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737 Fuel System  
User currently offlineDiego From Italy, joined Apr 2001, 135 posts, RR: 0
Posted (14 years 8 months 3 weeks 2 days 1 hour ago) and read 5980 times:

Hi folks, going over a 737 fuel eng diaghram I noticed that as the fuel leaves the HP pump part of it goes directly to the MEC while the remaining is routed to the fuel heater and then back to the MEC, what is the purpose of splitting the fuel flow in two distinct paths since the fuel has been preheatet before getting into the high pressure section for anti icing purposes? And then why do you need two IDLE speeds, HIGH and LOW, and why while the LOW IDLE is fairly constant does the HIGH one change according to the flight characteristics decreasing as the temperature of the incoming air and the airplane's speed decrease?

2 replies: All unread, jump to last
User currently offlineNKP S2 From United States of America, joined Dec 1999, 1714 posts, RR: 5
Reply 1, posted (14 years 8 months 3 weeks 1 day 18 hours ago) and read 5940 times:

I can't remember the schematic routing off the top of my head, but I'll check an engine myself later and re-post my findings ( not too easy to remember every system for a half dozen types of A/C ). Keep in mind that not all fuel from the MEC is metered fuel going to the nozzles- some high pressure fuel is used for actuation of the VSVs and VBVs, TCC and other uses. -- Yes, the CFM56, as well as most high bypass engines have 2 idle speeds...and the terms vary with different A/C manufacturers: 1) High, flight, or approach idle - 2) Low or ground idle. High-bypass engines accelerate poorly from ground idle ( usually about 62% N2 ) due to the mass/inertia of the fan. De-energizing the flight idle solenoid on the MEC raises the idle to about ehh...72-74% N2. This signifigantly shortens the acceleration time of the engine if power must be added quickly during descent ( windshear, go-arounds etc. ). The idle solenoid is energized on the ground, and de-energized in the air automatically. It's for "fail safe" in flight idle.

User currently offlineNKP S2 From United States of America, joined Dec 1999, 1714 posts, RR: 5
Reply 2, posted (14 years 8 months 3 weeks 1 day 7 hours ago) and read 5932 times:

...Back again with a refreshed memory. -- The fuel enters the low pressure pump...goes through the fuel heater/oil cooler and fuel flow transmitter first..then...into the high pressure pump...then into the MEC to be metered/distibuted to the nozzles for combustion, or servo fuel for the systems I mentioned in the previous post. I'm not sure what schematic you have, but it may or may not show the various check valves and filters...the lines to which make it look more complex than it is for the purpose of your question...although the MEC is a super complicated beast in it's own right.

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