The US Federal Aviation Administration (FAA) has asked Boeing to address an issue it has identified while reviewing the manufacturer’s software update for the troubled 737 Max.

Boeing handed over the software fix for the aircraft’s Manoeuvring Characteristics Augmentation System (MCAS) to the FAA in May and the regulator has been examining the system since.

However, the FAA issued a statement on Wednesday saying it had found a “potential risk that Boeing must mitigate”, though it did not reveal details of the fault.

In response, the manufacturer said it “agrees with the FAA’s decision and request, and is working on the required software”.

The news is another setback for returning the 737 Max to service after it was grounded in March following two disasters that killed a total of 346 people – one on a Lion Air flight in October and a second on an Ethiopian Airlines service in March.

Shortly after Boeing announced it had completed the initial software update, FAA director general Dan Elwell said the regulator could not put a timeline on when the grounding order would be lifted.

Earlier this month, Boeing issued a warning to its airline customers that a third-party component of the entire family of 737s’ wings could be faulty. It was in the process of working with carriers to replace the part to avoid damage to the aircraft.

The FAA said it “will lift the aircraft’s prohibition order when we deem it is safe to do so”. It is also responding to recommendations from the Technical Advisory Board (TAB), an independent review panel the regulator has asked to evaluate its work on the 737 Max issues.

The setback may be unwelcome news to airlines that are having to cancel and rearrange flights meant to be operated by the 737 Max, such as American Airlines and Southwest Airlines, which have both extended cancellations through September.

A recent survey of travel buyers by the GBTA revealed the majority are nervous about the aircraft’s return to service and believe their travellers may rearrange travel plans to avoid flying on the 737 Max even after it has been deemed safe by authorities.

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