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The worst nightmare of any helicopter pilot is having a rotor detach from the chopper. It's not yet clear what happened to the Israel Air Force's Cobra helicopter that crashed over the Lower Galilee during a training exercise yesterday, but eyewitness testimony and the information collected by IAF accident investigators suggest that a technical problem was to blame, apparently involving a detached rotor. It's too early to say which of the two propellers was damaged and apparently became detached.

While the wings of an airplane are what help it get off the ground, helicopters fly thanks to two propellers. One revolves around the upper part of the helicopter, and the second operates above the tail. A helicopter pilot controls the two rotors through the steering and the foot pedals. The movement of the blades is what allows the helicopter to go up or down, or to hover in place.

A technical problem in which a rotor gets detached from a helicopter is fairly rare, but there have been such incidents, mostly caused by structural fatigue. The rotor blades are supposed to be examined and replaced periodically, based on the number of hours flown. If those procedures were followed, nothing else could have been done to prepare for the possibility of propeller detachment. The air force commander ordered all Cobra helicopters grounded until the end of the accident investigation, and the next few days will presumably see meticulous examination of all the rotor blades of all the Cobra helicopters. In such cases, the manufacturer is also sent a notice about the incident and in turn updates all the air forces that use Cobra helicopters.

The helicopter that crashed yesterday was on a military readiness mission, and was therefore equipped with weapons - which exploded on contact with the ground and caused the fire to spread rapidly. It is not yet clear with which weapons the helicopter was equipped, but it may have been carrying missiles. A missile-carrying helicopter holds 24 missiles, 2.75 inches in diameter (seven centimeters), on each side, along with a rapid-fire cannon and about 1,000 bullets.

The Cobra carries a two-person team: a pilot and a navigator, the latter of whom operates the weaponry. The pilot sits behind and above the navigator. It appears that, unfortunately, when the malfunction took place, the pilot and navigator were unable to regain control of the helicopter. If the accident did indeed involve detachment of a rotor, there was no way to prevent the crash.