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Author Topic: Beta Followup  (Read 908 times)

donv

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Beta Followup
« on: July 13, 2019, 06:36:26 pm »
I was involved in a discussion about engine failure after takeoff, mostly relevant to King Airs. However, this led me to do some searching in the NTSB database, as I was curious if my intuition that way more PT6-powered turboprops seem to have engine failures on takeoff versus 331s (intuition was correct, although not adjusted for more hours flown in the PT6 fleet).

I did, however, come across one interesting MU-2 accident that those of us flying Commanders should keep in mind.

A refresher: when we all learned to fly multiengine airplanes, we probably learned "identify, verify, feather" in the event of an engine failure just after takeoff. Identify, often using "dead foot, dead engine"; verify-- by pulling the suspected dead engine's throttle back, and then feather.

Once we transitioned into Turbo Commanders, we should have been taught that "verify" does not mean pulling the affected engine's power lever back-- instead, we should verify using engine instruments, leave the power lever where it is, and go ahead and feather.

The explanation for this, at least where I trained, was "beta followup." Specifically, when a 331 fails, the NTS system will automatically drive the prop almost to feather with no pilot intervention. If you pull the power lever back to idle, the underspend governor is going to sense low rpm and try to send oil to the prop hub to drive the prop out of feather, against the NTS system. At least, I hope I explained that correctly.

Well, here is an MU-2 accident, where something very similar happened (and they crashed):

https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20060630X00859&AKey=1&RType=HTML&IType=FA

Quote from: NTSB

Based on an analysis of evidence from the wreckage and technical data from the airframe and engine manufacturers, a likely scenario for the accident sequence is as follows:  Shortly after takeoff, and after being instructed to change frequencies, the pilot may have perceived a loss of power in the right engine and an associated rise in rpm.  The right propeller then went into a feathered position about 3 seconds later.  The pilot then reduced the right engine power lever, contrary to the AFM procedure.  At this point, the fuel flow decreased, leading to a decrease in power section rpm.  The propeller governor then sensed an under-speed condition. As a result, oil was routed to the propeller by the propeller governor, causing the propeller to come out of feather toward a flat pitch (increased drag) position. The pilot may not have been aware that the propeller came out of feather. As a result of the increased drag condition on the right side of the airplane, the airplane yawed and rolled to the right and entered a spin. In an attempt to control the airplane, the pilot reduced power on the opposite (left) engine.  However, at this point, the airplane was not at a sufficient altitude to recover.

The investigation revealed that a TPE331 engine gearbox uncoupling event is an unusual engine failure that results in substantially different engine indications to a pilot in comparison to a typical flameout event in which the NTS system in operable.

According to the engine manufacturer, there have been five incidents of similar TPE331 ring gear support cracks during about 29 million engine hours of service history. All of the cracks originated at the high-speed pinion cutout detail. Three of these incidents were shop findings, one incident resulted in an in-flight shutdown at altitude followed by a safe landing, and the other incident was this accident.


Something we should all think about, on each takeoff!

Steve binnette

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Re: Beta Followup
« Reply #1 on: July 15, 2019, 07:13:11 pm »
Thanks Don, it would be hard to not touch the power lever if that was happening.

donv

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Re: Beta Followup
« Reply #2 on: July 15, 2019, 07:24:36 pm »
This is what FlightSafety said about Beta Followup:

Quote from: FlightSafety

In the event of an engine failure during flight, the negative torque sensing (NTS) system automatically moves the propellor blades toward feather, resulting in minimal drag. If the NTS system also fails, the propellor of a failed engine will remain in flat pitch, and the propeller governor tends to flatten the blade angle further. The cam mechanism in the PPC sets a minimum blade angle, dependent on power lever position. This is called Beta followup. If the power lever is retarded, the minimum blade angle setting will be reduced, increasing the aerodynamic drag.

WARNING

The power lever of a failed engine must not be retarded before the propellor is manually feathered.


In the case of the MU-2, the NTS system worked fine but the propellor became detached from the engine due to gearbox failure. When the pilot pulled the power lever back, as mentioned above, the prop governor moved it out of NTS. Bad day...

Steve binnette

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Re: Beta Followup
« Reply #3 on: July 16, 2019, 01:03:23 am »
Just shut it down is the right move. 

This where rushing is a mistake.

I try to remember what they taught the WW2 pilots. First step in a emergency is wind the clock!

Adam Frisch

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Re: Beta Followup
« Reply #4 on: July 16, 2019, 09:27:42 am »
Good advice to keep in mind. I'll try to remember: Just use the condition levers and don't touch anything else.

What was the difference in statistics in shutdown between PT6 and TPE?
Slumming it in the turboprop world - so you don't have to.

donv

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Re: Beta Followup
« Reply #5 on: July 16, 2019, 11:13:52 am »
I didn't find that data. I was specifically looking for accidents involving an engine failure on takeoff, and I believe I could only find two 331 accidents in the last 20 or so years-- both MU-2s.

One was the accident I mentioned above, which is worth all of us thinking about.

The other one was actually right here at HIO (a friend saw it go in). However, it involved an extremely sketchy pilot/owner, a bunch of engine work done by sketchy mechanics (NOT Aero Air!), and supposedly issues with the NTS system on the previous flight. Sketchy operator + sketchy pilot + sketchy maintenance = accident-- not that surprising. And by "sketchy pilot" I'm really not kidding... this guy was a piece of work.

donv

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Re: Beta Followup
« Reply #6 on: July 16, 2019, 11:14:50 am »
It's also worth noting that 331s are used in a lot of applications which aren't visible to us as general aviation or even airline pilots. UAVs (Predator), ag aircraft, SEATs, etc.

av8r

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Re: Beta Followup
« Reply #7 on: October 20, 2019, 01:03:23 pm »
The primary engine component in the scenario under discussion is the PPC (prop pitch control) not the underspeed governor. The underspeed governor is a fuel control component even though it has direct connection to both the condition lever and the prop governor.

The PPC is linked directly to the PL (power lever) and the prop. The PPC connection to the prop is through the beta tube which controls either the increase or decrease of the the oil pressure in the prop hub which affects all prop pitch changes. When you reposition the power lever the PPC cam assembly repositions a unit called the PPC sleeve.  The beta tube, housed in the PPC sleeve, is then repositioned forward or aft. As the PL is moved aft the PPC Sleeve is lengthened, increasing prop hub oil pressure and moving the Beta Tube Forward. The affect of these actions decreases the prop blade angle toward flat pitch (finer). As the PL is moved forward the PPC Sleeve is shortened allowing some prop hub oil pressure to return to the case and the feather spring increases the blade angle toward feather causing the Beta Tube to move aft. The affect of these actions increase the prop blade angle toward feather (coarser pitch).

BETA FOLLOW UP:

ē A back-up system that acts as a hydraulic stop to limit minimum blade angle and can be
utilized in the event of a Prop Governor or NTS System Failure.
ē Minimum blade angle, as set by Beta Followup, is decreased if the PL is retarded. If the PL
on a failed engine with an inoperative NTS System is retarded, windmilling drag will be
increased significantly.
ē During inflight engine shutdown, DO NOT retard the failed engine PL prior to feathering the propeller.





Adam Frisch

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Re: Beta Followup
« Reply #8 on: October 25, 2019, 12:06:44 pm »
I gotta be honest, the beta system on airplanes of this era is not so easy to get your head around in the beginning from an intellectual standpoint. All of a sudden your prop levers are no longer prop levers, but your PL's are...? I understand why it was done that way, but I think if you'd design it today, you'd put reverse on the prop levers and then have them control the power, not the other way around.
Slumming it in the turboprop world - so you don't have to.

Bruce Byerly

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Re: Beta Followup
« Reply #9 on: October 25, 2019, 12:16:32 pm »
I gotta be honest, the beta system on airplanes of this era is not so easy to get your head around in the beginning from an intellectual standpoint. All of a sudden your prop levers are no longer prop levers, but your PL's are...? I understand why it was done that way, but I think if you'd design it today, you'd put reverse on the prop levers and then have them control the power, not the other way around.

Itís definitely a miracle of slide rule engineering, no doubt, but at the end of the day, your power levers are always your thrust levers and so itís extremely simple for an average pilot like myself to fly. And if you want to avoid screwing up the whole beta follow up scenario, all you have to remember is to push the levers forward (if anything) and never retard them. Thatís what we have always trained to do and it doesnít require understanding the path the oil takes to safely operate imho.