§ Mr. SwayneTo ask the Secretary of State for Defence if he will make a statement about the E5 fault code found on the FADEC of RAF Chinook ZD576. [158185]
§ Mr. Spellar[holding answer 23 April 2001]: The E5 fault code which was found in the memory of the DECU of RAF Chinook ZD576 indicated that a soft fault had occurred at some point over the life of the DECU since delivery. A "soft fault" cannot affect the safe operation of the control system and is therefore one that has no impact on the normal control of the aircraft.
The rotational speed (N2) of each engine's Power Turbine Spool is detected by two sensors. The E5 code would be displayed either because of a power interrupt (switching from the Auxiliary Power Unit to main generators, or the reverse) or if a discrepancy of 5 per cent. or more is detected between the two N2 signals. When such a discrepancy is detected the lower N2 signal is disregarded (or locked out). If the other signal is then lost, the original locked-out signal will be re-acquired Should both signals be lost, the fuel to the engines will be maintained at the pre-failure rate.
The "E5" fault on the pre-production software that caused the Wilmington incident was significant as the system at that time operated differently. The team undertaking the test at Wilmington did not appreciate the significance of the E5 signal, which was displayed due to a discrepancy of 5 per cent. or more between the two N2 signals, causing the lowest signal to be latched. The team then removed a connector which resulted in the loss of the only remaining N2 signal. With the one signal latched, and the loss of the remaining N2 signal, unlike the system now, the pre-production software did not re-acquire the original locked-out signal. Instead the DECU "saw" an N2 value of zero, and in order to restore the N2, the system provided more fuel to increase the speed of the power turbine. This caused the rotors to accelerate which seriously damaged the aircraft.
After the Wilmington incident the software was amended hence a similar incident could not have caused the Mull of Kintyre accident. Moreover, as I informed the hon. Member on 21 February 2000, Official Report, column 730W, the Air Accidents Investigation Board's report of their technical investigation into the accident found no evidence of a technical malfunction that could have contributed to the accident with the possible exception of a radar altimeter system fault.