Which option is NOT one of the three main HMU inputs affecting fuel flow to the combustor?

• A. Boost pump discharge pressure
• B. Collective through the LDS
• C. PCL through the PAS
• D. DEC through the torque motor

Answer: (A)

Fuel flow to the combustor is controlled by signals that represent commanded power and rotor load, which the HMU converts into a metering action. The three inputs that drive this control are the PCL position signals (through the PAS), the collective position signals that provide the load-demand information (through the LDS), and the DEC signal delivered via the torque motor, which directly adjusts the metering valve to set fuel flow. These inputs together determine how much fuel is metered to the combustor.

Boost pump discharge pressure, while important for delivering fuel to the HMU, does not directly set the amount of fuel that enters the combustor. It’s a supply parameter that ensures enough pressure to feed the HMU, not a control input that modulates fuel flow. Therefore boost pump discharge pressure is not one of the HMU inputs affecting fuel flow to the combustor.