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Beta Follow-up??

Checks

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Joined
Dec 23, 2001
Posts
447
Total Time
7,000
In my SA-227 operators manual, there is a note under Engine Failure-Takeoff Continued that states the following:
NOTE
In the event of an engine failure and an NTS system and/or feather valve malfunction, setting the power lever fully forward will drive the propeller blades toward the feathered position (Beta Follow-up), yielding a lower windmilling propeller drag.

Here is what I do know: Engine oil drive the blades to low pitch and there is a spring that wants to feather the blades. With no oil the prop blades would go to feather. Above the flight idle the power levers control the manual fuel valve so how can pushing the power lever fully forward effect the blade angle?

Thanks in advance!
 

avbug

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Dec 14, 2001
Posts
7,602
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I believe the beta follow-up to which you're referring is the beta tube. During normal operation, the beta tube functions as an oil transfer tube. The feathering valve is really a bypass valve: it intercepts the oil flow from the propeller governor to the propeller assembly. The feathering valve is used both for NTS action by varying the oil to the prop to control negative torque, and is also used to actually feather the engine.

The propeller moves toward feather under the influence of spring pressure, and counterweights.

Engine oil is ported to the prop through a positive displacement type gear pump, located in the prop governor assembly. This pump maintains a positive output pressure approximately 350 psi higher than engine oil lubrication pressure. Oil passes from the governor pump through the pilot valve (the one you control in the cockpit to control propeller speed) to the feather valve, and then to the prop pitch control.

When working in reverse, the beta tube is flooded with high pressure oil from the governor, and a sleve surrounding the tube is controlled by the pilot to determine just how far into beta or reverse the propeller is allowed to go. During this time, the fuel control unit manages the fuel flow automatically based on the underspeed governor's inputs. In some aircraft this is a flat line...one underspeed value, but in others the value varies with the amount of reverse prop in use.

The term beta tube is somewhat of a misnomer, because it's true function is similiar to what a hollow crankshaft in a piston engine does; it's the means of oil delivery and return from the propeller to actuate the propeller in flight. It's called the beta tube because positive pressure oil must be used to drive the propeller internal piston into the beta range...and in reality increasing the blade angle toward feather is done by relieving oil pressure...back out the beta tube.

In the event the feather valve doesn't open to dump oil pressure from the propeller dome, pushing the power lever all the way up after an engine failure has the effect of opening the pilot valve in the prop governor...you're telling the engine you want more power and consequently a greater prop pitch, and the engine can't deliver because it's failed. What you're doing is causing the pilot valve to open, and dumping remaining pressure in the propeller dome that might have been otherwise trapped.

I can tell you that having experienced a total oil loss in a TPE-331, and also having been counselled to go to full power lever forward to feather the propeller...it doesn't necessarily work. In my case, a failed turbine bearing seal caused the loss of all engine oil in very short order. The engine ran perfectly, and according to Garrett/Honeywell, will do so for a half hour with no oil. The problem is that one loses control over the propeller.

One might think that it simply drives itself to feather, but this isn't necessarily the case, and it didn't happen with me. I pushed the power lever forward to find that I had normal engine response, just no torque. Engine exhaust temperatures were normal, power lever position was normal...everything appeared normal except for the torque...no oil left to move the propeller.

Running out of ideas to salvage the situation, and being at only 150' AGL and inside the bottom of a box canyon when the problem started (in a single engine airplane), I elected to push the power levers forward. I'd had the same counsel when undergoing training in type, and I'd run out of other things to do. The engine temperatures went through the roof. I wasn't willing to fuel chop it right away until I had some evidence that I was driving the prop one way or the other(and given the limited time and resources, felt I stood the best opportunity with the engine running and some potential oil available)...and thereafter I was setting up for a short glide and a forced landing on a hillside.

Long story short, while the engine may be featherable by pushing up the power lever in the event the engine doesn't feather by itself or by normal procedures...don't count on it. It didn't work for me. I've only had the one occasion to try it like that on the TPE-331, and given the circumstancs at the time, I'd rather not repeat the experiment...but like I said, don't count on it feathering. I did enjoy reduced drag, but never did come close to feathering. Once on the ground and shut down, the prop did slide into feather.
 
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