turboprop question.

[scubabri]

Hi,

I'm not quite sure I understand the purpose of the condition lever in a turboprop.

Other than fuel control (off/on) what other purpose does it serve. I'm not quite sure what the low-idle/high-idle does.

Can someone explain for me?

thanks

b

 

[KnowledgeSeeker]

The two purposes the condition lever serves is what you have said: It is a fuel cut-off lever and low/high idle lever. Meaning much like a mixture cut-off when the lever is in the fuel cut-off position no fuel SHOULD be delivered to the engine (burner can). Although, unlike the mixture in a piston, it does not control the mixture of the air and fuel. The added bonus you have with a turboprop is that you can now also control the Idle of the engine. Basically it is control over the miniumum power setting (idle) that you will get with the power levers all the way back. It would be like having an option to control the idling of your engine inside your car. I see you have flown in a BE90. That plane is usually set so that engine idle (N1) is 52% +/- 2% with the condition levers in the detent (lowest idle setting). You can then move the condition levers through any setting up to 70%. Meaning that if you set the levers full forward on the ground and you put the power levers to idle you would still be at 70% (N1). This is used for a number of reasons some being to compensate for high generator loads on the ground and for landing so that the engine is already spooled up for a go-around (power is not as instantaneous in a jet engine). I hope that clears some questions up, if not I would be happy to answer any more! (if you have any questions about N1 or whatever)

 

[scubabri]

So if I understand the answer, basically there are 3 settings, cutoff, low idle N1=%52, and high idle N1=%70.

Low idle is used basically for starting the engine, and high idle is everything else.

Ok, so if I have this right, now, what are the exceptions? Is there anytime this would be adjusted in flight?

b

 

[Andy Neill]

Let me muddy the water by introducing another aircraft. The engines are started with the CL is the Feather position and then moved to Min when the Nh has stabilized at around 62% (this produces a prop speed of 50%). Before takeoff, the CLs are moved to Max with an idle speed of 65% on the props and 100% props at takeoff power.

In cruise flight, the condition levers are brought back to a desired prop speed of 80-85%. Finally the CLs are put back to max as part of the before landing check.

Finally, the CLs are moved to Fuel Cutoff to shut the engines down.

You could say the following in summary:
At lower power settings, the condition levers are used as fuel control. At higher power settings, the are used to govern prop speeds.

 

[scubabri]

Ok, I assume this isn't on the King Air's then

b

 

[Singlecoil]

Ok, I assume this isn't on the King Air's then

b

Right, he's talking about Pratt P119, 120, etc. type engines on E120's, and Dash-8's. Two levers, a power lever, and a condition lever which acts as a prop lever, and a fuel cut-off when pulled all the way back.

 

[pilotyip]

Allison's

Fixed shaft Allisons T-56 and D-501 (civilian T-56) did not have condition levers, just one lever like a jet, push forward more power at 1050 prop RPM, pull back and less power at 1050 prop RPM. Engine always turns at 13,820 RPM, except when you select low RPM on the ground, it is done electrically

 

[Snoopy58]

T-56 in the Herk

Side note on the T-56, which powers the mighty Herk, which has condition levers. In that installation, the condition lever has 4 positions. From aft to forward, they are feather, ground stop, run, and airstart. Normally, the levers only are in "ground stop" (fuel electrically cut off to the engines, functions only on the ground) or in "run" (fuel shutoff valve electrically open, ignition can fire when the engine is turning fast enough). Feather position (used for inflight shutdowns & all "emergency" shutdowns ground or flight) cuts off fuel and commits the prop to feather (both tasks accomplished through both electrical & mechanical means). As was pointed out, the engine will run at a constant RPM (regulated by the fuel control on the ground, and by the prop in flight) most of the time. A separate control gives you "Low Speed Ground Idle." That control only works with the throttles fairly near the ground idle detent -- it would have no effect in flight. The airstart position changes the blade angle to allow a feathered prop to unfeather & begin to rotate in flight. Once it reaches sufficient speed, the ignition & fuel happen automatically (based on a speed sensor) & the engine lights.

One other note: on a constant speed arrangement like the Herk, power response is QUICKER than in a jet, because as quickly as you add fuel (push forward on the throttle), the prop pitch will change (electric throttle anticipation circuit at work) and you have extra thrust RIGHT NOW.

But you never have to worry much about the blade angle in a Herk -- push the throttle forward, and the power is there for you.

 

[pilotyip]

Forgot about C-130's

I forgot about the Herk, I was thinking of P-3's, Electra's and CV-580's I have flown, they just have the single power lever on the Allisons

 

[jetdriven]

I see you have flown in a BE90. That plane is usually set so that engine idle (N1) is 52% +/- 2% with the condition levers in the detent (lowest idle setting). You can then move the condition levers through any setting up to 70%. Meaning that if you set the levers full forward on the ground and you put the power levers to idle you would still be at 70% (N1). This is used for a number of reasons some being to compensate for high generator loads on the ground and for landing so that the engine is already spooled up for a go-around (power is not as instantaneous in a jet engine). I hope that clears some questions up, if not I would be happy to answer any more! (if you have any questions about N1 or whatever)

The 1900D low idle is 67% and the high idle is 72%. You usually use low idle on the ground taxiing as it minimized the use of brakes and beta to control taxi speed.
The high idle setting of 72% is for generator loads above 50% (like when you do a cross-generator start for the second engine and to recover the battery from the first start, or if you have an inop generator.) Also, the minimum speed for the VCM (A/C) compressor is 72%. Also the ITT limit for idle is 750, using high idle will lower the ITT a little.
We use low idle if any of the above conditions are not present. An added benefit is high idle gives you more drag in Beta range, and in case of a rejected takeoff you have more drag. Selecting reverse gets you the power sooner because the engine does not have to spool up as much. Same thing with a short field landing, high idle allows the reverse to build a couple of second sooner.

You can land and stop a light (<14,000#) 1900 in 1000'.

I like high idle because the condition lever at low hits my knee and the forward detent will clear it.

 

[msw]

I like high idle because the condition lever at low hits my knee and the forward detent will clear it.

*****************************************

Well, there's another good reason for when you get to move over into the left seat!

 

[KnowledgeSeeker]

Scubabri,

Just finished a long day in the B-90 so sorry for not getting back to you. Yes you are right that there are 3 settings, but (more precisely) there are an infinite number of settings between low and high idle (52%-70%). It's not just 52 or 72 but everthing in between.

Quote: "Low idle is used basically for starting the engine, and high idle is everything else"-

In a real general sense yes, but what Jetdriven said adds more to this. Basically like he said if a certain number of conditions are met (no A/C, low gen. loads, etc.) one can use low idle for different needs (slow taxi for example or a faster decent rate in flight).

Myself, I fly at low idle in flight for the use of lowest power setting and highest decent rate.

By the way...Are you doing some right seat time. Is that the reason for the questions. If you have more... fire away! And congrats if your getting some B-90 time. Its a great tank.

 

[scubabri]

Thanks for all the info. I am getting some right seat time, and I'm trying to make sure I understand the systems well enough that my time actually means something.

When in flight, in decent, with the A/C on at low idle, what does that do to the ITT?

The other question I have, the last B90 I was in was a 1969, according to the performance charts, the engines should have been putting out about 900fp's (or close to that) at 18K, we were running quite a bit less than that, around 650fp's. I asked the pilot about it, and he said that was all it had, and that he didn't want to push the PL more because he didn't want the ITT to go to high. If I understand correctly, as long as it stays below the limitations, it's ok. I don't recall what the limitation for ITT in cruise is, but I know we were below it.


I'm trying to figure out why that was happening. The first time I was in one, I didn't bother to notice power settings, now that I am learning more, more things catch my eye and cause more questions.

b

 

[B190Captain]

Also, the minimum speed for the VCM (A/C) compressor is 72%. Also the ITT limit for idle is 750, using high idle will lower the ITT a little.

You are right for the most part but the VCM (in the D model)does not require 72%N1 to run. In the B-C models where low idle was 58% N1, it was required to keep the right condition lever at or above 65% for the AC compressor to engage. This is not necessary in the D model since low idle is already at the 65%-68% range.

Fly Safe!!

 

[KnowledgeSeeker]

Scubabri,

a 69' king air, eh. must of loved telling the ladies that...I fly a 68' myself (missed it by one year). Sorry the juvinile is coming out in me.

When in flight, in decent, with the A/C on at low idle, what does that do to the ITT?

Basically realize that turning on anything that puts a large load on the electrical system with a low gas generator speed (N1) creates an increase in ITT. This will be less aparent with high N1. You can see this by looking at the ITT when the capatain turns on the Inverter (high electrical load) after engine start. If the N1 is at low idle you will see an increase in ITT. You get to a point after putting enough load on the system where you will have to increase N1 if you are going to keep the ITT within limits. Which is why the AFM calls for high idle when loading the electrical system on the ground.

In flight being at low idle is less of worry in regards to ITT because of the airflow cooling and also if you notice when you start a decent, even with the power at idle, you are still well above 52%N1. Therefore you don't fall into the catagory of high electrical load and low gas generator speed. Its all about ITT control ya know.

If you have a chance go to the airport early and get out the AFM and browse through it. There is a wealth of info in there. If you can look at the maintenance manuals the old king air ones are very readable and a lot is written very well. I myself thought they would be too detailed of reading but there is a lot of great info in there in laymans terms.

If I understand correctly, as long as it stays below the limitations, it's ok. I don't recall what the limitation for ITT in cruise is, but I know we were below it.

You are correct. but I wouldn't want to question your captains decision. (S)he may have been taking other things into consideration. Top of the green arc for ITT in the 68' B-90 is 700, but I never push past 680 on a continuous basis (10-20 degree changes can happen quick in a climb through different temps- don't want to overtemp and not catch it).

I hope this helps some more and if ya need more...