Maintenance procedure lapse left gust damage to 737 Max undetected

Southwest jet released to service after operational test missed impairment to fin and rudder unit.
Southwest Airlines maintenance personnel did not follow correct procedures during an operational test of a Boeing 737 Max’s standby hydraulic system, which left damage to its vertical fin and rudder control unit undetected.

The damage had originally been caused by gusting winds after the aircraft was left parked in a storm at New Orleans on 16 May 2024 – a week before the test, and two days before a 25 May rudder oscillation incident which raised the suspicion of its pilots.

According to the National Transportation Safety Board’s newly-issued final report, the maintenance test should have involved a visual confirmation of rudder position, adding that the damage would have manifested as a rudder that would not return to its neutral position.

Video of the test showed the rudder travelling to the right but not the left. No-one was present at the aft end of the aircraft to verify the rudder movement.

Southwest personnel “did not visually confirm” the rudder position, the inquiry states, but nevertheless signed off that the aircraft had passed the test.

“Investigators could not determine if the existing vertical stabiliser rib damage was further damaged during this maintenance activity,” it adds.

The effects of the damage initially emerged during the aircraft’s fourth flight on the following day, 24 May, but the crew of the service to San Jose believed the yaw damper had malfunctioned.

The damper was disabled and the aircraft carried out another flight on the same day, the last before the incident on 25 May when pilots of a Phoenix-Oakland service experienced a slight ‘Dutch roll’ and noted movement of the rudder pedals.

This led them to conclude that the oscillation was not due to the weather but rather the aircraft.

Inspections on arrival at Oakland discovered damage to the vertical fin trailing-edge ribs above and below the standby rudder power-control unit.

This damage was traced to the aircraft’s exposure to thunderstorms and wind gusts of up to 73kt while the jet was parked overnight at New Orleans nine days earlier.

The aircraft subsequently carried out 44 flights before the discovery, says the inquiry.

It points out that the 737’s rudder system does not feature a gust lock, and that the winds in New Orleans moved the rudder sufficiently to damage the fin ribs to which the rudder control unit was connected.

The aircraft maintenance manual, at the time, did not define a specific high-wind threshold for conditional inspections.

Nor did Southwest Airlines have a process to notify maintenance personnel if the carrier’s aircraft might have been exposed.

Southwest Airlines implemented several changes in the aftermath, including a fleet wide inspection of 737 Max vertical fins – which did not reveal any findings.

It also revised its test procedures for the standby hydraulic system, and implemented a process to notify maintenance control when parked aircraft experience an extreme high-wind event. Boeing also set a 65kt wind threshold for triggering a conditional inspection.

The post Maintenance procedure lapse left gust damage to 737 Max undetected first appeared on FlightGlobal.

Source: www.flightglobal.com — article syndicated from the publisher’s feed; all rights remain with the original publisher.