Aircraft Engine Age Deterioration

In Service Condition Inspection of N6937Y, PA23-250

• Compression: Excellent
• Oil Consumption: Good
• Oil Analysis: Clean
• Oil Filter Examination: OK
• Engine Operation: Smooth

Based on above findings aircraft was operated on June 23, 1996. Aircraft crashed during flight killing all aboard.

• Fatigue crack at corrosion pit between cylinder fins
• In flight fire
• Wing separated in flight
• 5 fatalities

Lycoming connecting rod with rust. Stress Corrosion Cracking  danger

Overhaul Condition Inspections that WERE NOT DONE but recommended (Factory overhaul inspections after 12 years; engine time-in-service: 21 years):

• Corrosion and Pitting Inspection
• Magnetic Particle Inspection
• Florescent Penetrant Inspection
• Eddy Current Inspection
• Visual Inspection
• Dimensional and run-out inspection
• Wear and surface damage

These more thorough inspections were not performed presumably because In Service Condition Inspections were all that were required (in the opinion of some airplane owners and mechanics) to determine the safety of the engine. Unfortunately, In Service Condition Inspections were not adequate given the age of the engine. "Bad practices that result in no immediate ill effects wind up becoming the norm."

 Age related deterioration may result in sudden and catastrophic engine failure as this example illustrates. This accident, and other age-related-failures, may be prevented on high calender time engines by using more thorough Overhaul Condition Inspections that better inspect for corrosion pitting, fatigue cracks, and other deterioration both external and internal. Stress corrosion cracking is not detectable from the outside of the engine; doesn't have to "look bad" to be bad;


Links to accidents related to corrosion pitting:

Lycoming fuel injector lines

Crankshaft Failure of antique engine

Navion Crankshaft Failure - pitting on fillet

Fractured landing gear leg from Stress Corrosion Cracking. Doesn't have to look bad to be bad.

A 150 µm deep (0.010 inch) corrosion pit
A post-mortem examination found evidence of soot  in the airway of the pilot, which indicated that he had  been breathing during exposure to smoke.  Toxicology  results showed the presence of cyanide in the pilot’s  blood at a significantly elevated level; cyanide is a  common combustion product of some materials  found in aircraft construction. 


Cessna 152 Cylinder Failure

Engine Failure/Fire Piston Pin Pitting

Propeller Loss "Had the failed engine been overhauled within the manufacturer’s recommended time of 2000 hours, or even within 2200 hours had it met the manufacturer’s 200-hour extension requirements, the overhaul would have occurred before the flange cracking had reached a critical stage and the crankshaft should have been scrapped."


The occupants were fatally injured. "A fatigue crack developed in the engine crankshaft as a result of corrosion pitting and the absence of a case-hardened layer on the fillet radius of the number six connecting rod journal. The fatigue failure of this section of the engine crankshaft resulted in a complete loss of power."


 Both occupants were fatally injured. The helicopter was completely destroyed in the post-impact fire. Enstrom F-28C Helicopter C-GVQQ total time in service of 611 hours over the 27 years since the 1982 overhaul. "The fracture of the check ball retainer in the exhaust valve hydraulic tappet..."

Calender time and Hours (stress cycles) are both considerations for continued safe operation

I0-520 Cylinder with combustion chamber cracks

"Old designs are never proven for fatigue simply by virture of their longevity. Fatigue is wear-out. There is no guarantee that future failures will be confined to those seen in the past." Steve Swift,GNATS AND CAMELS - 30 Years of Regulating Structural Fatigue in Light Aircraft"


Crankshaft Fatigue Failure in TIO-540 Engines operating past TBO "The inspection method to detect crankshaft fatigue cracks is conducted using a magnetic particle inspection (MPI), and the opportune time is at engine overhaul"

Cessna 152, N6565L - Piston failure at 2,762.5 hours since overhaul - 2 fatalities
Interesting accident in that the 100 hour inspection did not detect worn spark plugs, broken piston rings, and burnt piston. Obviously, not all inspections are done to proper standards; unfortunately, for the flight instructor and student they had had no way of knowing. However, if the instructor had followed Lycoming's recommendations as to TBO time, the accident would not have happened.

Cessna 152, N5314B Engine failure for unknown reasons, 1 fatality. Engine past TBO

Liability and owner negligence

Loss of Strength due to corrosion pitting:

The chart below is from Timkin Practical Data for Metallurgist.and shows endurance (fatigue) strength for various surface conditions. If we use the polished Lycoming connecting rod in the picture above and assume that the strength is 120 psi for a polished surface. With corrosion pitting the endurance strength drops from 70 psi to below 20 psi.

Read the note at the bottom!

What is "endurance strength". Typically, when we think about strength it is how much force is required to fail the part -- either break or bend. This is important for our connecting rod material must be strong enough to  resist the loads. However, our connecting rod is subjected to repeated loads. Given enough of these repeated loads the rod can fail at a very different stress level than a one-time applied load. This is the endurance strength.  Obvously then, we should be very concerned about endurance strength as our rod is subjected to many, many repeated loads.However, a bolt for example might only be subjected to the initial load when it is tightened so in this example, endurance strength is not an issue. 

Added 11/8/2012
My Personal Bias:

In reply to a question about an engine:


Does your engine need an overhaul?
1. Yes, based on the manufacturer's recommendation.
2. Is your engine in need of an overhaul based on its condition? How would I know?

I know of no accurate method of answering question #2 based on "other parameters". I have given a lot of thought on this over the years -- You see I am an internal engine guy who spent time inspecting the insides of engines. Most mechanics are outside guys who inspect the outsides of engines. It always amazed me how we would find serious internal problems in engines that the outside guys never knew existed. Flat camshaft lobes, bent pushrods -- how did the engine fly? -- "smooth as silk, hated to overhaul it". "Your camshaft is junk"  --but It just want through a "pre-purchase inspection!" Based on my experience, how can I trust the "outside guys" to get it right? 

Pendulum Detuning is another concern I have about internal engine wear.

My personal opinion, I have never owned, flown, or will fly in an aircraft who's engine is past manufacturer recommended tbo. But then again I am old fashioned.
-------------------------------------
Hours and calendar time are by their nature arbitrary and can be wasteful. Kind of like "replace hoses every 5 years". My nature is to rebel against such absolute statements. Another absolute is to tear down an engine after any propeller strike. In many cases all of these can lead to an expense that might not be necessary, if we had adequate knowledge before. But we don't so we inspect. Its not a process of guessing or opinionizing; either we have adequate information or we go get it through an inspection process.

My background is: "I will inspect and then report on the condition. That is what an engine shop does. The decision to take the engine to the shop in the first place is made by someone else. I was never in that business of guessing on what the condition might be. I'm therefore biased toward doing the inspection.

For many years we did about 12 propeller strike tear-downs a year. Boy, I thought, if I could come up with a way of inspecting for damage without tearing down the engine I would be a rich man. I couldn't and I don't think anyone else can -- for prop strike inspections.

I knew that "statistically" many engines were OK -- but I never knew which ones those were -- Mechanics deal with individual aircraft airworthiness -- statistics deals with a population of aircraft. I therefore cannot condone the use of statistics as a reason to determine airworthiness on individual aircraft.

Now can someone come up with a comprehensive method of inspecting the engine for calendar time deterioration. I think it can be done -- but no one is doing it- at least at the level that I believe is necessary. The engine inspections done today are not comprehensive enough to adequately address some of the issues that arise when engines are in operation over a long period of time. 

Addendum:


The concept behind DETECTABLE DEFECT

Continued airworthiness through inspection (rather than airworthiness through life-limit components) relies
on the concept:

1. A defect will appear before catastropic failure
2. That the defect will be detected

This is the basis of most aircraft maintenance and it is enshrined into the certification
standards of all large aircraft and given a fancy name "Damage Tolerance Analysis (DTA). If failure can occur without a detectable defect then some other form of intervention (life-limit) must be applied. In summary, if failure can occur in the absence of a detectable defect, the application of DTA is invalid.

My position is that DTA is invalid when applied to establishing piston engine service life. From experience the following is a short list of either undetectable defects or often-missed detectable defects, that can result
in sudden engine loss of power. This listing is not complete but used as examples of possible items. The experience of others would likely add to this listing and that is part of the problem. Not all catastrophic failure modes are known. DTA must include all possible failure modes that would result in rapid loss of  engine power as being detectable. The detectable defect must also rest benign until the next inspection interval -- a unrealistic assumption for several types of defects).

1. bearing fatigue failure (sudden loss of engine oil pressure resulting in sudden engine failure)
2. crankshaft fillet fatigue cracking (engine failure)
3. counterweight detuning due to bushing wear (engine failure)
4. loss of main bearing clearance due to crankcase fretting (sudden engine failure)
5. excessive main bearing clearance due to crankcase fretting (crankshaft fracture at fillet)
(its ok if 4,5 seem contradictory -- both situations may arise)
5. low power caused by camshaft lobe deterioration
6. accessory failure (magnetos, fuel pumps, hoses, etc.)
7. impulse coupling spring failure on D2000/D3000 magnetos
8. SCC of fuel injection lines
9. cylinder barrel to head failure (example of detectable defect that is often not detected due to inadequate inspection technique0
10. SCC of stainless steel fuel injector lines
11. Internal fluid hose liner deterioration