Altitude sensing control apparatus for a gas turbine engine

Altitude sensing control apparatus for a gas turbine engine of the type including a compressor section which receives an input airflow and develops a pressurized output airflow, which comprises:

• (a) means coupled to ambient air for developing a first pneumatic signal representative of ambient air pressure;
• (b) means coupled to said compressor section for receiving a portion of said pressurized output airflow and developing a second pneumatic signal representative of said pressurized output airflow;
• and
• (c) pneumatic valve switching means for receiving said first and second pneumatic signals and developing a control apparatus pneumatic output signal in which said pneumatic valve switching means includes a first stage having a poppet valve for driving a second stage, including a drive piston, said piston being driven in a first direction for ambient air pressures greater than said predetermined value and being driven in a second opposing direction for ambient air pressures less than said predetermined value, said piston second stage including an output port, said pneumatic output signal at said output port comprising said first pneumatic signal when said piston is driven in the first direction and comprising said second pneumatic signal when said piston is driven in the second direction, said poppet valve being coupled to a bellows through a medially pivotable connecting link, said piston including a pair of opposing head portions separated by a rod portion and being sealingly located in said housing, a first region being defined between one of the opposing head portions of said piston and said housing, a second region being defined between said opposing head portions of said housing and a third region being defined between the other opposing head portion of said housing, wherein said second region is coupled to receive said second pneumatic signal, said first region is coupled to receive said first pneumatic signal, said third region is coupled through said poppet valve to a region in said housing at ambient air pressure, said one opposing head portion including a servo orifice therethrough coupling said second region to said third region with said third region developing a third pneumatic signal representative of a servo pressure for controlling the movement of said piston, wherein:
• (i) said pneumatic output signal comprises said first pneumatic signal for ambient air pressures greater than a predetermined value which is representative of a predetermined altitude, and
• (ii) said pneumatic output signal comprises said second pneumatic signal for ambient air pressures less than said predetermined value.