Main Checklist - Engine Shutdown
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The siphoning system is always active regardless of the eng feed knob position - so long as at least one reservoir tank has no air in it, the FFP will still be getting fuel flow, even with the eng feed knob in off, and the DEC will still be getting fuel for cooling.
The fuel system displays usable fuel, so a totalizer reading of 0 lbs is the minimum operating fuel level.
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…first thing I ever learned in flight training was that all fuel indication systems lie.
It’s flow that’s important in this case, not quantity.
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Well its not going to indicate perfectly like it does in BMS, not during maneuvering flight…
USAF regs have minimum fuel level permissible for landing as being 1000 lbs - I vaguely recall reading somewhere that the 8 FW regulated bingo was 2400 lbs? IIRC that was so you could make it to Osan in case there was an emergency and the ALS could not be set up in time, and still have more than emergency fuel over Osan.
Back on the topic of the DEC getting adequate cooling, it still gets fuel pumped to it with the eng feed knob in off - setting it to norm turns on the electric pumps, but there are also pumps running off engine bleed air at all times regardless of the knob position. So you get the siphoning system, electric fuel pumps, and bleed air driven pumps… I somehow suspect someone put a bit of thought into the engine design there.
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Although, the wire should not prevent you from rapidly moving it as required.
Little off topic, but that’s why it’s not “safety wired”, instead of using safety wire (metal, used to secure nuts and bolts), break wire is used. To make sure the operator does not change the switch accidently, he will have to use some additional force to change the switch position if he want’s to. And like you said, the wire should not prevent movement.
(I know, just details, but as a technician, hearing somebody say to safety wire a switch gives me the shivers )
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Well its not going to indicate perfectly like it does in BMS, not during maneuvering flight…
USAF regs have minimum fuel level permissible for landing as being 1000 lbs - I vaguely recall reading somewhere that the 8 FW regulated bingo was 2400 lbs? IIRC that was so you could make it to Osan in case there was an emergency and the ALS could not be set up in time, and still have more than emergency fuel over Osan.
Back on the topic of the DEC getting adequate cooling, it still gets fuel pumped to it with the eng feed knob in off - setting it to norm turns on the electric pumps, but there are also pumps running off engine bleed air at all times regardless of the knob position. So you get the siphoning system, electric fuel pumps, and bleed air driven pumps… I somehow suspect someone put a bit of thought into the engine design there.
…heh…yeah, I did…back when I had that job at GE. It’s not just about the DEC…there are a number of things hanging on the engine that the fuel flows through/past that contribute or could cavitate if they run dry. 1000 lbs on-deck is certainly reasonable, though I can agree that the RTB considerations you mention can also be factored in per location requirement. I couple of jets I’ve operated have hard on-deck fuel requirements which are primarily aimed at engine/system health - the “health” min for the Viper is probably something around the low fuel warning in that regard. When I was operating Harriers the guys used to take pride in the low fuel light coming on as the jet was chocked…that’s 200 lbs for that jet, which also has a DEEC.
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DEC should be the only fuel cooled bit on there. The engine schematics show the three items on the fuel cooling loop being the DEC, the pyrometer, and cooling fuel shutoff valve. There’s a bunch of fuel operated stuff on there - like half the systems at least seem to be hydromechanical and most of the rest being electrohydromechanical… using fuel pressure to run things is a neat concept.
And on the topic of pressure, all the fuel operated (but not fuel cooled) systems get additional pressure from the engine fuel boost pump as well, regardless of the eng feed knob position.
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…like I said…there’s more to it than just cooling the DEC. From an engine standpoint. Yes, the working hyd fluid in the engine is fuel (and in some cases it and/or oil are also used for heat exchange or pre-heating)…and keeping it circulating and at proper pressures can also incur mechanical consequences. When new engines are shipped they are not shipped “dry”. And in some cases motoring the engine to “prime” it is required post-maintenance - depending on what was done - before actually running it up. But that’s a really short evolution.
One of the things I was really surprised by is the discussion elsewhere of “Viper wine” due to the Pratt nozzles being electrically actuated…I’m not really sure I believe that. But I wouldn’t put that past Pratt.
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PW220 and PW229 nozzles are actuated by high pressure bleed air.
It looks like we agree that running the engine dry is probably bad for it. I think we agree it is working optimistically in BMS at present during flight with either reservoir less than full? I mean, if you have the eng feed knob off, and a reservoir less than full, then any negative G could starve the engine of fuel. Which would be bad.
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…bleed air? That’s even worse, IMO…ever seen the damage a bleed air leak can do?
Yeah - I was thinking mostly about ground ops, though. That’s probably where a mistake is most likely to happen. Unless you secure the feed knob airborne for some reason - I can’t think of one…other than in flight fire, maybe?..
There are a LOT of things working optimistically in BMS…which surprises me. A bit. Probably my own fault - I think I’ve over-hyped it to myself before actually working with it extensively. BMS still rocks, though.
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…bleed air? That’s even worse, IMO…ever seen the damage a bleed air leak can do?
Yeah - I was thinking mostly about ground ops, though. That’s probably where a mistake is most likely to happen. Unless you secure the feed knob airborne for some reason - I can’t think of one…other than in flight fire, maybe?..
There are a LOT of things working optimistically in BMS…which surprises me. A bit. Probably my own fault - I think I’ve over-hyped it to myself before actually working with it extensively. BMS still rocks, though.
Actually, no on the bleed air leak. Got any good videos?
For an in flight fire, there is no mention of moving the feed knob at all. For a ground fire, its mentioned to turn it to OFF if external power is applied.
BMS rocks, but there is always room for improvement. Fuel systems being a little less (not a lot less) optimistic would be one of those areas. I have to admit, I am still very impressed with how many of the things I thought needed improvement in 4.32, are implemented/improved in 4.33. I guess I didnt really look at the fuel system as being something in need of improvement back with 4.32.
So, with both reservoirs full, and the engine feed knob off, negative G should not present much of an issue (short term). With a reservoir less than full, it (along with maneuvering flight) should present engine starvation problems. High fuel flow with reservoirs less than full should probably present similar issues.
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Yes. Usually there is some sort of restriction on duration for negative G, and this is why - that, and limitations on the engine oil sumps. I rather suspect that at high fuel flow the Viper’s reservoirs may tend to stay full as long as pumps are doing their job. Interruptions in that flow can be catastrophic too - I spent my initial stint in AB Controls investigating a pressure spike that blew the main fuel inlet off the engine in F-16XL (F110DFE at the time) during a throttle transient from idle to max…that happened because the AB boost pump would scavenge dry at part power, and at turn on the demand spike re-filling the cavity was strong enough to back-flow through the whole system and blow the fuel inlet elbow off the engine…jet landed on the lake bed, my boss at the time had the broken elbow on a plaque over his desk…
No…no videos, but some personal experience with a couple jets that have had to divert for bleed air leaks - I think pilots fear bleed air leaks second only to flight control failures or actual developed fire. In general a bleed air leak will result in a fire (something like 700C+ degree air usually)…and depending on just what the air impinges and/or how soon the crew take action the fire can do less visible damage…one such jet that I know of that had a leak that appeared to have not done a lot of damage was recently struck from inventory after it was learned that the heat warped the spine of the structure. Brand new jet, too.
One of the worst things I’ve seen happen to an engine is a titanium fire - usually due to a case rub. When I was working at a GE ASO I used to see CF6-50s that came in with such; at one time those engines had steel compressor blades and titanium cases. Blade would rub the case and the case would catch and burn like magnesium. Looked like someone poured molten lead down the intake of the engine. They stopped that by changing materials from Ti.
I sort of skipped BMS 4.32 because I figured 4.33 was coming as I began my pit project…in my transition from FAF to BMS and because I’m building a pit and PC for BMS I really don’t want to spend a lot of time on it just yet. It does do a LOT of things really, really well…which is another reason I get surprised when it falls short in places. Like I mentioned, I think I over-hype it to myself…but it does really impress me.