High Idle / Low Idle

[mach5]

What is the use of high idle and low idle in a turboprop engine and when should a pilot use it ?

[Tinstaafl]

Not all turbo props have a high idle/low idle option. It depends on the engine type & the aircraft manufacturer's design decision. A Piper Cheyenne I flew just had 'stop' & 'run' whereas Kingairs I've flown have high low idle settings on the condition lever yet they all had PT6 engines.

In the Kingairs I generally leave the condition lever to low idle unless needed higher. All it sets is the min. idle RPM for the gas generator which is fine for most operations. High idle sets the min. RPM rather higher than low idle. Turbines can take some time to go from idle power to high power so high idle is useful if you want to have a shorter spool up time. Beech specify high idle for short field landings, for example, where the time to spool up after selecting reverse thrust on landing is a consideration. Trouble with doing it on final is that there will be a lot of residual thrust at idle. I prefer to select high idle & max RPM immediately after touchdown & prior to selecting reverse

A couple of other times that I commonly use more than low idle include:

* Airconditioner on: There's a min. RPM below which the aircon. is automatically switched off. It's higher than low idle so if I wish to have a working airconditioner while on the ground I need to set a higher idle RPM (but not as much as 'high idle' would give).

* When there's a load on an engine such as a generator so I can minimise temps eg after starting one engine and am recharging the battery before the 2nd engine start. The generator sucks a lot of power so increasing to high idle gets a lot more air flowing through the engine. Also during the 2nd engine start using a generator assisted start. When the running engine's generator is switched on there's a high load on that engine so high idle helps with temps.

* Really gusty approaches where I may want a faster engine response to throttle movements.

[mach5]

Thank you Sir

mach5

[JammedStab]

I believe that for PT-6 engines, it is a good idea to have high idle for turning on the generators after start. The King Air and 99 had the high and low idle position but the Twin Otter did not so we went to idle Ng+15% before turning on the generator. I always assumed that this was to keep the engine speed from going below a minimum set value. Usually 51% Ng although that should vary with density altitude. Confirmation anyone?

Of the other turboprops, I only flew the PW100(no high/low speed) and Allisons. The Allison 501 had a low speed and high speed. I believe that initially they did not but this feathure was installed to reduce the chance of the propeller decoupling due to excessive negative torque on shutdown(at the fuel chop) which would be more likely with a higher headwind headwind.

Once again...confirmation anyone?

[mach5]

May I know what is Propeller Decoupling ?

[Tinstaafl]

Can't say I've noticed an idle RPM drop in the PT6 with the generator going on. The fuel controller is supposed to maintain the preset idle speed by scheduling more fuel to increase torque for the added load. The end result is a temperature increase. Increasing to high idle beforehand will keep the engine cooler by shoving more air through it.

[Old Fella]

Mach5, on the Allison T56 engine/gearbox combination the Propeller De-coupling device is essentially a back-up to the Engine Negative Torque system. The ENTS system causes the propeller blade angle to increase, when in the flight range, so that the propeller does not drive the engine. In other words, it loads the engine. The Propeller de-coupling device requires much more negative torque to cause a de-couple, around 6000 inch pounds of torque, such as might be experienced if the core engine were to suffer a seizure for instance. If that should happen the propeller will de-couple thus reducing drag giving the crew time to feather the prop. As for Low Speed ground idle on later T56's, it was as much to do with reducing noise during ground operations and to make speed control during taxy easier. The T56 is a constant speed engine and essentially runs at the same RPM throughout all flight range throttle positions. Advancing the throttle increases fuel and the propeller blade angle increases to absorb the added power and maintain the RPM within defined limits (98-102% in flight) Ground idle (Normal is 93.5 to 100.5%). Max Reverse is 96.5 - 105%. Low Speed ground idle is approx 72% RPM and will automatically cancel when the throttle is moved above about 30 degrees PLA. Hope this helps answer your questions.

[mach5]

Thanx for the information , actually this is the first time I heard about Engine Negative Torque System , Do the PT6A engines have ENTS , as far as I know they dont , Is there any particular design/technical reasons for that ?

[Tinstaafl]

The PT6A is essentially two separate components: The gas generator, which is used to produce lots & lots of hot gas from burning fuel, and the power section which extracts energy from the hot gas & uses it to spin the propeller. Functionally speaking, the two sections aren't connected by any mechanical device. The gas generator is the gas turbine bit ie it has intake, compressor, combustion & turbine sections, along with a power take off to drive a generator, fuel pump & the like. Pretty much a self contained unit. A little way downstream is another turbine, used to extract power from from the gas produced by the gas generator and pass it via a reduction gearbox to the propellor. The power turbine isn't mechanically connected to the gas generator's turbine. You can even feather the propeller while leaving the engine running. If the gas generator should seize then the power turbine & propeller are free to continue turning.

Turboprops that don't use the free turbine design (free as in not connected) such as Allison engines, have the propeller on the same shaft as the gas generator's rotating parts. What ever happens to the gas generator happens to the propeller. This design needs to have a negative torque sensor or similar that can activate protective systems if the turbine engine should fail. There's a huuuuuuuge amount of drag produced from the prop. trying to turn the rest of the turbine engine, unlike a free turbine design like the PT6A where the prop. would only drive the power turbine.

[mach5]

Thank you Sir, your explanation of the concept was very simple and impressive.

[Tinstaafl]

A mildly interesting side note on the difference between the free turbine PT6A designe and say, a Garrett TPE331 as fitted to a Metro:

Have a look at the propeller blade's position after shutdown. The Garrett will have the blades in fine pitch to reduce the load on the starter for the next start. There's even an anti-feather mechanism to ensure the blades don't move to the feather position on shutdown. Remember that the propeller is directly connected to the engine so turning the engine means turning the propeller too, just like a piston engined aircraft. The load on the starter trying to spin blades that are feathered up to turbine starting RPM would be excessive. The PT6A proplellers, on the other hand, will be feathered. The PT6A starter motor only turns the gas generator so no need to worry about having an anti-feather system.

[CV880]

I recall my Allison instructor explaining the NTS and Safety Coupling many years ago like this.
At full power the turbine produced 10,000hp; the compressor absorbed 6,000hp leaving 4,000hp for the prop and accessories. If an engine flamed out shortly after take off the prop governor would try to reduce pitch to maintain governed rpm and could in theory momentarily absorb close to 6,000hp from the airstream to drive the compressor alone. There were various features in the prop and engine to prevent this with Negative Torque Sensing and a safety coupling being 2 of the engine installed features. The NTS would nudge the prop toward feather limiting the negative torque being absorbed to about 250 -350hp. This was sensed mechanically within the reduction gearbox with indicator lights in the cockpit and was checked on every shutdown.
As a final back up the Safety Coupling would mechanically disengage the reduction gearbox from the core engine at about 1,500 negative hp.

[rigpiggy]

wrt the pt-6, you can get start locks, used mainly on aircraft on floats. First Air DHC-6 - YouTube like this they are just spring loaded pins that keep the props from moving forward on shutdown.

The Garretts, Allison's and Darts need to have the props as fine as you can get to reduce power required for start. The 331, Darts, and t56 both have NTS/autofeather systems, so that in the event of an engine failure you don't roll over due to prop drag. And yes I know some Pratt powered airframes have autofeather as well.

[JammedStab]

And yes I know some Pratt powered airframes have autofeather as well.

Including the Twin Otter. Reduced published VMCA by 4 knots.

[Old Fella]

Rigpiggy. You say that the T56 has NTS/autofeather. Certainly agree with the NTS feature but none of the T56 installations I have operated incorporated an auto feather function. I have only operated the T56 on C130's (A-E-H models) and am interested on which types using the T56 auto feather was fitted.

[mach5]

Why do we RESET the generator , before bringing it on-line and what happens when we place the switch to TRIP position . In Caravans there are three position for the generator switch.
RESET
ON
TRIP

[JammedStab]

Rigpiggy. You say that the T56 has NTS/autofeather. Certainly agree with the NTS feature but none of the T56 installations I have operated incorporated an auto feather function. I have only operated the T56 on C130's (A-E-H models) and am interested on which types using the T56 auto feather was fitted.

Same with civilian Herc but I have heard of autofeather on the Electra. Confirmation to follow.

[nopax]

Why do we RESET the generator , before bringing it on-line and what happens when we place the switch to TRIP position . In Caravans there are three position for the generator switch.
RESET
ON
TRIP

The generators on the PT6 King Airs, Caravans etc., goes offline when the engines are shutdown, and the engine speed goes below a preset value (approx 35% Ngas). This also allows you to start the engine, as the starter-generator is in start mode below approx 35%, and generator mode above it.

I'm not familiar with TRIP, but on the King Airs, it's OFF - probably the same thing - when an engine is running well, and a generator, and/or electrical system has a fault (GEN TIE etc), you may want to turn it off to prevent a bad situation developing.

[Old Fella]

Mach5. Basically, when you RESET the Generator you provide Excitation Voltage to the fields of the Generator (in those generators which are not self-exciting) to enable it to produce electricity, "ON" simply closes the Generator Breaker (contactor) to connect the Generator output to the Electrical Bus. "TRIP" simply opens the Breaker to take the Generator off the Bus. Some installations have a mechanical Generator Disconnect which removes the driving medium from the Generator in the event of a generator failure.