Boeing 707-321C accident at Anchorage, Alaska, 26 Dec 1968

Accident narrative

Pan American World Airways Flight 799, a Boeing 707-321C cargo and mail flight, crashed on takeoff from Runway 23 at Elmendorf Air Force Base in Anchorage, Alaska, at approximately 0615 on December 26, 1968. The aircraft was destroyed, and all three crewmembers—the only occupants aboard—were killed.

### The flight

Flight 799 was a regularly scheduled trip from San Francisco, California, to Cam Ranh Bay, Vietnam, with intermediate stops planned in Tokyo, Japan, and Da Nang, Vietnam. A refueling stop and crew change were scheduled at Anchorage International Airport.

Because local weather was below landing minimums at Anchorage International, the arriving captain elected to land at Elmendorf Air Force Base. The outbound crewmembers traveled to Elmendorf and conferred briefly with the arriving crew, who advised that they had experienced some difficulty with the reverser on the No. 4 engine. The outbound crew consisted of a captain, a first officer, and a flight engineer.

### Sequence of events

Flight 799 experienced several delays before departure, including a fuel density computation discrepancy that required additional fuel, and difficulty getting a jet starter unit to provide power. The engines were started at approximately 0555, and the flight departed the ramp at 0602.

The flight's clearance void time was extended six times by the Oceanic Control Coordinator. The final void time was set for 0615 to prevent a conflict with a Northwest Airlines flight cruising at Flight Level 310. The controller stated that if Flight 799 missed the 0615 deadline, they would have faced a 45-minute delay or been forced to accept a lower cruising altitude with excessive fuel consumption.

The crew initially received clearance for Runway 05 but requested Runway 23 because of its greater effective length. Because the crew was unfamiliar with the airport and a portion of the taxiway lights was out, a "follow me" truck was used. The cockpit voice recorder (CVR) captured the crew reading the taxi portion of the cockpit checklist, which included a callout for "wing flaps." The captain advised that he had raised the flaps, to which the first officer replied, "Oh, okay, let's not forget them." As taxiing continued, the crew was absorbed with radio communications regarding the void time, computing fuel burnoff, and controlling the aircraft on slippery taxiways.

The flight reached the takeoff end of Runway 23 at approximately 0610 and held for landing and departing military traffic. At 0614:30, they were cleared for takeoff. The crew had computed takeoff speeds based on a 14-degree flap configuration: a V1 of 148 knots, Vr of 154 knots, and V2 of 168 knots.

The flight engineer challenged the pilots on the final pre-takeoff checklist item, and takeoff power was applied. The captain called out airspeeds at 129 knots, V1, and Vr. Shortly after V1, the CVR recorded the noise of the stick shaker, which continued to the end of the recording, followed by numerous popping noises.

Witnesses observed an unusually long takeoff roll, a slow climb reaching an estimated 10 to 200 feet, and flames from the engines while the aircraft maneuvered in nose-up and wing-down attitudes. The aircraft crashed in a steep right bank with the nose low.

### What the investigation found

The right wingtip first contacted the snow-covered ground 94 feet left of the extended runway centerline. The aircraft became inverted and was almost completely destroyed by impact and a large ground fire.

Investigators found that the landing flaps were in the retracted position at the time of breakup. Eight of the ten flap drivescrews were found at or near full retraction, and the left outboard aileron lockout mechanism was in a position consistent with zero degrees of flaps. The landing gear was extended, and the elevator screwjack was set properly for a takeoff with 14 degrees of flaps. The engines showed evidence of rotation at impact with no signs of in-flight fire or overtemperature.

The Board noted that the air carrier's cockpit checklist included a wing flap item on the taxi portion, but no provision on the pre-takeoff portion to remind pilots to lower the flaps. The Board believed the captain had retracted the flaps in compliance with company cold weather operating procedures, which recommended leaving flaps up during taxi to prevent snow or ice from freezing the flap screws.

The Boeing 707-321C was equipped with a takeoff warning system designed to sound a horn if the thrust levers were advanced while the flaps were not in a takeoff position. No horn was heard on the CVR. The investigation found that in January 1967, Boeing issued a Service Bulletin warning that during cold weather operations, the warning system might not operate because takeoff engine pressure ratio could be reached before the thrust levers were advanced enough (42 degrees) to arm the system. Boeing recommended adjusting the actuator setting to 25 degrees.

Pan American had reviewed this bulletin but decided it was not applicable, noting that no specific temperature was given, no incident was cited, and the FAA had not made it mandatory. The temperature at the time of the accident was 6°F. The Board concluded the warning horn failed to sound because the takeoff power setting did not require more than 42 degrees of throttle advancement.

Because the takeoff was made with retracted flaps, insufficient airspeed was attained at lift-off to maintain lateral control. The aircraft left the ground close to or below stick shaker speed. The Board noted that outboard ailerons are designed to be locked out at zero degrees of flaps, reducing roll capability. The rapid attitude changes near the stall caused momentary compressor stalls, explaining the popping noises and flames seen by witnesses. The Board concluded that airframe icing was not a causal factor and that crew fatigue was not involved.

### Probable cause

The Board determined that the probable cause of this accident was "an attempted takeoff with the flaps in a retracted position. This resulted from a combination of factors: (a) inadequate cockpit checklist and procedures; (b) a warning system inadequacy associated with cold weather operations; (c) ineffective control practices regarding manufacturer's Service Bulletins; and (d) stresses imposed upon the crew by their attempts to meet an air traffic control deadline."