Pratt JT9D-7Q compressor stall question
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Pratt JT9D-7Q compressor stall question
I have a question for some engineers expert with the Pratt JT9D-7Q engine. Please if you're not an expert I don't need your "opinion."
Had an engine go into unrecoverable deep stall - low fluctuating EPR, spike in EGT, retarded power to idle and still would not recover, although EGT came down, but EPR continued to fluctuate. High 30s in altitude and .84 mach, very gentle level-off with little/no throttle movement. Shut the engine down and returned to the airport. Mx attributted the stall to a "clogged filter", however the filter never went into bypass as would be evidenced by an "icing" light.
Maintenance's expanation sounds like BS to me, but I'm a pilot and not an expert in JT9Ds.
Had an engine go into unrecoverable deep stall - low fluctuating EPR, spike in EGT, retarded power to idle and still would not recover, although EGT came down, but EPR continued to fluctuate. High 30s in altitude and .84 mach, very gentle level-off with little/no throttle movement. Shut the engine down and returned to the airport. Mx attributted the stall to a "clogged filter", however the filter never went into bypass as would be evidenced by an "icing" light.
Maintenance's expanation sounds like BS to me, but I'm a pilot and not an expert in JT9Ds.
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Which filter? Possibly not the fuel filter they were refering to. It may have been a clogged EVC filter which could have the effect of putting the vanes out of schedule.
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After looking in my Flight Engineer Tech manual, I see the EVC filter. It is located downstream of the main fuel filter. How could the EVC filter get plugged up unless the main went into bypass?
Also, does the Q engine have the same EVC system as the J's?
BTW, the engine was borescoped and checked-out good.
Also, does the Q engine have the same EVC system as the J's?
BTW, the engine was borescoped and checked-out good.
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Roadtrip
Personally my first point of call would have been the EVC 'golden filter' especially if there had no previous engine problems, then if there had been another 'event' time to look deeper (trim run?). IIRC the -7 could on rare occassions suffer engine steady state stall at high altitude I think caused by inlet spillage
and IIRC the -7R4G2 got round that with the auto recovery system that opened 2 of the 3.5 BV's.
The EVC system is different in as much as the -7Q has a combined Engine Vane Bleed Control Unit that has an added refinement of mach probes (3 off) at the outlet of the LP compresor.measuring mass and velocity of the air entering the N2 comp.
The EVC system of the -7J was basically the same as the -7A and -7F.
that's excersized the brain cells a bit
Personally my first point of call would have been the EVC 'golden filter' especially if there had no previous engine problems, then if there had been another 'event' time to look deeper (trim run?). IIRC the -7 could on rare occassions suffer engine steady state stall at high altitude I think caused by inlet spillage
and IIRC the -7R4G2 got round that with the auto recovery system that opened 2 of the 3.5 BV's.The EVC system is different in as much as the -7Q has a combined Engine Vane Bleed Control Unit that has an added refinement of mach probes (3 off) at the outlet of the LP compresor.measuring mass and velocity of the air entering the N2 comp.
The EVC system of the -7J was basically the same as the -7A and -7F.
that's excersized the brain cells a bit
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Originally Posted by Roadtrip
Still wondering how a filter downstream of the main could plug up with lint and other junk.
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Its hard to tell what is the cause until you get to look at the engine and see if there is an history of that eng playing up. A blocked filter could do it but which one? Besides filters don't just clog up for no reason.
It sounds like a component has bitten the dust. Either in the fuel system causing either an erratic metered fuel flow to the engine or more likely the EVBC system causing disrupted airflow through the engine. You'll be needing some engineers to have a good look and do some engine runs.
They've just fobbed you off with a blocked filter because they have no idea what the cause is from the information you have. Certainly a partiall blocked filter could do it,although that would be too simple a fix and rather unlikely. If it was a partially clogged filter that had blocked up over time then that eng would have a history of increasingly errattic behaviour leading up to your event. If it suddenly blocked up then that would be an indication of a component failing
Either way the result is the same. Do exactly what you did and get someone to have a look at it.
It sounds like a component has bitten the dust. Either in the fuel system causing either an erratic metered fuel flow to the engine or more likely the EVBC system causing disrupted airflow through the engine. You'll be needing some engineers to have a good look and do some engine runs.
They've just fobbed you off with a blocked filter because they have no idea what the cause is from the information you have. Certainly a partiall blocked filter could do it,although that would be too simple a fix and rather unlikely. If it was a partially clogged filter that had blocked up over time then that eng would have a history of increasingly errattic behaviour leading up to your event. If it suddenly blocked up then that would be an indication of a component failing
Either way the result is the same. Do exactly what you did and get someone to have a look at it.
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I have to agree - if there is lint in the fuel, why would one only donk suffer? (Although, maybe this is only the worst-affected filter - ask your mx why ALL the filters weren't inspected? 
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All the filters were changed (so they say), and the bad filter was sent out for exact analysis of the debrie. The engine was test run after the filter change and checked out ok. It's my understanding that the fuel used as hydraulics in the EVC system is first filtered through the main engine fuel filter. Is that correct? Is the main filter screening less fine than the EVC filter? How could something as large as lint get through a main fuel filter?
