The speed required for an aircraft to take-off varies with wind, air density, weight, and flap or slat positions. These are a few of the many factors that are taken into account before a pilot can get an aircraft off the ground.
The Jomo Kenyatta International Airport has an elevation of 5,327ft (1,624m) which means that the air is very thin and an aircraft needs as much area along the wingspan as possible for air to flow over it. A pilot can achieve this by extending the leading edge slats of the wings creating a larger wingspan causing lift at a high enough speed when taking off.
On 20 November 1974, a normal day in Nairobi, Lufthansa Flight 540 flight crew did their pre-flight checks and taxied for take-off. They were on the final leg of their Frankfurt-Nairobi-Johannesburg journey with 157 souls, luggage and freight, including some airmail, on board.
Shortly after take-off the pilots experienced random force vibrations. Unsure of the cause of the vibration, the captain continued to climb, and suspecting a wheel imbalance, he retracted the landing gear. The Boeing 747 was unable to maintain its altitude and the stall warning system light came on. The pilots did not have enough time to figure out the tragic error of Engineer R Hahn. In the pre-flight checklist the pneumatic system must be turned on as this allows the slats to deploy during take-off. Flight 540 was doomed before its wheels left the ground because the pneumatic system switch was off, which resulted in the aircraft becoming airborne in a partially stalled condition.
The aircraft grazed trees. The left wing struck an elevated road and exploded. The fire spread to the fuselage, completely destroying the aircraft. It was the deadliest air crash on Kenyan soil, with 59 lives lost. Miraculously some airmail covers did survive this horrific accident.
One such example is a cover from Denmark to Pretoria bearing a strike in violet of the English/Afrikaans cachet: “RECOVERED FROM AIR CRASH ON 20 NOVEMBER 1974 AT NAIROBI / HERWEN VAN LUGRAMP OP 20 NOVEMBER 1974 TE NAIROBI.”
This amazing airmail crash cover is available on filat.ch.
The investigation concluded that not only was it human error that caused this accident, but also the lack of adequate warning systems in place that could have alerted the pilots to the problem. After this first fatal accident, and their third hull loss for a similar error reported, Boeing added warning systems for unopened slat valves.