Flying helicopters IFR with a sling load presents unique challenges, requiring specific skills of the pilot. One must obviously be able to control the helicopter without any outside visual references. Less obvious, one must also be able to correctly interpret the instruments, which reflect both the behavior of the load and the orientation of the helicopter. A Class B external load (sling load) is one that is free of the earth’s surface and is attached to the helicopter by a synthetic or wire line. The pilot is “flying” both the helicopter and the load, which at times can seem to have a mind of its own.
Today slinging IFR is not a common practice, though there was a time on the North Slope of Alaska where it was employed regularly. I thought it might be interesting to look at this operation in some detail.
Controlling the sling load
Normally one can see the external load, and make the necessary corrections. Lateral swinging is more common than a fore-aft motion or a circular motion, so we will focus on that. A quick lateral cyclic input towards the load, just as it reaches its apex, moves the aircraft over the load neutralizing its motion. You are essentially moving the aircraft over the load after it has swung out to the side. This dampens the movement of the load and stabilizes it. However, when flying IFR the instruments must be used to indicate the loads’ position and movement. The best way to learn how the instruments reflect the movements of the load is during VFR flight, when the load and gauges can be seen together.
Flying IFR with a sling it is important not to make corrections reflecting the gauges as one normally would, but instead understand exactly what the load is doing beneath you. The attitude indicator reflects rhythmic changes in bank angle from the load tugging the helicopter laterally side to side, as does the ball in the inclinometer. The inclinometer is used to indicate when and how much lateral cyclic input is necessary for a correction, though there is a natural lag. The load will reach its apex prior to the inclinometer, and the pilot must compensate for this natural lag. When the ball starts to swing out of center to the right and is about half way from its apex, the load is almost at its apex to the left, the pilot then uses left lateral cyclic as a correction. The rhythmic oscillations in the attitude indicator and inclinometer reflect the movements of the load, and the average of these movements are the actual orientation of the aircraft. The pilot learns to mentally average these oscillations in order to control the pitch, roll, and yaw of the aircraft itself.
“An ounce of prevention is worth a pound of cure”, so one learns to make flight control inputs very smoothly so as to not aggravate the load. Turns are initiated slowly, and half standard rate turns are sometimes prudent.
Determining cruise airspeed
Another consideration is determining the target airspeed at which to fly. This must be greater than Vmini (minimum IFR speed) and less than the loads effective Vne. While the aircraft will have an external load airspeed limitation, this may not be possible if the load is unstable at a lower speed. Many loads cannot be flown at the external load Vne, and the effective Vne must be determined. As the pilot slowly accelerates during takeoff, the load is carefully watched prior to IMC to determine what airspeed above Vmini the load can be flown at. Once that airspeed is determined, it is maintained for the entire flight.
Should the load show signs of instability below Vmini or only slightly above so as to not provide a safe and adequate airspeed window, the takeoff is aborted while still VMC.
One should be sure of a load’s stability and capability at a safe airspeed prior to IMC, and one should only fly known loads in IFR or at night. A known load is one that is similar to one previously flown during the day. The load characteristics are predictable and stable.
Autopilots and external load operations
Autopilots and external loads don’t usually mix, and many Rotorcraft Flight Manuals prohibit autopilot coupling during external load operations. The autopilot can be too abrupt in pitch attitude and roll, particularly when initiating and terminating turns. A pilot can make changes with a more gentle touch; such as slowly entering a half-standard rate turn when necessary. The autopilot can be used for stability augmentation; it just shouldn’t be coupled to the flight director directly controlling the aircraft.
Horizontal and vertical situational awareness
Class B sling loads can be jettisoned, either intentionally or unintentionally. The hook release is typically electric and controlled by the pilot. Under normal operation the load is released once it has been placed gently on the ground; however, in the case of an emergency the pilot may opt to release it in flight. Due to the possibility of the load being released in flight, persons or property are never overflown. This requires horizontal situational awareness; easy enough VFR, but IFR is another matter. Fortunately, the North Slope of Alaska provides assurance due to its desolate nature.
Vertical situational awareness must also be considered, not just for the helicopter but also for the load hanging underneath. With the typical 25 to 50’ line, the altitude of the load isn’t a factor in cruise flight; however, during the instrument approach it must be considered.
The Instrument Approach
As much fun controlling the helicopter and load may be in IFR conditions, eventually we do need to land. For that we need to fly an instrument approach. Let’s stick with the North Slope of Alaska, using the Deadhorse (PASC) ILS 05 as an example, using a little simple math.
A load 5 feet high hanging on a 50 foot line would require a 55 foot adjustment factor to the decision altitude. For the Deadhorse ILS, this means increasing the decision altitude of 267 feet to 322 feet, and ALS conditional altitude of 167 feet to 222 feet. It would also be prudent to include this 55 foot altitude adjustment into your preflight IFR planning.
While flying slings IFR is no longer common, the training for IFR slings still occurs. Having the skill and confidence to be able to fly a sling IFR is vital should unforecasted adverse weather be encountered, not unheard of on the Alaskan North Slope where the weather can change quickly. Airports and options are few and far between north of the Brooks Range of Alaska. These skills also translate well and are employed for night sling operations, which are still done on a regular basis.