View Full Version : Ag Flying - PT6 Operation

Rarely Dble Amber
4th Sep 2005, 09:01
Landing a PT6 powered Air Tractor, is it best to avoid reverse thrust as much as possible and let the brakes do the work?

Or Is it ok to use as much Reverse thrust as you want (providing you have ample forward speed)

When do most of you AT Pilots crack the power lever past the gate?..

Any opinions would be appreciated, its something I have been wondering about lately.

4th Sep 2005, 11:16
Each to their own, personally -

Hardly ever need it.

Use it once in a while to stay current, as the book says, All wheels on the ground, tailwheel locked, surface considerations etc.

Sure others will have variations on the theme:E

4th Sep 2005, 12:44
I'm happy to use it whenever I need it.
I feel you should only ever use as much power as you need. Braking power included.
My understanding is that when the powerlever is back at the stop it is at the 'Primary Blade Angle'. Forward of the stop is the Alpha range (normal flight). Depressing the button and moving the lever back puts the blades into the beta range. Moving the lever further back the blades remain in Beta and do nothing more than provide increasingly effective drag until you leave Beta and start to go into reverse. You'll know because thats when your TQ/TOT starts to rise. Up until that point you are only in Beta.
You'll find that you can use Beta to regulate your descent if you need it and then if you need to, as Currawong says, when all 3 wheels are planted, the tailwheel is locked, you can pull the thing into reverse. Smoothly of course.
Bear in mind that you will throw up a fair bit of debris if you carry this to a stop so, like the airline guys, get the lever back into Beta as you slow. I use Beta instead of brakes because it saves brakes and doesn't hurt either the engine or the prop.
Like I said the above is my understanding only and may not be completely correct or even best practice.
Would be interested to hear if any body else sees this differently.

4th Sep 2005, 13:20
I'm with LLD, thats the way I use it.

Don't be in a rush to use it if your not too experienced, either Beta in the air or Beta/reverse on the ground, the rapid decrease in airspeed and loss of rudder effectiveness may catch you unprepared. Just gradually start using a bit when you aren't under pressure and have plenty of room, to see the effects.

What does everyone do with flight/ground idle?

4th Sep 2005, 14:14
Hey Turboman
Although I know a lot of experienced pilots use ground idle all the time, I use Flight Idle from the time I leave the pad to the time I pull back up on it.
Keeps it simple and if you need more power on approach for a go-around its there sooner. BTW thats what the book calls for.

What I forgot to mention in the last post but is critical is to always check that prop is set to full fine on final so that when you do go for Beta/reverse, you get it. The way the system works, even though the lever is back beyond the gate it won't happen till prop RPM falls below what is selected.

If you panic and pull it too far back because nothing is happening, when your Prop rpm falls below selected it will come on in a big rush. If you're not planted when that happens, the a/c response could be uncontrollable.

I know I haven't explained that near well enough. Anybody care to help me out here?

4th Sep 2005, 14:37
CAUTION flight idle ground idle.

If jumping into a machine for the first time, they can be rigged quite differently to what you might be used to. So ask the last pilot if he is a ground or flight idle type.

If he is a ground type, and you try to land in flight, you may land long. And vice versa.

Some people have them set up so at a certain point the :mad: drops out of them.

Which we all knew - just in case someone comes across this that did not know. Safety first, you see;)

4th Sep 2005, 15:03
I'm sure that some pilots do in fact have their aircraft set up to suit their own personalities/ flying styles. Though I can't believe that any engineer worth his pay would tolerate this.
Frankly, I think thats total BS. Theres only one way to set up ANY aircraft and thats 'By the Book'.
Now if you always set the thing to 'flight idle' and the aircraft tends to want to keep going, pull in a little Beta. If not no problem. That way it doesnt matter how a previous pilot set up Ground Idle. Which is no doubt one of the reasons the factory wants us to have the Condition Lever at Flight Idle when we're in Flight Mode.

4th Sep 2005, 15:43
Agreed. Unfortunately not always the case. At an average 100 hr inspection I doubt any engineer would pick up that someone had tweaked the prop.

But, on the bright side, someone might read this thread and not break something as a result.

What are you doing up this time of night anyway? I'm off before I start to look like this bloke:8

5th Sep 2005, 01:08
You know flight idle v's groung idle must be one of the most argued points on PT6AG use. For some reason it has become a personal point! Now I may be a bit of an old fart but when I did my Thrush/Airtractor turbo rating, the boss actually didn't leave it up to me at all.
He said, this is the way you wiil fly it and these are the reasons.
After all it is his cheque book isn't it!
I'm taxing the old 386 a bit here but his primary reasons for wanting me to fly it in Flt idle and not to use Grd idle untill I was pretty much stoped were.
1- If your taxying around all over the place in Grd idle just watch what the ITT is doing. The temp will be up and down and all over the place. Yep you will wear out brake pads in this mode probably twice as fast but if it was my PT6 I think I would be happy enough with that. My first PT6 boss was!!
2- Hands up who has been caught on short final with a heep of sink and you give the power leaver a quick nudge and that sinking feeling stops and you just make it over the fence. Phew!!
Well guess what? if you were in Grd idle you've got a bit of fencing to do!! Now a bit of fencing is pretty cheep hey. But whay if its a bit of a tight strip with a hugh craggy old gully at the threshold about 10 feet deep. Not a good position. This hapened to a good mate of mine many years ago and he cleared the gully by 6 inches, I kid you not. He had the thing firewalled and nothing was hapening, he had to pull back and she stalled as it crossed the gullly and there was an almighty dint in the ground where the mains hit. He never flew in Grd idle again.
All food for thought guys.

5th Sep 2005, 03:43
Would you believe someone experienced told me many years ago to pull it back to ground idle when on final. Not quite sure why he was pushing up into flight idle in the first place if he was going to do that. I have shut down twice pulling into ground idle on the ground. Not something you really want to happen on final. If your going to do anything with that lever in the air push it forward (on final is a good time as they creep back), DON'T pull it back.

5th Sep 2005, 04:57
Yeah that piece of stainless that acts as the gate on the condition lever is really 'mickey mouse' and should not be relied upon.

Super Cecil
6th Sep 2005, 12:03
For shorter strips, ground idle works better. I always used ground idle all the time, it's slower spooling up but any engine you don't just bang on throttle so treat it like you own it.

As for prop set ups, they come out of the shop and some slow up and some don't. An odd one if you throttle right back will really slow the Aircraft up, it can be usefull if your aware. It has to be a fairly short strip to worry about through the gate.

It's just basic flying really, know how your Aircraft handles. Every Aircraft is different even if they come out of the same jig, just get used to and know the one your flying.

Rarely Dble Amber
6th Sep 2005, 14:01
I Agree with the prop setup being different for each aircraft. I have flown PT6 powered aircraft rigged exactly by the book only to discover the settings almost unworkable. So of course it then comes down the the engineer asking the pilot what he wants.. more go, or more slow. The settings then end up being dictated to how the pilot wants it.

However, when it comes to Ground/ flight Idle operation, my rule is, if its moving then it is in flight idle. Ground Idle should be called start Idle.

8th Oct 2005, 08:41
I agree that each installation varies in its operation both on the ground and once airborne, with particular referance to the approach phase and how much deceleration is achieved with the lever on or near (or through) the gate with the normal prop rpm selected.

My personal preferance with the condition lever is to use gnd idle during taxing as i find it gives greater control over ITT temps during minor power adjustments, usually select FLT idle during pre-landing checks and I agree it's a dangerous practice trying to go-around from GND idle....it's a loooong wait for ot to spool back up!

Whatever our personal differences in operating it, I reckon the PT6 is Gods gift to Ag' aviation.

8th Oct 2005, 20:34
There's absolutely nothing wrong with operation in ground idle. The ONLY thing that going to flight idle does is reset the fuel controller for idle operation Ng speed. Above idle, the engine doesn't know the difference.

I believe someone mentioned using beta to control descents. Not a good idea.

Personally, I don't use beta or reverse much.

9th Oct 2005, 01:11
So if you're operating at ground idle in an AirTractor you're not complying with the procedures outlined in the Flight Manual. No biggie till you have to explain to the accident investigators why the condition lever was at ground idle.
What are the engineering reasons for avoiding beta or reverse?
Taxiing in Flight Idle and using Beta to control speed has less effect on ITT than taxiing in ground idle and using Alpha. It mightn't sound that great but I haven't had any engineers tell me its bad for the plane.
I happily use beta on late, late finals if I'm a little hot over the fence. It's an asset in the 802 because its such a heavy aircraft (more inertia) and you're better off having a little more rather than less speed on the approach.
My point here is that there is nothing in the Flight Manual to say 'Don't use Beta or Reverse' but it clearly states that Flight Idle should be selected prior to Take-off. So far I haven't heard a convincing argument for anything different.

9th Oct 2005, 04:35
Look in a Pratt manual, or consider the operation of the fuel controller itself. What are you accomplishing other than resetting the idle fuel? Nothing.

Further, for landing, having a reduced power also reduces the landing distance and allows the airplane to decelerate slower.

If you're going to be using reversing, you're going to have more reverse and beta available if you're in flight idle. However, I seldom if ever use reverse or beta in a single, and therefore have no need for flight idle position on the fuel control.

On other aircraft, such as several different pt6 powered twins, I will select flight idle as I enter the runway. I don't have to push the power up to get the aircraft moving as resetting the fuel controllers and the increased idle speed does that for me. I also have max reverse available to me, quickly, with the controllers reset. Where reverse will be used for takeoff protection or for landing to shorten the distance, then yes, there's an advantage to setting the controller to flight idle.

Otherwise, if you're not going to be using the reverse on the ground on the rollout, or if you're going to use it but not go deep into reverse, then ground idle is much preferred.

I've never heard of an accident investigator faulting a pilot for failure to use flight idle. Ever.

I've known of a few dingbats who came over the gate into the ground range in flight, and one who stacked up an airplane doing that...but there it won't matter if you're in ground idle or flight idle.

10th Oct 2005, 12:03
I seem to remember a landing accident involving overshooting into various bits of machinery , where the investigation concluded that had flt idle been used the loader driver would not have had to dive out to avoid the oncoming accident.

11th Oct 2005, 07:47
An approach at ground idle is going to have less idle thrust, and is going to make a shorter landing than an aircraft approaching at flight idle, all else being equal.

Pretty hard to make the case that landing at flight idle, read higher power setting, makes for a shorter landing.

If you're landing on something that short into obstacles, the real fault is failure to leave yourself an out. If you need to land that short and need deep reverse, you're setting yourself up for control issues by blanking out the tail.

Pick something longer on which to land. Think of it this way. You can always land in less distance than you takeoff, loaded. If you have enough room to takeoff with a load, then you certainly don't need heavy reverse and flight idle on an empty approach to landing.

Perhaps better control of the approach and associated airspeed might be the order of the day, rather than trying to use reversing as a crutch to compensate for a bad approach.

Did I suggest that because I haven't seen it it isn't true...of course not. However, I find the idea that an engine suffers damage during taxi from turns when the airframe does not, laughable.

11th Oct 2005, 09:23

I believe someone mentioned using beta to control descents. Not a good idea.
Could you please explain why you think so?

Further, for landing, having a reduced power also reduces the landing distance and allows the airplane to decelerate slower.

How can a slower decelerating aircraft possibly have a reduced landing distance??? :\ If you mean't to say 'faster deceleration' your still wrong for a fully reversable PT6.

With a fully reversable propellor, engine speed is independent of forward thrust. Granted, if you never go into Beta or reverse ground idle is going to give you the shortest landings, but I'm sure your car comes with a reverse gear. Do you choose not to use that as well.

Flight idle will give you more control everytime (if you use the rest of the power quadrant like 99.9% of us) and if your doing Ag on marginal strips with 50+ landings a day it is just a smart place to be.

Pretty hard to make the case that landing at flight idle, read higher power setting, makes for a shorter landing.

With all due respect you just don't know what your talking about. Don't forget this is an Ag thread.

11th Oct 2005, 10:12
I understand it's an ag thread, and I'm approaching it as an ag pilot.

Could you please explain why you think so?

Why do I think coming over the gate into a ground or reversing range in flight is a bad idea? Are you serious?

How can a slower decelerating aircraft possibly have a reduced landing distance??? If you mean't to say 'faster deceleration' your still wrong for a fully reversable PT6.

My wording was off. I meant to say "Further, for landing, having a reduced power also reduces the landing distance and allows the airplane to approach slower float less or not at all, and stop sooner."

I've seen few applications for going into deep reverse in an ag airplane. For one, I don't do anything with reverse with the tail still in the air, and once it comes down, I'm stopping fairly quickly anyway.

Granted, if you never go into Beta or reverse ground idle is going to give you the shortest landings, but I'm sure your car comes with a reverse gear. Do you choose not to use that as well.

My airplane is not a car. The comparison is senseless. Do you put your car in reverse while moving forward? Of course not...the closest comparison you might make is using your brake in a car, to using reverse in an airplane. However, the airplane already has brakes.

On loose surfaces, I avoid reverse. On approach, I have a slower approach, less float, and put the wheels on the ground more easily at a slower speed with ground idle, than flight idle. This being the case, I have less energy to dissipate. Once the mains are on the ground and I start to apply forward pressure, I bring up the flaps and the tail settles. I can use some reverse there, but I seldom need much. I have no need to be in flight idle.

11th Oct 2005, 13:10
Your choice, the aircraft owners too I hope. You may change your mind when you have to go around in a hurry. Kangaroos, sheep, cattle, etc will make you do that.

Now back to my question of "Please Explain?" for your statement...

I believe someone mentioned using beta to control descents. Not a good idea.

"Are you serious?" is just not making me understand why. We are talking about beta here NOT reverse.

P.S. touch down speed should be the same whether the engine is in ground idle, flight idle or not running at all. Stall speed is independent of what the engine is doing. You seem to be to be associating airspeed directly with engine speed. It depends on what position the power lever is in.

12th Oct 2005, 11:49
To be honest SN, I also used to land in gnd idle until I had to go around with a fair load still on board, how bloody silly I was !

I heeded the advise of more experienced pilots and learned to bring the power lever through the gate on short final in order to reduce the thrust and increase the prop braking ( airborne) as desired/required.

It makes for faster turn-arounds when spreading/sowing and costs nothing in mechanical wear.

13th Oct 2005, 09:55
In short, I feel like I have more control when in Flt Idle.
The thrust you have at Grnd Idle at the gate is exacly the thrust I have in Flt Idle, just (ever so slightly) through the gate. Our approaches would look the same to an observer on the ground. Its just that I won't have to turn around (putting massive loads on the prop;)) to get back to the pad and my brake discs will last longer than yours.
I ALWAYS 3-point, so the tailwheel is locked. Guess I can see where you're coming from if you wheel on, but don't see why you would.

13th Oct 2005, 10:39
Geeez' lowlvl, lol, there's another one we can all argue about; 3-point or wheel it on!!

Rarely Dble Amber
13th Oct 2005, 12:36
3 point all the time.... cannot get the damn tailwheel on the ground without it shimmy'in itself to death otherwise..

Surely everyone else 3 points?... :}

13th Oct 2005, 13:14
Sounds like your tail wheel locks aren't adjusted properly. Should only unlock in the last 1 inch of forward stick.

I 3 point also(or low tail wheeler), but heard many arguments against. In 10-20knot gusty wind have found it easier to wheel, also on slippery surfaces.

I have an article on "why it is safer to wheel", not that I necessarily agree, but I'll try and post it.

13th Oct 2005, 15:37
There's one difference...right now I'm flying a turbine Dromader, not an air tractor.

When coming across the gate, you're physically removing the low pitch stops. At low power settings, the prop is windmilling, and has experienced an airflow change on the propeller disc. In the helicopter world it's called an airflow reversal, where airflow is striking the blade back rather than the face, and the slipstream is imparting more energy to the propeller than the engine.

In such a case, especially at higher airspeeds, you may experience a prop overspeed. Someone had mentioned using beta or reverse during a descent. I can't imagine why someone would do that in an ag airplane (how far have you got to descend), but I do a lot of fire work where we do have extended descents, and steep ones. I have known people in single engine airplanes to apply reverse on the descents, or attempt to use BETA, and not only is it pointless, but dangerous and represents a lack of understanding of how the system works.

Outside the normal governing range for the propeller, such as beta and reverse, governing takes place by fuel topping using an air bleed orfice in the propeller governor. This airbleed orfice reduces control bleed air to the fuel controller, or allows it to increase by restricting the orfice.

When your engine is trimmed for reverse capability on the ground, it doesn't take into account airflow through the propeller, driving the propeller. The fuel limiting function of the governor slows the propeller by slowing engine speed, and assumes that you're traveling at landing speed or less, decelerating.

If you're flying through the air, you can experience a propeller runaway. If your prop speed runs away while out of the governing range, your governor is trying to take fuel away, but it's not fuel driving the prop, it's airflow, and the only thing protecting it has been removed; the low pitch stops (when you came over the gate into ground range).

Yes, your drag can increase exponentially and yes, it can form an effective "airbrake" (because it's absorbing so much energy from the airflow that's driving it)...but folks have found out the hard way that it may not come back out, and the aircraft may be less controllable (--or uncontrollable) as the overspeed worsens.

As the overspeed worsens, it also affects airflow over the tail, which includes both the vertical and horizontal stabilizers. This means less rudder authority, but it also means less elevator and horizontal stab download authority. As most conventionally loaded aircraft, ag or not, utililize a download on the horizontal stab for stability, this download is interrupted, and the result can be a loss of pitch control. As the downloading effect is interrupted by the lack of airflow, the nose can pitch down, and there may very well be insufficient time to recover.

That's not conjecture, it's happened.

What does your engine manufacturer, airframe manufacturer, and propeller manufacturer say about using reverse and ground range in flight? Ever wonder why?

As for needing to go around in ground idle vs. flight idle...planning ahead for that event and keeping the engine partially spooled up to begin with is proper technique in any turbine powered airplane. It responds better when slightly spooled. If you're at idle and having to bring in power quickly to go around, count on a lag regardless of weather you are flying in ground idle or flight idle.

Someone else asked me if the owner of this aircraft (which isn't me) knows I fly the engine the way I do. Yes, he does, and he flies it the same way. So does his father, who has flown more ag than I probably ever will, and who like us, is also a mechanic.

Use it where you need it, but even in daily operations in extreme mountainous terrain, I haven't found a need to come out of ground idle yet.

14th Oct 2005, 06:16
Remember this is a forum for everyones opinion TURBOMAN. Maybe it is time you sat back and listened.
I wonder if an apology to SNS3 is comming ? After all he does know what he is talking about in this AG thread.
Happy learning :D

Rarely Dble Amber
14th Oct 2005, 12:01
As for needing to go around in ground idle vs. flight idle...planning ahead for that event and keeping the engine partially spooled up to begin with is proper technique in any turbine powered airplane. It responds better when slightly spooled.

.. so why not just operate the thing in flight idle?:confused:

doesn't keeping it slightly spooled negate the original reason for operating in Ground Idle?

14th Oct 2005, 12:50
Thanks for your reply to my question SNS.

I have never heard of, or experienced a prop overspeed in Beta with a PT6. Are you refering to a Garrett or PT6 installation?

If you where in the first stages of an overspeed and were loosing airspeed and performance wouldn't you automatically push the power lever forward thereby returning the blades to a primary blade angle and positive thrust, or are you saying the blades may not come out of Beta?

Thanks for bringing me to my senses Reddust:ok:

14th Oct 2005, 23:14
I'm talking specifically about PT6A operations.

Once in beta in flight, you may not be able to come out. Once the prop starts to overspeed, the governor will try to control it by opening the bleed orfice, reducing engine RPM. As engine RPM is probably already at idle, it can't be reduced any farther, and the engine speed is no longer relevant because the slipstream is driving the propeller.

You're assuming you can get it out of beta or reverse, or that you'll have time. Chances are, you won't. When the prop starts to overspeed, you lose control...you'll be busy, and if you're close to the ground (when in ag are we not?), you probably won't get the chance to rectify the mistake. Even if you do, you'll likely be nose down, probably steeply.

When the prop starts into overspeed, it becomes self feeding. Adding power may only worsen it. You may not bring it out, you may just increase the Ng speed and increase the overspeed. Sometimes in an overspeed the only option is to go for feather, if you can feather it, and then come back out.

What does your airframe, engine, and propeller manufacturer say about using ground ranges in flight?

To address your other question...why not approach in flight idle rather than approach carrying power with the power lever? Simple. Your residual thrust is lower crossing the fence when you do come back to idle if you're already in ground idle. If you're in flight idle, your minimum Ng will be approximately 70%. If you're in ground idle, you can come back to approx 54% Ng, with less residual thrust, greater braking power from the propeller absorbing the airstream at idle, and reduced stopping distance.

15th Oct 2005, 05:58
Lets bring this back into context. We are talking about using Beta on short final, not reverse, or Beta at any other stage of flight.

Regarding an overspeed I’m still confused how one can develop in Beta. We are through the low pitch stops into Beta. The prop is disking. There is no positive angle of attack for the slipstream to drive the prop and we have a rapidly reducing airspeed. If there was any negative angle of attack the slip stream would be slowing the prop down. Why would the prop fail to come out of Beta? If there was a high risk of this occurring I’m sure certification with a Beta/reverse range would not have occurred.

I can understand an overspeed in a twin, when the live engine is maintaining airspeed and the governors fail on the other with the prop on the low pitch stops, but not on a single with reducing airspeed and the prop at a zero degree blade angle.

Regarding your last paragraph, there is no residual thrust. You land with slight Beta in flight idle, add a bit more Beta or reverse for free braking, when you’re settled and slow enough back to ground idle, push the power lever back through the gate, and roll up to the pad with minimal or no wheel braking. This does assume that the engine is correctly rigged of course. The prop is spinning faster but its not producing anymore thrust, it's in Beta, waiting to provide you with reasonably instantaneous forward or reverse thrust if you want it.

15th Oct 2005, 06:54
Lets bring this back into context. We are talking about using Beta on short final, not reverse, or Beta at any other stage of flight.

You are talking about using ground range, be it beta or reverse...a point outside the governing range of the prop with the low pitch stops retracted.

If there was a high risk of this occurring I’m sure certification with a Beta/reverse range would not have occurred.

Where do you find authorization in the certification of your airplane to operate in the ground range in flight? Your aricraft was certificated with this in mind? I've asked you repeatedly...what does your airframe manufacturer, engine manufacturer, and prop manufacturer have to say about ground range operation in flight?

You land with slight Beta in flight idle,

Asked of you again and again, but not answered...where do you find this authorization in youir approved aircraft flight manual, type certification, supplemental type certification, or field approved operating limitation modification?

I noted previously several posters contended with me regarding operating in ground idle, saying that certainly the pilot would be found at fault for operating in ground idle. Of course, there is no such restriction. Yet there certainly are restrictions in virtually all aircraft with reversing capability of operating in the beta or reverse in flight.

What do you say?

15th Oct 2005, 10:46
OK SN, in the interests of the discussion ( and please let's keep it a discussion and not a slanging match) , I've found an FAA approved fright manual for the 502 dated Oct.15 1996 and scanned through for details on use of condition lever in flt and during landing plus use of power lever behind the gate.

Fistly the condition lever on landing,

Under NORMAL PROCEDURES, sub section APPROACH AND LANDING( NORMAL-EMPTY HOPPER) ;(won't bother with all the points).

2. Start lever S- Flight Idle Position( 68-70% Ng)

(this was a point insurance companies in Aus' were keen to highlight a few years back)

On use of power lever through the gate in flight I could only find the following reference under Emergency Procedures, sub section ILLUMINATION OF CAUTION LIGHTS(AMBER).

Prop in Beta Range: Indicates prop' pitch is at or below the
minimum fine pitch that is safe for continu ous in-flight control. During ground opera tions it indicates prop' is positioned for ap plication of reverse thrusting power.

The way I read that it's a cautionary indication based on aerodynamic considerations (quite true and demonstrably so with over-eager fistfulls of beta in flight) not a powerplant engineering matter, if it were a possibility of being unable to come back out of light beta in flight I'm sure something more impressive would be mandated by the FAA.

BTW I agree that landing in flt idle without using Beta will most definately result in long floats in ground effect, which tends to make the case for light Beta even stronger.

I guess we'll all keep operating the way we are so long as everyone's happy.

15th Oct 2005, 18:10
That caution light is there for a reason. Operation off the ground with the prop in the ground range is unsafe, unwise, and not approved. In fact, only a handfull of installations in the world have ever been approved for any kind of reverse or ground operation in flight, and your aicraft isn't one of them.

Again, you feel that operation in ground idle, a fuel controller function that presents NO safety of flight concerns, could cause you big legal troubles because you *think* it's not an approved operating condition. It is an approved operating condition.

Conversely, you're willing to enter an unapproved engine operating condition, bringing the power lever over the gate into the ground range, and don't seem the least bit concerned about the safety or legal implications.

Write to the manufacturer and ask if they approve it. Let me know what your response is.

No "slanging" necessary.

16th Oct 2005, 05:51
Sorry but I guess what I was saying is that nowhere in the FAA approved flight manual does it say it is not an approved procedure ( unsafe and unwise are your judgements and you're entitled to them).

I'm reminded of footage showing Porters chasing skydivers vertically down and I suspect they are using the big brake up front in a big way, they're definately braver than I ,however seem to live to fly another load of jumpers.

The whole reason I changed my personal way of using the condition lever was safety based as the go around I mentioned earlier was from over the threshold when a four legged conveyance bolted out from a shadeline, waiting in ground effect as the PT6 spooled up from 51%Ng gave me time to re-think things.

As I said earlier mate , I reckon we'll all keep on doing what we do as long as It's safe.

16th Oct 2005, 07:41
Porters have a sightly different cam assembly up the front which varies fuel and blade angle scheduling. As I understand it they are approved for this operation, but I could be wrong.

I have always presumed that Beta in flight has never been formally approved in ag aircraft as it would present a liability issue one day when somebody (as agair3 pointed out) pulls in a heap of beta/reverse at the wrong moment, looses pitch and yaw control, and destroys the aircraft. Manufacturers have also never approved operations at the weights we are operating at, or approved diesel (of standard grade) for use in turbines.

Amongst a lot of reasons, one purpose for fast, slippery jet aircraft to chuck all those high drag devices out on approach is because they need a much higher power setting to overcome the drag. What does this mean? All the engines are half spooled up for the go around instead of sitting at idle.

Maybe thats why they invented FLIGHT idle. Doing 20 to 50+ landings a day exposes us to a much higher risk of needing to go around than the airlines. It's all about minimising risk at an affordable cost. However if the operators and insurance companies are fine with ground idle thats their choice.

17th Oct 2005, 07:24
Blast from the past...the following I obtained from a different web site three years ago, and have since posted it on several different forums for discussion. It may apply here. Notice not only the commentary regarding the potential problem that the author notes, but the results of several aircraft entering beta and/or reverse in flight. Specifically the rapid pitching moment when the download on the horizontal stab is lost and/or the tail is blanked or starved of driving propeller airflow.


by Lee Coffman
Aero Services Unlimited
Santa Fe, NM

Has the impossible happened to you? For over twenty years the turbine engine manufacturers and the aviation industry have said it was impossible for a PT6A engine to go to beta or reverse without the pilot initiating the command. To initiate the command the pilot has to move the power lever behind the gate toward beta and reverse. An uncommanded beta or reverse would be one in which the pilot did not move the power lever behind the gate.

I have now interviewed the pilots and operators of six (6) aircraft that went to uncommanded beta or reverse in flight. When these pilots try to talk to people in the aviation industry about what happened the word “Impossible” keeps coming up.

Facts about these occurrences
An AT-802A with a PT6A-67AG engine went to reverse when the pilot reduced the power to descend out of 10,000 feet during a delivery flight. The aircraft pitched over very steeply and the pilot discovered that pushing forward on the power lever made matters worse, causing the aircraft to pitched over even steeper with the tail stalled the aircraft went over on its back. The pilot slammed the propeller and power levers forward forcing the propeller out of reverse. The pilot recovered at 5,000 feet. The pilot flew over an airport and shut down the engine and feathered the propeller before attempting to land, dead stick. After landing, the engine was started and an attempt to taxi was made. When normal propeller operation could not be obtained the aircraft was towed to the shop. The propeller governor was changed and engine/prop operations returned to normal.

An AT-502B with a PT6A-34AG engine went to reverse when the pilot reduced the power to lower the flaps before landing. The aircraft pitched over 45 to 50 degrees. The pilot slammed the power lever forward. The aircraft was about 300 feet high when it went into reverse. The aircraft hit the ground about one half mile from the end of the runway, bending the main gear. The aircraft landed in tall soybeans and stopped in 150 feet. The engine was still at full power with the power lever full forward when the aircraft stopped. The propeller blades were all bent aft indicating the engine was making high power and the propeller blades were in reverse. The propeller blades did not hit the ground. The propeller governor was found to have heavy corrosion under the beta valve cap.

An AT-502B with a PT6A-34AG went into beta or reverse when the pilot reduced power during a turn to land at about 100 feet. The aircraft pitched over and the pilot pulled back on the stick and was leveling the wing as he hit the ground. The beta valve was found to be very stiff from corrosion under the beta valve cap. The beta valve was cleaned up, lubricated and returned to service.

An AT-502B with a PT6A-34AG went into reverse at about 15 feet as the pilot reduced the power to land. The pilot slammed the power lever forward. The aircraft hit the ground, stopped and started to back up. The pilot pulled the power lever to idle and tried to figure out what went wrong. The pilot then tried to taxi to the hangar, but the aircraft would only go backwards when the power lever was pushed forward. The beta valve was found to be very stiff and heavy corrosion was found under the beta valve cap. The propeller governor was changed and operation returned to normal.

An AT-502B with a PT6A-34AG started going into beta or reverse when the final power reduction was made just before landing. The aircraft slammed into the runway instead of making a normal touchdown. The rigging and other items were checked and when nothing wrong could be found the propeller governor was changed. The operation retuned to normal. When the beta valve was removed from the old governor, heavy corrosion was found. During the same season the operator had another AT-502B with a PT6A-34AG that slammed into the runway when the final power reduction was made. Another propeller governor was installed and operation returned to normal.

Common Facts
The aircraft propeller goes to beta or reverse when the power is reduced to the point that the air load is driving the propeller and the propeller governor goes to an under speed condition.

In the 1980s, Beta valves had a leak or seep problem until propeller governor manufacturer, Woodward, increased the “O” ring size to eliminate the problem.

All the PT6A-34AG propeller governors that were checked had corrosion under the beta valve cap and there was no evidence of any seepage or leakage. The beta valves appeared stiff or appeared to stick. When the propeller governors were bench tested they passed the test.

The PT6A-67AG governor was not checked or tested. However, Woodward has issued a service bulletin that applies to some of governors on –60 series engines to replace the beta valve packing with the old style because of a possible controllability problem. The service bulletin says this will contribute to leakage.

Air Tractor issued Service Letter # 172 dated 10/19/98 to address the possibility that a sticking beta valve could prevent the propeller from coming out of beta when power was applied.

I feel that the Air Tractor SL # 172 should be complied with at each annual. I feel that turbine oil should be applied to the spacer as a lubricant to prevent corrosion since we did not have a corrosion problem when we had seepage.

I am seeking information from any pilot or mechanic that has seen the problem or been involved with the repair of the problem. “Please contact Lee Coffman of Aero Services Unlimited” at 505-820-1476 or [email protected]<hidden>. Any input will be greatly appreciated.

Rarely Dble Amber
18th Oct 2005, 00:16
From that I would say there isn't a whole lot we can do about our PT6's going into beta or reverse all by themselves. None of those guys were selecting beta/reverse in flight. They were reducing power. Follow SL# 172 and hope for the best huh? :}

I can see your point about not going into ground range in flight Guppy, but I am also not going to operate in ground Idle, I work mostly off a short one way airstrip and If I need power to go around, then I need it right NOW. :oh:

It seems that most guys that work in flight idle need to grab a bit of beta on late final. By your reckoning, that is a bad thing, and sure enough the book tells us to not do it. Perhaps we should have our props setup to have less idle thrust?? So that we can operate in flight idle and not need to bring it behind the gate to get a slow short landing.?.?. :bored:

18th Oct 2005, 17:57
From that I would say there isn't a whole lot we can do about our PT6's going into beta or reverse all by themselves. None of those guys were selecting beta/reverse in flight.

That might be all you get out of it. Or if you read it again, you might also get that after entering ground range in flight, you may not come back out. Or you might get that the aircraft can become uncontrollable in ground range in flight. Or you might get that the aircraft can get worse or go deeper into ground range once it enters...remember that the prop is going to a flatter pitch in beta and a higher autorotative state, the prop governor is operating underspeed and no longer capable of controlling the prop speed (it's now a fuel controller issue).

Get what you want out of it. Better yet, contact the mechanic who wrote it, if he's still there, and see what he thinks.

Takeoff always requires more distance than landing, assuming proper speeds are used. Certainly a loaded takeoff. If you can takeoff loaded from a runway, you can certainly land back on it. If you were able to get off that short one way strip, you can certainly land, reverse or not.

Never land on something that you can't stop on without the reverse. Use it if you need it, of course...but a basic principle of airmanship is always having a second plan, always leaving yourself an out. If you're flying in and out of a location that absolutely must have reverse to stop, one has to question how you took off there with a load, and what you're going to do if that reverse fails. And if a go-around is such a critical issue, and you're going to try it from beta...flight idle isn't helping your case at all.

You decide. I find the idea that the prime reason several posters cling to the need for flight idle is because the aviation authority in their area found fault with operators who used ground idle to be a very interesting counterpoint...to the fact that the same operators use beta in flight contrary to every publication, instruction, and wisdom from manufacturers, government, and any reliable source on turboprop operation information you're going to find. On the one had they do something because they don't want to displease the government and act contrary to official data (might get into trouble if there's an accident...), and on the other they completely ignore it anyway. This just doesn't make sense to me. Clearly it's a matter of "we're going to do whatever we're going to do, no matter what."

If the power is retarded to flight idle on approach with the fuel controller set to ground idle, far less residual thrust exists crossing the fence to land. Without consideration for reverse or braking, you're not going as far down the runway.

If the power is retarded to flight idle on approach with the fuel controller set to flight idle, more residual thrust exists crossing the fence to land. Without consideration for reverse or braking, you're going farther down the runway.

Add braking. If the power is retarded to flight idle on If the power is retarded to flight idle on approach with the fuel controller set to ground idle, far less residual thrust exists crossing the fence to land, and the brakes do less work slowing the airplane, you use less landing distance.

If the power is retarded to flight idle on approach with the fuel controller set to flight idle, more residual thrust exists crossing the fence to land, the brakes do more work, get hotter, and experience more wear.

You add reverse to the equation. With ground idle fuel control setting you get slower response to reverse, and generally less reverse. with the fuel control set to flight idle, you get faster response, and greater Ng, resulting in greater reverse thrust.

A case of use it where you need it, and certainly an operator option. In other words, your choice.

Beta range in flight is not your choice. You can do it, but then there's a lot of things you can do that are unwise...nobody will say a word until they sweep you up.

The question then arises...is max reverse necessary? Are you applying this with the tail still in the air, or getting it on the ground where you have some tailwheen authority before you bring in the reverse? My guess is that most who are using a lot of reverse are doing it as the airplane touches down and are doing so on the mains, only. That can work a hundred or a thousand times...but sooner or later a brake will grab, or a brake will fail, and then you're going for a ride. I watched a thrush get destroyed that way once. I lost my hydraulics this summer...had I tried reversing when that happened with the tail off the ground, I'd have gone for a ride, too.

Do what you feel is best, when you need it, of course. But never assume that because we do the same thing every day that it must be okay because it's never hurt us, or that we're immune from it biting us. We aren't. Complacency kills.

19th Oct 2005, 23:09
After 30,000 Beta landings in well maintained aircraft with no problems I feel the technique I was taught was safe and effective.

I agree, constant vigilance is the key to a long, successful career in AG.

22nd Oct 2005, 13:30

I am flying a Cresco's and have had some similar questions, regarding PT6, below was the response from the Pratt Instructor, which proved a number of thing i was taught to be BS.

Here is another question, if you start the PT6 with the Power Level full forward ( max power ), will it be a normal temp start, i.e. is the FCU still able to schedule in such a way ?, from start to full flame. ( I can not see why )

Reply from P&W, enjoy:

Q: Why does the Compressor have Axial stages, then, one Centrifugal Stage prior to the combustion chamber ?, what is the purpose of the Centrifugal stage ( why not Axial ) ?.

A : To achieve the requested ration of 7:1 we would need too many axial stages which are less efficient at lower speed . The total ratio of the 3 axial stages is 1.80:1 and the centrifugal is 3.9:1 . The centrifugal is more efficient at higher speed . I guess the engineers in 1958 have found the best mix of axial/centrifugal .The purpose of the centrifugal stage is to accelerate the compressed air ( by the axial stages ) then the diffuser ducts reduces it to increase the pressure prior to combustion .

Q: Will a Centrifugal Compressor be able to tolerate smalll excursions of airflow in the Super-Sonic region ?, ( airflow then passes into divergent duct, and could become sub sonic prior to combustion ) ?.

A : Yes , it is designed for that . The air is travelling at 2000 ft/sec at the inlet of the diffuser ducts and at 200 ft/sec at the exit thus
increasing the pressure prior to combustion .
( note to self, speed of sounds is around 1,100ft/sec at sea level )

Q: Beta / Reverse Operation in Flight, my understanding is that this is forbidden, Why ? ( aircraft such as the Pilatus Porter are used in Parachute operations all over the world, Beta / Reverse is used very much common place in descent ).

A : I don't see anywhere that this is forbidden . You are right , parachute designed aircraft have to go back down fast after jumpers are out and they are using flat pitch to slow down the aircraft during descent or agricultural operators are also using a lot of beta /reverse . This info could be found in POH ( pilot handbook or Airframe manuals ) because the are airframe matters , not engine matters . You are not damaging the engine
at all if you go reverse in flight . It's a question of rigging which is
determined by the OEM ( depending on the mission of the aircraft) . Some people adjust the beta/reverse tighter than others . Of course charters are adjusted smoother .

I hope this answers your questions . Feel free to contact me if you have
any other concerns .


Bruno Bourdages
PT6 instructor
Pratt & Whitney Canada
phone : (450)468-7881
fax : (450)468-7834
e-mail : [email protected]<hidden>

23rd Oct 2005, 22:54
An interesting response Left Handed rock Thrower! A word from the horses mouth so to speak.
I've been thinking about SNS Guppys reasons for not using Flight Idle, Beta and reverse a fair bit during the last week and because I've been flying a 402 during that time have had plenty of opportunity to try the different techniques. (Except wheeler landings.)
If the big danger is that the prop won't come back out of beta I don't see that as a concern the way most of us use it. Beta is used to select a thrust setting that gives the required approach speed and rate of descent particularly on late finals and to eliminate float following the flare. Most of the time the power lever is just coming progressively back until we're landed and slowed to a normal taxi speed. In a controlled approach, once the power lever comes back through the gate there's no reason to let it forward. Used this way, you wouldn't even know you'd had a failure until you couldn't make power to taxi to the pad.

I will however rethink my practice of deep reverse descents from high altitude ferries in future.;)

24th Oct 2005, 10:29
Well Guppy this has all been very very interesting. NOT.
I suspect you know your PT6 machanics pretty well, but I also suspect that your PT6 ag time is relatively low or at least narrow.
Page after page of verbal dribble. Believe me if all these beautiful Aussie AG pilots were pulling fistfulls of reverse as the mains touch down, there wouldn't be a turbine ag aeroplane left down under. So I suspect that you suspect wrong.
Most of the guys down here would do more ag landings in a year than you guys up there would do in a decade. So maybe you should just tread softly softly and just listen, sssssshhh.
Pretty much everything you have said not to do we have been doing very well for a long long time old son.
Actually I was at a clearing sale the other day and I purchased an old pair of wire strainers pretty cheeply with you in mind. PM me with your mail details and I'll post them over. Your going to need them as soon as you get away from those big long flat strips you must work off.
Happy fencing AgwaGGon.

27th Oct 2005, 08:54
Cheeky bugger, lol !

Chimbu chuckles
28th Oct 2005, 19:55
Hmm...in PNG we were taught 'Beta approachs' during our Twotter endorsement...and then told 'don't play with it'...which of course we all heeded:E They were done in the Porters too..which had identical -20 or -27 PT6s.

Not only were we descending in beta but full blown reverse...as far into the reverse range as we could get it...flaps 20, full reverse and descend it 80kts virtually straight down and all the way to touchdown...was a lot of fun...not sure I would do it nowadays:}

Interestingly the series 200 Twotters with the -20 PT6 and skinny props did a MUCH better beta approach than the more powerfull -27 or -34 powered 300s with the paddle bladed props...never worked that out.

It was never an issue of damaging the engine/gearboxes and control was still wonderfull (I used to delight in doing multiple aileron rolls going straight down) but they stopped teaching them when the CP scared himself rigid when he thought he had an engine that wouldn't come out...the big bang he heard was actually just the tailstand crashing around the rear baggage compartment and both engines reacted normally..when his shaking hands finally applied enough pressure on the thrust levers to move them forward out of reverse....and of course those of us already doing them from time to time for 'currency' stopped doing them immediately:E


1st Nov 2005, 08:10
And people reckon ag pilots are crazy?!

Chimbu chuckles
1st Nov 2005, 13:46
I don't reckon ag pilots are crazy.....:ok:

Oh and standard landing technique at short strips in the Twotter included selecting reverse a foot or two above the ground...it became so ingrained that you would get mildly chipped for it on 6 mthly checks when you did it on 10000' of blacktop:cool:

Not once in many, MANY thousands of landings did I ever get even a hint of assymetry...she slid into reverse, already spooled up because you just cocked your wrist and went straight from 8 odd PSI in forward to something similar in reverse in one fluid motion...the high thrust lines of the Twotter gave you a gentle pitch up and the big low pressure tyres rumbled onto the grass effortlessly.

Just about as horny a feeling as three pointing a Tiger Moth on grass....you could even have the big window open clear down below your left elbow for the open cockpit feeling.

Genuinely better than sex!!!


2nd Nov 2005, 07:11
Hmm.. too many mountain girls i think lol.

3rd Nov 2005, 17:56
Cheeky is probably the right word for it agwagon, but up here we're not nearly so polite.

My PT6 time is fairly broad. It's not the in the air tractor, but it also extends to a lot of aircraft outside Ag as well as in, both single and multi engine. And to having been a Director of Maintenance twice, as well as an inspector and mechanic. As for long broad flat strips...almost all my flight experience has been in VERY mountainous terrain, and seldom on long strips...thanks for commenting, however.

Getting back to the question I asked before...what do the manufacturers say? Not verbal drivel...nothing thus far introduced has been less than fact, so lets stick to that basis, shall we?

We have someone introducing an informal comment by a pratt rep regarding the company....how about the prop manufacturer and the airframe manufacturer...how does Air Tractor feel about coming over the gate in flight?

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