More than the prop and crankshaft
The other thing to consider is all of the accessories on the engine or parts of the engine that are spinning at high speeds that could come to a sudden stop. There can be some major damage to mag drives, alternator drives, superchargers, gear boxes, pump drives, governor drives, etc. as a result of sudden stoppage.
DeJoris explains, "Take for example the prop governor drive — the rotating mass in the prop governor, mainly the flyweight assembly can really be problem if suddenly stopped. There's a tremendous amount of energy stored in the rotating flyweight, and when all of a sudden you stop the prop and engine, that energy has to be dissipated somewhere."
"The result is that those flyweights that are hanging on by 1/8-inch pins can easily bend, twist, or damage those pins. We see flyweights and connecting hardware, in some designs, brutalized quite frequently by sudden stoppage. The pins are more than adequate for running loads, but they are totally inadequate when you subject them to sudden stoppage. Not every prop strike results in sudden stoppage, but it doesn't take much to damage some of these accessories. Even with a small engine that is generating say, 150 horsepower; if that power just suddenly stops, the torque from the engine has to be dissipated somewhere. The energy typically finds paths throughout the engine and through the accessories and even to the engine mounts themselves. On severe prop strikes, the mounts can be easily twisted and loaded in a way that it was not designed to withstand," he says.
"You naturally think of the crankshaft and things that are directly connected to the engine, but you don't think of other accessories. The oil pump on the Continental IO-360 is driven by a very small shaft, for instance, and can be easily sheared as a result of a strike," says DeJoris.
Some elbow room?
Mahlon Russell, production manager for Mattituck Aviation Corporation, an engine overhauler in Mattituck, NY, says, "The only pertinent FAA definition that I have been able to find for Ôsudden stoppage' is in Advisory Circular 43.13-1A. It defines a sudden engine stoppage as; stopping an engine in one revolution or less for any reason, be it from propeller impact or from an engine failure of some sort. Both major engine manufacturer's have service literature that explains the desired course of action after accidental propeller damage and, in the case of Teledyne Continental, defines what their interpretation of a propeller strike is."
He continues, "T.C.M.'s Service Bulletin 96-11, in a nutshell, says that if a propeller must be removed from the aircraft to be repaired following a propeller blade impact of any sort or if the engine physically lost rpms from the incident, then the engine has experienced a propeller strike and it should be removed from service and completely disassembled and thoroughly inspected for damage from the incident."
But it's getting harder
Just a few short months ago, Textron Lycoming's Service Letter L-163C recommended taking the engine apart for inspection following any incident involving propeller blade damage. However, they have the caveat that the inspecting mechanic may override that position and return the engine to service without disassembly and inspection if he feels that it is the prudent and responsible thing to do.
Yet, the company recently published a new Service Bulletin, SB No. 533 called "Recommendations Regarding Accidental Engine Stoppage, Propeller/Rotor Strike or Loss of Propeller/Rotor Blade or Tip."
The bulletin reads as follows:
On numerous occasions, Textron Lycoming has been consulted about recommendations on whether to continue using an aircraft engine that has been involved in the separation of the propeller/rotor blade from the hub, the loss of a propeller/rotor blade tip or sudden stoppage following accidental propeller/rotor damage (such as propeller/rotor strike).
Conditions which surround accidents are many and varied; therefore the circumstances of the accident can not, in our opinion, be used to predict the extent of the damage to the engine or assure its future reliability.
FAA: Special Airworthiness Information Bulletin on All Lycoming Engines (Lycoming) Direct-drive Reciprocating Engines,
Lycoming engines must be in compliance with Airworthiness Directive (AD) 2004-10-14 if they have had a propeller strike.
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