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Metal fatigue is a dynamic phenomenon. A helicopter in storage will not develop fatigue cracks!
Generally speaking, in order to occur it requires moving parts, of which a helicopter has plenty in comparison to a fixed wing aircraft.
These moving parts represent a mechanism for the application, and subsequently removal, of a force or a load from one component to another i.e. On..Off...On..Off... On..Off.. On..Off..etc.
Each one of these “On..Off’s” represents one fatigue cycle. It is these fatigue cycles that are responsible for the slippage occurring in the atomic planes that we call metal fatigue.
What is particularly noteworthy is the relatively small magnitude of applied load required for a fatigue crack to occur.
This load, and hence the accompanying stress applied to the component, need only be around 30% of the ultimate load or stress the component is physically able to withstand.
This load, however, is generally required to be applied many thousands of times (fatigue cycles) in order for fatigue cracking to take place.
The fatigue crack, once developed, will continue to grow, seemingly with a life of its own, up until a certain point. This crack growth rate can be anywhere between 0.0000001 - 0.001 mm per fatigue cycle.
Eventually, the crack becomes so large that the component is no longer physically able to carry the load for which it was designed.
The remaining cross-section of the component i.e. the area as yet unaffected by the fatigue crack, subsequently fractures, due to overload, and the component fails.
Such a failure can have significant safety of flight implications.
Enter the term coined by TC – “Metal has Memory”.
All the different metal components that make-up a helicopter “remember”:
· every single load and stress (fatigue cycle) that they have ever been subjected to
· for every single hour, minute and second of operation
· for every one of its previous operators – including you.
The net effect of all these fatigue cycles is cumulative and non-reversible throughout the life of the component.
It is for this reason that the helicopter’s total number of hours in service, as opposed to time since new, is so important.
Recording In-service (Maintenance Release) hours is one of the most effective ways of measuring the fatigue life of metal components.
What Causes Metal Fatigue?
Many different factors contribute to the particular fatigue crack propagation rate in any given component.
Each different size of every different type of metal or alloy has its own unique and individual susceptibility to fatigue.
The most significant factors affecting fatigue include
Mustering operations are likely to subject the helicopter airframe to higher stress levels that that of normal flying operations.
For instance, even one particularly steep (high G) maneuver could have the equivalent fatigue effect on the helicopter’s structure or rotors as a week’s - or even a month’s - worth of normal landings and takeoffs.
A given component’s fatigue life, and hence time to failure, is in this way reduced by a proportionate amount of time to what it otherwise might have been if the sever maneuver had not occurred.
Similarly, conducting flying operations on a regular basis will subject the airframe to more stress and hence cumulative fatigue cycles, than would be the case if the helicopter was only flown on an infrequent basis.
It is for these reasons that the use of bogus or life-expired airframe components should never be contemplated.
Although from external inspection, the component may appear to be in reasonable condition, only detailed metallurgical analysis can accurately determine the true state of its structural integrity.
The installation of a component of this nature also brings with it more than you may have bargained for: its memory.
Such a component could be the weak link in the safety chain.
By installing this component, you also get its entire collective history of accumulated fatigue cycles and any damage, despite the fact that it may “look all right” from the outside.
An easier way to assist with understanding the nature of metal fatigue is to think of it as a modest inheritance in your bank account.
Unfortunately, there is no mechanism for making deposits and topping up your account.
There is only so much money in the bank. When it’s gone ….. it’s gone.
How the Manufacturer Fits In
Nobody knows better than the manufacturer where its product’s potential fatigue “hot spots” might be.
The manufacturer is obliged to go to a great deal of trouble, from an early stage, to conduct simulated fatigue crack tests on the helicopter’s flight critical components.
This is not only done in order to verify that the design data used is correct, but also to try and establish whether or not any potential problem areas exist.
This process commences long before the in-service fleet approaches the equivalent number of fatigue cycles and is undertaken by installing the components on test rigs.
Most of us are familiar with the term “Certification”. In essence this means that a manufacture has proven the safety of his product – including fatigue data - to a Regulatory body such as the FAA or CASA.
Should a particular component on test develop a fatigue crack, it then becomes the subject of close scrutiny.
Such details as the number of fatigue cycles required before the crack first appears, as well as the number of fatigue cycles to when the cracks size threatens the structural integrity of the component are accurately recorded.
Accordingly, an appropriate Service Bulletin, servicing schedule amendment or maintenance instruction is promptly developed and issued to industry.
These instructions will typically specify that at a certain number of in-service hours, which is equivalent to the number of fatigue cycles the test piece was subjected to on the test rig (including a safety factor), certain inspection, maintenance or replacement action should take place.
The importance of conforming to this critical fatigue management information is absolutely paramount from a safety point of view.
Similarly, this kind engineering feedback can also be gained by the manufacturer from accidents and incidents in the field as well as from the advice of operators with high time, in-service helicopters.
This paper was intended as a brief introduction into the nature of metal fatigue.
From an operational point of view, it is not necessary to have an in-depth knowledge and understanding of the subject - that’s for the rocket scientists.
However, it is extremely important that every one involved in the operation of all helicopters complies fully with everything these rockets scientists have to say.
To do otherwise is to act at your own peril, as well as those who may operate the helicopter after you - inheriting every one of your fatigue cycles.
The Golden Rules of Fatigue Management
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