What happened
On July 31, 2015, a resident in Saint-Georges sur Meuse discovered an aircraft component in a private garden located near the approach path for runway 05R at Liege Airport (EBLG). The object, identified as an aircraft part, was reported to the police and subsequently to airport inspection services.
Following the discovery, investigators cross-referenced the part with aircraft present at the airport. While most aircraft were found to be intact, a Boeing 73 37-4Q8 operated by Tailwind Airlines was identified as a potential source. Later that day, air traffic control in Brussels noted that a preflight check on the same aircraft had revealed a missing flap fitting. The aircraft had landed at EBLG at 11:35 UTC and departed for Liège-Tartege shortly thereafter.
The investigation
Technical analysis of the recovered part confirmed it was a left-hand inboard flap/inboard seal plate, matching the specifications for a Boeing 737 assembly. Upon inspection of the attachment mechanism, investigators found that the fasteners holding the plate adaptor were broken at the base of the bolt heads. Some of these bolts exhibited signs of corrosion, while others appeared shiny.
Maintenance records for the Boeing 737-4Q8 (registration not provided, but msn 27628) showed that while Tailwind Airlines had not performed work on this specific part during its period of operation, the component had been removed and reinstalled by previous operators on several occasions. Investigations into the hardware revealed that NAS 517 bolts had been used to secure the assembly instead of the Boeing-prescribed BACB30NN5K23 bolts.
Findings
- The detachment of the left-hand inboard flap/inboard seal plate was caused by the failure of the attachment bolts on the plate adaptor.
- The primary cause was the use of incorrect NAS 517 bolts rather than the required titanium aluminum pigment-coated BACB30NN5K23 fasteners.
- Unlike the prescribed titanium bolts, the NAS 517 bolts are cadmium-plated alloy steel. The loss of this sacrificial coating can lead to corrosion and a subsequent reduction in bolt strength.
- Examination of the remaining bolt heads revealed evidence of both corrosion and fatigue beach marks.
- Because the fasteners are located beneath the seal plate, their condition is not easily detectable during standard maintenance inspections.
Safety action
- The AAIU(Be) issued a safety message advising Belgian Part-147 Maintenance Training Organisations to utilize the findings of this report to emphasize the critical importance of strictly adhering to Illustrated Parts Catalogue (IPC) prescriptions when selecting fasteners.