A Boeing &47-406ERF was operating a cargo flight from Johannesburg-O.R. Tambo International Airport (South Africa) to Harare-Robert Gabriel International Airport (Zimbabwe), onboard a crew of two.
The location where the forflap used to be (© Dutch Safety Board)
The weather was good as the aircraft established on an ILS approach to runway 05 at Harare-Robert Gabriel International Airport;
Wind - 170º at 6 knots
Visibility - >10 kilometers
Cloud base - 5000 feet
Temperature - 25º Celsius
The crew configured the aircraft on the approach as per valid procedures. at approximately 5 nautical miles from the runway, while at a (radio) altitude of 1995 feet and flying at an indicated airspeed of 156 knots, the crew selected the flaps from 20º to 25º. While the flaps were travelling to their newly selected position a loud bang was heard in the cockpit and the aircraft showed a tendency to roll to the right.
Annotated picture of the righthand inboard flaps of the incident aircraft (© Dutch Safety Board)
This was corrected by the crew with a fair amount of left aileron (roll) input. The approach, however, remained stabilised, with the aircraft controllable and following the (approach) flight path, and adhering to the approach criteria. Shortly afterwards the aircraft made a safe landing.
A post-flight inspection revealed that the right half of the right-wing inboard foreflap was missing (the foreflap had effectively broken in 2) and that the right aft fuselage was damaged at several places. The left-hand side of the foreflap was deformed but still attached to the flap carriage. The missing (right-hand) part of the foreflap came down approximately 5 NM from Runway 05 near the extended centreline, close to the town of the Chitungwiza, Zimbabwe. The flap part was approximately twelve feet long and two feet wide. No injuries to persons or damages to property were reported.
Examination of the recovered right-wing inboard foreflap parts revealed that the outboard foreflap fitting had failed (see Figure 2). Visual inspection of the outboard fitting revealed that the lug of the fitting was broken with signs of fatigue and overload failure. It could not be determined when exactly the outboard fitting lug had failed.
The recovered part of the forflap that broke off the aircraft (© Dutch Safety Board)
The loss of the forflap section was investigated by the Dutch Safety Board (state of registry of the aircraft), they 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 over a long period of time. The full investigation report with detailed failure analysis is available by clicking on the field below;