Question 1098-99

  Lord Bowness sought clarification of the interrelationship of FADEC and icing trials, and asked how this tied in with a draft memo from "AOCinC to Air Staff" about the cessation of flight trials at Boscombe Down. Sir William Wratten in his response commented that he did not take up his appointment as AOCinC until September 1994 and had not seen the draft memo referred to. This draft memo was also submitted to the Public Accounts Committee (PAC) by Tony Collins of Computer Weekly. The Committee may be interested to see a copy of our response to the PAC about this document and this is attached at Appendix I (not printed). The draft memo was not provided to Mr Collins officially, and in view of its status and nature, we asked the PAC to publish neither the document nor our response to it. The PAC agreed to our request and I hope that the Committee will also recognise our request for privacy.

  The Department's response to the draft memo explains that the memo from AOCinC to Assistant Chief of the Air Staff was part of wider correspondence about potential delays in the full Controller Aircraft (CA) release programme to evaluate the aircraft's maximum weight performance envelope. The Committee will recall that the Chinook MkII was released to service in November 1993 with certain operating restrictions which ensured the aircraft remained within the safe single engine flight regime. These included a weight limit and icing restrictions. Icing trials, related to both the performance of the aircraft in certain conditions, and the engines, ceased on 1 June 1994. The reasons being that to undertake such trials safely, the capabilities of the engine must be properly determined. Given that the FADEC software, which, through the regulation of the fuel flow, controls the power of the engines, had not been fully cleared (by Boscombe Down) the capability and the necessary levels of engine performance could not be established. Until the FADEC, and therefore engine performance parameters were cleared, all trials including the icing trials were suspended.

Question 1102-1104

  Lord Brennan asked Sir William Wratten to consider if a determination that negligence had occurred by the waypoint change, irrespective of whether or not a technical malfunction had occurred which might explain the crash, was not too severe an application of the question of negligence. Sir William accepted that it was an extremely severe application but that both he and Sir John Day had not, despite lengthy and detailed staff work, found any reason that withstood scrutiny against such a finding. In many respects the environment, circumstances and manner in which Service personnel are required to perform their duties and conduct themselves are for very good reasons different to those of civilian life. The duty of care responsibility is such that it is specifically mandated in the RAF Manual of Flight Safety, Air Publication 3207, Annex G, Chapter 8, paragraph 4 which states:

"The duty to take care varies according to the operation being performed and a duty to take a very high degree of care is rightly imposed upon a person flying an aircraft or responsible for its maintenance or its control. In such circumstances what might be trivial in other fields may, when associated with aircraft operations, amount to negligence which justifies severe criticism".

  The Reviewing Officers were well aware of this particular stricture.

27 November 2001

TURNING PERFORMANCE OF CHINOOK

  1.  There have been a number of occasions during the evidence sessions undertaken by the Select Committee on Chinook ZD576, when either their Lordships, or witnesses, have discussed the turning performance of the Chinook. For example, Lord Tombs referred to his flight in the Chinook and the impression that he gained that to steer 12 degrees to port at the Waypoint the aircraft "cleared the land quite easily"[1]. Others have hypothesised that the crew could have been flying in visual meteorological conditions as they approached the cloud covered Mull, and that their intention was to turn left and "handrail" the aircraft along the coast to Waypoint B[2].

  2.  The ability of the aircraft to perform such a manoeuvre is a key element of the determination of negligence. The following analysis of the turning performance of the Chinook is provided as further support to their Lordships' understanding of how the two Reviewing Officers concluded that, if the crew were planning to fly along the west coast of the Mull of Kintyre peninsular, and bearing in mind their high groundspeed, they should have started to turn before the Waypoint change.

  3.  Underpinning this determination is the essential fact that when flying with forward speed, a helicopter's turning performance follows the same principle of mechanics as a fixed wing aircraft. The radius of the turn being a function of the aircraft's airspeed (velocity), and the radial force applied (lateral "g" force, which is a function of bank angle). The track over the ground is modified by the wind speed component and the aircraft's ability to sustain a given rate of turn is governed by the excess power available.

  4.  The radius of turn is given by the formula:

  It follows that, for example at 150 knots airspeed, a Chinook turns at the same rate and radius as would a fighter jet or airliner for the same speed and bank angle. From the above equation, and assuming 135 knots of airspeed and a wind speed component of 25 knots, the radius of turn at 30 degrees angle of bank is 1200 metres and at 60 degrees angle of bank is 400 metres.

  5.  Relating this to the flight of ZD576, the crew planned to fly from RAF Aldergrove direct to the Mull of Kintyre lighthouse (Waypoint A) and then on to Corran (Waypoint B). This is shown on Map A (not printed—see slide 11 on p 131 of HL Paper 25(i)). Had their intention, as some have postulated, been to "handrail" along the coast of the Mull having reached Waypoint A, then the closest potential track to cloud covered high ground a reasonable pilot, exercising an appropriate degree of care, would have planned to fly is depicted by the Blue line on Map B (p 75 below).

  6.  The weather conditions at the time of the crash precluded flight over the landmass of the Mull, in the vicinity of the lighthouse, above the height of its base. Any turn away from the Mull, therefore, would have to have been conducted solely in the horizontal plane to remain in visual meteorological conditions. Lord Tombs is mistaken, therefore, in extending his impression gained during his Chinook flight on a clear day, which allowed a track to be taken over the landmass and so use both the horizontal and vertical planes in order to turn a modest number of degrees. The cloud conditions at the time of the accident would in fact have required a horizontal turn of up to 50 degrees heading change to avoid the landmass.

  7.  Map C (p 76 below) is taken from the Board of Inquiry, Annex AQ, onto which has been added the potential track to avoid cloud covered high ground, indicated again by the Blue line. The accepted maximum bank angle used in helicopter operations when carrying passengers on a non-operational sortie is 30 degrees. From Map C it can be clearly seen that a turn on to the potential track had to be established before the position of Waypoint change at Point K. The distance required to roll to the required bank angle and establish sufficient rearward cyclic movement to establish a horizontal turn then needs to be added to this point. This can take between 1 to 3 seconds depending on the bank angle required.

  8.  As Sir John Day pointed out in his evidence[3], to conduct a horizontal turn in perfect conditions, the crew would have needed to have started a 30 degree angle of bank (AOB) turn at the point they made the Waypoint change if they were to stand a reasonable chance of not striking the cliff face ahead of them. This is depicted by the Orange 30 degree AOB line on Map C. Moreover, and critically, they were only in the order of 2 seconds away from the point where a 60 degree AOB turn would have to be initiated to prevent the aircraft, with its 25 passengers on board, from striking the cliffs. A 60 degree AOB turn is the maximum bank angle allowed in the Chinook and, furthermore, there would have been insufficient excess power for it to have been sustained for more than a few seconds; this turn is depicted by the Purple 60 degree AOB line on Map C.

  9.  The Department's position remains that had it been the intention of the crew to remain in visual meteorological conditions and follow the coastline towards Waypoint B, and not over fly the landmass of the Mull, then given the prevailing cloud base and their high groundspeed, they should have commenced their horizontal turn prior to the Waypoint change. By leaving their manoeuvre any later they would have placed their aircraft, and its passengers, in a perilous situation. This amounts to gross negligence.

  10.  If on the other hand the aircraft was unable to remain in visual meteorological conditions, it should have either have been at safety altitude, or conducting an emergency abort, whilst turning away from the high ground.

11 December 2001

2   Ibid. Witness A in answer to Question 836. Back

3   Ibid. Answer to Q1032. Back