PPRuNe Forums - JT8D-9A slow to spool.

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JT8D-9A slow to spool.

Need help trying to figure out why our JT8D-9A is slow to spool. It is taking about 9sec to spool up to 1.2EPR. Fuel Control(replaced), trim checks good, PRBC-Replace, P&D valve replaced and all bleed valves are opening and closing. All PS4,3 lines and moisture trap checks good. I am at a loss. Also, combustion chamber boroscoped and no problems noted. This is a duplicated gripe and it has followed with a new engine being installed. Any ideas?

Not being familiar specifically on JT8D -

Are there weep holes in the PS tube(s)? Possibly oversize, leading to low sensed pressure at the FC?

How does it check out at TO? EGT, N1/N2 & FF normal?

If you have changed the motor out and the problem is still there it may have something to do with the EPR indication system ... try the EPR transmitter???

Just wanted to let you guys know, we changed the PRBC and it fixed the slow to spool.. Thank you for all the inputs.

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ust wanted to let you guys know, we changed the PRBC and it fixed the slow to spool.

OK, you have my curiosity piqued. Can you elucidate, provide a little detail about cause-and-effect?

Hi,

If I am not mistaken, here a small detail:
the bleed valves (acting as anti-surge valves) will stay open during the acceleration if the PRBC is faulty. This will cause a slow acceleration and it may also cause slow start.

Slow spoolup above idle, yes I can buy that if bleeds are open. However shouldn't EGT be high too at takeoff EPR? Maybe a N1/N2 mismatch?

Further, bleeds should normally be open during start, wouldn't they?

As I said, I'm no JT8D guru, but I'm simply applying principles from other engines.

,Hi

Quote:

What you said is right but it may not apply 100% for JT8D especially after SB 5425. But it’s also my mistake, I didn’t choose the right word. I should say the bleed valve didn’t close at the right time. Here a quote:

Quote:

Operation (Post SB 5425):
As the engine RPM increases during engine start, Ps4 pressure opens the start bleed control valve and closes the 13th stage bleed valve. The 13th remains closed until the engine is shut down. When the engine is shut down, Ps4 pressure decreases to close the start bleed control valve and open the 13th stage bleed valve.
I guess the 13th stage valve is open at the beginning of engine start and it will close during or at the end of the engine start while the 8th stage valves are open until idle.

Operation (Pre SB 5425)
Engine Start and Idle. During this period compressor air pushes the bleed valves open when the muscle valve in the PRBC is (1) closed to the flow of Ps4 and (2) opened to atmospheric vent.
Above Idle. As the engine is accelerated above idle, the increasing Ps3 overcomes the combined force of Pt2 and spring. This forces the poppet valves to reverse position, allowing Ps4 to reposition the muscle valve. The repositioned muscle valve allows Ps4 muscle pressure to flow to the compressor bleed valves to force them closed.
Engine Shutdown. With the engine shutdown there are no active forces on the compressor bleed valves. Since they open at idle power they would remain in the open position after engine shutdown.

I guess this quote apply at what you said before but prior the SB 5425.

Quote:

As I said, I'm no JT8D guru, but I'm simply applying principles from other engines.

I am not JT8D guru but engine systems may be slightly different or different: recent engines like CFM 56 are fitted with VSV and VBV. The VBV are located at LP compressor which is fitted with 12 VBV while JT8D (low bypass without VSV) is fitted with one 13th stage valve and three 8th stage valve (post SB 5425) and located at the HP compressor. The VBVs are closed about 80% N1 or above on CFM 56 while for JT8 D I guess the 8th stage valves are closed before that RPM.

I will suggest you to read the troubleshooting of certain faults like: slow acceleration, slow start, bleed non operable or off schedule, hung start, impending hot start, low ilde N2 to have an idea. Schematic or excerpts from manuals are better but not available.

Link:Engine & APU specs, troubleshooting & maintenance tips

I stand corrected, hopefully a reel guru will give us more information.

Regards