So in essence, there are bound to be assumptions (which is what I was trying to point to in a round about way as well)..then why the questions? Why not just accept that assumptions are going to be a part of such a complicated calculation process and move on? Why question the assumptions? Is to come up with an alternate set of assumptions as these ones were not representative enough of the potential use of the aircraft? And who's to say those won't be questioned? In which case it sounds like a never ending story..
The reason may be that in this case, some of the assumptions may have been made after consulting with the customer. In Boeing's case, they know their equipment and its time before breakdown or failure, so they can give a good ballpark figure of how much it costs to keep a particular piece of equipment in service before a scheduled check comes up.
In this case, the IAF has no way of knowing when a particular equipment on board the Rafale may fail except for asking Dassault and AdlA to provide some data that they could use in their calculations. Some equipment may not even be in service, so how is one to come up with their longevity or their maintenance headaches? When we got the Kopyo for the Bison, its advertised MTBF was one thing, and its real MTBF quite another. Which would mean that had they done a life-cycle cost analysis of how many man-hours it would take to keep the Kopyo in service and how many spares would be required for that, they'd find that their calculations would've been wrong because they had gone with the manufacturer's data.
Of course, this doesn't mean that the calculations are skewed in Dassault's favour- the same is true of every other manufacturer as well who would've been consulted and asked to give data that was used in the life-cycle costs calculations.
Companies like GE, that provide turbofans to airlines have to keep a very very tight watch over their predicted failure rates, since they basically provide warranties for each turbofan that end up costing them a bundle if the engines fail for any reason that cannot be blamed on the user. So they do very detailed failure analyses using methods to statistically determine the odds of a particular failure occurring over a very large number of simulations (Monte Carlo simulations where thousands of cases are run with each variable being randomly varied). If the odds of it failing are very low, they are more comfortable making claims of a certain piece of equipement lasting as long as they claimed it will when they sold it to the customer.
If the IAF is going to introduce clauses in its contract that hold the manufacturer responsible for penalties if their equipment fails to meet the MTBF data they've given for the life-cycle cost calculations, then they may find out to their shock that the MTBF numbers may suddenly drop dramatically. After all brochure figures are one thing, and in-field figures are another. This is also what has happened to the IA, with the T-90, where the manufacturer's claimed performance and reliability specs are hugely bloated and are routinely exposed as being unreliable when the equipment is used in conditions that are dramatically different from bench test conditions or the relatively benign environments of the OEM nation.
That is one of the reasons why some Air Forces are now literally renting out the hours for operating some aircraft, such as trainers (Israel is also following that route, with an IAI/Elbit JV called TOR to buy the M-346 Master and then rent out each flying hour to the IsAF). Why be responsible for all the hassles that arise out of the equipment failing prematurely if another company is willing to do it for you, make sure that your reliability goals (which are paramount for any service) are met AND go run after the OEM when equipment doesn't meet reliability requirements?
Another model is where the OEM offers to meet the customer's reliability requirements with support contracts, but this is again basically money in their pockets for doing something that they claim their aircraft can do anyway. For e.g. Boeing offers a warranty with the C-17 that the user can buy, which basically takes care of this situation arising- they offer the guarantee that a specified range of requirements will be met. If they cannot meet that requirement, they stand to pay for not doing so.
Reliability and maintainability are two outstanding benefits of the C-17 system. Current operational requirements impose demanding reliability and maintainability. These requirements include an aircraft mission completion success probability rate of 92 percent, only 20 aircraft maintenance man-hours per flying hour, and full and partial mission availability rates of 74.7 and 82.5 percent, respectively. The Boeing warranty assures these figures will be met.