On February 25, 2009, Turkish Airlines flight 1951 from Istanbul to Amsterdam crashed just short of runway 18R while on final approach. There were 128 passengers onboard the Boeing 737-800, the world's best selling aircraft. Nine people died in the crash - including all three pilots and also three Boeing engineers. Investigators were puzzled - passengers and witnesses said the plane dropped out of the sky. Why did this happen?
The Boeing 737's fuselage cracked and the engines separated from the wings (Source: flyaway simulation.com)
The cause of the accident was determined to be a faulty radio altimeter on the captain's side of the flight deck (in total, the aircraft has three altimeters). A radio altimeter is "an instrument that determines elevation, usually from mean sea level, by measuring the amount of time an electromagnetic pulse takes to travel from an aircraft to the ground and back again."  The Boeing 737-800 has four external antennas to assist the altimeter readings - two that send out the signal and two that read the signal back.
At about 8,000 feet above the earth while descending, the captain's altimeter began to show a reading of -8 feet. The pilots were obviously aware that this altimeter's reading was incorrect and ignored the warning horns that started going off in the cockpit to put down their landing gear. So if the pilots were aware of the fault, why did they still crash?
There are two automated systems on airplanes that greatly assist pilots - one is the well known autopilot, that controls the yoke, and the other is the lesser known autothrottle, that controls the engines. The autothrottle's altitude information is supplied by the captain's side altimeter. Since this reading was showing the plane to already be on the ground, the autothrottle brought the engines power to idle. Following the instructions of their air traffic controller, the pilots (captain, first officer, and a third pilot assisting the training of the new first officer) were at a stage in their descent where they would have needed the engines to be near idle. This is not typical of most descents, but it is typical at busy Amsterdam, where ATC tries to bring the airplanes in faster by requiring a quicker/steeper descent. Because of this unique approach, the pilots did not realize that the autothrottle had actually kept the engines permanently at idle. With the loss of engine thrust, the airplane kept losing speed and at 460 feet above the ground, it stalled. The pilots were unable to recover the throttles in time and the Boeing dropped from the sky into a muddy field.
Above is the episode from the show Mayday that discusses Turkish Airlines flight 1951.