Aircraft Engine Time-Between Overhaul - Image

Five Reasons Your Aircraft Engine Doesn’t Reach TBO

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It's Not Rocket Science: How to Achieve Your Aircraft Engine's Time-Between Overhaul and Beyond.

Many aircraft engines don’t achieve their stated time-between overhaul (TBO), necessitating premature teardowns—often well before the published benchmarks.

TBO represents the manufacturer’s recommended number of running hours or the calendar duration before an aircraft engine or another component necessitates an overhaul.

While the reasons for not achieving TBO can be multifaceted, they usually boil down to five common causes.

Reason #1: Corrosion

Problem

The primary reason many aircraft engines don’t reach TBO is internal corrosion. Key engine parts, such as cam lobes, lifter faces, and cylinder walls, are susceptible. Sporadic operation is the main culprit behind this corrosion. After the engine runs, it’s shielded by a thin oil film. But with time, this protective layer depletes, exposing engine parts to atmospheric elements. If your aircraft is stationed near large bodies of water or in humid areas, corrosion risk spikes.

Solution(s)

Though the threat of corrosion is alarming, there’s a simple solution: fly your aircraft regularly. A flight every week or two restores the oil film on engine parts and evaporates water in the oil system, a byproduct of combustion.

Some pilots and/or owners think that manually turning the propeller or doing engine ground runs suffices to deter corrosion. This notion is misguided. Manually turning the propeller can strip away the protective oil, while ground runs can introduce more water and acids into the oil, causing more harm than good.

Storing your aircraft in an insulated, temperature-controlled hangar is the best defense against corrosion, both internal and external. Hangars not only shield from weather elements like sun, rain, and hail but also curb condensation — a major catalyst for metal corrosion and degradation of rubber and plastic components.

Using top-quality engine oil and additives, such as ASL CamGuard, is another wise move. While we champion single-grade oil, you might need to switch to multi-grade oil based on temperature fluctuations. Always choose engine oils approved for your specific aircraft engine.

If you anticipate not using your aircraft for an extended period (over 30 days), it’s vital to take engine preservation steps. For detailed guidance, refer to your aircraft engine manufacturer’s manual.

To guard against cylinder wall corrosion, consider installing chrome or nickel-plated cylinder barrels.

Reason #2: Cold & Dry Engine Starts

Problem(s)

The second most common reason aircraft engines don’t reach TBO is due to excessive wear. In terms of damage, cold and dry engine starts can be worse than 500 regular cruising flight hours.

Cold engine starts wreak havoc primarily because the materials in aircraft engines have varying thermal expansion coefficients. This means they expand differently during the warm-up, causing significant wear. Starting an aircraft engine below 0°C without preventive measures is ill-advised.

Dry engine starts are harmful due to insufficient lubrication. This problem arises from extended periods of inactivity, which deprives key engine parts, like cam lobes, lifter faces, and cylinder walls, of the essential oil film.

Solution(s)

To mitigate wear during cold starts, it’s crucial to pre-heat the engine. The goal isn’t just to warm the oil, but also to heat the entire cylinder heads. The most straightforward method is to house the aircraft in an insulated, heated hangar for several hours before starting. Another efficient approach is to equip the aircraft with an electric pre-heating system with heating elements for each cylinder. Pre-heating using forced air can also work, provided it’s done correctly and with ample time allocated. Alternatively, you can use natural methods to warm up the engine by placing your aircraft in the sun.

To avert dry starts, operate your aircraft regularly, ideally every week or two. If you foresee a long hiatus, ensure you preserve the engine as previously discussed. If you find yourself with an unpreserved engine that’s been idle for a while, you can still reduce dry start damage. Consider removing the top spark plugs and spraying light oil into each cylinder. Connect the aircraft to a ground power unit and crank the starter with the magnetos/ignition off until a stable oil pressure shows. After replacing the spark plugs and securing the engine cowling, start the engine as usual.

Reason #3: Contamination & Deposits

Problem(s)

Other reasons why your aircraft engine falls short of TBO are contaminated oil and induction air, as well as (lead) deposits.

Contaminated oil and induction air can lead to excessive wear and corrosion due to abrasive particles and acid in the oil.

Deposits form due to excessively rich operation. Byproducts, such as lead deposits, can accumulate on exhaust valves, pistons, and cylinders, potentially necessitating premature part replacement.

Solution(s)

Contaminated oil and induction air can be easily identified by regularly performing diligent oil analyses. Oil and oil filters should be replaced every 50 hours or every four months. If your aircraft isn’t equipped with a proper full-flow oil filter or if you operate in particularly harsh environments, consider changing the oil and oil filter every 25 hours. We recommend installing a full-flow oil filter if one isn’t already in place.

Regularly check, clean, or replace your induction air filter and inspect your induction system for leaks—especially if the oil analysis indicates an increased amount of silicon. The induction system can be easily assessed with an in-flight test procedure called the “Induction Leak Test”.

Lead deposits can be avoided through proper leaning techniques. There are only two times you should operate the engine full rich: during engine start and take-off. At all other times, the aircraft engine should be leaned, especially on the ground.

Reason #4: Overly High Temperatures

Problem(s)

It’s crucial to avoid excessive cylinder head (CHT) and turbine inlet temperatures (TIT) if you aim to achieve your engine’s TBO. The stated CHT limits by Continental and Lycoming are arguably too high for regular operation. It’s advisable to operate the engines below 400°F (Continental) and 420°F CHT (Lycoming), respectively. Always adhere to the stated TIT limits, aiming for at least 50°F below the maximum TIT.

Solution(s)

We recommend installing an engine monitor unit (EMU), CHT probes for each cylinder, and a TIT probe if your engine is turbocharged. Set the CHT and TIT limits in the EMU and operate your aircraft engine based on those guidelines.

To maintain optimal temperatures, you can:

  • Adjust engine power
  • Modify airspeed (opt for a cruise climb)
  • Utilize cowl flaps
  • Adjust the mixture (refer to Leaning Your Aircraft Engine Correctly)
  • Regularly inspect and maintain your fuel system setup and engine cooling baffles

Reason #5: Excessive Stress

Problem(s)

Excessive stress is detrimental to your aircraft engine. This may seem obvious, but it bears emphasis: reducing stress contributes significantly to engine longevity.

Solution(s)

By now, it should be evident that operating the aircraft engine with moderate CHTs, staying within TIT limits, and leaning aggressively to avoid (lead) deposits will enhance your engine’s lifespan. Power setting and engine handling also play a vital role in managing stress levels.

The impact of power setting on longevity largely depends on whether the engine is considered highly-stressed or de-rated. A highly-stressed engine, defined by a maximum take-off power (HP) to displacement (cubic inch) ratio of 0.6 or greater, may benefit from more conservative power settings (65% power or less). On the other hand, a de-rated engine with a maximum take-off power (HP) to displacement (cubic inch) ratio of 0.5 or lower can comfortably operate at 75% power for extended periods.

Regardless of the power-to-displacement ratio, it’s essential to handle the engine gently. Smooth power changes (for both throttle and propeller) are crucial, and shock cooling should be avoided. Always allow your engine to warm up prior to flight and cool down post-flight, especially with turbocharged engines. However, avoid unnecessary ground operation.

Conclusion

Ensuring Your Aircraft Engine's Longevity

Aircraft engines are marvels of engineering, and their performance and lifespan are significantly influenced by how they are maintained and operated. Throughout this article, we’ve touched upon some of the key factors that can shorten the time-between overhaul (TBO) of an aircraft engine.

  1. Corrosion: It remains the most prevalent reason for premature engine overhauls. Regular flying and storage in a controlled environment, complemented by using high-quality engine oil, can significantly reduce the risk.
  2. Cold & Dry Engine Starts: These starts can be as damaging as hundreds of flying hours. Pre-heating and consistent usage are the keys to countering the negative effects of cold and dry starts.
  3. Contamination & Deposits: Regular oil analyses and filter replacements are paramount. Ensuring proper leaning techniques and maintaining a clean induction system are steps in the right direction.
  4. High Temperatures: Overheating is a silent killer of engine longevity. Using an Engine Monitor Unit (EMU) and controlling various factors can keep temperatures in check.
  5. Excessive Stress: Gentle and considered operation, alongside understanding the nature of your engine—whether it’s highly-stressed or de-rated—can prolong its life.

In conclusion, reaching and potentially exceeding your engine’s TBO is not solely a matter of luck but largely a consequence of informed care and proactive measures. By understanding the factors that contribute to wear and tear and actively countering them, pilots and owners can ensure their engines not only reach their TBO but do so in a state of optimal performance.

About Quest Aeronautics

Quest Aeronautics is a state-certified engineering office for aviation, dedicated to shaping the future of general aviation by providing innovative and cost-effective solutions to enhance aircraft performance and operations. With a focus on CS/FAR-23 and experimental/amateur-built (E/A-B) aircraft, Quest Aeronautics provides a range of services including flight testing, aircraft operations and maintenance consulting, high-quality aviation products, and tailored support for E/A-B projects. Collaborating with industry-leading partners, Quest Aeronautics is committed to delivering unparalleled support and expertise to individuals and organisations in the general aviation market.

About Author

Sebastian, the founder of Quest Aeronautics, is a driven and enthusiastic individual with a passion for aviation. Before delving into aviation, he gained valuable experience as a chemical process engineer and laboratory technician. Sebastian holds a Master of Science in Engineering and a commercial pilot licence, with several fixed-wing aircraft ratings under his belt. He has also completed an introduction course for fixed-wing performance and flying qualities flight testing at the National Test Pilot School in Mojave, CA and is compliance verification engineer for flight.