Dynamic hoisting – what, when, how?
What are the real differences between a dynamic and static hoist, and are there any particular benefits to the dynamic approach? Mario Pierobon and Mandy Langfield spoke to the experts to find out
According to Adam Davis, Manager of SART/TAC Priority 1 Air Rescue (P1AR), professional helicopter hoist training organizations should be intimately familiar with the fundamentals of load force physics as it applies to helicopter hoist operations, and through their training be able to articulate to their students how physics impacts hoist operations. “It is also important to demonstrate how this corelates to the benefits and proper application of dynamic hoisting,” he says.
Brad Matheson, President of P1AR, agrees: “Aircrews must understand the difference between artificially generated forces, such as rotor wash or helicopter movements, versus environmentally occurring forces, such as wind, terrain features, and gravity,” he says. “An example of this is the core understanding that hoisting from a static location over the target may generate greater winds below the aircraft, which could then impact the hoist load and create the drivers that cause spins or oscillations.”
The term dynamic as it relates to hoisting makes sense; a process or system characterized by constant change, activity, or progress. The path to successful hoist operations and the premise of dynamic hoisting involves harnessing the continued movement of the aircraft and hoist load. This imparted movement and its negative effects also must be considered as part of any operation. From pilot and hoist operator input, down to aircraft rotor flow, these drivers need to be understood.
The benefits of dynamic hoisting
Being able to assess the environmental winds as well as the terrain can help determine the best method for approaching a hoist, highlights Matheson. “With dynamic hoisting, the fact that the aircraft is moving at altitude while conducting the hoist may offer better options for the aircrews in the event of aircraft emergencies, especially relating to power,” he says. “Dynamic hoisting can provide better options for the pilot during aircraft emergencies including flyaway with single-engine performance for twin-engine aircraft, as well as less exposure time for single-engine aircraft and the corresponding ‘dead man’ curve.”
Dave Callan, Operations Manager and Flight Instructor for SR3 Rescue Concepts in British Columbia, prefers dynamic hoisting for several reasons: “The load being hoisted is much more stable when moving in flight. The momentum involved tends to keep the load moving in its current direction, which makes it much easier for the hoist operator to keep under control and reduces swing. The forward airspeed also almost completely eliminates any spin, typically upon reaching airspeeds of 20-30 knots. We like to use this technique as much as possible to eliminate spin, particularly with litter operations. There is a turbulent flight zone where the rotor wash converges below the helicopter, which is typically at its worst about one rotor diameter below the fuselage. Raising or lowering the load through this zone while in flight allows the airspeed to stabilize the load and prevent the spin.” Furthermore, he explains, the aircraft is much more stable in flight. “As a pilot, it requires much more effort to hold a hover, particularly for extended periods of time. Forward flight, even at a slow airspeed, is much less of a workload for the pilot.”
The load being hoisted is much more stable when moving in flight
Conducting multiple hoist evolutions on large rescues can become very fatiguing, and dynamic hoisting is a great way to minimize this. “The aircraft is also much better set up to deal with almost all emergency procedures when in forward flight,” he adds. “Whether speaking of engine failures, hydraulic failures, tail rotor failures, or almost any other system or mechanical failure, having forward airspeed is going to be hugely beneficial in almost every circumstance.”
Static hoisting offers fewer opportunities for multitasking
With static hoisting there is less multitasking, which means that the pilot, with the support of the hoist operator, first focusses on getting the helicopter plumb over the target, and then the hoist operator can focus on getting the hoist load to the ground, observes Davis. “Conversely, the fundamental task for the hoist operator during dynamic hoisting is coordinating their cable speed and load deployment while directing the pilot to achieve the optimal descent and closure as the aircraft approaches and descends to a hover directly over the hoist target,” he says. “The most common problems for aircrews when practicing this technique is not causing a pendulum when coming to a hover or more seriously over-shooting the hoist target especially while operating in a confined area with trees or other entanglement hazards.”
Priority 1 Air Rescue has long emphasized a crawl, walk, run approach to teaching part and whole task mission training. “Part of this is understanding limits and capabilities while also ensuring a system is in place that can delineate between proper dynamic movements of the helicopter versus unnecessary ‘hotdogging’ of the helicopter,” says Davis. “We would strongly recommend having an experienced training organization, or partner agency that has a robust and comprehensive training program in place, provide a proper course of learning on dynamic hoisting before attempting to do it, or any other hoisting for that matter. Like any high-risk activity, one should not just attempt dynamic or static hoisting without being well educated and understanding the factors and procedures that apply.”
Trainers and training locations
Priority 1 Air Rescue has been training the theory and principles of dynamic hoisting to its commercial, para-public, and military customers for over 23 years. “We have trained over 10,000 students worldwide in live-flight at the customer’s venue and blended training at the Search and Rescue Tactical Training Academies (SART/TAC) facilities in Mesa and Bordeaux,” says Brad Matheson, President of Priority 1 Air Rescue. “Both dynamic and static hoisting techniques are taught either at the customers’ venues utilizing their aircraft in their area of operation, or at SART/TAC using our Advanced Aircrew Mission Simulators (AAMS) where we introduce the entire spectrum of situations and training evolutions that aircrews could encounter on a real mission, but are either too high risk or impossible to practice during live flight training.”
ARS teaches ‘fluid-dynamic’ hoisting: a focus on harnessing the movement, energy and inertia created through delivery of a robust curriculum based on the math, science and physics of movement during dynamic helicopter hoist operations.
You must be able to understand to a high degree a myriad of factors during an operation, movement is predictable when concepts from spiral slipstream, wall-jet, impact, disturbed flow, etc. are considered. Proactiveness is key. If you are reactive, you add unwanted movement, and ultimately chaos, to the load.
It is important to use synthetic training to introduce the principles and procedures of dynamic hoisting for both the rear crews and the pilots, according to Matheson. “We also employ dynamic hoisting in what we call our blended training approach, where we educate students on the tactics and techniques using our AAMS and hoist procedural towers, and then after completing that phase of training we then travel to the customer’s venue for the next phase of training to practice the evolutions in a live flight environment,” he says. “Utilizing synthetic training devices ensures demonstrable positive learning in how to apply techniques and tactics correctly, prevent possible cable entanglements, assess and act on emergency procedures such as shearing the cable, as well as introduce the site picture of what it would look like from the pilot and hoist system operator’s literal point of view.”
“Allowing participants to experience realistic situations and practice relevant dynamic hoisting procedures in the AAMS and hoist procedural towers provide tangible and positive metrics for their performance on actual SAR missions,” says Davis.
Swiss Air Rescue Rega crews regularly train to use the rescue hoist during the day and at night. “Training takes place exclusively in an operational scenario under real conditions, i.e. not in a simulator,” explains Media Spokesperson Mathias Gehrig. “Rega crews use dynamic procedures whenever possible for missions in which the rescue winch is used. The advantages of this, apart from saving time, are that they have to hover on the scene for as little time as possible and therefore have less exposure time at the same location.”
Risk assessment – when to apply the dynamic hoist approach
It is important to point out that while dynamic hoisting is beneficial in most circumstances, especially when encountering power issues or lack of environmental wind, dynamic hoisting cannot be employed when the risk of entanglement definitively outweighs the negative risks associated to the load and rotor flow dynamics. “In the event the aircraft has unlimited single-engine OEI performance and there are negligible environmental winds, it is important to assess the risk of spins versus accepting the risk of cable entanglement,” Matheson points out.
Spins caused by the turbulent flight zone can be resolved by effective emergency procedures or mitigated in some circumstances by correctly employing helicopter taglines with proper breakaway weak links. “The incorrect application of dynamic hoisting can be common with operators and new training agencies that do not fully understand the drivers, risks, and consequently why it is not always beneficial and why ‘standard’ dynamic hoisting can even be detrimental to employ in situations like operating in heavily forested areas, confined areas, vertical surfaces, ocean SAR, or certain circumstances encountered during night or night vision goggle hoisting,” Matheson adds.
For Callan of SR3, there are defined parameters of missions where a dynamic hoist would be a better approach: “Dynamic hoisting is best suited in more open terrain types where obstacles or large changes in elevation are not present,” he tells AirMed&Rescue. “This is not to say you cannot use dynamic hoisting in confined areas, however, there are times when we have to hoist statically in a hover.” An example of this would be very dense, tall trees. If the trees are confined and 100’ tall, then the crew would have to insert or extract from a static position. “What we prefer to use in confined terrain is a combination of both,” he adds. “As an example, we would begin lowering a rescuer out during an approach to a pocket of trees using the dynamic technique, and hold them just above them at a safe altitude. Once we come to a hover, we would continue the hoist in the static position. There are some situations where the terrain is so confined or steep that we do not hoist dynamically, because there just isn’t enough clearance to do so safely.”
Another example where SR3 sometimes do not use the dynamic technique is in very challenging wind conditions. If the winds are extremely gusty or there are lots of updraft/downdrafts, the pilot will sometimes hold the rescuer being inserted until the helicopter’s position over the target can be maintained.
Dynamic hoisting should be viewed as a tool in the toolbox and must be evaluated during the risk assessment and on-scene briefing, according to Davis. “While beneficial in many circumstances, it’s simply a method, no different than evaluating and deciding to properly employ a tagline because it is deemed the best tool for a given job,” he says. “Our ability to understand risks and make critical thinking decisions with the focus of prioritizing aircrew safety during an otherwise high-risk activity should always guide us in deciding what tools and methods we deem most appropriate to apply to the hoist situation presented to us.”
Understanding, experience and training will win the day
With hoisting most understand the what, but when it comes to explaining the how and the why, the physics of hoisting can be less comprehensible than that of aerodynamics.
With a moving load, as you decrease the cable length you will experience an increase in both movement velocity and size of the movement. Cable length is inversely proportional to building or decreasing centripetal force. In other words, the quicker you bring an unwanted movement in, the greater the velocity of the movement will become, the wider the apex to apex will be, and the Ft experienced will increase to the point of rapidly becoming uncontrollable. This is a key first lesson. Remember do not bring unwanted movement to the aircraft, or things can become challenging.
At the end of the day, says Gehrig from Rega, for hoist operators and also for pilots, dynamic hoist operations are more challenging than static operations. They all need good situational awareness and must always think ahead. Experience in the field and regular training help them to assess situations correctly.