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RPM FLUCTUATION


munirabbasi
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Hello Expertises

#2 ENG RPM DROP FROM 99.8% to 97.5% on SYN mode , when selected #3 Eng as master ,like wise selected # 2 ENG as master ,# 3 eng drop to 97 .5 % ,start fluctuating along with torque (2 & 3) ,electronic control box, valve housing,Pulse generator coil,SYN connectors & TACH generator were replaced. NTS gap checked found SAT, problem still existing.

Munir Abbasi

Home of hercules pakistan

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That's a new one on me. If the pulse generator circuit was bad on #2 propeller, it would make the #1,3,4 prop RPMs drop from 100% to 0% (can only follow 2 %). When you switch master to #3, #2 prop RPM only should rise, thinking it is trying to increase from 0%. Since both masters are dropping, and the outboards are not, I think the problem is wiring. This is assuming all other propeller checks are good (throttle anticipation w/in 5 seconds, and no RPM difference between mechanical and normal, pitchlock check). I suggest verifying all propellers are set to a Mechanical RPM of 100%, +-0.2%, then perform a reindex and try again. If the problem persists, I believe the problem is between the essential DC bus, the master select switch, and the synchrophaser. If the master select switch is shorted internally so that a master is always selected, it will cause a problem such as this. If the propellers do not pitchlock, you may have a problem with the Essential A/C bus voltage going to the synchrophaser. The A/C power is used as a reference signal, and further amplified once error signals are given to the synchrophaser to decide how far to make the speed bias servo motor move. If the A/C power is dirty, the synchrophaser will do crazy things.

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That's a new one on me. If the pulse generator circuit was bad on #2 propeller, it would make the #1,3,4 prop RPMs drop from 100% to 0% (can only follow 2 %). When you switch master to #3, #2 prop RPM only should rise, thinking it is trying to increase from 0%. Since both masters are dropping, and the outboards are not, I think the problem is wiring. This is assuming all other propeller checks are good (throttle anticipation w/in 5 seconds, and no RPM difference between mechanical and normal, pitchlock check). I suggest verifying all propellers are set to a Mechanical RPM of 100%, +-0.2%, then perform a reindex and try again. If the problem persists, I believe the problem is between the essential DC bus, the master select switch, and the synchrophaser. If the master select switch is shorted internally so that a master is always selected, it will cause a problem such as this. If the propellers do not pitchlock, you may have a problem with the Essential A/C bus voltage going to the synchrophaser. The A/C power is used as a reference signal, and further amplified once error signals are given to the synchrophaser to decide how far to make the speed bias servo motor move. If the A/C power is dirty, the synchrophaser will do crazy things.

Dear LKuest

Thanks. , during flight when FE selected SYN master# 2 or#3 ,2 Eng RPM raised from limit or some time decrease from limit # 2 eng RPM start fluctuating about 5 to 6 % but snags could repeat not on ground

Regards

Munir Abbasi

Home of hercules Pakistan

Edited by munirabbasi
Correction edited
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Does your synchrophaser run directly off the Essential AC Bus, or through a constant voltage transformer? You must make sure the Synchrophaser pin 34 is receiving a steady 115VAC/400Hz power source. If you source of power is dirty or malfunctioning, this could be the cause. If the issue happens in-flight only, I have heard a stray signal from calling on the radio can cause problems, so try to run again, and use the radio. Also, I have heard of the bottom strobe light causing a synchrophaser issue. During the ground run, I would recommend running all equipment your pilots use so that you can duplicate this problem. Once you duplicate it, turn off one system at a time until the problem goes away.

If you can confirm the power voltage is ok, I would recommend taking a synchrophaser from another aircraft that you know is OK, and putting it in this bad aircraft temporarily and see if it can fix the problem. Sometimes, we get new parts that are bad.

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