Helicopter hoist operations (HHO) can involve either humans or cargo. In either case, the load is suspended under the hovering helicopter and unless specific measures are taken to counter it, there is a strong possibility that the load will start to spin. Apart from being very unsettling, especially to the human at the end of the line, there is the possibility that some very real dangers develop from the spinning load. “Spinning is a potentially very dangerous situation that could lead to, at best, extreme disorientation and/or injury to the winchman/casualty or, at worst, the hoist cable being cut, with little control over the consequences to the winch man/casualty,” said Andy Tillion, Senior Pilot at SAR training specialists HeliOperations.
Spinning can become violent and potentially cause damage to the cable and the helicopter
“Spinning can become violent and potentially cause damage to the cable and the helicopter. There is also a risk of the rescue swimmer/winchman losing consciousness due to rapid spinning,” explains Kim Gardberg, search and rescue (SAR) Standard Pilot at CHC.
The main cause of spinning during HHOs is the effect of the downwash from the helicopter on the individuals or the other loads attached to the hoist cable – usually in low-wind conditions – in combination with the load’s aerodynamic shape.
According to Bob Cockell, President of Air Rescue Systems (ARS), most of the time issues with spinning and conical rotation can be directly attributed to just a few factors:
Misunderstanding of the physical laws affecting the drivers creating unwanted load movement
Misunderstanding of the techniques involved
No training process to achieve desired results
Lack of technical expertise/experience
Organically grown training and operational modes – aka ‘the way we’ve always done it’.
He said: “We have to do a better job of educating the rescue community and providing a clear path to both solutions and alternatives for these input factors that affect their operations.” Rotor flow, he added, is a patterned output that can be anticipated and therefore the unwanted effects of it can be mitigated.
The risks of spinning
According to Rob Thomas of the School for Mountain Leadership in South Africa, the main safety risks associated to spinning are vertigo, disorientation and nausea, object strike or entanglement, centrifugal forces, skid/step rub and force inputs onto the aircraft. While vertigo, disorientation and nausea are unpleasant, they are not really life threatening – at least, not until a person is put down onto terrain that is itself hazardous, i.e. where the person either needs to fend off objects during the hoist, or needs to be agile and spatially aware once s/he is down. “Then the disorientation may become life threatening. It may also become problematic for the aircraft if the person is unable to fend off sufficiently to avoid entanglement of the hoist cable with obstacles,” said Thomas. “Nausea itself does not become problematic until a person is unable to protect the airways, such as when immobilised and strapped down on a stretcher, which is often the case during an air rescue. Apart from the obvious risk of drowning, there are other problems which may arise and impair or even cause the death of the patient, i.e. lung damage from stomach acids or lung infection from fermenting and partially digested foods.”
A spin that involves oscillation may result in an object strike for the load
HHOs very seldom occur in wide open spaces. Even maritime hoisting operations are subject to the hazards of a ship’s superstructure, masts, derricks and the likes. A spin that involves oscillation may result in an object strike for the load. “If it is cargo you may end up damaging the load. Having a human load suffer an object strike is undesirable – especially since a fair number of the people we load onto hoists are already injured and are being rescued. Equally undesirable is to have a stretcher attendant getting crushed between a stretcher and an object. Some spins can be so fast that an object strike could cause serious injury,” Thomas explained.
There is at least one recorded case of a patient being ejected from a stretcher because of a violent spin
As the loads spins faster and faster, the centrifugal force increases the forces exerted on the body. These forces can get to the point where a load can start to disassemble mid-air if the forces are experienced in a direction not anticipated in the assembly of the package. “There is at least one recorded case of a patient being ejected from a
stretcher because of a violent spin, resulting in a 30 ft ground fall and a prolonged stay in hospital,” noted Thomas. “Prudent rescuers now secure their patients in such a way that they cannot be ejected from the stretcher, regardless of the centrifugal force. A sufficiently violent spin can have such outward pull that any stretcher attendant is pulled horizontally outward from the stretcher and may sustain injury from attempting to hold on instead of relying on their tether to the stretcher package.”
When the load starts to spin, there is a possibility that the hoist cable will start to rub on a step or skid. The conical rotation is the most destructive aspect, and individual strands of the cable can be parted. “While the damage to these individual strands is not likely to cause the cable to weaken sufficiently to break, they do create flying ends. These strands might become sufficiently pronounced to snag on the hoist assembly during the retrieval of cable, rending the hoist unable to retrieve the full length of the cable and therefore making it effectively inoperable,” warned Thomas. “If you do not have a dual hoist installation it becomes particularly problematic if there is no opportunity to land nearby (i.e., within ±10 n.m.) to recover whoever may be on the hoist. It may even force you to abandon whoever is still down below waiting to be hoisted.”
At any point in the oscillation, an oscillating load generates a horizontal force, he explained: “This force will be in the same direction as the load is relative to the centre of gravity of the aircraft. While it may not be great, it is an additional ever-changing force to be countered in addition to having to keep the aircraft stable, often in already-demanding conditions. The smaller the aircraft, the greater the effect of this force input will be.”
Spinning prevention techniques
The techniques to use to avoid or prevent spinning during HHOs fall mostly in the prevention category and include a safety assessment, a reduction of the surface area and making the load symmetrical.
the common preferred technique employed is to deploy the winchman in an offset position with the aircraft moving forward
According to Tillion of HeliOperations, a safety assessment of the downwash effects should be made as part of the pre-hoisting brief. “If, due to the wind conditions and the given situation, the downwash effects are manageable, the hoisting will be conducted in the normal fashion without any modification to the profile. If, however the downwash is considered an issue, the common preferred technique employed is to deploy the winchman in an offset position with the aircraft moving forward to effectively create wind and push the downwash behind the aircraft.”
Any surface area provides something for the airflow to push against. This surface area can be reduced either by making it permeable to air or making it smaller. As an example, Thomas points out that a person being hoisted has a better profile for downwash if s/he is sitting upright with his/her legs hanging down and the arms at the sides than if s/he is angled backwards at 45° with the legs sticking out straight in front and the arms outstretched.
“Another example is a rescuer with a backpack who presents a smaller profile to the downwash if the backpack is suspended from the hoist hook but hangs directly under the rescuer’s backside than if they have it on their backs,” said Tillion. “A wire basket stretcher allows air to flow directly through it, while a solid plastic tub stretcher acts as a scoop for the wind. An empty stretcher hoisted down to the ground in a vertical orientation offers a smaller surface area than a stretcher already horizontally configured to carry a patient. Similarly, where possible cargo loads should be configured to present a small surface area to the downwash. Cargo nets allow for better airflow than cargo bags,” he says.Bags that fly like doors, rescue vests that mean the victim is lying back too far, and the use of litters that were designed for ground rescue are all factors contributing to spin, said Bob Cockell. He told AirMed&Rescue: “Good rider position should bring the body’s centre of mass in towards the centre axis of the cable with the head at the bumper, arms in and legs tucked under your butt to create a cannonball so to speak. It is hard, if not impossible, to get a cannonball spinning because of the concentric, balanced nature of its mass.”
using a high line will give the rescue swimmer/winchman something to hold on to or make a stretcher fixed during the hoisting
According to Gardberg, there also ways to avoid the downwash altogether. “The more wind, the further aft the downwash will move. At around 10-20 knots, the effect from the downwash is almost completely mitigated. If there are light winds, you can add some forward speed to the helicopter after the lift (after confirming that the rescue swimmer/winchman are free with no obstacles in the way). A forward speed of about 10-15knots will in most cases eliminate the risk of spinning or even stop the spinning if it has started. There are also dynamic hoisting techniques, which require flying with the rescue swimmer/winchman already in the hoist to the pickup position, leaving them in the static hoist position as short a time as possible,” he says. “Moreover, using a high line will give the rescue swimmer/winchman something to hold on to or make a stretcher fixed during the hoisting. When hoisting with a stretcher, there is also a small triangle (like a sail) that the rescue specialist can use to stop the spinning by contracting it with the ‘sail’. Even fins can be used in the downwash at times to control the movement.”
Making a load symmetrical as a technique to manage the risk of spinning may sounds easy, but sometimes the implementation can be tricky. “In order to find the centre of gravity of a stretcher with a patient in it, for example, we have to lift it off the ground. That can be tricky if the patient weighs over 100kg. Furthermore, a stretcher may be symmetrically configured on the ground, but hoisting it together with an attendant will nearly always tilt the stretcher towards the attendant. People being hoisted need to configure themselves (or be configured) for symmetry. When hoisting in pairs, it helps to try to match pairs for size so that there is not a big size discrepancy,” advised Thomas.ARS has designed a product specifically to counter the risk of spinning – the Spin-Fin, said Cockell, is ‘100-per-cent effective’ and works on the majority of bag and litter type devices on the market. Designing a rescue device, he said, should result in a product that specifically solves a problem. “For example,” he added, “as stated before, there are bags that are designed like flying doors and have huge problems when it comes to unwanted load movement. Problems when these poorly designed and tested systems are exposed to the large rotor-flow influence of modern SAR aircraft become immediately apparent and can catch teams off-guard.”
Handling an emergency
In any case, according to Tillion, there is little the winchman can do to prevent the spin once it has started; the responsibility lies with the hoist operator and the options open to him are to recover the winch man/casualty to the aircraft or, if possible, put the winch man/casualty on to the surface to stop the spinning. “With the advent of modern SAR helicopters with a higher rotor disc loading, this issue has become increasingly more prevalent and SAR operators have had to adapt their winching techniques accordingly,” he told AirMed&Rescue.
“If spinning occurs, there are three major actions determined by the hoist operators evaluating each specific situation. The procedure requires, as needed, asking for speed forward if possible (discussed during pre-briefing), lowering the rescue specialist back into the water or down to the surface, and continuing to pull the hoist up as quickly and/or carefully as possible,” according to Gardberg.
Sharing best practice
There are some organisations that discuss SAR issues, such as the European Rescue Swimmers Association (EURORSA) and Helioffshore, which has recently created a SAR forum for a range of technical discussions. “The greater SAR community is dedicated to sharing information, identifying best practises and setting common standards,” concluded Gardberg.