AAIB Role
I had been trying to stay out of the circular arguments and the repeated mantra of Caz and JP, although I think SFFP got it right with his last comment - the AAIB investigation was very limited in terms of the information made available to the investigators as it was the BOI's responsibility, in which in my view they failed - partly for reasons beyond their control, fully to investigate the accident. The original AAIB report certainly did NOT give any grounds for a Gross Negligence verdict, and Mr Cable made this even clearer in his evidence to the House of Lords - to remind Caz and JP he said:
Evidence of Mr Cable, AAIB
98. As already mentioned, Mr Cable in evidence to us stressed that throughout the investigation the evidence was "remarkably thin" (QQ 956, 968, 1013). While the evidence available to him pointed strongly to the engines operating normally, i.e. without distress, at the point of initial impact, he conceded that this did not necessarily mean that this was in accordance with pilot commands (QQ 181-4). He further explained that the possibility of an intermittent fault prior to impact could not be dismissed (Q 182).
99. He further explained that the detachment of the pallet inserts and the components carried by them could possibly cause a restriction or jam. "It would be very difficult - impossible - to dismiss the possibility that there had been a restriction and evidence had not been found" (Q 196). This explanation is readily understandable given the crowded equipment in the broom cupboard. A balance spring is some 6 inches long by 1½ inches in diameter and its mounting bracket about 1½ inches long.
100. The only positive evidence of a fault possibly contributing to the accident was a radar altimeter system fault (AAIB statement para 7.2.17 and conclusions 48-9 and 52). However, in the light of all the evidence before us, we do not consider that this fault is likely to have been relevant.
101. Mr Cable summed the situation up thus:
"Where there is no fault found that does not mean that there was not a fault present. In this case I found it probable that on the engineering side as far as I could see there was not a fault highly relevant to the accident, but I certainly could not dismiss that possibility" (Q 264).
102. As already referred to in paragraph 58, the AAIB investigation disclosed a considerable quantity of very small metallic particles in residual hydraulic fluid in parts of the boost actuator for both the pitch integrated lower control actuator (ILCA) and the thrust lower control actuators (LCAs) together with the presence of four fine metal slivers up to 0.2 inches long on one of the servo valve screens of the yaw ILCA boost actuator. This contamination was thought to have been present prior to the accident (AAIB statement para 7.4.4).
103. In evidence Mr Cable expressed the opinion that a failure of both lower control actuator systems due to hydraulic contamination would be unlikely to be a major problem as it would merely reduce the boost on the pilot's control to the upper boost actuators which drive the rotor blades[29]. A jam of an upper boost actuator would be a very different matter (QQ 204-10).
104. The US Army, however, who operate very large numbers of Chinooks, take a different view. In a report of June 1997[30] on an incident when a Chinook turned upside down at about 1100 feet and righted itself at about 250 and where no exact cause could be established, hydraulic contamination was considered to be a possible cause. The recommendations section of the report[31] referred to "uncommanded oscillations, flight control movements, and flight attitude changes" possibly related to the performance of the upper boost actuators and metal contamination in part thereof.
105. The recommendations continued,
"An additional critical area is the integrated lower control actuators (ILCA). The metal contamination and moisture found in the pitch, roll and yaw ILCAs are considered critical to FLIGHT SAFETY. The amount of contamination found in the pitch and roll ILCA were considered sufficient to cause a disturbance in the normal operation of these components at any time. One solution may be to establish a drain point for each system 2 ILCA, since the corrosion and moisture contamination appears to be primarily found in system 2. CCAD shop personnel reported that some ILCAs arrive with secondary valves jammed due to internal corrosion. This means the unit is operating on the primary control valve with no back-up or secondary valve available. If the primary valve jams, in this situation, the capability to direct hydraulic fluid flow ceases.
The upper boost actuators and ILCAs deserve immediate and positive action, since these two areas are CRITICAL TO FLIGHT SAFETY, PERSONNEL SAFETY, AND EQUIPMENT SAFETY." [32]
Based on this evidence, and without in any way denigrating the work they carried out at the time "with the information made available to them", I suggest that use of the AAIB's much more extensive capabilties to look at the full causes of the accident was deliberately kept very limited, and even sticking just to their original report and their technical and forensic analysis of the wreckage they have been mis-quoted far too many times on this thread. Given what we now know the situation was it would be very interesting to have the AAIB carry out a FULL investigation of all the facts relating to the part played by the now well publicised deficiencies in the procedures for the introduction to service of the Chinook HC2, and the then airworthiness of the fleet and ZD576 in particular, in this accident. Perhaps the MOD will include them in their "promised" new investigation.
JB