bundle issue … and we are working with the FAA to perform the appropriate analysis," said Boeing spokesman Gordon Johndroe.
The company said, however, it is too soon to know whether it will need to make any design changes, such as moving the wiring bundles farther apart.
Boeing says it believes other safeguards, including circuit breakers and insulation around the wiring, could be sufficient to prevent a short-circuit from leading to another crash.
Boeing built and delivered nearly 400 Max jets before the plane was grounded in March after two crashes that killed 346 people.
Since then, another 400 or so jets have rolled off assembly lines, although they can't be sent to airlines.