It is well known that flight into instrument meteorological conditions without the proper instrumentation and training is almost certain to lead to loss of control. But controlling a rotorcraft in degraded (but not zero) visibility is a problem that is not well understood.
There is a reason helicopter pilots get to gaze through great big cockpit windows, and it’s not so everyone can see who the best — or worst — pilot is. In visual meteorological conditions, that big window provides our brain with plenty of constantly changing information, or visual cueing, which allows us to make the necessary corrections to keep the aircraft stable and doing what we want — or as we say, “Closing the loop.”
If precise hovering is required, say for a platform landing or in a confined area, the task becomes tighter, requiring more frequent, quicker inputs to correct small errors and precisely guide the machine. This is known as a “high-gain” task.
The problem at hand is that a helicopter with satisfactory handling qualities in a good visual environment (such as daylight or feature-rich terrain) can quickly become dangerous or downright deadly when enough of those visual cues are lost.
Studies have shown that degraded visual environments (DVE) — such as rain, haze, fog, snow, over water and/or with night-vision goggles on a moonless night — tend to obscure adequate fine-grain detail, like blades of grass and small stones. It is this “microtexture” in the pilot’s near field of view that would normally provide the primary cueing required to stabilize the aircraft in hover and low-speed flight.
While the visual scene appears to be adequate, it’s really missing the subtle cues that are necessary for precise attitude and position control. Without them, most of the pilot’s workload involves maintaining aircraft control. Little is left to maintain adequate situational awareness, and risk of collision rises dramatically.
It turns out that, to the pilot, flying in DVE manifests as degraded rotorcraft-handling qualities. This important realization has led the military to revise its specifications for the design of its rotorcraft, to include extensive evaluation of aircraft handling qualities in DVE. To help understand why this happens, consider how we control a helicopter.
The controls in a basic helicopter are rate-response systems in all axes. In other words, control inputs produce corresponding angular or vertical rates. So more stick deflection commands faster roll/pitch/yaw.
To maneuver or stabilize the helicopter in pitch and roll, the pilot must close the attitude loop by removing or reversing cyclic control inputs once the desired attitude is reached. This task becomes difficult and dangerous with a lack or lag of visual feedback to the pilot, causing him or her to increase the gain of the control inputs, potentially getting out of phase or beyond what the aircraft can respond to and follow.
Research has shown that, as the usable cue environment degrades, a better method of control tends to be attitude command attitude hold (ACAH). With ACAH, the aircraft attitude follows the pilot’s cyclic stick. A step input to the cyclic produces a step response in roll/pitch attitude.
As visuals get even worse, such as during a brownout landing, the ideal control response might progress from ACAH to translational rate command (TRC). This means the translational ground speed will now follow the cyclic. More stick deflection commands faster translation, and centering the cyclic zeros out the ground speed.
For existing rotorcraft with hydromechanical controls, these types of desired responses can be achieved by building complex control laws into existing automatic flight control systems. For example, modern control laws of the variety mentioned above have been designed for aircraft such as the Boeing AH-64D Apache and Sikorsky UH-60 Black Hawk.
In any type of aircraft, control is everything. Pilots can often cope without stability, but they must have control. With the advent of digital fly-by-wire flight controls, the difficulty in achieving this level of control has been greatly reduced. As these systems migrate into civilian helicopter operations typical of DVE (such as oil rig approaches), pilot workload can be reduced and the benefits to safety will only continue to be realized.