Ag Flying - PT6 Operation
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I understand it's an ag thread, and I'm approaching it as an ag pilot.
Why do I think coming over the gate into a ground or reversing range in flight is a bad idea? Are you serious?
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.
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.
Could you please explain why you think so?
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.
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.
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.
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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...
"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.
Now back to my question of "Please Explain?" for your statement...
I believe someone mentioned using beta to control descents. Not a good idea.
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.
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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.
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.
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SNS
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.
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.
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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.
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.
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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.
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.
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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
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
Thread Starter
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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.
doesn't keeping it slightly spooled negate the original reason for operating in Ground Idle?
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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
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
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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.
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.
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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.
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.
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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.
If there was a high risk of this occurring I’m sure certification with a Beta/reverse range would not have occurred.
You land with slight Beta in flight idle,
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?
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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.
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.
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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.
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.
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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.
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.
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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.
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.
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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.
IS IT REALLY IMPOSSIBLE ?
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]. Any input will be greatly appreciated.
IS IT REALLY IMPOSSIBLE ?
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]. Any input will be greatly appreciated.
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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.
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.?.?.
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.
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.?.?.
Last edited by Rarely Dble Amber; 18th Oct 2005 at 11:21.