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GTC UNDER LOAD PROBLEM


qasimzaheer
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i m facing a problem. the issue is regarding the gtc operating under load. in the absence of any external power, when the ATM is operated using the gtc bleed and then the engine is started, gtc start giving heavy surges and the bleed pressure drop to below 10 psi. but this defect doesnot repeat when the engine is started using the gtc bleed pressure in the presence of external power and ATM is not "ON". kindly give some suggestions

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I won't start an engine with the ATM on. I always use external power. Especially in hot environments, you want to decrease the bleed air load on the GTC as much as you can so you can start #3 as quickly as you can. Also, our tech data states that the ATM will normally be used after the first engine is started. If the GTC gives too much air to the airplane, it looses cooling air, which makes the Load Control Valve pinch off the air supply. As soon as the cooling air comes back and the TIT drops, the valve comes back open. This explains the surging you are seeing. As far as I know, your problem is not a problem at all, and you are observing normal operation of the GTC. The proceedure you are describing is what we call a "self contained start". Where I'm from, we are not allowed to perform them unless there are absolutely no power units available, which is never. APU's are different, but as long as you are using GTC's, just start #3 with external power connected, then start the ATM after you get #3 engine to High Speed Ground Idle. You will avoid a lot of trouble following this guideline. I say HSGI, because with the engine at LSGI, the GTC acually puts out more air, and you can flame your engine out trying to start another motor, especially during hot weather operations.

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First problem is starting an engine with ATM on - don't!! It's in the tech data ........

As to the surging, this is caused by the ACCELERATION thermostat setting being

the same or lower than the LOAD CONTROL thermostat setting. LOAD CONTROL is

set at 649°C - 663°C, while ACCELERATION is 677°C - 688°C.

Remember: simply installing new thermostats also won't fix the problem, as they

are factory set at about 590°C. You need to get the GTC onto a test stand .....

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Back in the day, self contained starts were shown during training so the students could experience one and knew the procedure. Inverters to -1 position, GTC and ATM on. Start #3, once you saw light off, ATM off. Once on speed ATM back on and check gages and meters again. Used this procedure many time in Alaska at the radar sites. Guess many of the old procedures and techniques have gone by the way-side. Guess the days of being a true airlifter as well.

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I believe they still teach that in the schoolhouse. I've seen them do it several times when an external power unit dies and they are in a hurry. It's just our maintainers that aren't allowed to do it. The main reason having A/C power during engine start is important is to have the suction boost pumps running during initial engine rotation. After that, the engines can provide their own hydraulic flow. If this wasn't the case, I'd just leave the ATM off and just run the engine ignitors off the battery. If you try that, you may cavitate the engine pumps, so like SEFEGeorge said, running the ATM during engine start until it craps out is the best practice for doing a self-contained engine start.

Me as an engine troop would rather just use an external power unit. The quicker the engine starts, the less stress is put on the starter, and the fewer starters I have to change. Starting with the ATM once in a while won't hurt much though.

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When did starting engines without EXT. PWR. and with the ATM stopped become SOP? I think I started more A models with the ATM/GTC than with EXT.PWR. If the ATM did drop off , hold Hot Day Start until engine was on speed. As far as the engine shop changing starters, in the 322nd A.D./317th TCW ,the C.C . changed them as well as generators and Hyd. pumps

.

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