HFS
Well Known Member
A have a three blade MT propeller (MTV-9-B/198-52) on my Rocket. It has been installed since 2002. It has had several "visits" to the local service facility, mostly for grease leaks from the hub, but also to comply with MT's "recommended" TBO of 72 calendar months.
It's OH in 2011 was completed with nothing more than new seals - at that time all parts of the prop passed specification as set down by MT.
Fast forward to August of last year (Condition Inspection). The 72 month calendar interval between OH's indicated another overhaul was due. Sent it off to the shop again - not expecting anything other than and inspection with reseal.
NOT!
The blades and aluminum ferrules are held together by 11 "Lag Bolts" - their description not mine. So on a three blade prop there are 33 lag bolts securing the ferrules to the blades. The first specification to be met as the inspection moves forward is to check the lag bolts for proper torque. If the fastener is advanced before the torque spec is met; then, that fastener is deemed to have "failed". There is probably some remedial process for the "recovery" of just a few failed bolts - but on my prop, 32 out of 33 of them FAILED!!
The interface between the blade shank (pilot) and the ferrule (register) is engineered to be an interference fit, and the intended purpose of the lag bolts is to secure blade and ferrule to each other to insure no relative movement between them.
Upon removing the lag bolts from the ferrules, they freely fell off the blade pilot - NOT GOOD! It meant there had been relative microscopic movement between
them indicating a "faying" process had begun, causing loading on the fasteners not designed for. Left in service in this condition over time, I would have probably experienced some sort of more extreme service failure (their words, not mine).
What had happened was that the blade pilot (wood) had shrunk to a smaller diameter, hence becoming "loose" in the register bore of the (aluminum) ferrule.
Why did this happen? The best guess of the OH shop is that I live in a very dry, hot (+110* in summer) climate, don't leave the airplane out much. It lives its life on day trips and comes home to a corrugated tin hanger at night, doesn't have much opportunity to recover any moisture lost to evaporation while in it's hanger.
How to fix the problem? When you bore an engine cylinder to an oversize condition, you have to install oversize pistons to make the engine whole again - Same thing here, I needed new, custom made ferrules (from Germany) to put Humpty Dumpty back together. OBTW - did I mention that the custom ferrules cost $1500 a piece - do the math! And, to add insult to injury, MT made the first set to the wrong specs - when the OH shop tried to install them, they fell off the blade pilot just like the originals did! Another 3 weeks shot to **** waiting for the second set to show up.
Got the prop back (after three months), installed it, and yep, still "weeps" a bit of grease out of one blade. MT told me this is normal after an overhaul, and to keep an eye on it - should stop after a while. The jury is still out on the leak stopping, but the prop is very smooth and I feel assured that I should be good to go for another 6 years.
If anyone wants more info they are welcome to call me: (559) 8216-0433
That's my story and I'm sticking' to it!
David Howe
HR II S/N 002
It's OH in 2011 was completed with nothing more than new seals - at that time all parts of the prop passed specification as set down by MT.
Fast forward to August of last year (Condition Inspection). The 72 month calendar interval between OH's indicated another overhaul was due. Sent it off to the shop again - not expecting anything other than and inspection with reseal.
NOT!
The blades and aluminum ferrules are held together by 11 "Lag Bolts" - their description not mine. So on a three blade prop there are 33 lag bolts securing the ferrules to the blades. The first specification to be met as the inspection moves forward is to check the lag bolts for proper torque. If the fastener is advanced before the torque spec is met; then, that fastener is deemed to have "failed". There is probably some remedial process for the "recovery" of just a few failed bolts - but on my prop, 32 out of 33 of them FAILED!!
The interface between the blade shank (pilot) and the ferrule (register) is engineered to be an interference fit, and the intended purpose of the lag bolts is to secure blade and ferrule to each other to insure no relative movement between them.
Upon removing the lag bolts from the ferrules, they freely fell off the blade pilot - NOT GOOD! It meant there had been relative microscopic movement between
them indicating a "faying" process had begun, causing loading on the fasteners not designed for. Left in service in this condition over time, I would have probably experienced some sort of more extreme service failure (their words, not mine).
What had happened was that the blade pilot (wood) had shrunk to a smaller diameter, hence becoming "loose" in the register bore of the (aluminum) ferrule.
Why did this happen? The best guess of the OH shop is that I live in a very dry, hot (+110* in summer) climate, don't leave the airplane out much. It lives its life on day trips and comes home to a corrugated tin hanger at night, doesn't have much opportunity to recover any moisture lost to evaporation while in it's hanger.
How to fix the problem? When you bore an engine cylinder to an oversize condition, you have to install oversize pistons to make the engine whole again - Same thing here, I needed new, custom made ferrules (from Germany) to put Humpty Dumpty back together. OBTW - did I mention that the custom ferrules cost $1500 a piece - do the math! And, to add insult to injury, MT made the first set to the wrong specs - when the OH shop tried to install them, they fell off the blade pilot just like the originals did! Another 3 weeks shot to **** waiting for the second set to show up.
Got the prop back (after three months), installed it, and yep, still "weeps" a bit of grease out of one blade. MT told me this is normal after an overhaul, and to keep an eye on it - should stop after a while. The jury is still out on the leak stopping, but the prop is very smooth and I feel assured that I should be good to go for another 6 years.
If anyone wants more info they are welcome to call me: (559) 8216-0433
That's my story and I'm sticking' to it!
David Howe
HR II S/N 002
