Dan Air Services B707 G-BEBP was engaged on a non-scheduled international cargo night on behalf of International Aviation Services for Zambian Airlines.
It was carrying a through load of palletised freight from London Heathrow to Lusaka International Airport, with intermediate stops at Athens and Nairobi, where there was a crew change. The flight from London to Nairobi was without incident and only minor aircraft unserviceabilities were recorded en route.
The aircraft took off from Nairobi for Lusaka at 071 7 h with a fresh crew on board comprising a commander, co-pilot, two flight engineers (one under training) and a loadmaster. In addition there was one passenger on board , a ground service engineer whose duty was to supervise ground handling during transit stops.
The flight proceeded normally and apparently without incident at Flight Level (FL) 310
At 0907:35 hrs the co-pilot contacted Lusaka Approach on 121.3 MHz and at 0911 :38 hrs G-BEBP was cleared to descend to FL 110 at the LW Non-Directional Beacon (NDB).
the right horizontal stabilizer and elevator of Dan Air Services B707-332 (G-BEBP) became separated from the airframe structure
At 0923 hrs the co-pilot reported that the aircraft was levelling at FL 110 at a Distance Measuring Equipment (DME) range of 37 nautical miles from Lusaka .
G-BEBP was then cleared by Lusaka Approach to FL 70 in 1000 foot steps following about five minutes behind another aircraft also bound for Lusaka International Airport.
- Shortly after the selection of 50° flap, with the aircraft on a normal approach the right horizontal stabilizer and elevator became separated from the airframe structure.
- The instantaneous loss of a significant stabilizer download caused an immediate nose down pitching moment associated with a large asymmetric load on the stabilizer centre section to fuselage attachment points.
- In the break-up, the elevator linkage was pulled to the full up elevator position , further increasing the asymmetric load on the stabilizer centre section mountings.
- The asymmetric load was sufficient to wrench the left stabilizer downwards twisting the centre section in the fuselage, fracturing the stabilizer jack screw and allowing the whole stabilizer to rotate in pitch to a large stabilizer nose up angle under the influence of the fully up elevator, which would tend to act as a very large servo tab .
It is also likely that crew reaction to the air craft nose down pitch change would be to hold the stick fully back.
- A very rapid pitch down resulted and the aircraft struck the ground in a 50° dive but with the pitch attitude at approximately 100° some 500m beyond the point of stabilizer separation.
The accident was caused by a loss of pitch control following the in-flight separation of the right hand horizontal stabilizer and elevator as a result of a combination of metal fatigue and inadequate failsafe design in the rear spar structure.
Shortcomings in design assessment, certification and inspection procedures were contributory factors.