(2) Three primary HMU functions affecting adjustment of fuel flow via the metering valve:
(a)
A LDS input adjusts the load demand schedule. The load demand schedule is manually adjusted by collective movements through the LDS. Raising or lowering the collective increases or decreases the fuel flow. The collective link to this schedule proactively attempts to match engine power output with the main rotor load. The term Proactive is used because power changes are made simultaneously with the main rotor load changes. This attempt to match engine power output to the rotor load is approximate, and requires further adjustment by the ECU circuits through the torque motor. In the event of a LDS cable shear, or a collective bias tube pin shear, an internal spring resets the LDS to a position that equates to full up collective; Np governing will limit back fuel if the engine is in governing with the ECU operating. If such a failure occurs at IDLE, the Ng speed on the effected engine will indicate higher than the other engine.
(b)
An ECU input through the Torque motor refers to trimming (fine adjustment) of Ng output that acts upon the power turbine. The torque motor accepts three inputs: TGT limiting, Np governing, and load sharing or torque matching. The torque motor acts as an interface between the ECU and the load demand schedule of the HMU. ECU trimming of the HMU reduces pilot load by automatically keeping Np and Nr constant (Np governing), keeping power outputs of the engines closely matched (load sharing), and preventing TGT from reaching destructive levels (TGT limiting). Torque motor trim of the collective schedule is reactive because schedule adjustments are made after parameters physically change. The pilot not on the controls may sever the ECU to HMU interface by locking out the torque motor using the PAS override function, and then manually trim the collective schedule with the PCL, effectively replacing the three ECU functions that have been locked out.
(c)
A PAS input sets the maximum power available schedule. Set by the PCL through the power available spindle. Sets maximum power limits that may never be exceeded by collective movements. PAS has no internal spring to set a default position, if the PAS cable shears (shot in two), power level does not change, however all PAS functions are lost.
(3) Additional fuel flow metering functions/features of the HMU:
(a)
Acceleration/Deceleration fuel flow limiting is an automatic mechanical function that prevents flameout due to rapid power changes (power changes called upon by PAS, LDS, or ECU). This limits the metering valve movement rate to prevent engine flameout due to excessively rich, or lean, fuel/air ratios.
(b)
The Ng limiting and Ng shutdown occurs when the Ng limiting limits the fuel flow (through Ng governing or maximum fuel flow) to control the rotational speed of the compressor/gas generator turbine rotors (actual limiting speed depends on T2). Keep in mind; TGT limiting may precede Ng limiting depending on the T4.5. As a general rule, more Ng is allowed in warm weather, less Ng is allowed in cold weather. Ng shutdown is engine shutdown (flameout) at 110 % Ng to prevent destruction of the Ng section due to excessive Ng speed. Ng shutdown occurs when a mechanical Ng speed sensing mechanism inside the HMU (centrifugal flyweights) stops all fuel flow to the engine when the Ng speed reaches 110 %.
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