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Tuesday, March 1, 2005

Military Spin

Lt. Col. Steve Colby

Brownouts have claimed more helicopters in recent military operations than all other threats combined. There are material and procedural solutions and combinations of both to cope with the crippling loss of visual references during this critical phase of flight. This month we'll examine available technologies that assist the pilot in brownouts.

Brownout occurs in varying degrees, dependent on several factors. Helicopter disk loading and rotor configuration, soil composition, wind, approach speed and angle all affect the dust cloud density and the resultant visual reference impairment. The main problem with brownout landing is the loss of visual references and induced drift just prior to touchdown. Most helicopters can handle forward aligned drift, but few can tolerate sideward or rearward movement during touchdown. The combination of a pivot point, a spinning rotor system and a rolling moment about the pivot point are the root causes of dynamic rollover.

Disk loading and gross weight determine the amount of dust, dirt and small stones that are recycled in the rotor air flow. The smaller the particulate matter, the more severe the dust cloud. Conventional rotor helicopters are most severely affected because the rotor's annular (donut-shaped) air flow creates the swirling dust cloud nearest the tips of the rotor where velocities and vortices are the greatest. In a tandem rotor configuration, the pilots are directly below the mast and may still have visual references close to the aircraft. Soil composition drives the intensity and persistence of the cloud. The powder-like dirt found in southwest Asia is extraordinarily aggravating. It blooms earlier, flies higher and is more persistent than that in most other environments. There are several technical means to counter the brownout threat. The material solutions are flight control, indicator, and proprioceptive devices. (Proprioceptors in your muscles, tendons, joints and inner ear sense motion and the position of your body and limbs.)

The most advanced systems are flight controls outfitted with a full-authority autopilot coupled approach. Some systems are equipped with software to accept a mark-point overflight and then execute a rectangular pattern to a slow, controlled approach to a fully coupled hover. This is clearly the safest technical means to counter brownout. Some aircraft have only hover couplers using Doppler, Kalman-filtered INS solutions, or velocity-smoothed GPS solutions to provide a reference signal. Still others have altitude hold, hover stabilization systems (AHHS) which are electric-servo, trim-based systems that use velocity or position references to help maintain a position or a constant velocity. The flight control systems described require a continuum of human-in-the-loop actions. Autopilot systems require the least intervention (monitoring) and AHHS the most pilot input during a brownout landing.

Several indicating systems are available to provide pilots with visual cues necessary to detect and correct for drift in a hover. Certain models provide cues on cockpit displays or night-vision goggle head-up displays (see "Panoramic NVG," June 2004, page 62) that have a position reference cue, a velocity cue and an acceleration cue. The symbols vary from a simple vector stick and a circle to a dynamically linked position cue with velocity crosshairs and an acceleration circle. For an intentional no-hover, run-on landing, the pilot aligns all the cues to ensure no sideward drift prior to touchdown. Some systems allow the pilot to effectively hover "heads in" provided he has a very accurate position reference for lateral control and an instantaneous altitude reference such as a radar altimeter to control descent rate. A highly useful display for brownout is a forward-looking infrared (flir) display that allows the flir sensor to be pointed straight down when stabilized in an out-of-ground-effect hover above the dust cloud. Often a flir picture can present the landing site in the middle of a brownout because its image is pointed in the dust cloud donut hole. An alternative to the vertical flir alignment is a Flight Path Vector mode. This allows the pilot on short final to transition from outside visual references to the flir as the cloud approaches the cabin. In nose-mounted flir installations, the flir can provide an unobstructed view of the terrain immediately in front of the aircraft for the roll-on landing after the tail wheel or skids make ground contact. The best of the indicating systems provide a combination of drift-cue symbology superimposed on the flir image.

A proprioceptive technical solution, recently tested by the U.S. Navy, is called the tactical situation awareness system. This is a vest with small, pneumatically or electromagnetically driven stimulators called tactors that provide a touch input to the pilot's torso or legs signaling a drift or required correction, depending on configuration setup. These systems take advantage of the human sense of touch, operating at a subconscious level. They provide a reduced-workload stimulus that is processed and responded to very quickly to control drift. Lower workload is critical in the intense, task-loaded brownout environment. For more, go to www.namrl.navy.mil/NAMRL_NEW_NEWS/ApproachMayJun04.pdf.

These technological tools can make the brownout landing a manageable threat, reducing terra firma's probability of kill to significantly less than 1.0. My challenge to industry is to develop these technologies in affordable packages for aircraft without highly advanced navigation systems or data-bus architecture. This technology can and does save lives in dust and snow.

The views expressed are those of the author and not an official position of the U.S. Air Force. This article has been approved for release by Air Warfare Center public affairs.

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