RPM DECREASE IN TD SYS AUTO

[sharif101]

dash 7b engine #2 eng LSGI TIT 700,FF 600.#3 eng TIT 700,FF 650.normal GI #2 eng TIT 700, FF 950.#3 eng TIT 600,FF 800.

x over #2 eng TIT 800 FF 1500 in auto, FF 1700 in null.during take off RPM 100%,FF 1950,TIT 977 then decrease to 97% and stable in TD auto with mech gov & normal but TD null RPM 101% with mech gov & normal no change.now what can i do?please help.

[Lkuest]

What is torque doing under all these conditions? Also, have you performed a TD system check using a test set?

[flhtcui98]

Bill Speicher at CAE Tampa ,instructor flight engineer , I did all the aircrew training for your pilots /engineers

Did you have an engine or a prop change on that plane recently . what happens to the torque when the rpm drops and increases .

Have you had it on a gray box and checked the instruments for calbration

[SEFEGeorge]

Bill Speicher at CAE Tampa ,instructor flight engineer , I did all the aircrew training for your pilots /engineers

Did you have an engine or a prop change on that plane recently . what happens to the torque when the rpm drops and increases .

Have you had it on a gray box and checked the instruments for calbration

Hey Bill, I'm with you. Engine or prop change recently? If not, then I'd go with Lkuest. Do a TD check with a test box. Even with a eng/prop change, I'd probably check the TD system on both engines just to be sure and eliminate that.

Not being an engine or prop guy, have you given any consideration to the fuel control or prop governor? Maybe blade angle on the prop?

I'd be curious as to what all 4 are doing. Compare 2 & 3 to 1 & 4 with them all running. At NGI it appears that #2 is higher than #3 so I might think fuel control at first.

Not bad for an OLD 32nd guy huh Bill?

[sharif101]

Hey Bill, I'm with you. Engine or prop change recently? If not, then I'd go with Lkuest. Do a TD check with a test box. Even with a eng/prop change, I'd probably check the TD system on both engines just to be sure and eliminate that.

Not being an engine or prop guy, have you given any consideration to the fuel control or prop governor? Maybe blade angle on the prop?

I'd be curious as to what all 4 are doing. Compare 2 & 3 to 1 & 4 with them all running. At NGI it appears that #2 is higher than #3 so I might think fuel control at first.

Not bad for an OLD 32nd guy huh Bill?

engine & propeller change few days ago.when rpm decrease at that time FF,TIT,TRQ rapidly decrease.td test found set,blade angle ok.

[SEFEGeorge]

engine & propeller change few days ago.when rpm decrease at that time FF,TIT,TRQ rapidly decrease.td test found set,blade angle ok.

Since you did an engine and prop change, checked blade angle and tested TD system, I'm pretty much out of ideas.

Since I've never worked in Herc maintenance I'm going to take a WAG here (that's wild a$$ guess), and mention the synchrophaser. It's been known to present some crazy problems.

I assume that when the engine and prop were changed that all associated parts were changed as well. Were any of the original components put back on? If they were I'd think about replacing them.

What might help is a 4 engine run and recording all the numbers for all 4 engines;

RPM 1 2 3 4

TIT 1 2 3 4

etc., for each power setting. It might help determining just what's going on.

But like I said, i'm not a Herc maintenance guy, just a FE. Recips didn't have all those new-fangled gadgets.

[Lkuest]

With RPM, TIT, FF, and Torque all low, I would think the problem is in the engine. The propeller's job is to maintain 100% rpm, no matter how much power the engine is putting out. If the propeller were to malfunction by increasing blade angle, the FF would not change noticeably, RPM would decrease, but torque would increase. If the prop were to decrease blade angle, torque would decrease, but RPM would increase until it hit the fuel control fuel governor where fuel flow would finally drop, but RPM would remain high. The problem would have to be in the engine fuel system. It sounds as if the TD System being turned on and off creates a difference, and the engine behaves normally in null, drops off power in auto. I would suspect a bad coordinator or bad wiring in the TD system. You might slowly swap TD system components between engines until the problem moves over to the other engine. I know it sounds silly, but with a strange problem like this, you'd be throwing money at it just changing parts. It may not make sense for a fuel system component to reduce RPM, since that is the propeller's job, but it may be taking so much fuel that the propeller drops down to 25 degrees blade angle and cannot maintain the RPM for the power anymore. Without running the aircraft myself, this is all that makes sense to me with the information provided. The problem cannot be caused by an air leak because the drop in torque would be accompanied by an increase in TIT.

[tenten]

Lkuest,

Once upon a time during FCF, whenever a full power is set for takeoff and as releasing the brake and aircraft start shaking and rolling, a sudden huge cut of power (Fuel, TIT, Torque) is taking place in one of the four engines, RPM was also seen dropping down of the 100%, takeoff was aborted.

Nothing was detected during checking/testing of the TD system, the defect as well it never duplicated during static full power, however FCU, TD valve, TD Amplifier coordinator...Valve housing, etc ....been changed, but defect remained !!

During the third/fourth... takeoff roll and while defect been acting TD was shifted to Null, power was restored to normal.

TD system defect was caught in the act, and to pinpoint as well as to confirm it, the thermocouple leads at the “Terminal Block†was swapped.

During the next rolling check with TD AUTO, TIT indicator alone suddenly hanged-up towards full scale!!

By changing both thermocouple harnesses defect never duplicated.

As for RPM drop-In my opinion, and as stated by Lkuest- if the TD system is at fault, in which leading to a big cut of fuel, it is normal for the RPM to dropdown out of the governing range.

[sharif101]

With RPM, TIT, FF, and Torque all low, I would think the problem is in the engine. The propeller's job is to maintain 100% rpm, no matter how much power the engine is putting out. If the propeller were to malfunction by increasing blade angle, the FF would not change noticeably, RPM would decrease, but torque would increase. If the prop were to decrease blade angle, torque would decrease, but RPM would increase until it hit the fuel control fuel governor where fuel flow would finally drop, but RPM would remain high. The problem would have to be in the engine fuel system. It sounds as if the TD System being turned on and off creates a difference, and the engine behaves normally in null, drops off power in auto. I would suspect a bad coordinator or bad wiring in the TD system. You might slowly swap TD system components between engines until the problem moves over to the other engine. I know it sounds silly, but with a strange problem like this, you'd be throwing money at it just changing parts. It may not make sense for a fuel system component to reduce RPM, since that is the propeller's job, but it may be taking so much fuel that the propeller drops down to 25 degrees blade angle and cannot maintain the RPM for the power anymore. Without running the aircraft myself, this is all that makes sense to me with the information provided. The problem cannot be caused by an air leak because the drop in torque would be accompanied by an increase in TIT.

TD valve swap with #1 problem solved but problem did not shift

THANKS TO EVERYBODY

[sharif101]

this engine same problem comes again & again.now when throtle forward to 850 degree that time rpm roll back.td null everything is ok.yellow box test only crossover 790 other range ok. thermocouple,harness,wiring,td controlling ck good.only crossover TIT 10 DEGREE more.co-ordinator check resistance high range.if change cordinator can i get good result.

[Lkuest]

Let me see if I understand your situation:

1. The engine will dramatically reduce power (RPM, FF, TIT, Torque) at 850 degrees TIT or higher only. Moving the TD Switch to Null corrects the problem.

2. The TD Valve has been changed to another motor, but the problem persists on the original engine.

3. The coordinator fails the crossover check by 10 degrees high.

To answer your question, the coordinator will indeed fix the high crossover temperature as long as the TD Test set verifies crossover temperature statically. In my experience, the engine actually runs between 4 and 10 degrees higher than the TD Amp setting at crossover anyway, so I like to see a TD Amp test set crossover temp of 770 or less. You can test the coordinator with a multimeter instead by disconnecting the lower cannon plug. At 60 degrees, resistance between pins A and B should be 385 +-30 Ohms. At 70 degrees, 595+-30. At 80, 800+-30. At 90, 1000 +-20. Resistance between pins A and C should always be 1000 +-20, and no variation of more than +-10 ohms between 60 and 90 degrees throttle angle. Never ohm check the coordinator potentiometer with the throttle below 60 degrees.

If this check passes, but the TD Amp test set shows crossover out of limits, suspect a faulty TD Amp or dirty power going to the TD Amp. I've seen dirty power make the amp go to full take before, but it usually does it on all 4 engines due to a bad invertor or other AC Instrument and Engine Fuel control power problem.

If it only happens at a certain throttle position, I would personally suspect a bad thermocouple harness. The E models are notorious for the Right Hand harness being cut into by the turbine overheat detector bracket as the engine torques within the nacelle. Even if it is not cut, I'd change both the harnesses AND the T-block for good measure. The T-Block is notorious for checking good when cold, but failing when it heats up. You could swap the TD Amp leads on the T-block with the Indicator leads as tenten suggests and see if the problem changes. If the T-block or thermocouple harnesses were bad, after the swap, you would see very high TIT during the malfunction, but no change in any other engine instruments. This is because the indicator would now be seeing what the TD Amp usually sees during the malfunction, and the Amp would be seeing the correct indication that the indicator usually sees.

[gmac]

Sounds like a bad coordinator. Check for continuity between pins D and E of coordinator connector at 66 degree throttle position - should be none when switch actuated.

[Lkuest]

this engine same problem comes again & again.now when throtle forward to 850 degree that time rpm roll back.td null everything is ok.yellow box test only crossover 790 other range ok. thermocouple,harness,wiring,td controlling ck good.only crossover TIT 10 DEGREE more.co-ordinator check resistance high range.if change cordinator can i get good result.

Just for my clarification, does the TIT hit a wall at 850, and the electronic fuel correction light stay on above crossover? If so, it might be a bad 94% switch, which is something you cannot check with the TD Amp test set.