The careful management of fatigue damage in aircraft is essential to ensure safe operations. In this article, we explore fatigue damage, provide a little bit of background, explain the process of fatigue and damage tolerance evaluation, and where Airframe Designs play a role in the continued safe operation of aircraft.

Understanding Fatigue In Aircraft

Fatigue is a common phenomenon in all metal airframes and components, occurring across all types of aircraft, from military to civilian. During flight, a plane is exposed to extreme conditions and pressures meaning the aircraft will start to age straight away. Frequent use and environmental factors result in the weakening of metal aircraft components (why you will sometimes see aircraft fatigue described as metal fatigue too).

Fatigue damage first occurs as tiny cracks that are not initially visible to the naked eye although over time they may become visible. If not detected or attended to early enough, these cracks can lead to the catastrophic failure of an aircraft during flight.

Every aircraft has an estimated number of flight cycles before it is retired because of fatigue damage, this is known as fatigue-qualified life. However, because different variables can accelerate fatigue, routine aircraft fatigue and damage tolerance (F&DT) evaluation is required on a scheduled basis to ensure the early detection of microscopic cracks. This ensures catastrophic failure will not occur before the end of an aircraft’s life span.

Importance Of Damage Tolerance Evaluation

Damage tolerance is a method used across engineering and not just specifically in aircraft, it relates to a structure’s ability to possess a certain level of damage that will not cause catastrophic failure before the damage can be detected and repaired. In aircraft, early detection means the affected part or parts, can be replaced or repaired, thus preventing a serious accident.

close up image of an airplane wingBackground And Regulation

Fatigue damage in the form of cracks that can compromise the structure of aircraft has been observed for many years. Over time, technologies and processes have been developed to help with the early detection of damage and improve safety outcomes.

A notable case in aviation history of fatigue damage was the 1988 Aloha Airlines Flight 243 incident. A Boeing 737-297 was flying across Hawaii when an explosive decompression occurred after a part of the fuselage broke off. The aircraft made an emergency landing, but one person sadly died, and 65 people were injured.

Upon analysis, it was found that the fatigue damage, alongside a poor maintenance history, was the cause of the incident. This led to research efforts and regulatory action around the globe to help better understand and control the risks posed by fatigue damage. Up until that point, fatigue damage in ageing aircraft hadn’t been a prominent safety concern.

One of the key recommendations after the accident from the Federal Aviation Administration was the requirement that: “all turbojet transport category airplanes certified in the future receive full-scale structural fatigue testing to a minimum of two times the projected economic service life. Also, require that all currently certificated turbojet transport category airplanes that have not been fatigue tested to two lifetimes, be subjected to such testing. As a result of this testing and subsequent inspection and analysis, require manufacturers to identify structure susceptible to multiple site damage and adopt inspection programs appropriate for the detection of such damage.”

In the United Kingdom, aircraft regulations are overseen by the Civil Aviation Authority, which has its own compliance standards about ageing aircraft and fatigue damage. See, Additional Airworthiness Specifications for Operations sub part 26.302 Fatigue and damage tolerance evaluation for more details.

The Process Of Conducting Fatigue Analysis

H135 helicopter in flightFatigue analysis can be performed at the design stage of an aircraft, when significant modifications are made, or on an ongoing basis to satisfy safety regulations. At the design stage, fatigue analysis can be incorporated to improve longevity, aid in better selection of materials, and improve design.

At Airframe Designs, our highly experienced engineers conduct fatigue analysis using either a stress-based (s-N) or strain-based (e-N) approach, developing the necessary analysis tools from first principles to suit the analysis requirements and needs of each customer.

A recent example of our fatigue analysis capabilities can be found in our Airbus H135 case study. In addition to static strength substantiation, our engineers established an adequate fatigue life for load-carrying components that sit between the H135 main lifting dual hook and a human cargo basket. For Airbus designed metallic parts, a loading spectrum was developed, and a safe life was determined for each critical feature by referencing material S-N data and applying an appropriate scatter factor.

Conducting Damage Tolerance Evaluation

Damage tolerance evaluation aims to identify potential damage to an aircraft in the form of cracks, and predict the growth of cracks, and their potential impact. This is incredibly important to ensure an aircraft can stay operational and avoid serious incidents.

Accumulated fatigue damage is inevitable in aircraft, but routine maintenance, including damage tolerance evaluation, can lengthen the longevity of an aircraft. Left unchecked, crack growth can result in severe damage to the structure and components of a plane. Whilst in flight, this could mean affected areas fracturing and breaking off, or the aircraft skin might peel off or ‘unzip’.

We evaluate damage tolerance at Airframe Designs by conducting crack growth assessments using our own in-house scripts. Client-supplied software or commercially available tools such as AFGROW or NASGRO are also utilised by our engineers.

Airframe Designs

We hope you found our short overview of fatigue and damage tolerance interesting or useful. You may wish to take a look at all of our analysis capabilities here.

Founded over ten years ago, Airframe Designs provides specialist engineering and analysis services to customers primarily in the aviation, defence, and space sectors.

If you wish to speak to us about fatigue analysis or damage tolerance evaluation, please don’t hesitate to contact us.