The US Federal Aviation Administration (FAA) said 106 Boeing 737 Max airplanes have been grounded worldwide by an electrical issue and said the American planemaker is still working on a fix.
Boeing disclosed an electrical power system issue on April 7 and recommended operators to temporarily remove these airplanes from service.
The problem involved the electrical grounding - or connections designed to maintain safety in the event of a surge of voltage - inside a backup power control system. The FAA said "subsequent analysis and testing showed the issue could involve additional systems”.
The FAA said in a formal notice to international air regulators that 106 airplanes are covered, including 71 registered in the US. "All of these airplanes remain on the ground while Boeing continues to develop a proposed fix," the agency added.
The FAA said Boeing's investigation showed the issue could impact the standby power control unit, a circuit breaker panel and main instrument panel.
The notice said the "FAA expects to issue an airworthiness directive mandating corrective action before further flight for all affected airplanes."
Boeing spokeswoman Jessica Kowal said "we concur with the FAA notice and continue to work closely with the regulator and our customers to address the issue."
The top three US 737 Max operators - Southwest Airlines , American Airlines and United Airlines - removed more than 60 jets from service following the notice from Boeing.
US carriers said they expect the issue will be resolved soon, potentially in the next week or two.
“We have a pretty good idea of exactly what the issue is and the remedies that need to be attended to," said American Airlines president Robert Isom.
The FAA said other carriers impacted include Cayman Airways, Copa Airlines, Gol Linhas Aereas, Iceland Air, Minsheng Leasing, Neos Air, Shanding Airlines, SilkAir, Spice Jet, Sunwing Airlines, Turkish Airlines, Valla Jets Limited, WestJet Airlines and Xiamen Airlines.
The FAA said it "verified all operators with affected airplanes have voluntarily taken those aircraft out of service”.
It said the production issue "is not related to recertification of the flight control system on the 737 Max, ungrounding of the aircraft, or its return to service”. Boeing has delivered more than 450 Max airplanes.