Propeller AD (June 1996) and Tach marking

[GregJohnson]

My 1976 Sundowner C23 has a Sensenich propeller (76EM8S5) that was overhauled in 2002 in accordance with current FAA Regs and Sensenich Manual SPRM590. Also at the same time the engine, O-360-A4K, was overhauled by Lycoming. In 2005 a new Mitchell tach was installed due to failure of the old one, and a note in the log indicates "Complied with AD 69-09-03 R3, Continuous Operation RPM, by marking new tach as stated in AD text". Therefore my tach is marked redline from 2150-2350 RPM (as well as 2700 and up. I've read the AD from the FAA website and unless I'm missing something it says that the AD paragraphs (a) and (b), which relate to the required marking, do not apply to propellers installed on solid crankshaft Lycoming O-360 series .... , -A4K, ...
Other requirements of the AD says the propeller is to be inspected and reworked. I would think that the overhaul would cover that part.
So I'm wondering if the A&P should not have redline marked my tach in the 2150-2350 range.
Comments?

Greg Johnson
N2241L

 

[wyofisher]

propeller ad (june 1996) and tach marking

From the text of the AD, I'd say that you are right, your engine model makes you exempt. As a mechanic myself, I am guilty of "speed reading" through the applicability sections due to lack of time, and lots of dry FAA english. You should ask the mechanic to correct this. Its probably a misunderstanding since the FAA usually lists what engine models do apply, not which engine models are exempt.

These type of AD's stem from harmonic vibrations set up on certain engine and propeller combinations. These vibrations can work harden the blades and snap off portions of the prop, or in other cases, crack and break the crankshaft. I've been told in my A&P school days that a spinning propeller at high rpms can be subject to upwards of 100,000 pounds of centrifugal force. Now consider this, if you should lose 2 or 3 inches of one blade, you could have over a ton of force trying to rip your engine from the mounts. Should the engine actually leave the plane before the pilot yanks the power back, the plane would be out of balance and you would not have any control of the aircraft. Happy landing! So, due to that, and the liability involved, the A&P is always going to error on the cautious side.

 

[wdnd5960]

This is the reason some aerobatic and airshow pilots use a short piece of chain running from the engine hoist ring (or a fabricated flange attaching to several engine bolts) to an engine mount / airframe bolt. After you loose a prop blade, the engine will rip itself from the mount before anyone could possibly react. The chain (hopefully) retains the engine as a nose anchor allowing a controlled glide (which the pilot could parachute from) rather than the fast approching flat spin (which is difficult to escape). I learned about this from Bob Cole, Cole Aviation, who was the mechanic for the US Aeorbatic in the 1970s & early 1980s. I spent many hours in Bobs hangar asking stupid questions during those years. Like, "What is this chain for?"

Bill