The aircraft involved was a Boeing 747-130 registered as D-ABYB and was named Hessen. It was the second 747 to be delivered to Lufthansa. It made its first flight on 30 March 1970 and was delivered to Lufthansa on 13 April the same year. The aircraft was powered by four Pratt and Whitney JT9D-7 turbofan engines. The aircraft had 16,781 flying hours at the time of the accident.
Crew
The flight crew consisted of 53-year-old captain Christian Krack, 35-year-old first officer Hans-Joachim Schacke and 51-year-old flight engineer Rudolf Hahn.
Accident
As the aircraft was making its takeoff from runway 24 at the Jomo Kenyatta International Airport in Nairobi, the pilots felt a buffeting vibration. The captain continued the climb and retracted the landing gear. However, as this was being done, the aircraft started to descend and the stallwarning system light came on. The aircraft continued to descend and approximately from the end of the runway, the 747 airplane crashed in the grass. It then struck an elevated access road and broke up. The left wing exploded and fire spread to the fuselage. 55 of the 140 passengers and 4 of the 17 crew members were killed.
Cause
The cause of the crash was determined to be a stall caused by the leading edge slats having been left in the retracted position. Even though the trailing edge flaps were deployed, without the slats being extended the aircraft's stall speed was higher and the maximum angle of attack was lower. As a result, the aircraft was unable to climb out of ground effect. The flight engineer was found to have failed to open the slat system bleed air valves as required on the pre-flight checklist. This prevented bleed air from flowing to the 747's pneumatic slat system and, since the leading edge slats on the 747 are pneumatically driven, kept it from deploying the leading edge slats for takeoff. The take-off warning system that would have sounded an alarm due to the flaps not being lowered did not have a separate warning that the slats' pneumatic valve had not been opened by the flight engineer. The faulty state of the slats should by design have been indicated by yellow warning lights: one for the pilot, and eight for the flight engineer. However, both crew members stated in court that these lights had been green. Three possible explanations have since been offered for this inconsistency: that the morning sun was blinding the cockpit crew and thus hampered color perception, that a construction error could have caused green lights despite the retracted slats, and that the crew lied. None of these possibilities could be conclusively proven. The flight crew was blamed for not performing a satisfactory pre-take-off checklist, but the accident report also faulted the lack of adequate warning systems which could have alerted the crew to the problem. Two previous occurrences of this error had been reported, but in those cases the pilots had been able to recover the aircraft in time. After this third, deadly incident, Boeing added systems to warn pilots if the slat valve had not been opened prior to takeoff. Captain Krack and flight engineer Hahn were dismissed from Lufthansa shortly after, but their dismissals were overturned by a labor court, as there was no investigation report available to rule out the chances of a technical defect. Flight engineer Hahn was charged with criminal negligence, but was acquitted in 1981.
