Investigation Summary
The accident aircraft, G-BEBP, was the first convertible 707-300 produced. During the development of the 300 series, Boeing enlarged the tailplane and stiffened the structure by using stainless steel skinning in certain areas. This increased stiffness changed the aeroelastic behavior of the stabilizer, forcing the spars to carry higher bending loads than in previous models. A fatigue crack initiated in the right horizontal stabilizer's rear spar upper chord, likely at least 6,000 flight hours before the crash. Because the crack was located in a concealed internal area not subject to routine close inspection, it remained undetected. On the day of the accident, the aircraft encountered aerodynamic gusts during the approach to Lusaka. The rear spar, already severely compromised by fatigue, finally failed completely. The load shifted to the front spar, which was not designed to carry the entire load under such conditions and subsequently fractured. The separation of the stabilizer was so violent that it likely fractured the vertical trim jackscrew, rendering the remaining left stabilizer ineffective for pitch recovery. Following the crash, a global inspection of the Boeing 707-300 fleet discovered similar fatigue cracks in 38 other aircraft.
Final Conclusions
The UK Air Accidents Investigation Branch (AAIB) concluded in report AAR 9/1978 that the cause was a loss of pitch control following the in-flight separation of the right-hand horizontal stabilizer and elevator. This resulted from a combination of metal fatigue and an inadequate fail-safe design in the rear spar structure. Contributory factors included shortcomings in the original design assessment, certification processes that relied on data from earlier 707 variants, and inspection procedures that failed to detect the growing crack.
Video Analysis
The DEADLY Secret That Ripped This Plane APART! | Dan-Air 707 — An analysis of the structural failure and fatigue issues that led to the loss of a Dan-Air Boeing 707 freighter in Zambia.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Systems & Failure Modes
Horizontal Stabilizer
The tail structure responsible for longitudinal stability; the right side detached due to fatigue failure of the rear spar.
Rear Spar
A primary structural member of the stabilizer; the upper chord suffered a fatigue fracture that eventually led to total structural failure.
Trim Jackscrew
The mechanism controlling the pitch trim; investigators believe it fractured during the violent separation of the right stabilizer.
Interesting Facts
- 01The aircraft was the first Boeing 707-300 series ever produced.
- 02The fatigue crack was estimated to have been present for at least 6,000 flight hours.
- 03Post-accident inspections of the global 707-300 fleet found 38 other aircraft with similar cracks.
- 04The failure was attributed to a design change where stainless steel replaced aluminum, increasing structural stiffness.
- 05The aircraft was being operated by Dan-Air on behalf of IAS Cargo Airlines under a subcontract for Zambia Airways.
Safety Actions & Advisories
Fleet-wide Inspections
Immediate inspections were mandated for the Boeing 707-300/400 fleet, leading to the discovery of cracks in 38 other airframes.
Damage Tolerance Requirements
The accident contributed to the industry shift from 'fail-safe' philosophy to 'damage tolerance' requirements in aircraft design and certification.