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07 Apr

Aviation Metal Fatigue: What You Should Know Now

If you’ve ever taken a paper clip and bent it back and forth until it breaks, you know at least a little something about aviation metal fatigue. It’s the cyclical application of pressure on the metal of the paper clip that eventually causes it to snap. The same can happen with aircraft.

Aviation metal fatigue describes the effect of cyclical pressure and stress on metal parts. It’s particularly dangerous for aircraft since much of the aviation today involves at least 1-2 human lives on board in addition to any cargo. When a plane can’t handle the repeated pressure of loading and unloading, crashes are a huge possibility.

In fact, recent cases prove what engineers learned long ago – aircraft metal can only handle so much loading and unloading. The exact number of cyclical loads an aircraft can accommodate can be pinpointed with scientific precision.

Testing to Determine Metal Fatigue Levels

It’s has not always been possible to determine exactly how long it takes before metal fatigue can cause dangerous damage to an aircraft. However, recent science and engineering has developed a tried and true method.

The first step involves determining a set parameter by which to assess maximum breaking point for a certain type of metal or aircraft. The parameter, called the Limit of Validity, or LOV, sets the standard for metal fatigue test evidence.

According to the Federal Aviation Administration, the LOV can be defined as “the period of time (in flight cycles, flight hours, or both) up to which it has been demonstrated by test evidence, analysis and, if available, service experience and teardown inspections, that widespread fatigue damage will not occur in the airplane structure.”

Basically, the LOV is a measure of the amount of time a particular aircraft has before widespread metal fatigue becomes an issue. It’s a pretty important measure for several reasons.

One is that metal fatigue can be extremely hard to detect without knowing the LOV. Also, the onset of widespread metal fatigue as tiny cracks around screws and other aircraft parts can sometimes happen so rapidly that it can’t be stopped in time to prevent dangerous crashes and accidents.

Aircraft Metal Fatigue and Dangerous Crashes

Crashes attributed to widespread metal fatigue have happened as recently as the 1980’s. These days, the FAA requires aircraft carriers to take a number of additional precautions in order to prevent widespread metal fatigue, especially with older planes. In fact, the FAA requires retirement of an aircraft once limitations containing the LOV have been reached.

One of the most well-known aircraft accidents to occur due to widespread metal fatigue happened in 1988. The flight became so legendary that it was picked up by producers and turned into a TV film in the 1990’s called Miracle Landing.

The inter-island flight, Aloha Airlines Flight 243, took a roundtrip course from Hilo to Honolulu, Hawaii. Everything about the trip began as normal, but along the way, the rapid growth of widespread metal fatigue cracks caused a portion of the roof to come completely off the aircraft.

Miraculously, the pilot was able to successfully land the aircraft at the nearest site, an airport in Maui.

Other notable crashes due to metal fatigue include the Comet jet crashes of the 1950’s. Comet jets, as many aircraft enthusiasts may already know, are the famous British precursors to the era of U.S. Boeing 700’s which began cornering the market in the late 1960’s.

The first jet passenger airplane was a Comet put into commercial flight in 1953. Just a year later, however,  a series of three crashes due to metal fatigue caused all of the famous jets to be pulled from service, although newly designed Comets did come back into service later.

A world-wide survey published in the International Journal of Fatigue in 1981, that that between the years of 1934 to 1981, a total of 306 aircraft accidents could be attributed to aircraft metal fatigue which resulted in 1803 deaths.

Metal Fatigue and the FAA

These types of crashes are much less frequent in modern times due to the FAA’s vigilance in determining accurate aviation limitations using the LOV parameters.

In fact, the FAA requires both manufacturers and operators of aircraft to submit information about LOV levels according to a set schedule. These FAA rules have been in place since 2011 and operate to ensure that aircraft approaching maximum levels of cyclical loading and unloading can be retired well ahead of accidents and crashes.

The FAA’s Aging Aircraft Program also plays a significant role in preventing accidents due to metal fatigue. The program began in March 2006 due to several concerns, including those brought on by the 1988 Aloha Airlines crash.

Mainly, the Administration was concerned that many aircraft were being operated beyond their design limits and that a number of inspection criteria did not cover age-related issues for aircraft.

 

As part of compliance with FAA rules, manufacturers also must adhere to several airworthiness directives (AD). These directives include their own thresholds for airworthiness and require manufacturers to makes costly inspections and repairs before these thresholds are reached. The AD focus mainly on identifying areas which are most susceptible to widespread metal fatigue even before the aircraft reaches LOV thresholds.

 

Even though these inspections and repairs are costly and require extensive scrutiny for the likelihood that metal fatigue will occur, the FAA is extremely adamant about enforcing the rules. Also, major carriers like Boeing have agreed not to support the use of planes that fail either AD inspections or that reach LOV thresholds.

 

Can Aircraft Metal Polish Prevent Metal Fatigue?

 

Can aircraft metal polish play a role in preventing or reducing metal fatigue? The answer to this question according to scientific study, is yes. Researchers have found that polished surfaces have a much higher fatigue life than rough, non-polished surfaces. This means that the kind of polish you choose can have a significant impact on the risk of metal fatigue, and the dangers it can cause, for your aircraft.

 

In fact, research suggests that components subject to high cyclic loading should definitely be ground and polished to ensure high fatigue life. Only the the best aircraft metal polishes can remove the scratches and nicks that could lead to widespread metal fatigue. For best results, and to ensure the safety of your aircraft, choose an aircraft metal polish that is aviation approved for trustworthy results.

 

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