Proposal for Aircraft Service Life Extensions Part 2: What Influences Aircraft Aging?
Statistical research on airplanes worldwide has shown that structural issues with aircraft and their components worsen over time. Most of these issues are connected to impacts like corrosion, brittleness, porosity, contamination, wear and tear, leaks and isolation damage.
The following list will help to clarify the complexity and challenges that airplanes face throughout their service lives by highlighting the factors that influence aging.
Operational conditions
An aircraft’s operational conditions typically change over time. When thinking about the potential for service life extension, it’s important to review the current operational conditions and potential future ones for both new and old aircraft, investigating and comparing them with the original design specifications.
This can be done through engineering assessments or, if the conditions cannot be established on a theoretical basis, by programs that measure operational loads. These programs can help determine life potential and current clearances, or they can serve as a foundation for additional qualifying testing. Since these programs can be costly and time-consuming, it’s crucial to implement them early in the evaluation process to ensure timely results on whether life extension is possible are available.
Usage history and life consumption
To review an aircraft’s usage history, it’s key to ensure that all necessary in-service documents are available before attempting an analysis.
However, gathering all the data required to evaluate life consumption and service life can sometimes be challenging. This can cause safety issues for components like wings that have limited designated lifespans, since it may be difficult to determine if the wing has reached the end of its useful life.
This challenge highlights one reason why the proposed research would take place in Jordan, as the usage history and life consumption of Jordanian aircraft are meticulously documented due to the strictness of the Jordanian Civil Aviation Regulatory Commission (CARC).
Environmental conditions
Aircraft are often exposed to harsh environmental conditions. Temperature, ultraviolet (UV) radiation and pollution all have a significant impact on how quickly an airplane ages and degrades.
For example, in Jordan, the air temperature difference that a Piper PA-44-180 SEMINOLE experiences while in service is about 65°C, transitioning from -15°C at a flying altitude of five kilometers to over 50°C in airports.
Other types of aircraft may have air temperature differences exceeding 100°C due to doubled ceilings and the geographical location in which they operate.
Additionally, the temperature of an aircraft’s coatings and skin surfaces is usually much higher than the air temperature, especially for dark colors. This means an aircraft with a darker coating could deteriorate more quickly than a light-colored aircraft, even if they encounter the same environmental conditions.
Wetness is also a known factor influencing the life and state of aircraft, meaning those in dry climates can often maintain better condition than aircraft in wet climates, even if they have similar usage histories.
Inspections and inspection efficiency
As airplanes age, they require more frequent inspections to ensure they don't deteriorate to a dangerous condition before the end of their designated service life. This involves choosing the right methods for testing, clearly specifying the necessary inspection protocols and confirming that trained staff are available to complete the inspections.
When aircraft don’t receive inspections at the right frequency or with the necessary scrutiny, they’re more susceptible to effects of aging and corrosion.
Corrosion as the main influence on aging
The deterioration of a material or its characteristics brought on by the substance's reaction with its chemical surroundings is known as corrosion. Depending on an aircraft’s operational environment and perceived demands, each operator approaches the danger of corrosion in a different way.
Corrosion and corrosion fatigue are key examples of visible damage and effects of aging on airplane structures. An aircraft structure's corrosion and corrosion fatigue failure processes are directly impacted by elements like load, material properties, corrosive environments and more.
The continuous safe operation of an aircraft is seriously threatened by corrosion and fatigue, both individually and when they occur simultaneously, compounding their effects on aging.
The Federal Aviation Administration (FAA) and the former Joint Aviation Authorities (JAA) have established standards about the proper construction and maintenance of aircraft to reduce the likelihood of fatigue-related failures.
However, airworthiness regulations and requirements provide fairly limited guidance on corrosion, despite the fact that it has a severe impact on structural integrity. They simply state that every component of the aircraft must be "properly protected against deterioration or degradation in strength in service due to any cause, including weathering, corrosion and abrasion.”
It has been historically challenging to evaluate how future corrosion, either working alone or in conjunction with fatigue, may affect an aircraft’s structural integrity. To mitigate this, work has been done to develop a framework for evaluating how corrosion and fatigue affect the structural integrity of an aircraft and its components.
For my example in Jordan, future corrosion should be, theoretically, simpler to gauge than in other climates. This is because Jordan’s climate is arid and not salty, which helps to limit common issues caused by corrosion in aging aircraft. Jordan’s climate is considered mild, which makes it an ideal environment for preventing the stimulation of rust.
This is a significantly more challenging issue in coastal and tropical areas, since aircraft that operate in those locations are exposed to constantly changing temperatures, humidity and salty air. Having gained more than two decades of expertise working as an engineer on different types of aircraft in Jordan and other countries, I can confirm that aircraft corrosion in Jordan is not rare, but it does have a much lower average rate in comparison to nations in harsher climates, making it an ideal base for this research.
In the next installment of this series, I’ll discuss how these factors and other stimuli directly impact the service life of an aircraft and its components in addition to general aging. This will showcase how the effects of aging caused by various environmental factors can cause structural fatigue and accelerated degradation that pushes an aircraft closer to its manufacturer-designated service life limit.
I’ll also provide more detail about service life limits set by manufacturers, with an example that highlights the need for the service life extension program being proposed.
