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Dangers of shock loading

All helicopter pilots understand the dangers and difficulties inherent in aerial winching and hoisting. One of the less understood dangers is that of Shock Loading – loads which exceed the static load caused by rapid movement changes such as swinging, impacting or jerking. CHRIS SMALLHORN reports.

There was a time, not so long ago, when the critical risks associated with the shock loading of helicopter hoist wires were not appreciated. I recently spoke with a colleague who has working as a crewman in military and civilian helicopters for more years than he cares to remember. He told me of some of his early experiences doing cliff winching where taking shock loads on the hoist was not all uncommon. While not intentional, a slip of a person from a cliff edge, over-control of the aircraft in poor visual conditions or perhaps over-eager winching techniques with some of the older equipment, often resulted in sudden application of excessive loads, otherwise known as “shock loading.” The associated risks and just as importantly, why the risks exist – were not taught in training. I would like to think that throughout the helicopter industry, the cautions relating to shock loading during winching or hoisting have become well known. Even if this is so, one can never have too much information on the “whys?” and “what fors?” applicable to our craft. Hoisting of personnel from beneath a helicopter is a risky business at the best of times. To minimize the risks it is the responsibility of all operators to understand them, and more importantly, to understand what mitigations are available, whether by way of technique or engineering.

What is shock loading?
Let’s look at a few common engineering terms used in cable dynamics and loads. A cable has what is called in the engineering world a Working Load Limit (WLL) which is the maximum load that should ever be applied under any conditions. Note however, that the WLL is based on the load being uniformly applied in a straight line pull. The flight manual rated loads applied for operations takes this into account and you may be reasonably assured that the load limit in your flight manual is significantly less than the cable WLL.

The “Breaking Strength” is an average loading at which cable samples were found to break under laboratory conditions in straight line pull, using a constantly and predictable increasing load. The Breaking Strength is not used for design or cable load rating purposes – the WLL is used instead.
Shock Loads are those loads which exceed the static (or simple hanging/straight line) load, caused by rapid movement changes such as swinging loads, impacting or jerking. I would assume that all readers who are somewhat experienced in hoisting or winching missions are thinking, “Yep, I’ve seen each of those at some time.”

Steel cables, such as those used on helicopter hoists, are actually like a rope, in that they stretch – although to a far lesser degree. The amount of stretch is directly proportional to the length of cable deployed and the load being carried. The more cable that is deployed and the greater the load, the more physical stretch is exhibited by the cable. The more stretch that the cable is capable of undergoing, the more it is able to absorb shock loading. In short, the more cable deployed the more the cable can absorb shock loading, and the shorter the cable, the more susceptible the cable will be to damage or catastrophic failure from shock loading. This brings up an important note – arguably the most critical part of the winching process where shock loadings may occur is in the last few feet of the recovery. Here the rescue crewman and/or survivor are trying to access the cabin where it is most likely that a slip or fall may occur, imparting heavy shock loads to the cable. Furthermore, the shock loads in this position are also likely to have a lateral aspect, with a small swing developing. Maintaining little to no slack in the cable is imperative during this phase of the recovery.

The Math
In order to understand the magnitude of force that can be applied to a cable under a shock loading condition we must first explore the math and physics of the problem. In order to simplify, we’ll assume our cable acts like a simple spring. This is not a leaping assumption, but an engineering reality. The cable,
like a spring, has a spring constant defined by the amount of “stretch” or elongation that will occur for a given static weight. The spring constant is a measure of stiffness of the cable and is a fixed value for any given length of cable. A typical helicopter winch cable is a wound 3/16 inch steel cable. A reasonable figure to work with for our purposes is that the cable will stretch approximately 0.334% of the payed out cable length with a static 600lb (272.15kg) mass hanging from the hook.
The spring constant is calculated from two fairly simple (metric) formulae:

EQUATION 1
F = k*x
or
k = F/x
where
F = Force (in Newtons, N)
k = spring constant (in Newton per meters, Nm-2)
x = cable stretch or elongation (in meters, m)

Note that mass must be converted to force – so:

F = m*a (Newton’s Law)
where:
F = Force in Newtons (N)
m = mass in kilograms (kg)
a = acceleration in meters per second squared (ms-2) and in the case of a mass being affected by gravity, the acceleration value equals 9.8066 ms-2.

Our 272.15 kg mass therefore equates to a force of:
F = 272.15 * 9.8066 = 2668.87 N

For a 61 meter (200ft) cable with 0.334% cable stretch, the cable will stretch 0.2032 meters (8 inches)

Therefore, for that 61m cable:
k = F/x = 2668.87 / 0.2032 = 13134.18 Nm

If we plot force v. elongation, the slope of the curve remains linear until the cable reaches its elastic load limit. The elastic limit is the point at which the cable will no longer return to its original length after a load is removed. At the elastic load the cable is said to “yield”. Now, unlike a natural fiber rope, steel once it has yielded, actually elongates faster for a given increase in load than it did before yield. Therefore, exceeding a yield limit in a steel cable is not where we want to be, and manufacturers fortunately ensure the yield limit is sufficiently high that our static loads don’t come close.

Accordingly, we need only consider the linear portion of the curve. Note that the spring constant, k, varies for a given length of cable. Equation 1 (on the previous page) shows the amount of stretch or elongation for a static 600lb (272.15 kg) mass for varying lengths of payed out cable. Using Equation 1, Figure 2 shows the spring constant (k) for varying lengths of payed out cable. Note that the spring constant becomes exponentially larger, the shorter the payed out length of cable, which makes sense as the amount of force required to affect a given amount of stretch will increase dramatically for a shorter length of cable, as there is less available cable to stretch.
So as you can see, there is very little stretch in a cable, and critically, the amount of stretch is significantly less when the cable is shorter; but what little there is, is all important in the business of absorbing shock loading. The more stretch, the more energy absorption the cable can facilitate. The level of stretch controls the critical issue of deceleration of a mass once a shock load is “taken up.” That is to say, the time taken to slow down a load that is rapidly applied to a cable is less – the shorter the cable. The longer the cable, the more spring is evident, hence it will take slightly longer to slow down the load. The resultant rate of deceleration is all important when it comes to the magnitude of shock load that is applied to the cable. The magnitude of the loads can be extraordinary, as we will now calculate in Equation 2.

The term “shock loading” is, in the purist sense, a little misleading – but suitable for our purpose. If we think of a shock occurring in a very short period of time, that is in a fraction of a second, then we are on the right track. When a mass falls it builds energy. The mass is accelerating towards the ground, and for the short periods we are talking about, drag and friction are negligible. The mass accelerates towards the ground at the acceleration provided by gravity, that being approximately 9.8ms-2. So a mass accelerating towards the ground for half a second would be traveling at approximately 4.9ms-1. Note that in that half a second the mass would fall 4ft (1.2m), which is pretty extreme. Provided good cable management techniques are employed, it is highly unlikely to occur except possibly on yacht rescues.

Firstly, let us assume that the crewman and survivor have fallen for half of a second, and once the cable begins to take the load it requires 0.15 seconds to stop the fall, or decelerate the mass to zero speed. The period of deceleration will vary depending on the length of the cable. It is this rapid deceleration time that causes the shock loads and the rate of deceleration can be calculated as follows:

To put this figure into context, it is the equivalent of 906 kg (1,998.6 lb) of mass, or 3.33 times the original weight of our crewman and survivor. It is this force or load, that the cable must absorb in order to carry the “shock load.”

Note that the deceleration time is critical and will be directly dependent on how much cable is payed out. In the previous example, had the deceleration occurred in 0.1 second the resultant dynamic force applied to the cable would be the equivalent of 1,360kg (2,994lb), or five times the original weight.

The calculations we have provided are relatively simple and valid only for the specific examples discussed. Here we have used simple Newtonian physics to give you an appreciation of the loads involved. The pure engineer will use work and energy relationships with a knowledge of cable elasticity. Results using this method will yield similarly alarming results.

In addition to the effect on the cable and hoist mechanism, an understanding of these calculations will show how dramatically shock loadings can impact on the flyability of the aircraft. It is not difficult to imagine the effect of a brief and almost instantaneous effective weight increase, equivalent to several extra people, applied to the helicopter when maneuvering in sometimes marginal conditions.

Beyond the Cabin
Hopefully, in light of these simple examples, you can appreciate how easily extraordinary loads can develop. Cable management techniques are critically important in ensuring that cable damage, or worse still cable failure, does not occur. In using the term cable management we should extend this beyond the cockpit/cabin to the maintenance arena. Like the rest of an aircraft, the cable is an aeronautical product that requires detailed and regular inspections, coupled with a rigid maintenance regime. In my experience most organizations have got this aspect well under control. However, here are some basic checkpoints to ensure your team is on the game.

Cables require:

• Washing and oiling – extremely important in salt environments
• Regular inspections. Traditionally this has been done visually looking for broken strands, kinks, corrosion etc. The visual method is still important – however equipment that x-rays the cable while washing and oiling it does the best job.
• Hoist inspections and pre-flights must include assessing the unit for smooth running – no binding or restrictions, and ensuring the cable winds correctly on the drum.

It is in the cabin, however, that the “rubber meets the road.” Cable management is a discipline that must become second nature to the winch- and wire-man alike. Ensuring that there is always minimal cable payed out (i.e. minimal slack), will go a long way to reducing the likelihood of a shock loading incident. Winching directly to yachts or small boats can be perilous and should be avoided, particularly in a rough sea state. The opportunity for a shock loading incident whilst conducting winches to a dynamic
vessel provides the opportunity for snag hazards and sudden weight application, if the vessel is to drop away suddenly or the wireman is to slip or fall overboard. The issue is further complicated at night, as you might imagine. Techniques such as Hi-Line and Floating Line access should be used wherever possible to reduce the possibility of a cable snag.

Fast Roping
Some military and para-military operators are, through necessity, attaching the fast rope to the winch hook – but this is not the preferred method. It must be underpinned
by rigorous engineering analysis, although it is unlikely that the manufacturer will agree that it is a good idea. Typically, manufacturers do not endorse abseiling or fast roping from a housed or extended hook. The ropes used in these evolutions will absorb much of the associated loads due to their being able to stretch. The residual loads, however, will be transferred directly to the hook and cable. With very little extended cable, referring back to our discussion so far, all of that load and more will be transferred to the cable. If this is the only option, and fast roping or abseiling must be done for the mission at hand, it is imperative to approach the manufacturer and ensure a thorough engineering analysis has been completed, providing the necessary safety margins to use this configuration.

Clutching the Solution
Hoist systems now coming on to the market are addressing the issue through engineering design. Breeze Eastern, for example, has introduced a Reactive Overload Clutch (ROC) that is designed to recognize a shock loading event and allow the cable to pay out commensurate with the load experienced. The best way to think of this system is that it operates similar to a drag device on a fishing reel.

Shock loading on cables may not result in the cable breaking, but will certainly have an affect on the life of the cable. There is no real way of knowing how much shock load has been applied to a cable during its life, or indeed on any given event. Good SOP is to educate crews on the dynamics of shock loading; how to manage it and to report it to maintenance when it does occur, so that an on-condition inspection can be made. The forces associated with shock loading can be very high and a mismanaged winch evolution, or snag during a rescue, can easily result in cable breakage. In that event, at best you’ve lost the SAR asset and can no longer fulfill the task at hand – at worst somebody was on the cable and the dire consequences of that scenario are obvious to all of us.