Inlet door state control system

1. An inlet door state control system for an auxiliary power unit (APU), comprising:

an inlet door configured to allow air into an inlet of the APU;

an actuator configured to open the inlet door;

a plurality of parallel-connected APU threshold switches connected in series to the actuator, the plurality of parallel-connected APU threshold switches configured to change from an open position to a closed position in response to a predetermined APU operating condition being satisfied, and

a plurality of parallel-connected door state switches connecting the actuator in series to the plurality of parallel-connected APU threshold switches, the plurality of parallel-connected door state switches configured to automatically change from a closed position to an open position in response to the inlet door being in a fully open position, thereby interrupting the actuator from opening the inlet door,

wherein the actuator opens the inlet door in response to any one of the plurality of parallel-connected APU threshold switches and corresponding plurality of parallel-connected door state switches being in the closed position.

2. The inlet door state control system of claim 1 , wherein the APU threshold switches move from the open position to the closed position independent of instruction from a controller.

3. The inlet door state control system of claim 1 , wherein at least one of the plurality of parallel-connected APU threshold switches is a thermal switch and the predetermined APU operating condition is at least one an exhaust gas temperature, bleed air temperature, oil temperature, and fuel temperature, each exceeding a temperature threshold.

4. The inlet door state control system of claim 1 , wherein at least one of the plurality of parallel-connected APU threshold switches is an air pressure switch and the predetermined APU operating condition is an air pressure within the APU exceeding an air pressure threshold.

5. The inlet door state control system of claim 1 , wherein at least one of the plurality of parallel-connected APU threshold switches is an oil pressure switch and the predetermined APU operating condition is an oil pressure within the APU exceeding an oil pressure threshold.

6. The inlet door state control system of claim 1 , wherein at least one of the APU threshold switches is a fuel pressure sensor and the predetermined APU operating condition is a fuel pressure within the APU exceeding a fuel pressure threshold.

7. The inlet door state control system of claim 1 , wherein at least one of the APU threshold switches is a speed sensor and the predetermined APU operating condition is a speed within the APU exceeding a frequency or rotation-per-minute threshold.

8. The inlet door state control system of claim 1 , further comprising a door position switch connecting the actuator to at least one of the APU threshold switches, the door position switch configured to automatically change from a closed position to an open position in response to the inlet door being in a fully open position, thereby interrupting the actuator opening the inlet door.

9. An inlet door state control system for an auxiliary power unit (APU), comprising:

an inlet door configured to allow air into an inlet of the APU;

an actuator configured to open the inlet door;

a first APU threshold switch connected to the actuator, the first APU threshold switch configured to change from a first open position to a first closed position in response to exceeding a first threshold, thereby opening the inlet door; and

a first inlet door state switch connected to the first APU threshold switch, the first inlet door state switch configured to change from a second closed position to a second open position in response to reaching a first inlet door state, thereby ceasing further opening of the inlet door,

wherein the actuator opens the inlet door in response to at least one of the first APU threshold switch being in the first closed position and the first inlet door state switch being in the second closed position.

10. The inlet door state control system of claim 9 , further comprising a second APU threshold switch and a second inlet door state switch,

wherein the first APU threshold switch and the first inlet door state switch are connected in series, the second APU threshold switch and the second inlet door state switch are connected in series, and the first APU threshold switch and the first inlet door state switch are connected in parallel to the second APU threshold switch and the second inlet door state switch.

11. The inlet door state control system of claim 10 , wherein the second APU threshold switch is configured to automatically change from a third open position to a third closed position when a second threshold is exceeded, and the second inlet door state switch is configured to automatically change from a fourth closed position to fourth open position in response to reaching a second inlet door state, and

wherein the second threshold is greater than the first threshold, and the inlet door is more open in the second inlet door state than in the first inlet door state.

12. The inlet door state control system of claim 11 , wherein at least one of the first APU threshold switch is configured to move from the first open position to the first closed position and the second APU threshold switch is configured to move from the third open position to the third closed position, both independent of instruction from a controller.

13. The inlet door state control system of claim 11 , wherein at least one of the first inlet door state switch is configured to move from the second open position to the second closed position and the second inlet door state switch is configured to move from the fourth closed position to the fourth open position, both independent of instruction from a controller.