Cause
The Boeing 707 320/420 series had an enlarged horizontal stabilizer (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 stabilizer spars entirely by themselves. This led over time to fatigue cracking in the right horizontal stabilizer'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, whilst 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 stabilizer.
The crack found in the failed stabilizer 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.
Read more about this topic: 1977 Dan-Air Boeing 707 Crash