Korean B744 at Frankfurt on Oct 8th 2014, dropped flap

Last Update: April 26, 2018 / 18:42:47 GMT/Zulu time

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Photo of Korean Airlines HL7400, Boeing 747-400 — Korean Airlines HL7400, Boeing 747-400 (Photo credit: Alec@B92 / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Oct 8, 2014

Classification
Accident

Airline
Korean Airlines

Flight number
KE-529

Departure
Moscow Sheremetyevo, Russia

Destination
Frankfurt/Main, Germany

Aircraft Registration
HL7400

Aircraft Type
Boeing 747-400

ICAO Type Designator
B744

A Korean Airlines Boeing 747-400 freighter, registration HL7400 performing flight KE-529 from Moscow Sheremetyevo (Russia) to Frankfurt/Main (Germany), was on approach to Frankfurt's runway 25L about to extend the landing gear when a part separated from the aircraft. The aircraft continued for a safe landing on runway 25L.

Germany's BFU opened an investigation into the occurrence, after examining the aircraft a part of the trailing edge flap was identified missing.

The aircraft was unable to resume its schedule but was able to ferry back to Korea on Oct 13th.

Police confimed observers on the ground reported to police on Oct 8th that an aircraft had lost a part while on approach to Frankfurt. The part was not found until Oct 15th, when it was located in a forest near Frankfurt Airport.

A part number on the debris, sized about 5 by 1 meters (197 by 39 inches) identifies the part as an inboard trailing edge foreflap.

The airline confirmed that one of their Boeings received damage on Oct 8th 2014, but cautioned that is yet to be verified the found part belongs to their aircraft. The aircraft was repaired and returned to Korea.

On Feb 24th 2015 Germany's BFU reported in their October bulletin, that there were four flight crew in the cockpit (captain and first officer operating the aircraft and two relief crew occupying the observer seats in the back of the cockpit). The aircraft was on final ILS approach to runway 25L on autopilot at about 2500 feet MSL, the crew had just selected the flaps from 20 to 30 degrees, when a sound similiar to an gear extension was heard by the crew and the aircraft unexpectedly rolled about 8 degrees to the left, airspeed was 160 KIAS. The captain (64, ATPL, 18,009 hours total, 13,671 hours on type) disconnected the autopilot, steered the aircraft back onto the localizer and continued for a landing without further incident.

The BFU reported that a post flight inspection revealed the inboard fore flap, dimension about 4.5 by 1 meters, was missing from the left wing. The missing flap was found and recovered from a forest near Frankfurt. The BFU reported that examination of the fractures identified the fracture began in an area of corrosion.

On Apr 26th 2018 the BFU released their final report concluding the probable cause of the occurrence 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:

According to the crew’s statement, which was confirmed by the FDR recording, the fracture of the flap occurred during approach to Frankfurt/Main Airport shortly after the flaps had been fully extended.

The fracture of the attach fitting was due to fatigue corrosion. The corrosion, in consequence of the used material, resulted in kerfs which, in correlation with stress, served as crack origin for fatigue fracture. The fatigue fracture propagated until the remaining section was destroyed by ductile forced rupture. Because of a wellpreserved fracture surface of the attach fitting the fracture origin and the fracture propagation could be established conclusively. Trigger for the fracture of the remaining section was the extension of the flap and the corresponding stress increase, which the section could no longer tolerate.

The corrosive damage is not uncharacteristic for the used material. Literature referring to corrosion characteristics of aluminium alloys, such as the cupreous EN AW 7075, describes the corrosion susceptibility due to their copper content.

The area of the lubrication groove of the bronze bushing showed a different surface condition which was clearly less susceptible to corrosion.

Due to the fracture of the attach fitting the flap lost its outer mounting. Subsequently, it flapped upward and fractured.

This resulted in additional damages in the periphery of the flap. The airplane itself was not hit or damaged by severed components. The FDR recordings showed that the loss of a flap part did not have any significant influence on the flight characteristics of the airplane. The PIC was able to manually counteract the loss of lift capacity during approach. The landing occurred without further problems.

There is an increased risk for third parties, because an airplane is usually in final approach to an airport and therefore often above metropolitan areas, when the flaps are extended. That in the past as well as in this case no persons or objects suffered injuries or damages is a stroke of luck. The BFU is of the opinion that there exists general need for action to improve the flap attach fitting for this aircraft type.

In November 2003 the manufacturer issued the Service Letter 747-SL-57-097, which describes the fracture of the inner attach fitting of the inboard fore flap. The described facts are also true for the outer attach fitting of this particular case. According to the manufacturer the improvement consisted of the change of aluminium alloy to the material 7075-T351. This alloy is already used for the outer attach fitting, as the present case shows. It is therefore possible that enhancement of corrosion resistance could be achieved by surface treatment. The investigation revealed that the area of the lubrication groove on the surface of the attach fitting was less corrosion susceptible. The BFU views this as a possible future solution method to improve corrosion protection.

The SB 747-57-2119 Revision 2 issued in June 1975 covered damaged and fractured outer attach fittings of inboard fore flaps. Back then different causes had been identified. There was no explicit description of damages and fractures caused by corrosion.