Your Engine is Approaching TBO: Now What?

March 13, 2006
As an engine gets older and builds operating hours and approaches TBO (either based on operating hours or calendar limits) owners start to ask questions concerning the decision to either continue flying, have a top overhaul or major overhaul performed, or exchange engines. The following nine points are offered by Lycoming to help owners and mechanics evaluate each engine and make such a decision.

As an engine gets older and builds operating hours and approaches TBO (either based on operating hours or calendar limits) owners start to ask questions concerning the decision to either continue flying, have a top overhaul or major overhaul performed, or exchange engines. The following nine points are offered by Lycoming to help owners and mechanics evaluate each engine and make such a decision.

1. Oil consumption — any unusual increase?
2. Engine history and calendar age.
3. How has the engine been operated?
4. Pilot’s opinion of the engine.
5. Maintenance — what kind has the engine received?
6. What does the oil filter tell?
7. What has been the trend in compression checks?
8. What do the spark plugs show?
9. Refer to the engine manufacturer’s service letter for engine life and recommended overhaul periods.

Lycoming discusses each point more specifically in the following.

1. Oil Consumption

Both the operator and mechanic should know what the general history of oil consumption during the life of the engine has been.

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. The mechanic should 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 the 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.

2. Engine History and Calendar Age

If an engine 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. A logbook inspection would reveal any accumulative record of engine repairs.

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. Engines infrequently flown do tend to age or deteriorate more quickly than those flown on a regular basis. Therefore, 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 checklist will help to determine if an overhaul or engine exchange is needed before the engine reaches these recommended limits.

3. Pilot’s Opinion of the Engine

The pilot’s opinion of the powerplant based on his or her experience operating it is another important point in our checklist. The pilot will have an opinion based on whether it has been a dependable powerplant, and whether or not he or she 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. The pilot should consult with maintenance personnel concerning their evaluation of the condition of his powerplant.

4. Operation

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.

5. Maintenance

Good maintenance should aid in achieving maximum engine life; alternately, poor maintenance tends to reduce the expected life. Lycoming says it has noticed that among the powerplants going back to the factory for rebuild 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 reflect the kind of maintenance provided the engine or engines. The mechanic who regularly cares for the engine will usually have an opinion about its health.

6. 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 changed, open it and carefully examine for any foreign elements, just as is accomplished at oil change when the engine oil screen is 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 be 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.

7. 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 percent loss or more, then trouble may be developing.

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 mechanics 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 mechanics 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.

8. Spark Plugs

The spark plugs when removed and carefully observed, tell you 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.
2. Black carbon and lead bromide may indicate low temperatures, the type of fuel being used, and possibly excessive richness of fuel metering at idle.
3. Oil fouled plugs may indicate that piston rings are failing to seat, or excessive wear is taking place.
4. The normal color of a spark plug deposit is generally brownish gray.
5. In high compression and supercharged engines, a cracked spark plug porcelain will cause or has been caused by preignition.

9. Engine Manufacturer 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.

Lycoming says it is surprised from time to time by owners who say 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.