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C-130 descent tecnique


wildweasel_pt
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Hi guys

As the dash1 says the descent on clean config should be done with idle power and speeds acording with the area we're flying in and then 250 KIAS from 20000' downwards. The thing is that we follow a squadron rule that says that we should set around 3000 lb torque for the descent. We were discussing that on one of those long flights... (we had already talked about the ladies :D) and no one really knew why was that because its been done like that for 30 years. The thing is that i couldn't find the reason for that teqnique. My AC was telling me that his guess would be for pressurization purposes, that in idle the capability of the engine to output the same flow via bleed valves would be diminished and the risk for compressor stal would be higher.

I didn't agree with it because eventhough that RPM limits at flight idle are lower than at cruise power (94,5% vs 98%) i think that we can disregard that little difference and assuming that the outflow on the bleed valves only depend on engine RPMs and not on torque (blade angle). Of course that the ram air is higher with higher blade angles/torque, but will that make the difference on bleed air output for the pressure manifold? What is your opinions on that? TIA

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I thought of that as well but the lower limit for the idle is 94.5% and acceleration bleed air valves open at 94% descending. And eventhough that the source for the speed sensing comes from different places i don't know to what extent those 0.5% may be enough. I was told that it depends on how the finetuning of the engine was made regarding RPMs. Nevertheless the tendency of the engine when you reduce it to idle is to accelerate and sometimes even the NTS needs to come on due to that sudden acceleration (normal due to the lower blade angle)...

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I think you are on track. I think it is a torque issue. The 94.5% is a ground limit for us. The RPM should be 98-102 for flight operations. The compressor bleeds should not be a factor. Also the Flight Idle engine power available may not be sufficient to maintain positive torque and that is what I think the descent technique is based on. During the descent profile at Flight Idle it is a common practice to increase the throttle as the altitude decreases due to more dense air. so I think your profile is a step to prevent NTS and the need to constantly monitor torque on descent.

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I think you are on track. I think it is a torque issue. The 94.5% is a ground limit for us. The RPM should be 98-102 for flight operations. The compressor bleeds should not be a factor. Also the Flight Idle engine power available may not be sufficient to maintain positive torque and that is what I think the descent technique is based on. During the descent profile at Flight Idle it is a common practice to increase the throttle as the altitude decreases due to more dense air. so I think your profile is a step to prevent NTS and the need to constantly monitor torque on descent.

Thanks. I had a hard time figuring out what the question was. It peaked my interest though.

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I just want to clear up this whole confusion on RPM limitations. Coming out of our maintenance manual, ground idle is 94% to 102%. Because of the low pitch stop and fuel scheduling at flight idle, it is 92.5% to 100.5%. Normal rpm in flight will be 98% to 102%. Of course, the upper and lower rpm limitations allow for an RPM gauge to have 2% RPM error in either direction, which is strange considering an RPM gauge is legally bad with 1% error. As measured with an Accurate Tachometer, ground idle is 96.3%-99.1%. Flight idle is 94.4%-98.3%. Normally in flight, RPM will be 99.8%-100.2% under no throttle movement. If you know your RPM indicator is dead on, RPM should never drop below 94% at any time because that is when the acceleration bleed valve are scheduled to open.

As for the manifold pressure, between ground idle and takeoff, it is possible for bleed air pressure to change from 70psi to up to 110psi on ground runs with only a minor RPM increase. This is measured with all bleed air operated equipment turned off and one engine outputting air-- not exactly something you'd check in flight, but with jet engines, bleed air equals power and temperature.

With all 4 engines at flight idle, some engines pull more torque than others. The engines that pull less torque will tend to NTS more often on decent. Because of the nature of the NTS system, the props will increase blade angle, then decrease back to the low pitch stop again when the NTS condition is gone. This can create an RPM and torque flux situation which probably won't make an engine go below 94%, but you'll wonder what's going on with it.

As for why your squadron rules differ from the -1 guidance, who knows. These planes have been around for a long time and it makes sense to change the way we do business based on experience. It would just be nice if whoever thought it up would change the manuals if it's that important. Otherwise it's just an opinion.

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Sorry 94% not 94.5%....

The NTS thing is one of discussion often... NTS is the reaction of the prop due to a lack of engine power (available power is insufficient to maintain 100% Prop RPM) no excess power equals 100% RPM and zero torque. Any excess power will result in a blade angle increase and torque rise. NTS does just that, it increases blade angle to allow the engine power to drive the prop, if this power setting is insufficient to maintain the RPM the cycle will continue until the throttle is advanced.

I would say at some point if you allow the oscillations to continue there might be a chance to flame out the engine but.......who knows (if you let it NTS that long I guess you have BIGGER problems!!)

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i just want to clear up this whole confusion on rpm limitations. Coming out of our maintenance manual, ground idle is 94% to 102%. Because of the low pitch stop and fuel scheduling at flight idle, it is 92.5% to 100.5%. Normal rpm in flight will be 98% to 102%. Of course, the upper and lower rpm limitations allow for an rpm gauge to have 2% rpm error in either direction, which is strange considering an rpm gauge is legally bad with 1% error. As measured with an accurate tachometer, ground idle is 96.3%-99.1%. Flight idle is 94.4%-98.3%. Normally in flight, rpm will be 99.8%-100.2% under no throttle movement. If you know your rpm indicator is dead on, rpm should never drop below 94% at any time because that is when the acceleration bleed valve are scheduled to open.

As for the manifold pressure, between ground idle and takeoff, it is possible for bleed air pressure to change from 70psi to up to 110psi on ground runs with only a minor rpm increase. This is measured with all bleed air operated equipment turned off and one engine outputting air-- not exactly something you'd check in flight, but with jet engines, bleed air equals power and temperature.

With all 4 engines at flight idle, some engines pull more torque than others. The engines that pull less torque will tend to nts more often on decent. Because of the nature of the nts system, the props will increase blade angle, then decrease back to the low pitch stop again when the nts condition is gone. This can create an rpm and torque flux situation which probably won't make an engine go below 94%, but you'll wonder what's going on with it.

As for why your squadron rules differ from the -1 guidance, who knows. These planes have been around for a long time and it makes sense to change the way we do business based on experience. It would just be nice if whoever thought it up would change the manuals if it's that important. Otherwise it's just an opinion.

these acceleration bleed valves you speak of, how do they know when the engine is at 94%? Also how do they know when to open and close?

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The acceleration bleed valves do not "know" the engine is at 94% they are controlled by the Speed Sense Valve (SSV) which is gear driven "air routing valve". The SSV is the component that “knows†engine speed.

Ambient air is on one side of the acceleration bleed valves and 5th and 10th stage compressed air is on the other. The 5th and 10th stage air pressure is higher than the ambient air pressure so the valves shift and allow the air to dump overboard.

When the SSV is rotating at 94% it routes 14 stage compressed air to the ambient air side of the acceleration bleed valves. The 14th stage air is a higher pressure than the 5 and 10th stage air so the valves shift closed and all compressed air is routed to the diffuser.

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Don't recall it written from so many years ago, but I always had the pilot bump the throttles up just enough to get it off of NTS. Didn't want the NTS system, if it might be faulty, to cause an engine failure. I seem to recall that this was addressed in the -1.

Its been a while but if I remeber there is a note saying that NTS operation is a normal condition and does not require immediate correction, right next to that statement there was a caution also that said that if the NTS system malfunctions, the engine could flame out. Had an engine flame out once on approach due to NTS sticking and the mech claimed it was one of the old "pre teflon covered" NTS systems.

Mike

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The NTS thing is one of discussion often... NTS is the reaction of the prop due to a lack of engine power (available power is insufficient to maintain 100% Prop RPM) no excess power equals 100% RPM and zero torque. Any excess power will result in a blade angle increase and torque rise. NTS does just that, it increases blade angle to allow the engine power to drive the prop, if this power setting is insufficient to maintain the RPM the cycle will continue until the throttle is advanced.

NTS stands for Negative Torque Signal. NTS isn't so much a function of engine power as it is a reactionary system to prevent prop overspeed, not to correct engine underspeed. Negative torque means the prop is turning the engine due to excess airflow for the blade angle. There are helical splines in the gearbox that, when the engine is driving the prop, screw together and basically lock in position. When the prop turns the engine, the helical splines un-screw and start separating. This action has no function in the gearbox itself, but the unscrewing action pushes a rod forward that interfaces with the NTS bracket, enabling the actuation of the feather valve, increasing blade angle, preventing the propeller from either making the rpm run away or causing a decouple.

The confusion here comes from the low pitch stop. This prevents the propeller from dropping below the pitchlock range in flight. The problem is that some pitchlock regulators are set at 25.5 degrees blade angle, some are set at 24.5 degrees, and the engine fuel control tuning differs from motor to motor. Flight idle, being the lowest power setting, as well as the prop hanging up on the low pitch stop causes the scenerio most likely to create an NTS condition on a single engine putting out the least amount of power, not because engine power is low, but that the airflow instead of the engine is driving the propeller. The power is lower, but the blade angle is low and airflow is going to make the RPM shoot up if it doesn't NTS. Now keep in mind the engine decouples at roughly negative 6000 in-lbs of torque.

The reason all of this is important is people need to know A) what causes what they are seeing at flight idle, and B) an NTS condition can exist at all power settings due to either a maintenance problem, wind gusts, or a nose down attitude. The NTS system requires airflow to function correctly which is why you cannot duplicate in-flight NTS problems on the ground and there is an in-flight NTS check every time there is NTS system maintenance. If you would like to read up on the NTS system, you can check out the 1C-130h-2-70gs-00-1, section II paragraph 2-2.1. Your maintenance CTK should have a copy.

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I thought of that as well but the lower limit for the idle is 94.5% and acceleration bleed air valves open at 94% descending. ..

In flight, it should never be below 98% - even accounting for 2% inaccuracy, that only gets you to 96% - no factor for accel bleed valves.

I just want to clear up this whole confusion on RPM limitations. Coming out of our maintenance manual, ground idle is 94% to 102%. Because of the low pitch stop and fuel scheduling at flight idle, it is 92.5% to 100.5%. Normal rpm in flight will be 98% to 102%. Of course, the upper and lower rpm limitations allow for an RPM gauge to have 2% RPM error in either direction, which is strange considering an RPM gauge is legally bad with 1% error.

I don't care about any of that - the only one that matters is inflight - 98-102. The rest of the time, I'm on the ground and can get it looked at if there's a question! :D:D

To distill down many words, at high altitude cruise, your blade angle is high. As you descend into thicker air, the blade angle will decrease to maintain RPM. If you set flight idle at altitude and began your descent, you will eventually get NTS as you descend. There is an institutional fear of NTS sending a prop to feather based on older NTS plungers that would do this, and there have been a very small handful since the teflon coating. Consequently, we tend to try to avoid NTS and that's probably where your SQ SOP comes from...

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Because of the low pitch stop and fuel scheduling at flight idle, it is 92.5% to 100.5%. Normal rpm in flight will be 98% to 102%.

As for the manifold pressure, between ground idle and takeoff, it is possible for bleed air pressure to change from 70psi to up to 110psi on ground runs with only a minor RPM increase.

I believe that for different fleets/operators we might have different limitations and reference numbers, but in flight idle you can never have 92,5% as a minimum RPM due to the fact that the acceleration bleed valves will open below 94% by "order" of the speed sensitive valve and the RPMs will decay because of massive bleed air output (sometimes that happen in very hot and low air density days just like in Afghanistan OPS - happened to me a couple times, sometimes would stabilize around LSGI some othertimes somewhere inbetween or even with the ignition going below 65%). Our TO states on Chapter 1 (Limitations) that at Flight Idle the limits are 94.5% to 100,5% in order to prevent the ACCel B VAlves from opening and flamingout the engine. You should take a look also at the procedures for multiple engine rollback/decay in torque regarding the steps to preclude these losses on rpms and torque available, but this is for situations way above FLT idle.

Regarding Bleed air output pressure, if its in normal it modulates for approximately 45 PSI, otherwise we can only reach those numbers of 70 if you go Override just like for engine start.

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I believe there are only two times while flying that you may see the RPM go below 98% where it is not some type of malfunction. These are from practical experience 1. During landing (100% Flaps) when the throttles are at Idle and the pilot floats it below landing speed, the prop has no ability to reduce blade angle and the airflow slows which can result in the RPM reducing below normal limits (still technically "in-flight") 2. during Stall maneuvers. during an FCF the FE watches the RPM gauges specifically because the RPM can decay before stall speed is reached. In this case the stall recovery commences based on RPM rather than airspeed. (only saw this during 100% flap stalls). Both of these situations are essentially the same. The first situation was not uncommon at the schoolhouse (16th)

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