Not all helicopters are equal when it comes to reliability. I am not referring to actual in flight performance reliability but instead I am referring to predicted reliability. In the case of predicted reliability this prediction is made mainly on large helicopters that will be certified for passenger service.
In the performance of the predicted reliability the analysts will use failure rates (when possible) that are at the flat portion of the so-called bathtub curve. They do not consider infant mortality nor, do they consider failures due to excessive use or cycles (old age). When it is not possible to use failure rates that have been proved in service then the analyst will draw from other failure rate databanks that contain failure rates that may or may not apply to helicopters or for that matter aircraft in general. In any case the analyst will crunch the numbers until he can show that the helicopter meets or exceeds the requirements set down by the certification authorities.
The factory will then market the helicopter on the basis of the predictions and the spares will be predicated on the predicted rates of failure. There is a remote possibility that under specific operational conditions that the helicopter might and I emphasize “MIGHT” achieve the predicted reliability. This achievement is based on operational conditions, the number of times the helicopter is operated under extreme conditions (High stress levels), the quality of the maintenance, the change out of parts at the recommended time and other contributing factors.
In most cases the repair manuals will have minimum content. The content is not expanded until service difficulties have manifested themselves. ADs will be issued and will be incorporated in the repair manuals at six-month intervals. These service difficulties can range from minor items to those that resulted in a crash. If a crash results from a single point failure the manufacturer will not be held liable by the certification authorities if they change the design or come up with a repair solution.
Nothing is ever as reliable as predicted. As many of you have said “Never fly the A model of anything”. However some failures do not manifest themselves until the models B, C, D…. Are flying.
Here is an example of operating under stress: The CH-47 because of its’ flight control system operates with both rotors in full collective with no forward cyclic being introduced. During this period the helicopter is usually operating at near max gross. The rotor heads among other dynamic components are sent back to the factory for overhaul and or repair. When the rotorheads are disassembled and inspected over 60% of the high stress parts are rejected due to overstress and resultant cracking. In another few hours of high stress operations the parts could fail. On a Sikorsky head the parts can be reworked and placed back in service for many more operating hours.
The next time someone asks what you do you can honestly tell him or her that you are a TEST PILOT.
But true.