The Boeing 737 Max passenger aircraft was grounded following devastating fatal crashes in October 2018 (a flight managed by Indonesian airline Lion Air) and March 2019 (an Ethiopian Airlines flight) which caused the deaths of all 346 people on the flights.

Investigations after the first crash found that the 737 Max’s new flight control software, the Manoeuvring Characteristics Augmentation System (MCAS), caused the aircraft to nosedive repeatedly, although the issue was not fixed in time to prevent the second disaster.

The 737 Max has been grounded worldwide since the Ethiopian Airlines flight crashed, causing anxiety across the aviation industry. In December 2019 – soon after the FAA suspended Boeing’s authority to issue airworthiness certificates for aircraft in the range – Boeing ousted its CEO and announced that it would suspend production of the model, beginning in January.

Although attention has mainly been paid to Boeing’s new flight control software, the FAA has now concluded that Boeing’s wiring bundles on all 737 Max models are “not compliant” with wiring safety standards intended to prevent dangerous short circuits. The FAA and other regulators are concerned that under some circumstances, a short circuit in certain wiring bundles could lead to the aircraft nosediving outside the pilot’s control.

Boeing had proposed that it would not need to move or otherwise modify wiring bundles in the aircraft, arguing that the bundles do not pose a safety threat given that the same wiring design in previous 737 models has caused no safety issues, in part because the circumstances in which short circuits would cause a pilot to lose control of their aircraft are so extremely unlikely.

A final decision is yet to be made on whether the wiring bundles will need to be reworked.

An FAA spokesperson said: “The FAA continues to engage with Boeing as the company works to address a recently discovered wiring issue with the 737 Max. The manufacturer must demonstrate compliance with all certification standards. The aircraft will be cleared for return to passenger service only after the FAA is satisfied that all safety-related issues are addressed.”

Boeing has indicated that meeting the FAA’s demands would not affect the 737 Max’s planned return to the skies. A spokesperson told CNBC that the company is in ongoing discussions with the regulator and that “our estimation for a mid-year return to service of the Max is unchanged.” According to Boeing’s plans, any required changes could take place while the company updates the approximately 400 737 Max models which it hopes to deliver later this year, provided regulators permit the aircraft to return to service.

The disagreement over the wiring bundles emerged just days after a US House of Representatives report criticised Boeing for excessive cost cutting, technical design failures, insufficient oversight and a “culture of concealment” over the 737 Max’s safety issues. The report concluded that Boeing “jeopardised the safety of the flying public” in its attempts to get the airliner approved by regulators.

The Boeing 737 Max passenger aircraft was grounded following devastating fatal crashes in October 2018 (a flight managed by Indonesian airline Lion Air) and March 2019 (an Ethiopian Airlines flight) which caused the deaths of all 346 people on the flights.

Investigations after the first crash found that the 737 Max’s new flight control software, the Manoeuvring Characteristics Augmentation System (MCAS), caused the aircraft to nosedive repeatedly, although the issue was not fixed in time to prevent the second disaster.

The 737 Max has been grounded worldwide since the Ethiopian Airlines flight crashed, causing anxiety across the aviation industry. In December 2019 – soon after the FAA suspended Boeing’s authority to issue airworthiness certificates for aircraft in the range – Boeing ousted its CEO and announced that it would suspend production of the model, beginning in January.

Although attention has mainly been paid to Boeing’s new flight control software, the FAA has now concluded that Boeing’s wiring bundles on all 737 Max models are “not compliant” with wiring safety standards intended to prevent dangerous short circuits. The FAA and other regulators are concerned that under some circumstances, a short circuit in certain wiring bundles could lead to the aircraft nosediving outside the pilot’s control.

Boeing had proposed that it would not need to move or otherwise modify wiring bundles in the aircraft, arguing that the bundles do not pose a safety threat given that the same wiring design in previous 737 models has caused no safety issues, in part because the circumstances in which short circuits would cause a pilot to lose control of their aircraft are so extremely unlikely.

A final decision is yet to be made on whether the wiring bundles will need to be reworked.

An FAA spokesperson said: “The FAA continues to engage with Boeing as the company works to address a recently discovered wiring issue with the 737 Max. The manufacturer must demonstrate compliance with all certification standards. The aircraft will be cleared for return to passenger service only after the FAA is satisfied that all safety-related issues are addressed.”

Boeing has indicated that meeting the FAA’s demands would not affect the 737 Max’s planned return to the skies. A spokesperson told CNBC that the company is in ongoing discussions with the regulator and that “our estimation for a mid-year return to service of the Max is unchanged.” According to Boeing’s plans, any required changes could take place while the company updates the approximately 400 737 Max models which it hopes to deliver later this year, provided regulators permit the aircraft to return to service.

The disagreement over the wiring bundles emerged just days after a US House of Representatives report criticised Boeing for excessive cost cutting, technical design failures, insufficient oversight and a “culture of concealment” over the 737 Max’s safety issues. The report concluded that Boeing “jeopardised the safety of the flying public” in its attempts to get the airliner approved by regulators.