There is a relationship between Ng limiting and Ng shutdown.  There are two types of Ng limiting (Ng governing or maximum fuel flow) that limit the overall Ng speed, a malfunctioning T2 sensor may cause the Ng governing to allow Ng speed to increase well beyond the maximum design parameters.  Test cell analysis places catastrophic Ng section failure at approximately 150-160 % Ng, thus Ng shutdown is a fail safe fuel metering function that prevents catastrophic failure in the event the Ng governing fails to properly limit the Ng speed.

(c)
The PAS override function is used if an ECU malfunction causes an uncommanded power reduction as low as idle.  The pilot can reclaim lost power through ECU lockout.  Advancing the PCL to the lockout position mechanically blocks off the torque motors fuel controlling passage within the HMU, effectively disabling all torque motor inputs, engine power saturates to maximum, and the PCL is quickly retarded to set power at the desired level.  Full manual control is available while in ECU lockout.  Moving the PCL to the IDLE detent reopens the torque motor fuel control passage restoring torque motor (ECU) operation.

(4) HMU functions not affecting fuel flow metering:

(a)
Variable geometry positioning- A variable geometry actuator extending from the HMU varies the angle of attack of the IGV's and the variable stator vanes, and opens or closes the anti-ice/start bleed valve. As the Ng speed increases, the IGV and variable vanes are sequenced open while the AISBV is closes.  As the Ng speed lowers, the IGV and variable stator vanes are sequenced closed, while the AISBV opens. The pilot has a cockpit indication of the AISBV position, when the AISBV is opened by the HMU (or pilot for A/I) the "engine A/I" advisory light will illuminate.  Actual positioning of IGV, variable vanes, and AISBV depends on T2 and Ng speed.

(b)
Vapor vent valve- The vapor vent valve allows the pilot to manually prime the fuel system passages from the fuel cell to the HMU case cavity.  Holding the PCL in the lockout position with engine not running vents the HMU inner case (normally full of pressurized fuel) to the overboard drain. Activating either the prime boost pump or the submerged boost pump will then purge the fuel system.  When fuel is observed exiting the overboard drain, located on aircraft belly, the prime maneuver is complete.  Manual priming is required when the fuel system components are changed due to maintenance, or if the TGT rise is not noted within 45 seconds during cold weather operation.  See chapter 8 in TM 1-1520-237-10 for current manual priming procedures.

(c) 
Pressurized Overspeed Unit (POU) - The POU is located on the aft side of the AGB; all main metered fuel from the HMU passes through the POU.  P-3 air is supplied to the POU for fuel purging.  The POU has three outlet tubes.  One leads to the start fuel manifold and two primer nozzles, another leads to the main fuel manifold and 12 fuel injectors, and a third serves as an overboard drain.  The POU has four functions: (1) sequences start fuel to two primer nozzles, (2) sequences main fuel to twelve main fuel injectors, (3) purges start and main fuel manifolds and nozzles/injectors, and (4) fuel flow cut back to provide Np O/S protection IAW the ECU input.
　
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