US investigators have concluded that a Northwest Airlines crew's decision to shut down the left engine on a taxiing McDonnell Douglas DC-9, leaving it with no hydraulic pressure after the right-side system had failed, resulted in its colliding with an Airbus A319 at Minneapolis.

The DC-9 (N763NC) had suffered a valve fracture that caused the loss of right-side hydraulic pressure, a situation that the crew noticed as the aircraft climbed out of Columbus on a domestic service to Northwest's Minneapolis/St Paul hub on 10 May 2005.

While the pilots carried out in-flight procedures to deal with a loss of right-side pressure, the captain became increasingly convinced, later in the flight, that the apparent pressure loss was a false indication. This view was reinforced by the normal lowering of the landing gear. Emergency teams at Minneapolis were nevertheless put on standby as the aircraft approached.

NWA DC-9 A319 collision 
© Cyprus Cambata   

The Northwest Airlines DC-9 collided with an Airbus A319 on pushback

The DC-9 touched down safely on runway 22 and the landing roll appeared to be normal, with brakes and reverse-thrust operating, and the aircraft exited the runway to taxi to gate G7.

Just over 5min after touchdown the flight recorders detected the shutting down of the aircraft's left-hand Pratt & Whitney JT8D engine - a technique employed to save fuel during ground manoeuvring - although the National Transportation Safety Board says the captain "did not recall" doing so.

The first officer later stated that he did not know that the engine had been stopped and that the captain had not mentioned it.

Upon nearing the gate the crew discovered they were suddenly unable to steer or brake the DC-9, and they activated the thrust reversers, which also stopped functioning within a short time and the aircraft began rolling forward.

The aircraft was travelling at around 14kt (25km/h) when it struck the wing of a Northwest A319 (N368NB) on pushback from gate G10. Both aircraft were subsequently evacuated, with eight occupants injured among the total of 145 on board the two. The DC-9 was transporting 94 passengers, the A319 had 39 on board.


Source: Flight International