This paper presents the results of a recent study undertaken by Barry. The objective was to determine the average residual strength of helicopter synthetic longlines which had been in service for various numbers of years. The purpose was to establish service life criteria for helicopter longlines and thereby provide guidelines and recommendations on retirement criteria for end users. The resulting data obtained was plotted and a graph was created to act as a reference guide found in the Barry helicopter longline User Instructions Manual.
"The objective was to determine the average residual strength of helicopter synthetic longlines."
The use of HMPE (High Modulus Polyethylene, such as Dyneema®, Spectra® or Plasma®) synthetic helicopter longlines for underslung external load transport is widespread and has been adopted for several decades by operators and regulatory helicopter agencies worldwide.
Synthetic longlines are lightweight and easy to handle. They do not corrode. New longline and jacket cover designs can compensate for the inferior wear properties of the synthetic fiber rope compared to steel cable, and take full advantage of the other fiber properties. They provide superior tension and bending fatigue performance compared to wire ropes. Thus, the new forms of synthetic longlines are ideal for many helicopter external load applications
Recently, there has been an increase in interest concerning the safety of materials used in the manufacturing of helicopter longlines, particularly since there were reported cases of failure of longlines resulting in significant loss of valuable goods. While the exact causes of these accidents are often nebulous and may be due to incorrect splicing methods, misuse or other factors, some are currently under investigation.
This brought about some questioning as to exactly when does a synthetic longline become obsolete?
"This brought about some questioning as to exactly when does a synthetic longline become obsolete?"
One of the determining factors affecting longline integrity is prolonged exposure to cyclic loading. Information regarding the "service life" of longlines is either sketchy or contradictory, and altogether not well documented.
The present study was initiated by Barry as a result of discussions held at HAI and HAC meetings, whereby several operators and industry officials requested the undertaking of a pilot project to investigate the residual tensile strength of helicopter longlines for external load operations.
The purpose of this pilot project was to establish the service lifetime of helicopter longlines based on actual testing, and to document and establish an inspection criteria and methodology which may be easily adopted by the industry.
Use time and identification of existing longlines is often problematic since proper inspection forms and logbooks are generally not used even though they are available. The lack of proper inspection and logbooks, combined with the lack of control samples, namely new and unused material, make longline users unaware and misinformed about whether their longlines are still safe to use.
The Barry study intends to provide a guideline to users to assist in determining whether their longlines are still safe to use based on their age and in-service use.
Of course, pre-use and after-use inspections are and shall remain an essential part of a longline documentation process and as such will continue to prevail. The actual lifetime depends on the intensity and frequency of use, as well as the environment of use. One exceptional circumstance might limit the product lifetime to a single use.
Barry obtained several used longlines from helicopter operators worldwide, that had been used for different periods of time under various environments in a multitude of missions, carrying loads ranging from 1000 lbs to 6000 lbs, thereby providing a cross section of uses throughout the industry.
The longlines received were between 1 and 11 years old and their diameter varied between 7/16" and 5/8". For this present study, the main focus is on 1/2" diameter longlines although data is also presented on the 5/8" for comparison purposes. All the tested longlines were made of HMPE fiber rope.
The samples were made by removing an end section from the worst looking side (usually the bottom end) of a longline. The cut end of the sample was re-spliced to perform the break test and obtain the tensile strength.
Figure 1 - Used longline spliced sample prior to break test
For some longlines, several segments of the same longline were cut and tested because there was no "worst looking" section. In those cases, only the minimum break test result was reported.
Initial (new) strength reference was taken from current published information by rope manufacturers.
The study demonstrates that as soon as longlines are put in service, they begin a gradual and irreversible loss of tensile strength over time.
"as soon as longlines are put in service, they begin a gradual and irreversible loss of tensile strength over time."
Some longlines, used intensively or incorrectly, had lost 40% of their initial strength after only 1 year of use.
At the other end of the spectrum, another longline that was never used (shelved) or maybe used once, had lost only 12% of its initial strength 4 years after its manufacturing date. This demonstrates that decay in the material properties must be expected even if the longline is never used.
As another example, even though this is outside the scope of this study, a 1-1/2" diameter heavy-lift longline had lost 58% of its initial strength after only 2 years of use. This longline had been used intensively carrying very high loads.
With the break test results displayed on a graph (age versus residual break strength), an exponential curve was then drawn to visualize the average loss of strength over time (figure 2).
Figure 2 - Average residual break strength of 1/2" longlines
To better illustrate the spectrum of longline conditions that may be observed on the terrain, two average curves were made (figure 3) by taking the best residual strengths (top edge of red section) and the worst residual strengths (bottom edge of red section) from the tested longlines.
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