### Requirements restated

By knowing the required probability of survival (95%) and the desired duration of the maintenance-free operating period (MFOP), we could calculate the minimum system mean (operating) time between failures (MTBF). Again, assuming the times between failures are exponentially distributed, the minimum MTBF for the system is 2924 hours (or approximately 0.34 failures per 1000 hour versus the 420 requirement on the *Typhoon*).

If we assume that a typical aircraft is made up of 5 systems, viz airframe, armament, avionics, propulsion and ‘general’ (see Figure 1) then we can calculate the survival probability for each of these in order to ensure the system achieves the required level. One way we could do this is to give equal weightings to each system. Another would be to apportion the requirement in line with other similar aircraft such that each system has to make a similar percentage improvement in terms of its mean time to failure.

**Figure 1: Aircraft configuration**

Taking the probability of survival, for a given maintenance-free operating period, to be 95% then each of the 5 systems will need to achieve very nearly 99% probability of surviving the period. If we use the method of allocation based on an existing aircraft, the probabilities for the airframe, armament, avionics, propulsion and general systems are 98.75%, 99.5%, 98.6%, 99.72% and 98.4% respectively.

Taking the propulsion system as an example, 99% equates to an MTBF of 14,925 hours (or 67 [failures] per million [flying] hours). With the alternative apportionment method, the probability is 99.72% which implies an MTBF of at least 53,500 hours or 19 per million hours. Now the propulsion system is made up of two engines and two sets of accessories (e.g. control units, oil pumps and, fuel pumps). If we arbitrarily assume that the accessories account for 90% of propulsion system failures we can determine that the required probability of survival for an engine is 99.986% giving an MTBF of 1,069,928 engine flying hours (EFH) or 0.93 per million EFH.

### Putting this into perspective

To put these numbers into some kind of perspective, the in-flight shut down (IFSD) rate on the Boeing 767 after its first 10,000,000 hours in service was around 20 per million. These actually represent only a small fraction of the engine arisings which require unscheduled maintenance. The B‘767 does not normally perform 9G turns, fly at Mach 2 at an altitude of 50 feet, climb vertically using reheat or any of the other aerobatics that a combat aircraft is expected to perform. Most of its flying is done at 35,000 ft (or thereabouts) at its cruise speed (approximately Mach 0.8) for several hours at a time. The difference is similar to that between a long-distance coach and a Formula 1 racing car. The coach’s engine typically lasts over million miles, the racing car’s often less than 200 miles.

It has been suggested that punitive charges may be levied against the manufacturers if they fail to achieve the required levels of reliability. One possibility is that they may be expected to perform any unscheduled maintenance at their own cost. This could mean having to hire a *Hercules* (C130) to transport a crew of skilled mechanics along with any special equipment and a spare (engine) to the aircraft’s location, possibly at or near to the front line.