MISSILE RELIABILITY
Analysis of the comparative military utility of missiles and aircraft must also take into account their operational reliability. The frequent claim of assured mission success by ballistic missiles overlooks the problem of mechanical failures, which can account for losses of missiles of the same magnitude as losses of aircraft to defenses.
In practice, at least 10% of ballistic missiles must be assumed to fail prior to reaching their targets. American strategic missile systems such as the Poseidon and Minuteman are reported to suffer failure rates of at least 5% to 10%.(37) The American Harpoon medium range cruise missile has demonstrated a reliability approaching 90%, following almost 50 test flights which initially demonstrated a 50% reliability.(38) And strategic nuclear exchange calculations use failure rates as high as 20%(39)
Space launch vehicles, many of which are derived from strategic rockets, also provide an indication of probable ballistic missile failure rates. These systems typically suffer approximately 10% failure rates, with 5% failure rates being unusually good, and 20% certainly not being unusually bad.(40)
Mechanical unreliability is not limited to missiles, of course. The probability that an aircraft will successfully attack a target is a function of both the probability of penetrating air defenses, as well as the reliability of the aircraft itself. While this later factor is poorly documented, the American raid on Libya provides some indication of the magnitude of aircraft reliability. Of the total of 83 aircraft participating in the raid (including 28 KC-10s and KC-135s, 24 F-111s, 5 EF-111s, 14 A-6Es, 6 A-7s and 6 F/A-18s), all but 7 completed their assigned missions, suggesting an 8% failure rate.(41) But 2 of the 14 A-6Es and 5 of the 18 F-111s that attacked Libya (the other 6 were unused spares) suffered mechanical problems that led to mission aborts, suggesting failure rates of between 14% and 28%.
from the following article....
http://www.fas.org/spp/aircraft/part03.htm