Uncommanded Reverse (PT6)

avbug

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The following short statement was recently posted on a different web site, seeking others who have experienced an uncommanded reverse. I have never experienced this on this engine (I have on a different engine, but due to something entirely different).

Anyone who may have experineced this should consider giving a call to the number at the end of the article. The author is genuinely interested in the problem, and could use all the input available. It's serious. These problems have been recorded in ag airplanes, but the potential exists for anyone flying certain PT6 combos.

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 turbine@newmexico.com. Any input will be greatly appreciated.
 

TurboS7

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That's why we(the consumer) pay those big bucks so you guys can take care of those soybeans. Better you than me.,,,,,
 

avbug

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I think the interest should extend beyond ag work. The individuals involved thus far have been ag pilots, and the results of an uncommanded reverse are more immediately apparent in that arena. However, this issue affects more than just PT6A's on ag aircraft.

Anybody experience an uncommanded reverse on a PT6?
 

chperplt

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I posted a thread a month or so ago about an uncommanded beta in flight.. It happened about 50 feet or so when the power levers were being brought to flight idle for landing.. The left engine went into and out of beta 3 or 4 times.
 

TurboS7

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I know that was a serious thread and I am sorry but your only chance in a twin would be to shut the engine down-NOW. Would prop windmill in reverse causing drag????? If the prop wouldn't feather that would be nasty. I had a friend that died years ago in a BE-18 when the HamStan acutally went into reverse, he went in in a hurry.
 

BluesClues

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I used to fly Twin Otters and remember hearing a story of an uncommanded reverse occuring in a Twin Otter up in Alaska that flipped it over and into the ground on approach. I don't have any details about the accident. It was one of those stories that circulated amongst the line pilots so I can't vouch for the accuracy. I can vouch for the reliability of the PT6-27A though. 1500 hrs flying the Otter and never had a major problem. I hated that autofeather system though, it never seemed to function properly during the preflight checks. (Glad I never had to rely on it)
 

avbug

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Yes, a prop winmilling in a reverse condition will still cause a great deal of drag. There is some possibility of bringing it out by moving the power lever forward and propeller levers forward, as several pilots have done. However, as noted with the AT-802 pilot, 5,000 of altitude was lost before he could recover. Shutting down the troublesome engine will likely be the only recourse, especially if close to the ground. Running up power will only aggravate the situation and could easily be fatal close to the ground.

The hydromatic ham standard prop on the BE-18 doesn't normally reverse, and must have experienced an internal failure to be driven to a condition providing reverse capability. There is a large metal stop which must have sheared internally to allow something like that to happen.

I had it happen several times in a P-2 when the microswitches failed; these are all that prevent the prop from being driven into high pitch and reverse. There are three switches, and all must fail...and frequently do. I've had it happen on the ground several times, too.

The issue on the PT6 sounds very preventable, however, with more frequent inspection and cleaning on the Beta valve. More disconcerting to me was the indication that the AD fix from P&W may have caused some of these failures, do to leakage at the o-ring seal on the beta valve itself. One of those cases where the cure was worse than the origional problem.
 

PHX767

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Yep, had a B1900D do it to me on a couple of occaisons when the power was brought back to idle during the pre flare phase. Only one engine, of course. Big yaw to the affected engine and real heavy sink rate increase.

Wrote it up and the outstanding maint department of course found no defect. I firmly believe the low pitch stop was mis adjusted and allowed the blades to move toward beta in an underspeed condition, rather than letting the RPM decay. Airplane later ended up ground looping into a snow bank on an aborted takeoff.

Had another B1900 that would surge really badly on one of the engines during stall practice when I was a training Captain. Finally figured out it was stalling and relighting. Not cool. Thank you, auto ignition.

Great presence of mind to firewall everything on the incidents quoted. This would put the governor in a positive pitch condition and bring the blades around out of Beta.
 

avbug

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Firewalling everything would work, so long as the beta valve wasn't completely stuck or heavily bypassing.

I'd encourage those who have experienced this to call the number in the first post. The gentleman at that number is not an attorney, and isn't trying to gather evidence for a law suit. He's a mechanic/IA who is pursuing the issue in sincerity, and can use all the information he can get as he seeks a fix and a good preventative measure.
 

TurboS7

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On the BE-18 the NTSB said it was a combination bad feather motor, gov, and internal prop problem. I guess my friend didn't have a chance.
 

avbug

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That's about right; it couldn't really get any worse. The only option at that stage would be to shut it down and drive it to feather with the feather pump. If the feather pump had failed, blown a line, or was bypassing internally or elsewhere, then there would be no options left. The feather pump bypasses the governor functions, but it's apparent that an internal gearing and ring failure probably caused the loss of the prop. Depending on what failed, even a functioning feather pump would not necessarily provide any relief. It must have been quite a failure. I've seen ham standard props have internal breakages and failures, but none of that magnitude. Very unfortuanate for your friend, and little consolation to the family.
 

Groucho

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PT6's Beta an other stuff

As a pilot and a mechanic I can tell you, and this is known by most experienced turboprop aviators, that the engine is fairly simple the propeller system is a b*^ch.

Jet engines are simple spin em, spark em and spray em and dey run all day long.

Ask any Convair or Allison mechanic or pilot will tell you, " The toughest oral I ever had was on the 501-D13 prop system." Thats the old convair 580, C-130 or Electra prop system. It has about 12 different low pitch and beta settings, NTS, NTS beta back up beta, several high pitch stops, feather stops etc. etc. etc.

The prop system of a modern turboprop airplane is far more complicated than the engine. It takes the brunt of the abuse and delevers the thrust from the engine to the air. When it screws up there is hell to pay. There is a new book out about the Comair Brasilia that had a prop runaway. The aircraft was unflyable and uncontrolable, not because of the engine, because of the prop. I'll take a simple flame out to a prop problem any day of the week.

To throw my .02$ to this question, Ag operators are known for trying to scrimp and save just like the old "MA and Pop" commuters of 20 years ago. They grew up on ole Prat and Whitney round engines that ran on gas lots of oil and guts. The old Hamilton Standard counter weight props were simple and didn't require lots of maintenance. Now the PT-6 is the standard and the props and engine prop rigging are much more critical. Are these operators up to it? How many have been to factory school?

This engine has been around too long to just start going into beta unannounced all of a sudden.

What is the operator background?
 

avbug

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Well, as an ag pilot and mechanic, I'll have to take exception to the notion that ag pilots in general are haphazard or scrimping on anything. I certainly never have, and have never worked with another ag pilot, or known another ag pilot who has. I certainly don't know everyone, and there may be unscrupulous or lacksidaisical operators out there. However, I certainly don't know anyone who would fit that description.

The aircraft involved were from different operators. Several other posters have responded so far with similiar comments, having experienced similiar issues.

New information crops up all the time, we all know better than to fall into the trap of saying that because something has been around for a while, we can't learn something new about it. That's why we still get AD's on old equipment. I've run some fairly old equipment, and have been invovled in the discovery of information that either no one had yet discovered, or had put in writing for everyone else to find out. (One such tidbit involved a run of A model hercs that had been put to pasture. We nearly lost a wing. We came to find out that the problem was a production flaw that the USAF knew about, thought had fixed, and had never issued any information following the fix, as it wasn't a fleet wide issue. It involved 7 serial numbers, and as luck had it, we got three of them that hadn't been updated.

I worked on the Hamilton Standard 54H60 props also, and while a bear at times, they gave us relatively little trouble, for the most part. We ran them on the -9 engine, and the engine gave us more trouble than the props. Most prop problems we had were maintenance related; that is, faulty servicing.

I'm not familiar with the details of the Brasillia mentioned, but I am with a C-119 that we lost a crew in. While the propeller was the issue; it ran away, it needn't have been fatal. The crew attempted to pull the power back, and the prop produced a lot of drag, the airplane descended. The airplane slowed, the power was reapplied, the prop oversped, the airplane climbed, and the cycle repeated itself over the course of a half hour until barely under control, the aircraft crashed.

A basic principle of a windmilling propeller, or any propeller which is driving the engine instead of the other way around, is that RPM is a function of airspeed. In the case of the C-119 crew, had they simply slowed the airplane down, they could have carried power on that engine without fear of it running away, and let the engine drive the prop, instead of the other way around. Not only would te drag have been relieved, but they would have had useable thrust, as well. In short, it was purely pilot error. They may have had a prop failure to a runaway situation and an overspeed, but what to do about it was up to them.

This isn't a fix on a constant speed engine such as the T-56, but it is on engines such as the PT6, and recips. In such cases, the failure is usually pilot knowledge of the system and the operation of the system. In the case of the C-119, that's exactly what it was.

As for radials running on guts, oil, and gas, it's not true. One can run any engine into the ground, but I've always found radial engines to be maintenance intensive, and I've always taken great care to be particular with them, both in maintenance, and in operation. Any operator that spends his time close to the surface, especially with just one engine out there between him and the powerlines and crop, doesn't simply pour in oil and go. For the most part, ag engines and airplanes generally get a greater degree of maintenance on a more regular basis, than most any other segment of the industry. It's critical, and necessary. The only reason that ag airplanes are the issue in the origional post is because the mech who is performing the investigation happens to work on ag airplanes, and got the info first hand. Reading posts here, and on a few other boards, I'm hearing from others who have had the same thing happen to them.
 

Fr8DoggyStyle

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part numbers

Anyone have any info on whether or not the part numbers on the -34ag and -67ag beta valve parts are the same. Also, are they the same as on other pt6's? if so which?

I wonder if the high cycle/ hour ratio has something to do with why this problem seems to be congregating around Ag planes. The prop system in an ag plane has a hard life....... then again ANYTHING in an ag plane leads a hard life (including the guy driving it). We probably would just have to wait to see it in other applications. Any thoughts on the chemicals somehow contaminating the beta valve and causing corrosion?

my experience with ag operators is that they are much more carefull than than those of us who only carry people or boxes. This is because they wouldnt last long if they weren't
 

sstearns2

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....

Didn't the Turbo Raven crash because of an inadvertant beta?

Scott
 
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