As an engine builds operating hours and approaches TBO, which may be either the manufacturer's recommended operating hours, or a calendar year limit before overhaul, the question arises concerning the decision to either continue flying, or top overhaul, major overhaul, or exchange engines. Here is a quick reference checklist to help make such a decision, followed by a brief explanation of the nine points.
1. Oil consumption—any unusual increase?
Along with the above quick reference checklist, as an engine manufacturer we would like to share our experience with interested operators by discussing the nine points:
I. OIL CONSUMPTION
The operator and maintenance people should know what has been the general history of oil consumption during the life of an engine.
A possible danger signal concerning engine health is a definite increase in oil consumption during the recent 25 to 50 hours of flight time. The oil screens and filter should be carefully observed for signs of metal. Maintenance should also take a good differential compression check at this time. They should also look in the cylinders with a gooseneck light or a borescope to detect any unusual conditions in the combustion chamber.
If you haven't looked at your air filter lately, it would be a good idea to carefully inspect it for wear and proper fit. This is all the more important when operating in dusty areas, and definitely could be a cause of increased oil consumption.
II. ENGINE HISTORY AND CALENDAR AGE
If a powerplant has been basically healthy throughout its life, this would be a favorable factor in continuing to operate it as the engine approached high time. Alternately, if it has required frequent repairs, the engine may not achieve its expected normal life. The engine logbook should contain this accumulative record.
Another important aspect of an engine's history would be its calendar age. Engine flight time and calendar age are equally important to the operator. We have observed that engines infrequently flown do tend to age or deteriorate more quickly than those flown on a regular basis. Therefore, Textron Lycoming recommends both an operating hour limit and a calendar year limit between overhauls. Service Instruction 1009 gives these recommendations, but other items in this check list will help to determine if an overhaul or engine exchange is needed before the engine reaches these recommended limits.
III. PILOT'S OPINION OF THE ENGINE
The pilot's opinion of the powerplant based on his experience operating it is another important point in our checklist. He will have an opinion based on whether it has been a dependable powerplant, and whether or not he has confidence in it. If the pilot lacks confidence in an engine as it approaches the manufacturer's recommended limits, this could be a weighty factor in the decision to continue flying or to overhaul it. He should consult with his maintenance personnel concerning their evaluation of the condition of his powerplant.
The basic question here would be how the engine has been operated the majority of its life. Some engines operating continuously at high power, or in dusty conditions, could have a reduced life. Likewise, if the pilot hasn't followed the manufacturer's recommendations on operation it may cause engine problems and reduce the expected life. This becomes a more critical influence on a decision in single engine aircraft, and also for single or twin engine planes flown frequently at night or in IFR conditions.
Good maintenance should aid in achieving maximum engine life; alternately, poor maintenance tends to reduce the expected life. We notice among those powerplants coming back to the factory for remanufacture or overhaul, that the smaller engines in general have had less care and attention, and in a number of instances have been run until something goes wrong. The higher powered engines have generally had better maintenance and show evidence that the operators do not wait until something goes wrong, but tend to observe the manufacturer's recommended operating hour or calendar limits to overhaul. The engine logbook should properly reflect the kind of maintenance provided the engine or engines. The technician who regularly cares for an engine will usually have an opinion about it's health.
VI. WHAT DOES THE OIL TELL?
Clean oil has consistently been an important factor in aiding and extending engine life. A good full flow oil filter has been a most desirable application here. When the filter is exchanged, ask the mechanic to open it and carefully examine for any foreign elements, just as is accomplished at oil change when the engine oil screen is also examined for the same purpose. Just as the spark plugs tell a story about what is going on in the engine, so the engine oil screen and the external oil filter tell a story about the health of an engine. Whether the engine is equipped with an oil filter or just a screen, oil changes should have been accomplished in accordance with the manufacturer's recommendations. These oil changes should have been recorded in the engine logbook.
If oil is analyzed, it should be done at each oil change in order to establish a baseline. Analysis is a tool which only gives useful information when a dramatic departure from the established norm occurs. (See "Spectrometric Oil Analysis" later in this section.
VII. COMPRESSION CHECKS
What has been the trend in compression in at least the last two differential compression checks? The differential compression check is the more reliable type and should be taken on a warm engine. If the differential check reveals 25% loss or more, then trouble may be developing.
Some operators are confused by the compression check and its application. A compression test should be made anytime faulty compression is suspected, anytime the pilot observes a loss of power in flight, when high oil consumption is experienced, or when soft spots are noticed while hand pulling the prop.
Many maintenance technicians do a compression check at each oil change, and it is also considered part of the 100- hour engine inspection and the annual inspection. Most experienced maintenance men feel that the differential compression check is best used to chart a trend over a period of flight hours. A gradual deterioration of charted compression taken during maintenance checks would be a sound basis for further investigation.
VIII. SPARK PLUGS
The spark plugs when removed and carefully observed, tell the skilled mechanic what has been happening in the cylinders during flight, and can be a helpful factor in deciding what to do with a high time engine:
1. Copper run out and/or lead fouling
means excessive heat.
IX. ENGINE MANUFACTURER'S RECOMMENDED OVERHAUL LIFE
Service Instruction 1009 is the Textron Lycoming published recommendation for operating hour and calendar year limits until engine overhaul as they apply to each specific engine model. The amount of total operating time on an engine will be a basic factor in any decision to either continue flying, change, top, or major overhaul the powerplant. Operators should be reminded, however, that the hours of service life shown in the service instruction are recommendations for engines as manufactured and delivered from the factory. These hours can normally be expected provided recommended operation, periodic inspections, frequent flights, and engine maintenance have been exercised in accordance with respective engine operator's manuals.
If an operator chooses to operate an engine beyond the recommended limits, there are factors to consider. The cost of overhaul is likely to be greater as engine parts continue to wear, and the potential for failure may also increase.
Operators who have top overhauled their engine at some point in the engine life invariably want to know if this extends the life of the engine. This is an important question. The chances are that if the operator applies the checklist we have been discussing and comes up with favorable answers to these questions about his engine, he can probably get the hours desired—with only a few exceptions. But a top overhaul does not increase the official life or TBO of the engine.
We are surprised from time to time to have owners tell us they top overhauled their engine at some point less than the major overhaul life for no reason other than somebody said it was a good idea. Unless the manufacturer recommends it, or there is a problem requiring a top overhaul, this is a needless cost. If the engine is healthy and running satisfactorily, then leave it alone!
One other point deserves attention here; there is no substitute or cheap route to safety in the proper maintenance or correct overhaul of an engine.
Apply all of these basic nine points concerning your engine or engines and then make a decision whether to top overhaul, major overhaul, exchange engines, or continue flying.