Asiana B744 at Frankfurt on May 8th 2009, lost part of flaps on final approach

Last Update: April 26, 2018 / 18:59:09 GMT/Zulu time

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Incident Facts

Date of incident
May 8, 2009

Classification
Report

Aircraft Registration
HL7419

Aircraft Type
Boeing 747-400

ICAO Type Designator
B744

An Asiana Airlines Boeing 747-400 freighter, registration HL7419 performing freight flight OZ-797 from Vienna (Austria) to Frankfurt/Main (Germany) with 2 crew, was on final approach to Frankfurt's runway 25L about half way between outer marker and touch down zone at a height of 1400 feet with an airspeed of 160 KIAS and the autopilot tracking the instrument landing system, when the crew selected the landing flaps to 30 degrees. Moments after the flaps lever was moved, the crew heard a knocking sound and the aircraft rolled left by about 1 degree countered by the autopilot. A safe landing followed with no further incident, the airplane taxied normally to its parking position.

The German Bureau for Aviation Accident Investigation (BFU) reported, that in a post flight inspection a part of the left landing flaps, the inboard fore flap sized about 4.5 meters by 1 meter (15 by 3 feet), was detected missing from the wing. The remaining fragment was twisted. Two connecting rods between the forward and center flap were torn out.

The skin of the fuselage at the left side between wing and rear freight door was pierced at a length of 3.5 meters, the inner pressure cabin was pierced as well. Longerons and ribs were severly damaged in that area.

The left side of the vertical finwas pierced as well.

The approximate location of the airplane at the time, when the part departed the wing, was determined by analysis of the flight data recorders and a search conducted on the ground, so far the missing piece has not been found however.


On Apr 26t 2018 the BFU released their final report concluding the probable cause of the accident was:

The flap attach fitting was destroyed by an extended fatigue fracture. It is highly likely that it started with a corrosion depression.

The BFU analysed:

The severe damages of the airplane, especially the punctured pressurised cabin, were caused by the fracture of the left inboard trailing edge fore flap. Due to these damages the occurrence was classified as accident. The fracture of the flap was the result of the fractured outer flap attach fitting. It fractured as a result of a fatigue crack, which had propagated over time to about half of the front of the bearing seat’s bar. This front bar is the area where the highest stress occurred. The residual fracture occurred as forced rupture. The starting point of the fracture could not give any indication as to the cause because it had later been mechanically damaged. Due to the corrosion found on the surface of the bearing seat, and the experiences from another case, it is highly likely that the corrosion formed depressions and holes, which provided a starting point for a fatigue crack at the area of the highest stress. Therefore, in this case it is highly likely that the corrosion of the bearing seat’s surface was the real cause for the fracture of the flap attach fitting.

The SB does not mention corrosion of the bearing seat. It mentions wear, stiffness, and possible miss-alignment of the flap mechanism as potential causes for a fatigue fracture. These are surely possibilities which may increase loads and stress on the attach fitting. The BFU is of the opinion that as root cause for fatigue cracks these attempts at explanation are insufficient. A fatigue fracture can occur anywhere, where a potential starting point is encountered. In this case the corrosion provided this starting point. If the starting point is there the fatigue crack propagates in the area of the highest stress. The stress level may be lower than during dimensioning of the component. Therefore a fatigue crack can start in a corrosive environment even if the flap mechanism is well adjusted and functions properly.

The laboratory results showed that the surface condition of the bearing seat, in which the lubrication groove of the bronze bearing was located, was different from the remaining area. The “track area” was slightly elevated and clearly less sensitive to corrosion. The major part of the surface showed depression and hole corrosion. Such an area provided the kerf where a fatigue crack could start.
Incident Facts

Date of incident
May 8, 2009

Classification
Report

Aircraft Registration
HL7419

Aircraft Type
Boeing 747-400

ICAO Type Designator
B744

This article is published under license from Avherald.com. © of text by Avherald.com.
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