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MT Propeller SB NO.30 R7

Bruce

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MT PROPELLER SERVICE BULLETIN NO. 30 R7
Submitted by Lorrie Penner on Fri, 2022-07-01 08:47

An AD will soon be released based on MT Propeller Service Bulletin No.30 R7, which was first issued in November 2014. The subject of the bulletin is “Replacement of Blade Lag Screws of a certain production lot.”

With a recent incident on June 19, 2022, in which a Great Lakes 2T-1A-2 experienced a blade loss, Gerd Muehlbauer, President of MT Propeller Entwicklung GmbH, is requesting an AD from the EASA who will then forward it to the FAA.

“At the time of the original bulletin in 2014, we assumed that an Alert Service Bulletin was enough to inform our customers and Service Centers about the need of replacement of lag screws in certain blade of different propellers according to the listing in the SB”, said Gerd. “After seven years we are now finding that a great number of affected blades have not been replaced or modified. Until the AD comes out, we would like to bring this alert to our flying community as soon as possible.”

The reason for a possible fatigue failure of the Lag Screw is a manufacturing flaw in the transition area from the thread to the shaft, which was only detected after a routine test of a particular batch, which we perform frequently. Again, this was more than 7 years ago.

The propellers in such airplanes could be the 2-blade MTV-15-B-C, the 3-blade MTV-9-B-C and the 4-blade MTV-14-B-C. Most of these propellers may start with Serial-Numbers 13XXXX and 14XXXX and a few from earlier years according to the SB. 15XXXX and higher or 12XXXX and lower are not affected, but see the SB.

The blades should immediately be inspected according to Owners Manual No. E-124, Section 6, Item 6.2.2.4(link is external) - if the silicone sealer is cracked, stop flying right now because this is the indication that the blade is already moving in the ferrule.

The SB no. 30 R7 covers action that aircraft owners and their mechanics must take to replace the lag screws in question.

Further information can be obtained by contacting MT Propeller support at: techsupport [at] mt-propeller.com .

https://www.mt-propeller.com/pdf/sbs/sb30r7.pdf
 
Some Background Info on This

This is an issue for us since a fair number of RVs are flying with MT propellers.
I know the instructor in the front seat of the Great Lakes that had the blade separation. He and the owner of the airplane prefer to remain anonymous, but did agree that I could relate some info about the incident.
As part of the normal preflight, the pilot shook both propeller blades and did not notice any movement or play and the prop seemed normal. Shortly after takeoff, at about 650' AGL, the engine and airplane shook violently. There was no vibration or any other indication there was a problem until the whole blade departed. Instructor knew right away what had happened and immediately pulled the throttle to idle. The engine stopped after about 2 seconds, he says, apparently by itself. Instructor took the controls and gently maneuvered the airplane to the runway. He was very gentle since he knew that a propeller blade had separated and was afraid the engine was just lying in the cowl. No injuries and the landing went fine. The blade is still AWOL, the G meter pegged at -4 G.
Enter Budd Davisson, who's says he spoke to the unnamed IA who removed the propeller. (yeah, it's third hand now. jd) IA is writing up a report and it should be published "on the Forum. (but he didn't say what Forum. jd) IA says the lag bolts "sheared", (I think he meant they failed in tension as there's no shear force to speak of on a propeller blade lag bolt. jd) They did not pull out of the wood. MT says there's a "surface finish" problem. Two of the approximately 12 bolts were still in place and bent. (so did those two pull out? jd) Threads appear to be cut, not rolled and they extend all the way to the heads and that's where they failed.
This IA is very experienced on Pitts airplanes. He goes on to say that the engine mounts on the Great Lakes are different from a Pitts and had this been a Pitts, the engine would have come off the airplane. The engine compartment was a disaster area. The alternator was both loose and broken, all the slip joints on the exhaust system were pulled apart, and the airbox was torn off. He could feel movement in the aluminum ribs in the tail surfaces, normally riveted to the steel framework. WACO is having the airplane disassembled and shipped to them for inspection and rework.

Now, I'm back, and here's my two cents: I've seen and heard about propeller blade failures and they're always traumatic events. I've seen and heard of several failures and so far and one of those was fatal. My wife had an adventure with an old wood prop on a Porterfield, of all things. Some stainless tipping went away from one blade and it shook like a wet hound dog. A friend of a friend was killed in an accident along about 1989 or so resulting from an external counterweight coming off a blade root on an early MT aerobatic propeller. It was kind of a dumb design, I think. We were already seeing leading edge protection screen failures from big bubbles in the epoxy laminates. The fatal accident moved my friend to remove the MT constant speed prop from his akro airplane, the sister of the accident airplane, and install a fixed-pitch metal Sensenich, which cost him some performance.
The MT Service Bulletin says to inspect the silicon seal at the blade roots. This does not seem like much of an inspection procedure but I'm not sure what all else a pilot could do on a preflight inspection. Actually, on some airplanes you'd need to remove the spinner to even see the blade roots. I would think that there needs to be a fairly short interval between times to disassemble the propeller and have a peek at those lag bolts or replace them... along with the blades? Draconian maybe, but the problems that ensue when they fail are not minor. It's going to be interesting to see what FAA does with this.
Meanwhile, if you're flying with an MT propeller, please take a look at the blade roots. Any indication of movement between the blades and the hub (cracked finish of any type, or gaps) or any looseness of the blades in the hub should make you believe that propeller and hence, that airplane is not airworthy and should not be flown, even for a ferry flight.
 
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My MT Propeller Saga - FWIW

I have a 3-blade CS MT installed on my HRII since 2002 (S/N 02477).

Followed factory recommended OH intervals of 72 calendar months without any untoward results until 2017, when 32 of 33 of the "lag" type socket screws holding the three ferrules to the blade shank pilot failed the factory spec torque test. That's a 97% failure rate - Ut Oh!

The initial operation for the overhaul of these props is to check the torque of these screws against factory specifications. If specs are met (by checking with a torque wrench, set at factory torque) - go on to the next op. With the level of failure of mine, it meant that disassembly of the ferrules from the blades was mandatory. When the screws were removed, the ferrules fell off the blade pilots - no more interference fits! These blades were in the beginning stages of failure through cyclical fatigue movement. Now comes the fun part - what to do?

MT recommended "custom" ferrules be made to re-establish the interference specs for each blade. New ferrules were made in Germany (3 month lead time) and shipped to the OH shop (Tiffen Aire in Ohio - good people). When installed, these custom ferrules fell off the blades too - oops! They were made to the wrong specs - which the factory admitted to. Back to square one, another 3 month wait for "Gen 2", which fit as required. But still - 6 months for an overhaul! Oh, and did I mention that each ferrule was $1500 & the total build for the OH was $7300+!

I am halfway through the next 6 year cycle, but I'll sure send it out for OH when the time comes.

As a "truth in advertising" note - I live in a very hot, dry climate (San Joaquin Valley - CA), the airplane lives in a corrugated tin hangar, and has almost no RON's. I believe these conditions, when taken together caused some residual moisture in the prop to be lost, thereby causing the shrinkage of the pilot. Just my .02.

YMMY - and I hope it does in a good way.

HFS
HRII S/N 002
 
‘Nother Comment

I was just informed that the Great Lakes that tossed the propeller blade had about 160 hours since new and never did any aerobatics. Woo.
 
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