1.  Vector BA is error in RNS 252 SuperTANS position at powerdown.

  2.  Vector JK is most likely error in RNS 252 SuperTANS position at point of leg transfer to WP B .

  3.  Track KA is most likely aircraft track to impact.

  4.  Track LH is possible closing track to WP A assumed by the crew.

INTRODUCTION

  1.  The principles of sound airmanship dictate that there are fundamental differences in the way that helicopters are flown in good weather, marginal weather and bad weather. For the purposes of this short paper, I define these 3 weather states as follows:

(a)  Good Weather. At least as good as the Visual Flight Rules which apply to flight at Indicated Airspeeds greater than 140 knots. This requires a visibility of greater than 5 kilometres, 500 feet vertically clear of cloud and 1500 metres horizontally clear of cloud.

(b)  Marginal Weather. Weather conditions that are no longer good enough to permit flight at Indicated Airspeeds greater than 140 knots. In practice, this may mean the visibility reducing towards 1 kilometre or the general cloud base reducing towards 250 feet above ground or sea level.

(c)  Bad Weather. Worse than 1 kilometre visibility or general cloud base lower than 250 feet above ground or sea level. This is also known as Instrument Meteorological Conditions.

FLYING IN GOOD WEATHER

  2.  When flying in good weather conditions, a pilot would have no qualms about flying his helicopter at high speed. In the case of a Chinook MkII, this might typically give a cruising speed of 135 knots or more, depending on aircraft weight, turbulence levels and vibration levels. When flying at low level in good weather, the pilot uses a combination of power changes and cyclic control movements to follow the contours of undulating ground and thus maintain a reasonably constant height above the ground and obstructions. This is known as "contour flying". The pilot would also use the cyclic controls to roll and, thus, turn his helicopter in order to avoid buildings and steeply rising ground. This is part of the "contour flying" technique, which may be summarised as the pilot using all the flying controls to fly the helicopter visually in direct relationship to the ground and obstructions. To do this, he must be able to see the ground clearly and be able to see far enough ahead to fly his aircraft safely. Speed and forward visibility are the two vital factors in achieving safe flight.

FLYING IN MARGINAL WEATHER

  3.  When flying in marginal weather conditions, the pilot must take into account the fact that the weather has deteriorated below that required for flight above 140 knots Indicated Airspeed. Potential hazards such as other aircraft, large buildings, masts or pylons, or rapidly rising ground, would appear at unacceptably short notice if the speed of the helicopter is not reduced to take account of a reduction in visibility. At low speed, a helicopter can be turned or brought to the hover quite quickly, but this is not possible at higher speeds. In this respect, an analogy can be made with a motor car. The greater the speed, the longer it will take to stop. The turning circle will also be greater, and the longer it will to take to turn through, say, 90 degrees.

  4.  When low flying, a reduced cloud base will almost always cause a reduction in the pilot's forward visibility for a number of reasons. First the helicopter is "contour flying", so its height above mean sea level is varying, and thus its vertical distance from the cloud base is varying. Generally the closer it is to the cloud base, the worse the visibility will be. Secondly, the cloud base above mean sea level goes up and down as a result of the effect of wind and topography. Thirdly, sea spray and/or additional moisture from large concentrations of vegetation causes a significant reduction in cloud base when the general cloud base is, in any case, low; this is manifested to the pilot as a significant reduction in visibility. For all these reasons, good airmanship dictates that pilots must reduce their forward speed when low flying in marginal conditions.

FLYING IN BAD WEATHER

  5.  When flying in bad weather, defined as Instrument Meteorological Conditions, the pilot must fly by sole reference to the aircraft's instruments. Heights, speeds and headings cannot be varied rapidly. For this and many other reasons, flight in Instrument Meteorological Conditions requires that the pilot maintains a vertical separation from the ground of at least 1000 feet. This is known as the Minimum Safety Altitude.

SELECT COMMITTEE'S CHINOOK SORTIE TO THE MULL OF KINTYRE

  6.  When the Select Committee flew to the Mull of Kintyre in a Chinook, the weather was good. The pilot was therefore able to "contour fly" as he replicated the track taken by ZD 576 on 2 June 1994. He was able, through relatively minor adjustments to the cyclic controls in both pitch and roll, to vary the aircraft's flight path to match the contours of the terrain. Whilst this would have felt perfectly safe in good weather conditions, it would have been very frightening and dangerous in marginal weather conditions, and would have been considered reckless in Instrument Meteorological Conditions.

Air Chief Marshal Sir John Day

6 December 2001

RELIABILITY OF EYEWITNESS EVIDENCE

  In his note of 9 October 2001 to the Committee, which was published at the end of the first volume of evidence[4], Capt J D Cook questioned the reliability of the eyewitness evidence. The comments of the eyewitnesses with regard to the weather, and the noise of the helicopter, were summarised by ACM Sir John Day in his critical analysis (forwarded to the Committee on 12 October)[5] in response to the scenarios put forward by Capt Cook and Mr Tapper at the evidence session on 28 September. We have examined thoroughly the witness statements against both the aircraft's track and the AAIB determination of the dispersion of the wreckage and the location of the crew and passengers after the crash. The deduced positions of the eyewitnesses are shown on the attached Appendix. All the eyewitnesses were within 400 metres of the aircraft's final track. There seems no reason to question the accuracy or veracity of any of their evidence, which show no inconsistencies.

POSSIBLE FAILURE OF THE UPPER BOOST ACTUATORS

  Capt Cook ended his note of 9 October 2001 to the Committee describing a scenario in which there was a failure of the upper boost actuator, possibly whilst Chinook ZD 576 was in straight and level flight, that became apparent to his son Rick at the point off the coast when he wanted to turn left.

  Helicopters are inherently unstable and require constant control inputs either from the pilots or the onboard stability system (Automatic Flight Controls System (AFCS)) even when flying straight and level. We therefore totally reject Capt Cook's scenario. These control inputs require the upper boost actuators to maintain control of the rotor system and any failure of the upper boost actuators would be readily apparent to the handling pilot. In such circumstances, the pilots would not have made a manual waypoint change.

11 December 2001

5   HL Paper 25(i) p 153. Back