I have moved that PDF to local storage – here (it uses a rather unusually named PDF, with round brackets in the name, which breaks the text processing here).
It has almost certainly been referenced here before.
It doesn’t matter if the engine was not producing power, because any impact of a prop blade with something hard sends torque shock waves backwards through the drive train, via the crankshaft initially, through the conrods, through the accessory gears, etc. Consequently sometimes cracks are found in those gears. So the inspection involves a check for cracks in all the moving parts. Plus the crankcases which also get whacked pretty hard.
I don’t know the legal reference for following that Lyco document, however.
I was presented with a similar situation back in 2002, G-reg, and a maintenance shop where I was hangared (long since defunct) offered me the option of cutting the last 2cm off each of the 3 blades 
The plane was new, with 1hr on it (!!) and the insurance was paying the 20k bill anyway, so I got a new prop etc.
I too don’t quite follow the pilot’s explanation and unfortunately aircraft owners do tend to downplay what actually happened. I know of one such situation, from c. 2005, where a bird got chopped up by the prop (blood etc on the cowling afterwards) and technically that also required a SL inspection, but it wasn’t done on the basis that no damage (not even paint damage) was seen on the prop.
Back in the dark ages when those maintaining aircraft could use a bit of discretion the action after a minor prop strike involved a good look over the engine, checking the run out on the crank and monitoring the oil filters over the next 100 hours to so.
I suspect all of this was based on WW2 wisdom when pilots had no reason to play down any damage done as they did not own the aircraft so the maintainence people would get an accurate picture of the nature of the damage.
Things have moved on and most private owners will try to play down any damage to keep the costs down so it is difficult to exercise any sort of judgment on the severity of the likely damage when the data is questionable. Add this to an environment full of circling lawyers who are just waiting for mistake to happen and you will see why anyone who wants to keep their maintenance licence will revert to the Lycoming SB as the default position despite what engineering logic might say.
The Lycoming SB is Lawyer rather than logic driven.
Another factor is that Lyco will absolutely wash their hands of any warranty if that SL is not followed.
I can and have spoken privately to a really honest engine shop and they just say there is a 99% chance nothing will be found but do you want to be the 1% if you fly over mountains and water?
It is obviously different if you hardly leave the circuit or just fly over a lot of good fields.
A_and_C wrote:
The Lycoming SB is Lawyer rather than logic driven.
I don’t know. A prop strike is probably the worst thing that could happen to an engine. Even if there is no visual signs of damage, and the crank is nice nice straight, there is no way of knowing if the crank is OK without X-raying it. Depending on the severity of the strike, lots of other thing could also be damaged.
LeSving
The old way was a progressive inspection that looked at the state of the prop, the runout on the crankshaft and the oil filters to see if the engine started making metal over the next few hours in service.
For years this was the way things had been done and the ground was not littered with broken aircraft largely because if any major damage had been done the engine would start making metal and the enhanced filter inspections would pick this up before the engine failed.
The lawyers then got involved and rather than using engineering logic to put the decision making back in the hands of the engineers the engine companies discovered that this shock load inspection thing was very good for business.
As for X-ray inspection of crankshafts I think that magnetic particle inspection is the more usual form of NDT.
Under FAA regs, there is no requirement to disassemble, NDT & reassemble the engine per the Lycoming SB.
I have seen a simple log entry for a prop replacement and minor repairs on the airframe following a full gear-up landing of a FAA reg’d Piper Arrow.
Remember: Only TCDS, Chapter IV and AD instructions are obligatory. SBs are not.
That said, I did a tear down on a Conti TSIO-520 that suffered a broken crankshaft in-flight. Investigating the log-books showed that the prop was replaced with new 10 years & 1000 hours previously.
The crankshaft split open starting at the oil transfer hole that supplies regulate oil to the CS prop.
Continental & I believe that a stress riser was created from a prop strike but the crack took 1000 hours before it propagated and failed.
Here it is :
Although the prop strike could always be a contributing cause in the above crankshaft crack, it would be necessary to a do a full metallurgical analysis of the incident and evaluate Continental’s private records for the design, manufacturing, and failure history.
We investigate crank cracks in all types of engines, most of which are not in airplanes (and have not had prop strikes!!!). It is not uncommon to see a crack like this one, especially at an oil passage. If you find a metallurgical or processing defect at the crack initiation site, fine you have the cause. If you don’t find a defect, then you have to wonder what the design stress was in comparison to the service stress. Then look at the details of the geometry of the oil hole and the in service failure rate. For example… if there had been a bigger (or smoother) radius at the oil hole, would the crack have started? Also, given it is a turbo-charged engine, have the manifold pressure/RPM operational RPM limits been adhered to?
The manufacturer will always say it was abused, overloaded, etc. Only rarely will they settle (with confidentiality agreements). I suspect they only settle when they know about ‘multiple’ incidents. The previous prop strike is a ‘get out of jail’ card for the manufacturer and probably also reduces the likelihood that a significant sum of money will be spent getting to the real root cause. Although an outside party might be able to do the metallurgical work, they will not know the history of the design, or of any secrets hidden by the manufacturer.
vic wrote:
I do not believe it would require magic to produce an engine light enough for aviation to stand the test of destroying a wooden prop or bending half of an alu prop without cracking the case or crank.
Isn’t that what Thielert and Austro do? 
