Saturday, February 1, 2014
Looking for a Lift: Hoists 101
Stretching the limits of the vertical flight envelope.
Stretching the limits of the vertical flight envelope.
|Maryland State Police operates a fleet of 20 aircraft. The newest nine are AgustaWestland AW139s equipped with the UTC model 4413-2-3 hoist system.|
Any operator that needs to lower and raise people or objects from an area below can use a hoist, but one of the most common applications is in search and rescue (SAR).
Maryland State Police (MSP) operates a fleet of nine AgustaWestland AW139s and 11 Eurocopter AS365s, primarily in law enforcement and emergency medical transport roles. But each aircraft is equipped with a UTC Aerospace Systems (formerly Goodrich) hoist for the times when crews must haul boaters out of the waters of the eastern shore, or pluck injured hikers from the mountain tops of its western counties.
“We do about 10-12 hoist rescues a year,” explained Chris Lovejoy, MSP’s deputy director of aviation. “But when we have to do them, it’s a big, big deal, because lives are at stake.”
Not to be confused with a cargo hook system, which can neither raise nor lower the line attached to it independent of changing the aircraft’s altitude, a hoist system uses a drum to deploy and reel in a cable. And while it can carry a load externally from point A to B, hoist are primarily used as a way to get people and gear into a hovering rotorcraft, or onto the ground before the helicopter transitions back to forward flight.
A basic rescue hoist system consists of several main components: the external hoist assembly, the controls, the electrical system, and the hydraulic system.
The hoist assembly is normally housed in a fairing above an aft cabin door, and contains a spool of steel cable – often around 300 feet in length – with a hook attached to the end. Ratings can vary, but 600 lbs is a common on-the-hook lift limit. To make getting the hoist’s load in and out of the aircraft easier, some assemblies are mounted on a swing arm that can add additional clearance from the side of the aircraft.
Hoist assemblies are also equipped with a variety of safety devices, such as brakes, speed limiters, overheat detectors, and emergency cable cutters.
The controls for the hoist are generally found on the pilots’ cyclic, as well as on a handheld “pendant” control in the aft cabin. The pendant is connected to the system by means of a coiled power cord, so that the crewmember operating the hoist can lean out of the door to watch the load, reposition himself in the cabin, or pass the control to a helper. And, of course, the pilots can use their controls to override the aft switches. All UP/DOWN thumbwheels or switches can control the speed of the cable’s movement based upon the pressure applied by the operator, and are spring-loaded to return to the STOP position when released.
The electrical system for most popular hoist installations operate on 28 VDC, can draw up to 150 amps, and will allow the cable to deploy a 600-lb load at 260 fpm. (The retract rate is around 200 fpm, similarly loaded.) If primary power is disrupted, emergency power can be brought online, but will spool the cable at a much slower rate.
The laws of hydraulics tell us that forces can be greatly amplified when moved by an incompressible fluid. Therefore, hydraulic fluid is passed through the system to give it the power it needs to lift its loads. Sensors monitor the temperature of the fluid, and will activate a warning light if it becomes too high – an indication that the hoist is malfunctioning.
Altogether, a typical hoist system with a single, 300-foot cable can weigh in at approximately 110 lbs.
Working a hoist safely, especially in situations where lives are at stake, is not simple. Many things can go wrong if a crew is not properly trained in its operation.
“Our training evolutions include using the basket, deploying a soft litter, and rescuing people off of a boat,” said Mike Gartland, MSP’s chief pilot. “We train at least twice a year, and crew coordination and safety are stressed.”
To ensure that the hoist is used safely, the U.S. Coast Guard (USCG), an organization that conducts hoist operations in some of the most hazard conditions imaginable, uses a checklist that covers nearly all possible scenarios. It begins with regular inspection and service by maintenance personnel on the ground, and includes a thorough checklist aboard the aircraft.
|U.S. Park Police Bell 412 during a hoist rescue operation.|
Pilots make the performance calculations, taking into consideration the aircraft’s ability to maintain a hover both before and after the victims are brought aboard. Special attention is given to winds, pressure altitude, and the availability of visual references, to name a few. Yes, those things are always a consideration for any kind of mission, but can be particularly critical when the pilot has to hold a hover within the relatively short length of the hoist cable.
Emergency procedures discussions will not only include engine failures, but will address fouled hoist cables that could drag the aircraft down, jammed winches that may need to be cut free, and where the pilots will try to ditch if the worst happens.
Losing contact with the victim can create a variety of problems. The biggest one will normally be how much time can be spent trying to reacquire the victim before fuel reaches critical levels.
The last part of the briefing involves settling on a plan (and backup plans) for how to affect the actual rescue. If the victim is strong, a basket might be lowered. Once they climb inside, a member of the helicopter crew will haul them aboard. If a person needs to be grappled by the rescue swimmer, a sling could be the best tool to affix to the cable. And if a person is aboard a ship and not ambulatory, a litter might be the tool to choose.
Each method can create its own set of difficulties, however. A litter, for example, is more difficult to use, primarily due to its larger size and sail area.
Communication between crewmembers is vital in any search and rescue evolution, since hoist operators frequently have a better view of where the victim is than the pilots. For years, this meant the hoist operator had to shout commands to the flight deck, such as, “Up a little,” or “Come right some.” But aircraft manufactures now offer an optional aft cabin hover control. When activated by the pilot, the hoist operator can use it to move the aircraft in small increments from the aft cabin, thus using his sight advantage to keep the cable where needed.
Hoist systems and applications are numerous. But it can certainly be argued that when attached to a helicopter staffed by a skilled crew, they can stretch the capabilities of vertical flight exponentially.