Since 1999, the North American aero-medical transport industry, particularly its helicopter EMS community, has become increasingly introspective regarding an accident rate it sees as excessive and has focused its attention on night flights and operations in reduced visibility, which constitute a large portion of mishaps. An area of special interest is wider use of technologies to enhance and amplify the visual capabilities of pilots. Night vision goggles (NVGs) have garnered most of the attention in association studies and operator surveys, but infrared (IR) sensing is gaining a constituency.

A recent visit to enhanced vision system (EVS) supplier Max-Viz in Tigard, Ore., provided insight into the relative merits of helmet-mounted night vision devices and infrared energy-based display systems. Briefings on the three-year-old company’s business outlook and development programs in progress preceded an airborne look at the Max-Viz EVS, a standard option on the Cessna Citation X and Sovereign, to cap off the day.

The demonstration flight in Max-Viz’s Riley Rocket turbine Cessna 421 began northbound from Aurora State Airport toward 8,365-foot Mt. St. Helens (until 1980 a 9,671-foot volcanic peak). On this bright, sunny afternoon the short-wavelength IR sensor of the EVS-2000 system picked up the energy closer to visible light and provided an image that closely matched the outside view. The long-wavelength sensor mounted beside it on the twin’s nose clearly displayed heat sources such as the flanks of Mount St. Helens and contrasting surfaces such as lakes at the mountain’s base, the Columbia River and adjacent terrain.

After the aircraft circled the crater to provide a direct view inside it, the thermal activity under a fast-growing lava plug showed on the EVS-2000’s flat-panel display as a bright white spot. Later, on the northwest leg of the flight toward Astoria, the Columbia and its tributaries were plainly identifiable despite a slight haze nearing the coast that was reducing unaided visibility.

The smoke plume from a controlled burn near Astoria was plainly visible on the display. It was joined by a white dot at its base as the aircraft drew closer.

The flight demonstrated the dual IR sensor system’s ability to show terrain features and heat sources simultaneously in a brightly sunlit cockpit. The EVS-2000 also accurately annunciated denser obscurants such as thick smoke and cloud through which thermal energy from beyond could not penetrate.

NVG and EVS Cover All Bases

Chuck Crompton, Max-Viz field sales manager, said there is a growing sentiment in the EMS community that the best way to make night helicopter operations safer while achieving higher mission completion rates is through combined use of both NVG and infrared EVS. He noted that years of experience by the military have proved the advantages of using both NVGs and infrared sensors.

“There are recorded occasions during which [potentially] fatal accidents were avoided when obstacles that were visible in IR were simply not ‘seen’ by those wearing the NVGs,” he explained. Each technology takes a different path to gathering and presenting information; each has capabilities the other lacks.

The Max-Viz representative described some obvious advantages to having both NVG and IR sensing available in night helicopter operations. “The pilot may choose to use only the EVS after coming to a hover during a dark landing or brown-out, dust-obscured landing. With EVS you avoid the orientation problems of NVG transition from dark flight to lighted landing zones, with flashing lights on emergency vehicles. With infrared you ‘see’ the hot vehicles but not the flashing lights.

“During an approach to a landing, EVS would give you the ability to sort out ‘hot from cold’ on vehicles and other aircraft in the landing zone…very useful in an active landing zone or at an EMS auto accident scene. With EVS on a cold night, people in the landing zone will be highlighted as ‘white hot.’ Having both NVGs and EVS available makes possible a complementary combination scene of lights and thermal (non-light-emitting) targets and the surrounding environment,” he said.

Despite the additional cost of providing EMS helicopters with both NVG and EVS capability, there is anecdotal evidence suggesting it could pay off by lowering the rotary-wing EMS accident rate, the highest in civil aviation. In the preliminary NTSB report on a fatal helicopter EMS accident near Chico, Calif., on the evening of Sept. 22, 2001, the NTSB said, “Ground witnesses who were assisting with landing [at an accident scene] stated that the helicopter’s approach appeared normal. [At] about 30 feet agl their vision became obscured due to dust and dirt kicked up by the main rotor blades. They reported that because of the dirt and dust they did not see the impact sequence.”

The NTSB narrative continued, “One of the flight nurses stated that he was sitting behind the pilot using a pair of handheld night vision goggles to assist in making sure that the landing area was clear. He indicated that the approach was normal. [Then at] about 10 to 12 feet agl during the flare for landing their vision became obscured by a dirt/dust storm (brown out) initiated by the main rotor blades. The flight nurse stated that he kept asking the pilot if he could see anything but did not receive a response. He heard the engine power up with no problem and then remembers hitting trees.”

Clearly, an EVS operating in the long-wave portion of the IR spectrum, if available to complement the NVGs, could quite possibly have prevented this mishap and spared the resulting death and a serious injury. Long-wave IR is able to penetrate the “brown out” phenomenon, which blinds both the naked eye and NVGs, because dust particles small enough to be suspended in air are smaller than the IR wavelength and thus allow IR energy to pass.

The Importance of Particle Size

Crompton explained that electromagnetic energy is attenuated by airborne particles equal to or greater in size than the energy’s wavelength. Thus short-wave IR energy is less able to penetrate dust, haze, mist and other small-sized particles for the same reason that visible light (which has a wavelength of slightly less than one micron) is blocked by solid particles and liquid droplets. Longer-wave energy, in the 8- to 14-micron range, will pass through, however.

On the other hand, the long-wave IR sensor detects energy from the visible light spectrum poorly or not at all (if the light source generates no heat). A dual-sensor EVS that covers both long- and short-wave (three to six micron) IR bands will do better on targets such as runway and approach lights–especially incandescent.

A primary difference between night vision goggles and infrared sensors is that the goggles sense visible light and amplify it electronically, while the IR sensor detects electromagnetic energy emitted from heat sources at lower frequencies. The former greatly enhances the eye’s ability to see; an infrared EVS can display targets that do not emit or reflect visible light and even some features that would be invisible to the human eye regardless of light level.

An EVS will provide situational awareness with imagery gained from differences in thermal content and reflectivity even where the visible light level is at or below the threshold of NVG detection. During ground operations in fog, EVS will detect objects at substantially longer distances than NVGs will, and at four to 10 times greater distance than the naked eye.

NVGs have the advantage of following the pilot’s head movements while the fixed EVS sensor stares straight ahead. The latter has a somewhat wider field of view, which helps the pilot maintain perspective and situational awareness. With their reduced field of vision (typically 40 to 60 degrees of the total vision field) NVGs require frequent head movement for effective scanning, especially in single-pilot operations.

As vision specialists G.J. Salazar and V.B. Nakagawara point out in a Federal Air Surgeon’s Medical Bulletin article, “In addition to reducing field of view, NVGs reduce depth perception,” limiting a pilot’s ability to determine closure rates on terrain or other aircraft when first detected. By amplifying low levels of ambient light, NVGs decrease some of the limitations inherent in unaided night vision, but by no means do they turn night into day, the vision specialists added.

While the typical NVG image intensifier can produce a useful image from starlight or low-level moonlight by amplifying it an average of 1,000 to 3,000 times, the NVG will always require some visible light.

Visual Acuity

Since the FAA issued the first STC for NVG use by a civilian helicopter EMS operator in 1999, aviation medical examiners (AMEs) have become aware of some basic operational issues and associated clinical implications. For instance, at a visual light level one-tenth of normal starlight, NVGs will allow the wearer to see with the equivalent of 20/40 daylight visual acuity, but 20/40 vision “will not let you see wires or poles unless they are shiny, large or very close,” Salazar and Nakagawara noted.

An infrared EVS, in contrast, “sees” active power lines by the heat that the current they carry creates. Also, they added, at 300 feet agl and above, the NVG wearer’s visual acuity diminishes. A medical helicopter safety report by the Air Medical Physician Association noted that more helicopter EMS accidents occurred during cruise (when NVGs are less effective) than during any other phase of flight.

Salazar and Nakagawara caution that “NVG-aided acuity of 20/30 or 20/40 assumes perfect cockpit lighting, properly focused goggles and ideal weather conditions.” In addition, flickering or flashing aircraft position and anti-collision lights can cause NVG wearers operational problems.

Most important, NVGs are subject to interference from rain, clouds, mist, dust, smoke and fog–the same elements that hinder natural human eyesight. “Any of these will severely degrade performance of the equipment. Landing in dusty areas can cause ‘brown outs’ with the goggles. Particularly hazardous are obstacles or terrain features masked in shadows. Experience and recurrent training are crucial to using NVGs safely.”

The researchers concluded by advising, “Pilots must be carefully examined to ensure they have the physical ability to use the equipment. Prolonged NVG use may cause neck discomfort or general fatigue. AMEs also need to assess carefully vision and compatibility of current ophthalmic corrective devices for use with night vision goggles.”

In addition to acquisition cost, typically around $10,000 per pair for night vision goggles (two to three per aircraft), using the goggles requires initial and recurrent flight crew training.

In most current aircraft (and more than 90 percent of helicopters), operators must install special NVG-compatible cabin and panel lighting at costs that can range from $50,000 to the $77,000 cited by NVG STC pioneer Red Wing Aviation of Logan, W. Va., for a Eurocopter factory installation in a BK 117.

Crompton said a single-sensor MaxViz EVS-1000 system will run more than $100,000 installed but has no recurring training commitments or user certification, support and calibration costs.