An uncommanded rudder hard-over was the cause of the 1991 United Airlines Boeing 737-200 accident, says a just-published US National Transportation Safety Board (NTSB) revision of its original open verdict.

Now the NTSB says the cause of the Colorado Springs crash, which killed all 25 people on board when the aircraft went out of control on final approach, was the same as that in the 1994 USAir 737-300 accident on approach to Pittsburgh.

The USAir accident cause was reported as: "The rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit (PCU)." All 132 people on board died.

Since the rudder hardover phenomenon was determined, the NTSB says the US 737 fleet has been retrofitted with newly designed PCU servo valves, and Boeing has provided new rudder control redundancy.

Source: Flight International