MyCargo B744 at Frankfurt on Sep 16th 2018, dropped part of flaps
Last Update: August 5, 2020 / 15:51:06 GMT/Zulu time
A post flight inspection revealed the right hand inboard trailing edge foreflap was missing from the airframe, the leading edge of the vertical stabilizer showed a dent, the fuselage a hole.
On Sep 18th 2018 Germany's BFU reported, the occurrence was reported to the BFU, the occurrence was rated an accident and is being investigated. Due to the ongoing investigation no further information like, whether the missing flap could be found/recovered or whether the crew did notice any anomaly and when, can be provided. Those details will probably become available in the preliminary or interim report respective final report.
On Mar 4th 2019 the BFU released their September 2018 bulletin reporting larger parts of aircraft debris were found near the touchdown zone of runway 07R between taxiways R17 and M27, which were subsequently identified to be the missing parts of the flaps of the Boeing 747-400.
The BFU reported the three guide rails connecting the inner middle and fore flaps were damaged, the outer mounting bracket of the inboard fore flap was fractured. A number of previous similiar occurres had occurred due to problems with the guide rails during assembly and/or lack of proper lubrication.
On Aug 5th 2020 the BFU released their final report concluding the probable cause of the accident was:
The flap fitting was destroyed by extensive fatigue failure which originated from a corrosion cavity.
The BFU analysed:
The location of the flap fracture pieces indicate that the flap fractured during touchdown of the airplane on runway 07R.
The fracture of the mounting was caused by corrosion fatigue. The corrosion, caused by the material used, had a notching effect, which, given the respective stress, served as fracture origin of a fatigue failure. The fatigue failure progressed until the remaining cross section failed with a ductile forced rupture. Because the fracture surface of the flap fitting was well preserved it was possible to prove conclusively fracture origin and progress. It is possible that the landing shock and the subsequent load increase triggered the fracture of the remaining cross section.
The corrosive damage pattern is typical for this material group. Literature regarding the corrosion characteristics of aluminium alloy name aluminium alloys containing copper, such as the EN AW 7075, as corrodible due to their copper content.
Because the flap fitting fractured the flap lost it outer hold point. Subsequently it flapped upward and fractured. This resulted in other damage in the periphery of the flap. The rest of the airplane was hit or damaged by debris of the fractured components. Because the airplane had been on the ground when the flap loss occurred this did not affect the flying characteristics of the airplane. After leaving the runway the flaps were retracted, according to the respective procedures. Therefore, the loss of the flap was not noticed after the airplane had been parked at the stand and the walk-around conducted. In the morning the damages on the vertical tail were discovered and subsequently the extent of the other damage.
Since the airplane is usually on final approach to an airport and therefore above urban areas, there is an increased risk for third parties. That in the past persons were not injured or objects damaged is a stroke of luck. This is also true for this case where the flap had lasted until the landing. The BFU is of the opinion that there is general need for action to improve the flaps of aircraft of this type.
This case is already the third flap loss of a Boeing B 747-400F on approach to Frankfurt/Main Airport in the past 10 years. The two Investigation Reports (AX001-09 and EX007-14) elaborately describe the problematic which exists since the 1970s.
The maintenance documentation showed that since taking over the airplane the checks stipulated by the AD based on the manufacturer’s SB addressing the flap mechanics were conducted. These checks were ineffective, however.
The BFU is of the opinion that prior actions did not fully address the basic problem of corrosion at the ball bearing of the aluminium flap fitting and the dented bronze bush. If installed, this part where corrosion forms is not visible from the outside. The inside of the flap fitting’s ball bearing becomes visible if the bronze bush is removed. That is why this critical part is so hard to check.
If corrosion forms in this area on the aluminium a rugged surface develops which than functions like a number of grooves. Due to operating stress, these served as possible crack origin which developed into fracture and ultimately failure of the component.
This article is published under license from Avherald.com. © of text by Avherald.com.
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