Lufthansa Flight 2904 was cleared to land at Okęcie International Airport Runway 11 and was informed of the existence of wind shear on the approach. To compensate for the crosswind, the pilots attempted to touch down with the aircraft banked slightly to the right. Additionally they landed with a speed about faster than usual. According to the manual, this was the correct procedure for the reported weather conditions, but the weather report was not up-to-date. At the moment of touchdown, the assumed crosswind turned out to be a tailwind. Due to the tailwind of approximately and the increased speed, the airplane hit the ground at approximately and far beyond the normal touch down point. The aircraft's right gear touched down from the runway threshold. The left gear touched down 9 seconds later, from the threshold. Only when the left gear touched the runway did the ground spoilers and engine thrust reversers start to deploy, these systems depending on oleo strut compression. The wheel brakes, triggered by wheel rotation being equal to or greater than, began to operate about 4 seconds later. The remaining length of the runway was too short to enable the aircraft to stop. Seeing the approaching end of the runway and the obstacle behind it, the pilot steered the aircraft off the runway to the right. The aircraft departed the runway at a speed of and rolled before it hit the embankment and an LLZ aerial with the left wing. A fire started in the left wing area and penetrated into the passenger cabin. Two of 70 occupants died in this accident, including the training captain who died on impact and one passenger who was unable to escape because he lost consciousness as a result of the smoke in the cabin.
Causes of the accident
The main cause of the accident was incorrect decisions and actions of the flight crew. Some of the incorrect decisions were taken when information about wind shear was received by the crew. The wind shear was produced by the front passing over the airport, accompanied by intensive variation of wind parameters as well as by heavy rain on the runway itself. One additional cause was the lack of current wind information at the tower. For that reason no up-to-date wind information could be transmitted to the crew. Further additional causes were certain design features of the aircraft. Computer logic prevented the activation of both ground spoilers and thrust reversers until a minimum compression load of at least 6.3 tons was sensed on each main landing gear strut, thus preventing the crew from achieving any braking action by the two systems before this condition was met.
Aircraft systems
To ensure that the thrust-reverse system and the spoilers are only activated in a landing situation, the software has to be sure the airplane is on the ground even if the systems are selected mid-air. The spoilers are only activated if at least one of the following two conditions is true:
there must be weight of at least 6.3 tons on each main landing gear strut
the wheels of the plane must be turning faster than.
The thrust reversers are only activated if the first condition is true. There is no way for the pilots to override the software decision and activate either system manually. In the case of the Warsaw accident neither of the first two conditions was fulfilled, so the most effective braking system was not activated. Point one was not fulfilled because the plane landed inclined. Thus the pressure of 12 tons on both landing gears combined required to trigger the sensor was not reached. Point two was not fulfilled either due to a hydroplaning effect on the wet runway. Only when the left landing gear touched the runway did the automatic aircraft systems allow the ground spoilers and engine thrust reversers to operate. Due to the braking distances in the heavy rain the aircraft could not stop before the end of the runway. The computer did not actually know the aircraft had landed until it was already 125 meters beyond the halfway point of runway 11.
Passengers and crew
As a result of the impact a fire broke out and penetrated into the cabin, killing one of the passengers. The second pilot also died as a result of the collision. 51 people were seriously injured, 5 slightly injured.
