What is SMOH in Aviation? (Since Major Overhaul)

In the world of aviation, safety and maintenance are of utmost importance. Regular maintenance and inspections are conducted to ensure the airworthiness of aircraft. One term that is often used in relation to aircraft maintenance is “Since Major Overhaul” or SMOH. So, what exactly does SMOH mean?

SMOH refers to the time since the major overhaul of an aircraft’s engine. It is a critical factor in determining the engine’s remaining life and overall condition. During a major overhaul, the engine is completely disassembled, inspected, repaired, and reassembled according to specific manufacturer guidelines. It is essentially a comprehensive refurbishment of the engine to bring it back to its original or like-new condition. This process involves replacing worn-out parts, repairing any damage, and ensuring that all components meet the required specifications.

Why is Since Major Overhaul Important?

The SMOH value provides valuable information about the engine’s performance and potential lifespan. It serves as a reference point for aircraft owners, operators, and maintenance personnel to determine the engine’s condition and make informed decisions regarding maintenance schedules, repairs, and replacements. Here are a few reasons why SMOH is important:

1. Engine Performance and Efficiency

The major overhaul plays a significant role in restoring an engine’s performance and efficiency. Over time, engines experience wear and tear, which can lead to a decline in their performance. A major overhaul addresses these issues by replacing worn-out parts, cleaning and reconditioning components, and fixing any underlying problems. As a result, the engine’s performance is improved, and it operates more efficiently, reducing fuel consumption and enhancing overall aircraft performance.

For example, a major overhaul may involve grinding cylinder heads, reconditioning pistons, replacing piston rings, cleaning valves, and balancing the engine’s rotating components. These tasks ensure smoother operation, better combustion, and reduced friction, leading to improved efficiency and power output.

2. Reliability and Safety

One of the most critical aspects of aviation is safety. Aircraft engines are subjected to rigorous operating conditions and extreme temperatures. A major overhaul helps ensure the reliability and safety of an engine. During the overhaul process, all components are carefully inspected, tested, and repaired or replaced as necessary.

The meticulous inspection of the engine during the major overhaul helps identify any potential issues or defects that could compromise its reliability and safety. By conducting a thorough examination and adhering to manufacturer guidelines, maintenance personnel can address problems proactively, reducing the risk of in-flight emergencies or unexpected engine failures.

3. Residual Engine Life

SMOH is an essential factor in evaluating the potential lifespan of an aircraft engine. The major overhaul resets the engine’s time and provides an estimate of the remaining hours before the next overhaul is due. This information is crucial for planning purposes and helps aircraft owners and operators make informed decisions regarding their engines.

For instance, if an aircraft engine has a major overhaul that resets its time at 2,500 hours, and the recommended time between overhauls is 1,500 hours, the residual engine life would be 1,000 hours. This information helps determine when the next major overhaul will be due and allows for budgeting and scheduling maintenance activities accordingly.

It’s important to note that the SMOH value is not an absolute measure of an engine’s remaining life. It serves as a reference point and should be considered alongside other factors, such as the engine’s operating conditions, maintenance history, and overall wear and tear.


Since Major Overhaul (SMOH) is a significant term in aviation, particularly when it comes to assessing the condition and performance of aircraft engines. It provides valuable insights into an engine’s remaining life, reliability, and overall efficiency. The major overhaul process helps restore an engine to its like-new condition, addressing wear and tear and ensuring its safe and reliable operation. By considering the SMOH value alongside other factors, aircraft owners, operators, and maintenance personnel can make informed decisions regarding maintenance schedules, repairs, and managing their engines efficiently.


1. https://aviation.stackexchange.com/questions/19702/what-exactly-is-smoh-in-an-engine

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