Right beneath the spectator’s gallery in Frankfurt. Fortunately nobody got killed or injured.
What happened: Engineers had replaced a landing gear retraction system component and needed to leak check it. For this they needed to put UP pressure on the system. They installed the landing gear lock down pins, as required (which prevent the gear from retracting, even if there is UP pressure applied to the actuators) and asked the first officer to power up the hydraulic pumps and to put the gear lever into UP.
B747 landing gear pins:
The nose gear pin is not shown in the picture. it is smaller and doesn’t have the Fokker-needle safety clip, but instead a push button with two ball bearings securing the pin in its hole in the landing gear downlock.
As you can see the pins all have along streamer saying “REMOVE BEFORE FLIGHT” because keeping them in will prevent the landing gear from retracting and pilots really hate this.
Now on the B747 the nose gear doors are hydraulically actuated and will open and close when the landing gear lever is moved. Somehow the streamer of the nose gear pin got caught in the nose gear door and every time the gear lever was moved to UP or DOWN it pulled at the pin until it finally pulled the pin out, unnoticed by the ground crew. As soon as the pin was out and the gear lever moved to UP again, the nose gear retracted and the aircraft nose came down.
Lufthansa analysed the accident and designed a new gear pin, which prevents this accident from reoccurring.
Edit: Normally you can’t move the gear lever to UP while on the ground. The landing gear has proximity switches, which tell the aircraft through the PSEU (Proximity Switch Electronics unit) that there is weight on the wheels. Among other functions it will energise a solenoid in the gear lever module, which will lock the gear lever in the DOWN position. But this can be overridden on Boeing aircraft with a lever on the gear lever shaped like a pistol trigger.
By – Jan Krusat
