What happened
On May 27, 2021, a Boeing 737-7K5, registration OO-JAO, operated by TUI Airlines Belgium (TUI fly), was cruising at FL400 over southern Serbia during a flight from Santorini to Brussels. Approximately 50 minutes after departure, the crew received an indication of low oil quantity in the number one engine. While oil pressure and temperature initially remained within normal limits, the pressure began to drop significantly about 20 minutes later.
The crew evaluated the situation and decided to divert to Belgrade Nikola Tesla Airport. During the descent, the oil pressure dropped below normal limits, prompting the crew to execute the appropriate checklists and shut down the number one engine. The aircraft declared an emergency and successfully performed a single-engine landing in Belgrade. No injuries were reported among the crew or passengers.
The investigation
Post-flight inspections at Belgrade airport revealed damage to the number one engine' and the engine starter. Specifically, the starter housing was partially detached from its mounting, and several bolts were found to be missing or damaged. A detailed inspection by maintenance personnel later confirmed that all nine bolts (PN MS9556-32) securing the starter were damaged, with their heads broken off.
The investigation examined the maintenance history of the engine and the starter. The engine had been installed on the aircraft in July 2020, and the starter had been in service since its last workshop visit in January 2016. Investigators also reviewed the Boeing Maintenance Planning Data (MPD), noting that existing inspection requirements focused on magnetic chip detectors rather than the structural integrity of the starter mounting bolts.
Findings
- The primary cause of the engine shutdown was the failure of the engine starter bolts due to material fatigue caused by cyclic loading.
- The breakage of these bolts led to the detachment of the starter from the pneumatic supply, resulting in a loss of oil and subsequent engine shutdown.
- The existing maintenance protocols and Boeing MPD tasks did not include inspections for the integrity of the bolts connecting the starter to the accessory gearbox (AGB).
- The manufacturer, Honeywell, had previously identified this issue and issued a Spare Parts Bulletin (SB 80-0018) on May 25, 2021, recommending the replacement of the existing bolts with higher-strength, fatigue-resistant bolts (PN AS3236-34).