To: BIT
1. Firstly I fail to see how the presence or otherwise of a catastrophic event within the FMECA would directly change the reliability of an airframe. Surely reliability and availability are functions of MTBF and system redundancy?
In a proper Product Assurance program the first thing is to establish the failure rates for the individual piece parts that make up the elements of the components that are in a system. At this time an FMEA is performed at the component level and the failures of the individual piece parts within that component are related to the effect at the top level of that component. Then the FMEA is performed for the system. The end effects at the component level are the modes of failure at the system level. If a system is properly designed with adequate redundancy the effect of failure of a single component within the system will not migrate upwards to the aircraft level. So to answer your question proper design does effect the reliability and ultimate availability of the subject aircraft. The FMEA performs two major functions. 1) It drives the design by identifying the weak spots and 2) it serves to develop the troubleshooting instructions relative to diagnosing system defects / failures.
2. Secondly, if a catastrophic event were not mentioned at the design stage of an aircraft, which is not to say it would not be in the safety case to the airframe purchaser. Are you saying that there were no catastrophic events in the Merlin FMECA?
As indicated above one function of the FMEA is to identify the weak spots. The FMEA is normally performed at the very beginning of the design process so as to minimize the cost and time effects of changing the design. In the design process of the EH 101 the FMEAs identified all of the catastrophic single point failures that could down the helicopter. By definition a single point failure that could cause loss of the helicopter or death and or serious injury to the occupants can occur no more frequently than one time in a billion hours of operation of the aircraft fleet. (1 10-9).
After including all of these failures in the FMEA the department manager for whatever reason decided to take them out. Since they were not included in the FMEA they were never included in the Safety Hazards Analysis. By not being included in the SHA the failures were never mentioned in the training syllabus, the tech manuals nor were they included in simulator training. In other words these catastrophic failures would never occur.
3. Lets permit the Merlin to get some serious time in UK Service before we judge it. If after that if there are problems, it will be constructive criticism that will be required.
When these failures do occur the operators will collectively wonder why this type of failure was never addressed in the design.
[ 22 July 2001: Message edited by: Lu Zuckerman ]