Hydro Mechanical Units
Please forgive! I am desperately trying to figure out the HMU on Being 737-700/800. I appreciate that it senses aircraft parameters, power lever settings and air conditions and adjusts the fuel flow and bleed air according to its dear little brain. I don't understand the 'HARD' and 'ALTERNATE' settings. Is it on a hard setting that the power lever makes direct demand on the fuel system despite what the HMU says?
Point 2 - If the APU is running during the climb to provide increased power availability, would the HMU have to work harder?
Sorry if I appear dense - I have asked the same question on Tech Log forum but I would really appreciate an engineering answer too!
Cheers
Trux.