On this day in aviation history, an ATR 42-300 was operating a scheduled passenger service from Dublin (Ireland) to London Gatwick (England). It was the fourth and final sector for the crew. During the taxi-out from Dublin, a failure of the N0. 2 DC generator became apparent. The aircraft was stopped on the taxiway to attempt a reset of the generator and to consult the Minimum Equipment List (MEL).
The latter to check if the aircraft could be operated with a failed generator, should the reset not be successful.
In this case, the reset was successful and at 13.47 local time the aircraft took off.
Accident side overview (Source aaib.gov.uk)
Shortly after takeoff, the No. 2 DC generator failed again, as per the checklist for a DC generator failure the generator was switched off. There were no other actions or notes than just switching the generator off. The remainder of the flight was uneventful and at 15.02 local time the aircraft touched down on Gatwicks Runway 08R.
After vacating the runway the aircraft was received instructions to taxi to its designated parking position. As per company procedures, the left-hand propellor was feathered in anticipation of shutting the engine down after a cooling period of 1 minute. There were no instructions for completion of this procedure in combination with a DC generator failure, not in the normal procedures, not in the abnormal procedures for a generator failure. The shutdown of the left engine was completed 35 seconds after the propellor was feathered.
The taxi-in continued and just before turning on to the stand the aircraft brakes were checked and found functioning. (Taxi-ing is done by the captain in the left-hand cockpit seat, as the nose wheel steering tiller is on that side of the flight deck) Under guidance from a marshaller, the aircraft turned off the taxiway onto the stand and slowly rolled to its parking position. When that position was reached the marshaller signalled 'Stop' the Captain pressed the brakes, but found them unresponsive. He immediately notified the other pilot, while selecting reverse thrust. The other pilot also found the brakes, when applied from his side of the flight deck, Aircraft final vs. normal stop position to be unresponsive and immediately reached (Source aaib.gov.uk) for the emergency/parking brake.
However, the application of reverse thrust and the emergency brake was too late to avoid hitting a ground power unit which was positioned forward and to the right of the normal parking position. Two ground crew staff saw that the aircraft was going to collide with the ground power unit and attempted to run clear of the area. One of them was struck by the debris but not injured. The aircraft hit the ground power unit approximately 35 seconds after the left engine was shut down. Due to a large amount of smoke on the right-hand side of the aircraft and a master warning in the flightdeck (aural and visual) an evacuation was ordered and the aircraft was shut down in accordance with the Emergency Evacuation checklist, Buses were already waiting for the passengers and the passengers were brought to the terminal, two passengers sustained minor injuries.
View from the right-hand underwing area looking forward. (Source aaib.gov.uk)
The aircraft sustained major damage the Air Accident Investigation Branch report (available by clicking here) lists the following damage;
All four blades of the right propellor were severely damaged
Propellor pitch change mechanism damaged (causing oil leak)
Propellor blade cuffs damaged (causing oil leak)
Engine casing at the forward end of the compressor sustained a 360º crack.
Fuselage sustained debris impact damage between frames 18 and 25, with most damage between frames 20 to 23.
Impact damage to the right-wing inboard leading edge
Impact damage to the right-hand wing to body fairing
Part of the ground power unit had been propelled through the fuselage skin, causing damage to the aircraft structure and an air conditioning duct.
After system repairs were completed all systems were tested and found to be functioning normally. With both engines shut down no hydraulic pumps were functioning causing the hydraulic system pressure to drop rapidly, avoiding the normal brakes to operate. With the left-hand engine shutdown, the Blue Hydraulic pump stopped, causing loss of the Blue Hydraulic system, and with the right-hand DC-generator failed the Green Hydraulic pump was not operating.
Leaving only the emergency/park brake accumulator as a pressure source for the emergency/park brake In this accident, impact occurred before the crew could effectively apply the parking brake. Several safety recommendations were made in the report to prevent re-occurrence.
The Hydraulic systems after both engines were shut down (Source aaib.gov.uk)