Sub-idle speed control apparatus for an airplane turbine engine

#The turbine engine of an airplane runs at ground idle speed while the aircraft is taxiing into an airport station or taxiing out to the runway in preparation for take-off. Ground idle is a throttle position which maintains a moderately low thrust at which the engine may be operated suitable for specific ground conditions. Taxiing with the engine operating at ground idle thrust, however, should be kept to a minimum since the engine thrust is sufficiently high to cause the brakes to wear relatively fast. In addition, fuel consumption while taxiing at ground idle is also relatively high. Furthermore, the ground idle thrust may cause the aircraft to be difficult to control on slippery runways coated with ice or snow.

#A thrust reverser mechanism may be incorporated in the engine to help reduce the thrust of an airplane turbine engine to solve the above mentioned problem. However, a thrust reverser mechanism would have to be generally controlled by a separate set of control throttles which would mean more equipment in the pilot's already crowded cockpit. In addition to the fact that the pilot would be forced to become familiar with still another set of complicated controls, when the thrust reverser mechanism is utilized at the low ground idle speed, there may be a tendency for exhaust gases to reenter the air inlet which could result in compressor surge and an excessive exhaust gas temperature.

#Alternatively, the existing throttle could be modified to include another position corresponding to a lower thrust. This arrangement would entail further complicating the throttle. More particularly, safety precautions would require another detent added to the throttle to prevent the pilot from moving into the lower thrust position too soon, and thus losing power unnecessarily.

#FIG. 1 is a schematic diagram of a prior art airplane turbine engine fuel control system.

#FIG. 2 is a schematic diagram of the sub-idle speed control apparatus of the subject invention after activation at which time the pressure flow P.sub.3 is diverted through the restrictor.

#FIG. 3 is a schematic diagram of the sub-idle apparatus of the subject invention after activation at which time the pressure flow P.sub.3 is diverted through the restrictor.

#Referring to FIG. 1, a prior art fuel control system embodied in an engine 10 extends between compressor 12 and a fuel control 14. As noted above, the amount of fuel flow to the engine is a function of four parameters: compressor speed (N.sub.H) which is measured by a speed sensing means 16; air temperature entering the engine (T) which is measured by temperature sensing means 18; compressor discharge pressure (P.sub.3) which is measured by pressure sensing means 20 through fluid pressure line 22; and the position of throttle lever 24 of throttle control 26. In order to taxi the airplane in the airport, the pilot selects the lowest throttle position, which causes the airplane engine speed to run at ground idle. During taxiing, the thrust of the aircraft at ground idle speed is higher than what is actually needed, thus causing rapid brake wear and fuel consumption. To reduce the thrust, the amount of fuel supplied by the fuel control 14 must be reduced, which will reduce the engine speed. The throttle 26 already being at the lowest position, one of the other factors that controls the fuel flow must be reduced. The compressor speed N.sub.H at ground idle is also at its minimum level of 50% N.sub.H, which is required to run the alternator to supply electrical power to the aircraft for support functions while the aircraft is on the ground. However, if an auxiliary power unit is utilized to supply the necessary electrical power, the compressor speed can be reduced. Alternatively, in a multi-engine airplane, the compressor speed can be reduced in less than all the engines. Thus, if an apparatus could be supplied to reduce the compressor speed, the engine thrust would be significantly reduced.

#Accordingly, sub-idle speed control apparatus 28, as shown in FIG. 2, is provided which reduces the amount of P.sub.3 sensed by pressure sensing means 20 thus reducing the amount of fuel flow supplied by fuel control 14 thereby lowering the compressor speed and producing a lower engine thrust. Referring now to FIG. 2 sub-idle speed control apparatus 28 includes normally open valve 30 intermediate fluid pressure line 22 extending between compressor 12 and pressure sensing means 20. When the turbine engine 10 is operational, compressor 12 will discharge a pressure indicated by arrows P.sub.3 through fluid pressure line 22. In the condition of the sub-idle speed control apparatus 28 as shown in FIG. 2, the sub-idle apparatus 28 is deactivated and the aircraft engine operates under normal conditions. At normal operation, the normally open valve 30 is in the open position and pressure P.sub.3 flows through fluid pressure line 22 directly to pressure sensing means 20 as indicated by the arrows P.sub.3 in FIG. 2. Hence the actual level of P.sub.3 is sensed by pressure sensing means 20 and therefore fuel control 14 supplies the amount of fuel to the engine corresponding to the actual P.sub.3 pressure.

#A by-pass line 32 is connected to fluid pressure line 22 at location 34 between compressor 12 and normally open valve 30 and at location 36 between normally open valve 30 and pressure sensing means 20. Restrictor means 38 is disposed intermediate by-pass line 32. Preferably, restrictor means 38 is a multi-orifice restrictor which acts to drop the pressure flowing through the orifices when the sub-idle speed control apparatus is operational. Extension line 40 is connected to the by-pass line 32 and fluid pressure line 22 at location 36. A normally closed valve 42 is disposed intermediate extension line 40. Normally closed valve 42 is in the closed position when the sub-idle apparatus is not in operation and hence prevents pressure P.sub.3 from flowing through by-pass line 32. Spring loaded valve 44 is connected to the free end of extension line 40 for maintaining a steady pressure drop across the restrictor means 38. One end of spring loaded valve 44 extends outboard the airplane for releasing excess air pressure. At normal operation, spring loaded valve 44 is in a closed position, as shown in FIG. 2.