by John Schwaner
Regardless of the claims of marketing departments, you can zero-time a logbook but you can not zero-time the metallurgy. Engine parts have a wear life and they have a fatigue life. The primary reason components fail catastrophically in service is due to metal fatigue. With a knowledge of fatigue you will see the reason behind many maintenance practices and recommendations. The design engineer can design for metal fatigue, but it is up to the mechanic or operator to inspect for metal fatigue.
Fatigue fractures are the most serious and most common types of failure in machinery. Fatigue fractures can occur under normal operating conditions without excessive overloads. The Comet I aircraft failures in the early 1950s were the result of low-cycle fatigue caused by repeated cabin pressurizations. These are the earliest examples of catastrophic structural failure occuring on airplanes that have not been stressed beyond their limits. Other fatigue failures include the 1988 Boeing 737-200 fuselage failure on Aloha flight 243, and the F-111 wing that failed in 1968 after a few hundred hours (the estimated life was 4,000 hours). These failures demonstrate that because of metal fatigue, a safe load applied once becomes unsafe when applied many times.
More common types of fatigue failures include exhaust valve breakage in older 80-octane Lycoming engines, spalling of camshaft followers, failure of propeller blades in unlimited aerobatic airplanes, and failure of the crankshaft flange on Lycoming "lightening hole" crankshafts. Most of us use fatigue when we break off a piece of metal wire by repeated bending. The wire breaks because of a low-cycle fatigue fracture.
Fatigue failures are always brittle failures which occur without buckling. A wing or engine mount should buckle rather than snap. With the pursuit of lightweight vehicles, the advent of new lightweight designs and computer stress analysis, it is possible to create structures which are of minimum weight. As a result, more high-strength steels are used under high stress conditions. Unfortunately, increasing the hardness of steel decreases its ductility and it becomes more subject to brittle fatigue failure under service stress conditions. High strength, high stress and low ductility create catastrophic fatigue failure modes. As a consequence, the engineer must predict the fatigue life of the component and the component must be inspected for fatigue or removed from service.
Any time that we inspect a structure for cracks, a knowledge of stress and fatigue enhances our ability to examine that part. To pursue our understanding of metal fatigue, we will investigate some properties of materials...
Imagine you have just purchased that dream airplane. During the purchase, you did everything right. You had the airplane checked out, including a pre-purchase inspection. The engine was examined and an oil analysis sample was taken with normal engine wear reported by the lab. The airplane is in perfect condition.
Six months later, your shop tells you that the engine is contaminated with metal, the camshaft is shot, and the engine has to be disassembled. The cost of repair is no less than $3,000 and probably more.
Unfortunately, once or twice a year, I see one of these engines. What is more unfortunate in some cases is that the engine had already failed before the purchase; the normal pre-purchase inspections had failed to detect the failure.
An engine can operate normally, pass compression tests and oil analysis, and still have damage to the camshaft lobe and follower. Even when a camshaft lobe has ground down to 1/2 its original shape, the only indication is that the engine fails to produce rated power. Engines with a fixed-pitch propeller may not produce rated RPM. But on engines with a constant-speed propeller, lack of engine power may not be noticeable. Engine roughness occurs only when the camshaft lobe has been totally wiped out.
There is one specific type of inspection that detects this type of occurrence in the early stage without disassembling the engine: a thorough oil filter examination.
Proper inspection of the oil filter is the best method of detecting camshaft or cam follower spalling and failure. Cut the oil filter open and wash and rinse the filter media carefully into a bucket of clean solvent. The washing shifts particles into the bucket. Using a clean magnet, work it around in the solvent solution. Ferrous metal particles in the solution will adhere to the magnet, allowing inspection. A visual inspection of the cam followers is warranted if the end of the magnet is covered with metal slivers.
You will not detect early signs of cam follower damage by taking the filter media out of the can and spreading it out on a table. There is not enough metal at this stage of failure to be visible in the folds of the filter. Oil pressure forces the small slivers of metal into the filter media where they are not visible. A magnet concentrates the small abount of metal, making it visible. Unless the examination of the oil filter is done correctly, you will not get a reliable analysis.
Make the initial temperature change slowly. We want to start the temperatures going down early. Once you establish the downward trend in temperatures, there is far less likelihood of causing engine damage by fast cooling.
There are three ways in which you are going to cool the engine:
The wost thing you can do is to do all three of them at once. Start the initial cooling trend by lowering the nose by keeping the power and mixture the same. Try to make small changes in RPM and manifold pressure. Limit yourself to 1 or 2 inches of manifold pressure reduction at a time. To prevent rapid turbocharger cool down during descent, the desired EGT is 1350F.
Every flight, you should anticipate a rapid descent and start a cool-down before the let-down. Reduce power or open the cowl flaps part way to start reducing the temperature of the engine. When you have to do the fast descent, you can close the cowl flaps and be less concerned since you have already reduced engine temperatures.
You do not want to do a long let-down with the prop driving the engine. This causes ring flutter and broken piston rings. High RPMs increase the up-and-down inertia load on the rings in the piston grooves. Cylinder pressures help to keep the rings seated to the bottom of the ring groove. Reducing RPM to low cruise will be beneficial by reducing ring inertia and increasing cylinder pressures. Of course, at some time (as on final) the prop will be driving the engine and this is okay for a short time. But a long descent at high RPM and low manifold pressure is hard on the piston rings.
Prolonged engine idle during glide, particularly in cold weather conditions with a carbureted engine, may result in engine stoppage. The primary cause of this type of engine stoppage is the inability of the engine to receive adequate fuel/air mixture ratios. During idling flight, propeller windmilling induces an engine RPM that is higher than the ground idle RPM for which the mixture adjustment of the carburetor was set. This produces a lean fuel/air mixture ratio. When the throttle is opened rapidly, the fuel delivery by the carburetor lags behind the throttle motion due to surface tension and inertia in the fuel, and this futher leans the available mixture. The cylinders have inadequate fuel and the engine misfires or quits.
Our hose shop operates under FAA manufacturing TSO and also under FAA Repair Station Approval. Most customers send us the old hoses off of the airplane and have us make new hoses from them. I'll bet that we have seen more old hoses than anyone in the country. I thought I would share with you what we have seen and found out from looking at all of these old hoses.
Most hoses can be considered critical components in that their failure will cause component damage and engine loss of power. Yet most hoses are left on the airplane too long. When we see a 30-year-old hose arrive at our shop, we wonder how in the world that airplane passed annual inspection the last 20 years. All rubber hoses are life limited regardless of their apparent condition and should be replaced at regular intervals.
All hoses forward of the firewall should be firesleeved. Most Lycoming engines have two oil cooler lines that go from the engine to a remotely mounted oil cooler. If your exhaust stack cracks or fails in flight, the escaping exhaust gases will most likely be directed on these hoses. If they are firesleeved, the hoses will last until you can land the airplane at the next airport. If they are not firesleeved, then the lines will rupture in a matter of minutes. Engine oil will be blown overboard. Engine seizure will occur and you are coming down.
If you have a hose failure on a fuel hose that is not firesleeved, you have both engine stoppage due to fuel starvation and raw fuel leaking into your engine compartment. Even if the leak is not enough to cause fuel starvation, your choices are either to (1) keep the engine running but continue to leak raw fuel into the engine compartment and risk a fire, or (2) turn the fuel selector off to keep additional fuel from leaking into the engine compartment and thereby shut down the engine. If the hose was firesleeved, it would contain the leak and cause a fuel stain and the end of the hose that could be found at the next 50-hour inspection.
Firesleeve protects the hose from chafing. In a tight engine compartment, the hoses may rub against a mount or accessory that will cause the hose to fail in a short time.
Firesleeve protects the hose from high temperatures, resulting in a longer-lasting hose.
So firesleeve those hoses. It costs approximately $7.00 a foot, which is a good bargain.