Uncommanded Reverse (PT6)

[avbug]

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]

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]

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]

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]

....

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

Scott