Tony Cable's task as an independent investigator seemed perfectly straightforward. His job was similar to that of a forensic IT auditor. He was to establish what role, if any, computer problems had played in a major accident. .
Fadec, a safety-critical engine control system had recently been installed on Chinook military transport helicopters. The question was: had any of the known problems with Fadec affected the last flight of a Chinook helicopter which crashed on the Mull of Kintyre in June 1994, killing all 29 people on board?
It was an apparently straightforward task because Cable had only to ask the right questions and identify any software flaws with the help of the Ministry of Defence and Fadec's supplier and sub-contractors.
At that time, there was a vast amount of information that could have been made available on the Chinook Fadec because the customer and the supplier were at an advanced stage of legal dispute over Fadec.
Cable was unaware of the litigation. So he was also unaware that the MoD was claiming that the design of the Fadec contained "fundamental flaws". He was also unaware of the MoD's claim that the Fadec system was not airworthy.
Other information was kept from Cable. He was not told the Fadec's full history of problems. He was not given access to Squadron Leader Robert Burke, the Chinook community's most experienced unit test pilot, who had a specialist in-flight knowledge of Fadec. Burke said that he was instructed not to talk to Cable.
Cable was also unaware of the existence of the MoD's expert witness in the litigation over Fadec, Malcolm Perks. Perks could have told Cable that a flawed Fadec was capable of causing a catastrophe. In the end, however, Cable had only a limited knowledge of the system.
The issues facing Cable were similar to those that confront independent forensic IT auditors during routine investigations. The auditors are not experts in the software that is in use at the companies they are investigating. They can identify the cause of an incident and any related computer problems only with the help of the supplier, the customer or both.
But sometimes suppliers and customers are unenthusiastic about an independent investigation of their systems, the results of which could embarrass both sides or worse, provide ammunition for litigants.
These sensitivities are highlighted in a report by the US-based Rand Corporation, which has carried out a study of the relationships between suppliers and independent investigators.
In its report for the National Transportation Safety Board, which investigates aviation accidents, Rand highlights some of the difficulties that independent investigators face in trying to identify whether software was to blame for a major incident and, if so, what lessons can be learned.
Jim Hall, chairman of the Safety Board, which requested the report, said, "If Rand is correct, and we believe it is, that accidents will only become more complex, we'll need more - and better - data to help us determine the cause of such accidents."
He added, "I read with interest accounts of rocket accident investigations that found the causes to involve software problems - in one case a misplaced decimal point in the computer code. The accidents were attributed to errors in computer code overlooked by engineers and quality assurance personnel. Fortunately, the reason the causes could be determined was the existence of sophisticated data link recording capabilities on the spacecraft.
"Would an investigator be able to discover such a problem in the computer code of a crashed civil airliner?"
Rand pointed out that manufacturers are placed in a position of conflicting interests when helping in an investigation into whether their software was the cause of a major incident.
"The parties most likely to be named to assist in the investigation are also likely to be named defendants in related civil litigation," said Rand. "This inherent conflict of interest may jeopardise, or be perceived to jeopardise, the integrity of the investigation."
Yet the crash of a Chinook on the Mull of Kintyre and the subsequent investigation showed the extent to which an independent investigator is reliant on the manufacturers in identifying whether their software was in any way involved in an accident.
In his accident report for the RAF, Cable said that the Fadec system recovered in the wreckage contained only "nuisance" fault codes. This assessment was one made by the manufacturers.
Cable was asked later, at an official inquiry, what reliance he had placed on help from the manufacturers.
Question: "Did you have carried out or carry out yourself any independent checks on the integrity of the Fadec or the software used in the Fadec?"
Cable: "No, I did not."
Q: "Were such checks carried out?"
C: "Not to the best of my knowledge. Not as part of the investigation."
Q: "So in checking basically the reliability of the Decu [part of the Fadec] you went back to the manufacturers and checked it with them?"
C: Yes, it is a totally normal process.
Q: "Would it have been possible to have some alternative body from the manufacturers?"
C: "No, I don't believe so in this case."
Q: "Why is that?"
C: "I think it was far too specialised for that"
This reliance on the suppliers, which was highlighted by Cable's evidence, is not confined to the area of air accident investigation.
When MPs sought to investigate the causes of the IT disasters at the Passport Agency, the Immigration Service, and in the NHS, they interviewed all parties, including the suppliers, and were left with no clear idea of the causes, who was to blame and what lessons must be learned.
But if independent investigators are less likely in future to get to the truth in major incidents in which software is a suspected factor, it is also less likely that similar incidents and accidents will be avoided in future.
Key comments from the Rand report