1977 Dan-Air Boeing 707 crash

Investigation

A full investigation was launched by the Zambian authorities and the investigation was then delegated to the UK Air Accidents Investigation Branch (AAIB).

It was determined that the structure of the right horizontal stabiliser failed due to metal fatigue in the rear spar structure, and due to the lack of an adequate fail-safe structure or device should such an event occur. The investigation also identified deficiencies in the assessment of aircraft designs and their certification and in the way aircraft were inspected.

The Boeing 707 320/420 series had an enlarged horizontal stabiliser (tailplane) assembly compared to earlier 707 aircraft, and in the redesign the increased loads on the tailplane structure had been taken by replacing some of the aluminium skinning with stainless steel. In addition, the spar attachment fittings had been redesigned, making them both stronger and stiffer. This had the unforeseen effect of changing the way the tailplane structure handled gust loads, the stiffer fittings being no longer able to help in absorbing and transferring the stresses caused by gusts and other normal aerodynamic loads, the flexure (i.e., the bending loads) of the left and right horizontal stabilizers instead having to be carried by the stabiliser spars entirely by themselves. This led over time to fatigue cracking in the right horizontal stabiliser's rear spar, which, due to the concealed (internal) nature of the tailplane construction, was not noticed by maintenance engineers.

The 707 had been designed to a 'fail-safe' philosophy, and failure of the tailplane rear spar had been calculated to be insufficient to cause the loss of the aircraft, the remaining front spar being sufficiently strong to enable the aircraft to land safely, the damage then being expected to be repaired before the aircraft was re-flown.

However, the accident aircraft had developed a cracked rear right spar without the fault being detected due to its location within part of the structure not normally accessible during routine maintenance, and the aircraft had been flown for a considerable number of hours with the fault present. Over time, the crack grew until, it was surmised, the damaged spar was no longer capable of carrying its designed load, whereupon the load was then taken over entirely by the front spar. The accident aircraft encountered several strong gusts during the approach immediately before the accident which, while not dangerous to a structurally sound Boeing 707, exceeded the load capable of being carried by the remaining intact spar on its own, leading to the spar eventually breaking and resulting in complete structural failure of the entire right horizontal stabiliser.

Tests were conducted to determine whether or not the loss of a single stabilizer was a recoverable situation, and it was determined that recovery would have been possible with action on behalf of the pilots in the form of nose up trim. The wreckage recovered revealed that the elevator trim should have been sufficient to save the aircraft, but further analysis concluded that the failure of the stabilizer was violent enough to fracture the vertical trim jackscrew, which would have resulted in the remaining horizontal stabilizer pitching into a nose down trim position.

The crack found in the failed stabiliser after the accident was thought by investigators unlikely to have been detectable using normal testing means, such as fluorescent dye. It was also thought that the crack had been present for at least 6,000 flight hours before the accident and before the aircraft had been acquired by Dan-Air from Pan Am.

Inspections of the Boeing 707-300 fleet, made as a result of the crash, found another 38 aircraft with similar cracks.