Inquest: 'Rotorblade downwash did not cause plane crash'
Sasless - verbatim from the AAIB report:
Quote: It was concluded that there was no effect from the rotor downwash on the Cessna during the turns to the right and left on the runway. A maximum wind increase of 25 kt some 21 seconds after the helicopter applied 95% twin torque was recorded however at a data test point positioned where the Cessna would have passed over vehicle access road 'M'. This wind increase could best be described as a sudden gust.
Encountering wake vortex
The effect on the aircraft during initial touchdown caused by residual turbulence created by the Sea King when landing in the south dispersal was considered. From the evidence of the crew, it was estimated that the Sea King was over the southern taxiway abeam the arrester wire more than three minutes before the Cessna crossed it. This was above the two minutes minimum spacing for aircraft (including helicopters) with crossing flight paths specified in the Manual of Air Traffic Services (MATS) Part 1 for aircraft in the weight categories of 'small', for the Sea King, and 'light' for the Cessna. Furthermore, the rotor downwash trial proved that there was no effect on the Cessna as the Sea King lifted into the high hover taxi manoeuvre. The trials showed that the longest time for the increased surface wind to return to normal was 25 seconds.
Conclusion
It was concluded that the Cessna crashed following a loss of control during an attempt to carry out a 'go-around'. The activity of the Sea King and the loss of control by the pilot of the Cessna were two distinct, and for the most part, unrelated events occurring at the same time. The plot constructed of the relative positions of each aircraft considered in conjunction with the rotor
downwash trial established conclusively that the rotor downwash from the Sea King did not play any part in the Cessna leaving the side of the runway.
unquote.
So although the Sea King might have generated downwash sufficient to get to the roadway 'M' by pulling to 95% Tq, this downwash would have dissipated after 21 seconds and the Sea King had been in the high hover taxi for longer than that. The mown grass would have indicated any downwash proximity to the runway, taxiway or roadway 'M'. Since the only downwash effect they could measure was as a direct result of the increase in Tq and that downwash dissipated 21 seconds after the Tq increase (and therefore did not exist in the area of roadway 'M' at the time of the initial loss of control of the Cessna nor its crossing of the roadway 'M' a few seconds later), one can only come to the conclusion that the downwash played no part at all in the crash which was inevitable from the point at which the pilot elected to attempt a go around.
Quote: It was concluded that there was no effect from the rotor downwash on the Cessna during the turns to the right and left on the runway. A maximum wind increase of 25 kt some 21 seconds after the helicopter applied 95% twin torque was recorded however at a data test point positioned where the Cessna would have passed over vehicle access road 'M'. This wind increase could best be described as a sudden gust.
Encountering wake vortex
The effect on the aircraft during initial touchdown caused by residual turbulence created by the Sea King when landing in the south dispersal was considered. From the evidence of the crew, it was estimated that the Sea King was over the southern taxiway abeam the arrester wire more than three minutes before the Cessna crossed it. This was above the two minutes minimum spacing for aircraft (including helicopters) with crossing flight paths specified in the Manual of Air Traffic Services (MATS) Part 1 for aircraft in the weight categories of 'small', for the Sea King, and 'light' for the Cessna. Furthermore, the rotor downwash trial proved that there was no effect on the Cessna as the Sea King lifted into the high hover taxi manoeuvre. The trials showed that the longest time for the increased surface wind to return to normal was 25 seconds.
Conclusion
It was concluded that the Cessna crashed following a loss of control during an attempt to carry out a 'go-around'. The activity of the Sea King and the loss of control by the pilot of the Cessna were two distinct, and for the most part, unrelated events occurring at the same time. The plot constructed of the relative positions of each aircraft considered in conjunction with the rotor
downwash trial established conclusively that the rotor downwash from the Sea King did not play any part in the Cessna leaving the side of the runway.
unquote.
So although the Sea King might have generated downwash sufficient to get to the roadway 'M' by pulling to 95% Tq, this downwash would have dissipated after 21 seconds and the Sea King had been in the high hover taxi for longer than that. The mown grass would have indicated any downwash proximity to the runway, taxiway or roadway 'M'. Since the only downwash effect they could measure was as a direct result of the increase in Tq and that downwash dissipated 21 seconds after the Tq increase (and therefore did not exist in the area of roadway 'M' at the time of the initial loss of control of the Cessna nor its crossing of the roadway 'M' a few seconds later), one can only come to the conclusion that the downwash played no part at all in the crash which was inevitable from the point at which the pilot elected to attempt a go around.
Crab,
I wonder what effect this might have on a Cessna 5-6 feet off the ground moving along at about 50 mph/kts or so with a nose high pitch attitude?
The AAIB puts off the wing dropping to the onset of a stall....could it have been the gust that upset the aircraft at that point?
A maximum wind increase of 25 kt some 21 seconds after the helicopter applied 95% twin torque was recorded however at a data test point positioned where the Cessna would have passed over vehicle access road 'M'. This wind increase could best be described as a sudden gust.
The AAIB puts off the wing dropping to the onset of a stall....could it have been the gust that upset the aircraft at that point?
If the gust existed at all, it might have actually helped since the reason he stalled was the flaps were up and therefore the wing was not producing sufficient lift. A sudden gust of 25 kts would have taken the wing into normal conditions, not caused a stall.
The time difference just doesn't work - the helicopter pulled 95% Tq more than the maximum persistence time measured in the downwash trial before the Cessna got to the roadway 'M'.
Interesting that the Cessna pilot didn't acknowledge his landing clearance (if the transcript of the ATC tapes is complete) - an indication that he was already working at capacity just to land the aircraft, let alone make sensible judgements about other traffic.
The time difference just doesn't work - the helicopter pulled 95% Tq more than the maximum persistence time measured in the downwash trial before the Cessna got to the roadway 'M'.
Interesting that the Cessna pilot didn't acknowledge his landing clearance (if the transcript of the ATC tapes is complete) - an indication that he was already working at capacity just to land the aircraft, let alone make sensible judgements about other traffic.
