to generate thrust the high pressure compressor 12 further compresses the air and delivers it to a combustor 18 wherein fuel 21 is delivered and burns adding energy to the air in the form of heat the high energy fuel air mixture 22 is then ducted to a high pressure turbine 24 which extracts mechanical energy from the mixture and uses that energy to drive the high pressure compressor 12 through shaft 27 the exhaust 30 of the high pressure turbine 24 is ducted to a low pressure turbine 33 which extracts further mechanical energy and uses that energy to drive the fan 6 through shaft 36 the exhaust 35 of the low pressure turbine 33 is utilized to produce thrust to the extent possible assume that as in fig2 damage to the engine 3 occurs in the form of a hole 39 in the casing 42 which surrounds the high pressure compressor 12 this damage may be inflicted for example in wartime by a projectile which strikes the engine with the hole 39 present compressor efficiency is reduced because of the loss of air 41 the air 9 delivered to the combustor 18 is reduced in mass flow and pressure as a consequence thrust of the engine 3 will be reduced the invention detects the reduction in compressor efficiency that.