15th of December 1972
It was on this day in aviation history, in 1972 that Northwest Airlines B747-151, N602US, was taking off from Miami Airport, Florida at 16.55 local time. Al was well up to the moment of rotation when the aircraft encountered a flock of seagulls. Sounds of bird impact were heard by the crew and airframe vibration became apparent at the same time. The Flight Engineer reported maximum vibration indication on the #4 engine turbine vibration indicator. With the other three engines operating normally the captain reduced the thrust on the #4 engine and then shut the engine down. As a result, also the airframe vibration stopped.
The aircraft taking off in 1987
ATC was informed at 16.58 lt and that they intended to return to Miami. ATC vectored them on the approach for runway 27L and cleared them for an ILS approach.
At 1711 they were in contact with the tower controller while over the outer marker. ATC cleared the aircraft for landing and reported the wind as 120º at 10 knots which were confirmed by the crew. With some light rain at times, the approach was without any trouble. At 1000 feet the crew had the runway in sight. Flaps were set to 25º and Vref was determined to be 140 kts. The runway was wet from the showers moving through the area, as encountered by the crew on the approach
The touchdown was normal, reported as in the touchdown zone, and brakes were applied, and reverse thrust was selected on the operative engines. The reverser lever of the #3 engine would not move past the interlock position. This led to yaw as a result of reverse thrust only being available on the left-hand engines. As a result reverse thrust could not be used in the most effective way. Despite full braking and repeated attempts to increase the reverse thrust on engine #3, the groundspeed was hardly reducing. The aircraft was hydroplaning and could not be stopped before the end of the runway. The aircraft left the paved surface of the runway hit a concrete block (an old foundation under the grass) with the nose wheels, causing the nose landing gear to fold backward and finally came to a stop 611 feet (186 meters) past the end of the runway.
The aircraft the day after the runway overrun
The incident was investigated by the NTSB and an investigation report was published on the 30th of May 1973, click here to go to the full Aircraft Accident Report.
The investigation revealed, amongst others;
1. The birdstrike was on engine #3, not on engine #4, and cause extensive damage to the engine and the engine controls, limiting the selection of reverse thrust on landing
2. The maximum vibration indication on engine #4 was the result of a failure
3. Combination of the birdstrike coinciding with the vibration on engine #4 led to the logical action of shutting down engine #4
4. Touch down was 1475 feet past the thresh hold
5. Based on runway friction data, hydroplaning played a role in the limited brake effectiveness
The probable cause of this accident was the ineffective braking capability of the aircraft on the wet runway because of the low coefficient of friction of the new runway surface, and insufficient engine reverses thrust to decelerate the aircraft. The combined effects of the lack of the No. 4 engine reverse thrust and malfunction of the No. 3 engine reverser resulted in a directional control problem and restricted the use of Nos. 1 and 2 engine reversers.