I know many of you have your own birds and most probably do some maintainance on your own ships.
My Bi Plane, Marquart Charger, has an IO-320-B1A engine.
That engine and many 320 Lycomings have an AD, 98-02-08, that requires an inspection of inside of the business end of the crankshaft.
Not a bad process in itself but kind of a pain in the tailfeathers.
On mine, since it had only 1000 hrs since rebuild, was due for the first look see.
Pulled the freeze plug, cleaned all of the 1000 hours of sludge out of the bore, cleaned up the bore and inspected for corrosion.
Looked great!!!! (Good oil and regular changes helps to keep the bad stuff burned away, I hope)
With the AD is a form that has to be filled out and sent to the feds.
That form required ser. no. and part no. off the crank.
In the process of trying to find these numbers on the prop flange I noted some interesting marking on the webs between the lightening holes on the rear side of the flange. With a tight feeling in my gut, I commenced to clean up the back and front side of the flange with scotchbrite.
Once all was cleaned up and nicely shiney, the knot in my gut was quite a bit larger.
The marks were CRACKS!
There are six lightening holes, the webs are between the holes.
THREE adjacent webs had cracks hole to hole on the back side!
The center web was cracked to visable on the front side!!!!
My Crank was scrap!
Once I thought it out and figured better found now on the ground that when upside down or with the nose pointed straight up.
NEW Lycoming crank is on the way.
Crankshaft guru at Poplar Grove Airmotive told me that the crank in my engine was a very OLD seris crankshaft and was lighter than the new one will be. I hope the new one has a solid flange!!!!
SOOOO........Next time you have the prop off your bird, take the time to remove all paint and crud from the flange, both front and back, and take a REAL CLOSE LOOK for anything that might be a crack.
My cracks were pretty hard to see, smaller than a human hair.
Hopefully none of you will find anything but remember, that little flange is what keeps the wind blowing in your face and without it and the attached propeller, you might experiance the wind hitting you in the BACK of your head as you plummet rearwards towards the ground to that terrible hard stop at the end.
Take this seriously!
If you pay to have your work and inspections done, INSIST that the flange is inspected!
Interesting. Was any of that area of the crank included in the AD? Good find
The Crank Inspection.
Actually, SB 505B is a Mandatory Service Bulletin covering the inspection of the interior of the hollow Lycoming crankshafts on all 320 and most 360 engine models.
This inspection targets pitting and cracks to the interior surface of the prop end bore caused by oil byproducts collecting in this bore and subsequently causing corrosion. The only "requirement" with the flange is to look for PN, SN, and the stamping of "PID" which denotes that the crank a proper corrosion proof coating applied to the bore.
The crankshaft that failed was a very old version with an 1/4" thick flange and a large rear chamfer. The NEW replacement crank has no lightening holes and a 3/8" thick flange as well as the proper interior coating and PID designation on the flange. How many time have you had the prop off and not bothered to look the flange over VERY CLOSELY? This thin piece of metal takes an incredible amout of stress doing what it is intended to do and when it fails in flight can be the end for all involved.
I will be inspecting the flange of every engine I have removed the prop from and all crank flanges during Annual and condition inspections. (Including my NEW replacement)
The time is worth the piece of mind.
Those cracks in the crankshaft flange webs are very likely caused by the gyroscopic moments on the crank from a spinning heavy metal prop. These bending moments (torques) in the flange are proportional to the rpm times the vector rate of change of aircraft attitude (degrees per second) about a vector sum of the lateral axis (pitch) and vertical (yaw) axis.
These flange overturning moments can be high - like on the order of a few thousand ft lbs for an aircraft that is used in serious aerobatics.
Remember that serious aerobatics generate serious flange and front bearing stresses. Lucky you spotted those cracks.