Q: I have 210 hours on my engine, which was overhauled in November 2009 at 3,288 hours. Five of the eight studs are now broken on cylinder #3.
The motor had only a slight vibration during the last 3/10ths of an hour before discovering the broken studs.
I would appreciate your input as to how extensive an inspection should be made. Have you had experience with such an occurrence?
DOUG LUNDREN, via email
A: First, let me say that this is a very serious situation and could have resulted in a serious in-flight failure requiring an off-airport landing — or worse.
Since you didn’t mention anything in that regard, I’ll assume the problem was discovered while the aircraft was parked on the ramp or safely back in the hangar.
I’ve got to be honest with you and tell you this situation raises more questions in my mind, but I’ll just go with the information you provided. I can tell you from my experience this is a rather rare occurrence, but it has happened before and the causes and results vary.
I’d like to know exactly what maintenance has been done to the engine in the last 100 hours or so. Have any cylinders been removed, etc.? You mentioned that there was a slight vibration in the last 3/10th of an hour before the discovery of the broken studs was made. What, if any maintenance was done just prior to that time?
If nothing was done, then we need to look elsewhere for any contributing factors. I’d be interested to know how long you’ve owned the aircraft and if it has ever suffered from a prop strike that you are aware of? Did the crankcase have any weld repair during the overhaul in 2009 that may have led to this problem?
From my experience, cylinder base studs break from two major reasons, which are either a lack of proper torque or over torque.
If they are under torqued, the result is fretting between the cylinder base and the crankcase deck, which eventually loosens the cylinder enough that the studs fail by snapping off between the cylinder hold down nut and the cylinder flange.
If the cylinder base studs are over torqued where the stud is actually “necked down,” the failure occurs between the cylinder flange and the cylinder deck of the crankcase. In this case, the stud typically breaks off flush with the surface of the crankcase versus the under torque situation, where the stud breaks off on the outside of the cylinder flange.
With the information you’ve provided, I’d recommend nothing less than a complete engine bulk-strip and focusing on a detailed inspection of the crankcase. I’d especially be looking for fretting on the cylinder base deck and at the crankcase parting surfaces. This is where the results of loss of torque on the cylinders typically shows up. I’d suggest the crankcase undergo a complete, very detailed, inspection just as if it were going through an engine overhaul.
Doug, I hate to be the bearer of bad news, but this is — as you already know — a bad situation and the only safe thing is to bite the bullet and do the right thing. Unfortunately, there is no easy or quick fix for a situation like this and it’ll cost money.
When you stop and really think about what happened, you were very lucky that no one was hurt or killed. As my 98-year old mother says: If it can be fixed with money, it really isn’t all that bad!
Doug, I don’t take situations like this lightly. I’m sure you realize you dodged a bullet here. Even though the proper fix will cost you money, it needs to be done properly to assure safe future flight.