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- Dan-Air B707-300 at Zambia, Africa
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations / Policy / Background
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Precursors
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- Dan-Air B707-300 at Zambia, Africa
Accident Overview
On May 14, 1977, Dan-Air (G-BEBP) was on a non-scheduled international cargo flight carrying a load of palletized freight from London Heathrow to Lusaka International Airport, Zambia. The flight had two intermediate stops at Athens and Nairobi. After the second intermediate stop, the aircraft took off from Nairobi to Lusaka with a new crew, ground service personnel, and one passenger on board.
History of the Flight
The third flight of the day for the Dan-Air aircraft proceeded normally and without incident, until final approach at Lusaka.
The flight data recorder provided the following details:
- At 0907 hours the co-pilot contacted Lusaka Approach, and at 0911 the aircraft was cleared to descend to Flight Level (FL) 110 (11,000 feet altitude).
- At 0923 hours the co-pilot reported that the aircraft was leveling at FL 110 at a Distance Measuring Equipment (DME) range of 37 nautical miles from Lusaka.
- At 0928 hours the co-pilot reported that the airfield was in sight.
- At 0929 the co-pilot reported that the airplane was turning downwind.
- At 0932:02 the Lusaka Approach controller gave the aircraft a clearance to make a visual approach to runway 10. The co-pilot replied "Roger." This was the last transmission received from the aircraft.
- At 0932:53 hours the readout from the cockpit voice recorder (CVR) indicated that 50-degree flaps had been selected.
- At 0933:11 the landing checks had been competed.
Six seconds later, 24 seconds after selection of landing flaps, at 0933:17 hours, a loud "break up" noise was recorded with the CVR record terminating five seconds later, at 0933:22 hours.

Photo of right hand stabilizer in jig, showing clevis attachments at the front and rear spars
Photo taken from accident report
Eyewitnesses on the ground observed the Dan-Air flight on what appeared to be a normal approach to runway 10 at Lusaka International Airport. They saw a large piece of the aircraft structure separate in flight. The aircraft pitched rapidly nose down and dived vertically into the ground from a height of about 800 feet, approximately two miles short of the runway threshold, and caught fire upon impact. The accident occurred in daylight and in good weather. All 6 occupants of the aircraft were killed.
The complete right side horizontal stabilizer and elevator assembly were found 200 meters back from the airplane wreckage site, indicating the separation occurred in flight prior to the aircraft pitching nose down into its final dive. The cause of the stabilizer separation became the focus of the investigation.
Horizontal Stabilizer Structure
The horizontal stabilizer is a box structure, with the forward and aft sides of the box being the front spar and rear spar, respectively. The top and bottom surfaces of the stabilizer box are formed by skin structure. The stabilizer is attached to the fuselage by lug and clevis attachments at the front and rear spars. The center of lift of the horizontal stabilizer is located slightly aft, such that the normal load distribution is split between the front spar and rear spar at approximately 5% and 95%, respectively.
Aerodynamic Affect of Horizontal Stabilizer Failure
The diagram below shows the relative locations of aircraft center of gravity, center of lift, and balancing tail load of a typical transport category aircraft in flight:
- The design of transport category aircraft results in a natural tendency to pitch the airplane nose down
- Balance (longitudinal stability) is provided by the downward acting force of the horizontal stabilizer
- Loss of the horizontal stabilizer results in an unbalanced nose-down pitch and loss of control

Diagram of locations of center gravity, center of lift, and balancing load of aircraft

Diagram of Dan-Air accident, taken from accident report
Description of the Failed Structure

Photo of separation of Dan-Air right side stabilizer.
Accident investigation photo.

Photo of fracture face of Dan-Air accident aircraft right side horizontal stabilizer.
Photo taken from accident report.
Examination of the detached stabilizer revealed evidence of a fatigue failure of the top chord of the rear spar, initiating at the 11th fastener hole, which is used by both the rear spar upper chord and upper skin structure. The location of the fastener was 14.25 inches outboard of the attachment of the stabilizer attachment pin.
The cracking progressed in fatigue over approximately 60% of the chord, and then began a series of several tensile jumps, separated by small periods of fatigue. The total number of flights between the initiation of the fatigue crack and the final failure of the upper chord was estimated by the investigators to have been approximately 7,200 flights, with 3,500 of the flights being the duration to grow the crack across the exposed surface of the top chord.

Photo of top chord fracture face, with crack progression identified. Photo taken from accident report.
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Failure of the top chord was followed by fracture of the upper web, center chord, lower web and lower chord, leading to loss of the stabilizer and loss of control of the aircraft.
The investigation discovered no unique feature leading to the cracking at the 11th fastener hole other than high stresses existing in the entire inboard area of the rear spar. The location of the 11th fastener hole is indicated by an arrow in the photo below.
Post-accident inspection of the 707-300 in-service fleet discovered cracking occurring in the rear spar upper chord at the 11th fastener in three other aircraft. Cracks were also found in other fastener holes on either side of fastener hole 11, from holes 2 to 21. In total, cracks were found in 7% of the in-service fleet, equal to 38 of the 521 model 707-300s in operation.

Photo of fracture face emanating from the 11th fastener
Photo from accident report
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Diagram of stress concentration
Effect of a Fastener Hole in a Tensile Member: "Stress Concentration"
The top chord of the stabilizer rear spar is loaded in tension. A structural member of uniform cross section will have a uniformly distributed tensile stress across the section.
A structural member having a discontinuity, such as a fastener hole, will develop a stress field that is larger at the edges of the hole. This means that the stresses are higher at the edges of the hole than in the area away from the hole. This phenomenon is called a "stress concentration," and explains one of the causes of cracking typically found to occur at the location of the discontinuity.
Development of the Design of the 707-300 Stabilizer
The Dan-Air aircraft involved in this accident was the first model 707-300 produced by The Boeing Company. The Boeing 707 began with the KC-135 military transport and includes the 707-100, -200, -300, and -400 series of aircraft. The horizontal stabilizer on the early models, the 707-100 and 200 derivatives, are identical and have rear spars of two-chord design.
In order to certify the aircraft, the manufacturer demonstrated that the two-chord design of the horizontal stabilizer was "fail-safe." That is, with a failure of any single structural element, the remaining intact structure could carry the loads anticipated in flight. In the case of the horizontal stabilizer, the failure assumed most likely, and most critical, was the failure of a single lug. In the case of the early 707 two-chord stabilizer design, the Boeing Company demonstrated through a series of dynamic tests that the horizontal stabilizer rear spar upper chord could fail, and that the stabilizer loads were reacted safely by the stabilizer box and remaining chord attachment.
The fail-safe philosophy of the 707-100, supported by the testing performed on the 707-100 two-chord design, was extrapolated to support certification of the 707-300 horizontal stabilizer. However, there were design differences between the two models that made the early tests invalid for the later model.
When the model 707-300 was developed, the stabilizer assembly was extensively redesigned. The stabilizer span was enlarged to increase tail volume. To accommodate the resulting increase in loading on the larger tail, the intended fail-safe characteristic of the structure was achieved by the addition of a third chord, with a lug and clevis attachment point, located at mid-spar depth on the rear spar.
The analysis performed on the new three-lug design of the 707-300 assumed the same failsafe scenario as was proven during testing of the two-lug stabilizer design of the 707-100/200: failure of the stabilizer rear spar upper lug. Testing was not performed to validate the failure scenario on the three-lug design because it was felt the 737-100/200 tests were representative.
For substantiation of the 707-300, Boeing used analysis to show that in the event of the upper lug failure the load would be redistributed to the lower two lugs, which would be able to carry the flight loads safely.
Another design change was made to the 707-300 as a result of a discovery made during the flight test program. Flight testing indicated that the 707-300 horizontal stabilizer lacked sufficient torsional stiffness. In order to correct this deficiency, the stabilizer lower aluminum skin was increased in thickness, and a stainless steel plate was used to replace a portion of the upper aluminum skin. The location of the stainless steel plate is shown as the shaded area on the figure below.
The stainless steel plate was much stiffer than the aluminum structure to which it was attached. Without the plate, the aluminum horizontal stabilizer would deflect a certain amount under flight loading. With the stainless steel plate riveted to the stabilizer, the deflection was less because the steel plate resisted the bending. This bending resistance was transferred to the aluminum stabilizer rear spar through higher-than-expected loading in the fasteners.
The load increase on the stabilizer that occurred as a result of the stiffening of the skin panel led to the development of fatigue cracking in an area outboard of the lug. This outboard location, which was not considered to be a likely failure scenario, was not analyzed by Boeing.
These two design changes in the 707-300 relative to the 707-100 (addition of the third chord and stainless steel plate) became significant factors leading to the loss of the Dan-Air aircraft.
Post Crash Investigation

Diagram of rear spar fracture
(View Large Diagram)

Diagram of the horizontal stabilizer load spectrum
(View Large Diagram)
Dan-Air 707-300, G-BEBP, Basic Aircraft Information:
- Investigation of the pilot and co-pilot revealed no anomalies.
- Aircraft: Manufactured - 1963
- Total airframe flight hours at time of accident - 47,621
- Total airframe flight cycles at time of accident - 16,723
- Flight hours since last C-check - 662
- Flight cycles since last C-check - 176
- Dan-Air G-BEBP was the first 707-300 produced by Boeing.
After the accident, a fail-safe test was performed on a 707-300 horizontal stabilizer. The top chord was cut to simulate the fatigue damage on the Dan-Air airplane. During testing, the spar fractured completely at a load approximating the load estimated to have been acting on the Dan-Air aircraft at the time of approach and stabilizer separation. Application of up elevator during approach was shown to produce loads 20% greater than the test failure load. This load discrepancy, and the strikingly similar fracture characteristics of the test specimen, indicated that the failure occurred as a result of an inability of the remaining structure to carry the flight loads subsequent to the fracture of the rear spar top chord.


