Loss of part of flap, Boeing 747-406F ER, Harare (Zimbabwe)
On 8 July 2019 the Boeing 747-400F, with registration PH-CKA, was flying an ILS approach to Runway 05 at Robert Gabriel Mugabe International Airport in Zimbabwe for landing. While selecting the required position of the flap system, a part of the right wing inboard foreflap broke of and separated from the aircraft. The separation caused a slight rolling motion to the right; the pilots were able to keep the aircraft under control. The pilots continued the approach and landed safety. The broken foreflap part came down in a residential area 5 nautical miles from the airport, close to the approach path and was
recovered later. Nobody was injured.
Fatigue crack failure of the foreflap
The investigation found that the right wing inboard foreflap of PH-CKA failed and partly separated, because of a fatigue crack failure of the foreflap outboard fitting lug. The fatigue crack was caused by pitting corrosion. The pitting corrosion had formed because of moisture that had accumulated between the inside of the foreflap fitting lug and the outside of the fitting lug bearing for over a long period of time. The cause of PH-CKA’s inboard foreflap outboard fitting failure is similar to other investigated occurrences concerning inboard foreflap separations, that occurred with the Boeing 747 series aircraft in the past.
Boeing Alert Service Bulletin
Boeing issued Alert Service Bulletin 747-57A2367 on 15 November 2019 to further improve the reliability and the safe operation of the inboard foreflap and related parts of the inboard trailing edge flap system. This alert service bulletin requires replacement of the inboard foreflap outboard fitting lug at an interval that is well before the expected time of failure. On 30 March 2021, the Federal Aviation Authority mandated with Airworthiness Directive 2021-02-15 the maintenance requirements from Alert Service Bulletin 747-57A2367. This measure as imposed by the airworthiness directive has been incorporated after a prolonged period with various maintenance measures to improve the Boeing 747 inboard trailing edge flap system. The Dutch Safety Board considers that
the alert service bulletin requirements and the mandatory compliance as imposed by the airworthiness directive are adequate to prevent future inboard foreflap outboard fitting lug failures. Hence, the Dutch Safety Board does not issue recommendations.
