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Strange Flux


speich4fun
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So we changed a prop last week and during the maintenance run we discovered a flux. The flux only happens below crossover. Found it during a pitchlock check. Started when we came back 1% in locked. Flux is 2000 torque, 1-1.5% 50-100 TIT, 30 +-FF. When we actuate switches and pitchlock flux stops. Changed Valve Housing and problem continued. C130H-3 -15. Any Help?

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This sounds like a problem that one of those \'Old Heads\' that have been around for 40+ year might have seen before. I know I never have, but I\'d give it a shot.

Does this flux only happen only during the pitchlock check when in locked at 99% rpm, or all the time below crossover?

If only during the pitchlock check, you are basically pitchlocked at your first torque via your Low Pitch Stop, which takes the mechanical portion of the valvehousing out of the picture. When below crossover, you are basically in null since you have no electronic fuel correction unless you exceed 1077 TIT. Like Gmon said, go to mechanical and null and see if the problem goes away. If it does, then there is a problem in the TD system or Synchrophasing system that is making corrections when it isn\'t supposed to be. If the problem persists when in mechanical/null, you are looking at a fuel control issue.

If this flux is happening at all times below crossover, see what happens when you go to null. If it fluxes in null at all throttle settings, look at the fuel control. If it becomes rock solid at all throttle settings in null, I\'d change the coordinator and see if the problem goes away. I\'d be willing to bet that the coordinator has an internal short that is intermittently triggering fuel correction below crossover. This would only be the case if the fuel flux is erratic, like wires arking with vibration. Depending on the model of coordinator, you may not even get an electronic fuel correction light if it is doing this below crossover. This is just a wild guess as like I said, I\'ve never seen this problem before.

Definately though, eliminate as many systems as you can (TD, sync) to help troubleshoot the problem. Hell, it might even be something off the wall like a speed valve, so check CDP when this is going on. If CDP changes a lot during the flux, that\'s probably what it is.

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Yes is happens in null, auto, locked, mechanical and normal. After that we just started tring things like being in mechanical and Hitting the Governing switches, as we thought it did not pitchlock or flux go away. We ran the sync test and found nothing, so we took the valve housing cover off and ran all the test directly on it. Found nothing wrong, speed bias, pulse gen, aticipation...all good. Moving on to coordinator static check now and will see after that.

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Yea it does NOT flux when pitchlocked. Which leads my to the prop. However we sync checked and check out the valve housing directly and found nothing, and nothing on the prior valve housing before the change. We replaced the fuel control yesterday and are going to run this morning. Let you know if that fixes it.

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This sounds like a problem I had +-2 years ago. Problem is probably not in the prop at all (easily checked by removing the prop and doing a short run). Initially the problem was cured by changing the speed valve, I was venting intermitently, found during a man-on-stand check. The problem returned, becoming progressivly worse.

See if you can attach an air-line and 160psi guage to the bleed valve system DOWNSTREAM of the speed valve. Attach another hose and guage to the CDP fitting. Do a run and compare the pressures from normal to cross-over. There shouldn\'t be more than 10psi difference, and there shouldn\'t be more than 5psi flux on either guage.

Eventually my engine was found to have a bad tower shaft system, causing the flux.

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We had a simular problem a few years back up here in Kodiak. Above crossover the TD and prop did it\'s job and everything was fine but below crossover and even into reverse we had a FF and TIT flux. Changed fuel control(2), fuel pump and then the TD Valve. Still did it and we needed the plane so we had to change the engine. What finally was the problem was a 3 way elbow/valve assy thats on the bottum of the TD valve and was allowing fuel to flow through it(wrong diection). Of coarse when we did the TD valve change we took the old elbow off and put new orings and installed on new TD valve. What a waste of man hours but I will never forget that again and always put new on new if available!!

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duckhunter04 wrote:

We had a simular problem a few years back up here in Kodiak. Above crossover the TD and prop did it\'s job and everything was fine but below crossover and even into reverse we had a FF and TIT flux. Changed fuel control(2), fuel pump and then the TD Valve. Still did it and we needed the plane so we had to change the engine. What finally was the problem was a 3 way elbow/valve assy thats on the bottum of the TD valve and was allowing fuel to flow through it(wrong diection). Of coarse when we did the TD valve change we took the old elbow off and put new orings and installed on new TD valve. What a waste of man hours but I will never forget that again and always put new on new if available!!

I remember something similar many moons ago .... still want to know whether the thread-starter resolved his problem

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I think when you get the fuel control done you willl have fixed your problem...You say that the flux stops when you pitch lock the prop right? when you pitchlock the prop you also place the fuel control into fuel topping or a reduction in fuel flow based on the RPM level above 103.5%. With the fuel control on fuel topping there is no flux because you have disabled the fuel control metering section and are now limited as to the amount of fuel out of the fuel control.

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Its the other way around, when you do the pitchlock check, you hit the check switches and that cheats prop into not correcting RPM. Now the fuel control has to play its part in preventing an overspeed. With the propeller pitchlocked, the only thing left to control RPM is the fuel control. If the flux continues in a pitchlock, a fuel control would be suspect. If stops in during a pitchlock, more likely to points to the prop. Its not all definative either. Just when you do a pitchlock check the fuel control is only temporily in fuel governing.

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When the check switches are placed in CHECK the sync box tells the speed bias motor the prop is in a under speed condition and commands a blade angle decrease. This allows the prop to speed up and once the blade angle decreases the prop\'s ability to control RPM is lost (pitchlocked)and the fuel control\'s \"fuel topping\" overspeed protection starts.

Once pitchlocked (overspeed condition)the avalible fuel is reduced by aprox 900 PPH per 1% so if RPM is 104.5 basically 900PPH is being by-passe. The fuel control is in /on fuel topping any time the RPM is above 103%...

The charted fuel flow versus RPM chart tells you if the fuel control is bypassing the correct ammount of fuel based on the RPM. It charts specific fuel flows for a specific range of RPM, so once pitchlock is achieved the fuel control is bypassing fuel based on the RPM or in/on fuel topping.

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So, based on the last three posts, the prop cannot control RPM (since it is pitchlocked) and the fuel control can\'t operate as normal (since it is in governor mode).

It would seem to me then that, if we want to check out the fuel control action as this point, once we bring the throttle back toward flight idles until RPM is back at 100% and release the Check Switch so that the fuel control is no longer in governor mode.

At that point, if the flux continues, it is not the prop. If it fluxes at that point, it must be because of the engine fuel control(since the TD system swtich is in \"locked\" position).

Am I correct here?

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You are correct, Steve. Fuel govern/pitchlock checks goes follows.

throttles crossover

lock valve

retard RPM 1%

hit governing check switches

throttles to takeoff (this is topping/governing)

retard throttles slow back to retarded RPM. (no longer governing)

Torque increase of 500ibs let you know what you are pitchlocked, its normally higher than 500ibs. If its fluxing at this point the fuel control more then likely the cause.

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