Electrical grounding issues that led Boeing to recommend some 737 MAX operators park their aircraft have turned up in two areas besides the original standby power control unit where it was first discovered.

Sources with knowledge of Boeing’s work confirm that grounding issues on affected aircraft are also being found in the “P6” panel rack that sits behind the right-hand pilot seat and holds the standby unit, and in the main instrument panel. All of the problems are similar in nature—interrupted electrical ground paths that may inhibit components from operating correctly.

The issue appears to be caused by a coating or primer applied to the parts in or around fasteners that is interrupting the ground paths, the sources said. There are no issues with wiring or wiring insulation, the source emphasized.

Boeing declined to comment.

Revealed on April 8 to operators, Boeing said the grounding issue—originally flagged as affecting standby power control units (SPCUs)—stemmed from a production-line change made in 2019.

Aviation Week reported April 15 that the problem was turning up in more areas besides the SPCU. 

Boeing has not provided details on the production change that is causing the issues. It continued to evaluate the situation on April 16, talking with both affected operators and the FAA about possible maintenance actions that are expected to include inspections and repairs. 

In the interim, Boeing is recommending that the 90 affected 737 MAXs in airline fleets remain grounded. 

“The recommendation is being made to allow for verification that a sufficient ground path exists for” SPCUs, Boeing’s April 8 message said.

Information provided to operators suggests about 460 total aircraft are affected, including 370 that Boeing added to its inventory during a 21-month delivery pause related to the 737 MAX’s global grounding. 

While a handful of the affected aircraft were built before the global grounding started in mid-March 2019, none entered service until after the grounding was lifted in late 2020. Boeing said it discovered the issue recently, during normal production work. The problem has not been linked to any in-service incidents. 

The exact plan of action for in-service and undelivered aircraft affected by the latest issue remains unclear. One industry source with knowledge of the deliberations between stakeholders said that a range of options remain in play, including an inspection that could deem aircraft airworthy if no faults are found, although repetitive inspections would be needed.

Permanent repairs will likely require aircraft to be out of service for 1-2 days at least, the source said.

All undelivered aircraft would have to be repaired before airworthiness certificates are issued.