Enhanced Vision Systems

It was a dark and stormy night. The moon was obscured by low-lying clouds resembling mountainous peaks while a combination of mist and fog blanketed the hilly terrain. All a sudden, without warning, a shot rang out. All the while a private aircraft approached the nearby private runway. A well-versed captain and an eager but not nearly as qualified co-pilot comprised the cockpit crew and the flight deck they operated consisted of 1980’s vintage electronic flight instruments with some later technology enhancements.

The crew appeared unconcerned and even complacent about the current situation as they had flown this approach into the boss’s ranch on a regular basis. It had, so far, been an uneventful flight, but as usual the boss had been running late which of course delayed the departure, and meant arrival time would be well after sunset. As the crew sat listening to the drone of the twin turbines they contemplated the approach and landing.

The ranch runway, although still private, recently underwent a major refurbishment including improved approaches so the weather was more of a nuisance than a limitation to landing. At about 350 feet above ground level the aircraft broke out of the clouds and the crew made visual contact with the end of the runway. Unfortunately the combined effects of mist, fog, and darkness obscured the second half of the runway. The current situation did not alarm the occupant of the left seat as he knew the landing field better than anyone and had complete confidence in the stopping power of his aircraft.

The runway lights had been activated and sliced through the darkness enabling the crew to align the descending machine with the center line of the runway. Landing gear had been extended and verified down and locked, full flaps had just been selected and the final approach thrust settings were made. Touchdown occurred as planned and the throttles were set to idle in anticipation of deploying the thrust reversers. As the piggy back levers were pulled the crew observed the normal illumination of the in transit lights followed by the green deploy indication. Just as the request for an increase in reverse thrust was made, the co-pilot spotted several deer about two-thirds of the way down the runway. There wasn’t time to stop the aircraft nor was there time to stow the thrust reversers and get the engines spooled back up to enable a go around.

During the unplanned barbeque the next day, the chief of maintenance had taken some time away from repairing the impact damage to the aircraft and was talking with the ranch foreman who previously had been a career military intelligence gatherer. The discussion of course revolved around the recent events and how future situations could be avoided. During the conversation military surveillance techniques were reviewed including how “heat seeking” technology could easily unmask hidden targets. In fact, the comment was made: “You’d think they’d have something like that for aircraft”. Well in fact they do!

Infrared Radiation

Infrared radiation was first detected in 1800 by Sir William Herschel, who was attempting to associate heat with the visible light spectrum. These early studies proved that heat will radiate out from any object whose temperature is slightly above absolute zero. In fact absolute zero is -459.67 F and heat radiation will begin at -441.67 F and each degree has its own unique and predictable radiation pattern. Infrared means “below red” and in this case it is below the visible light spectrum producing a lower frequency and a longer wavelength. As it is not detectable to the human eye and is thereby not associated with sight, infrared radiation is referred to as thermal or heat radiation.

It is highly likely that almost everyone has already experienced this concept, by slowly reaching for an object; you can often tell if it is too hot to touch prior to making physical contact. Different temperatures have specific and identifiable radiation patterns, which mean far-off objects, can be identified from analysis of their wavelength and frequency. Radiometers are devices for measuring radiation and when specifically calibrated they can be used in the medical field for thermography which makes it possible to scan the human body and identify areas where temperature is higher or lower than normal. This technology is also the basis for heatseeking devices such as missiles or even laser thermometers.

Forward looking infrared (FLIR) sensors

Forward looking infrared (FLIR) sensors have been in use for many years in military applications but more recently they have found their way into general aviation. When put to proper use pilots will be able to see through darkness, smoke, fog, rain, and even snow. Considering that runway incursions are a leading factor in reported incidents and accidents, equipment such as this could provide a significant improvement to aviation safety.

There are currently several manufacturers of the devices used in general aviation with paraphernalia and installation costs anywhere from $10,000 to $100,000.

The components needed in most cases are a camera and a power supply. A means of viewing the generated image also needs to be considered. Some aircraft today have the ability to interface the video signal with existing flight deck displays while others will require a separate monitor. There is also the ability to project the infrared image to passenger cabin displays.

Selection and installation

Equipment selection and installation will require some upfront consideration when planning for an enhanced vision system. Some systems currently have United States Federal Aviation Administration (FAA) Supplemental Type Certificates (STC) available for various aircraft models while other hardware will depend on a local FAA Field Approval.

Proper location of the infrared sensor or camera is paramount and installation methods may have an impact on which system is selected. The ideal location is one that enables an unrestricted view of the area directly in front of the aircraft path. There are some variables here, such as the variations in aircraft deck angle, structural ability to support the camera, and interference with other equipment. One of the most common locations is in the leading edge of the vertical stabilizer. In an aircraft with a significant sweep to the stabilizer and a wing that enables a high deck angle during approach and landing, the sensor may not be able to provide the best possible situational view if located in the tail. Some have considered fitting the sensor on a retractable landing gear leg. This will still afford a bird’s-eye view during the takeoff and landing phases of flight but the operator will loose the value in being able to see surrounding terrain during night flying or poor visibility situations.

Some STCs provide an aerodynamic fairing to house the infrared sensor and in other cases the unit itself already comes encased and only needs to be attached to some strategic part of the airframe.

Electrical bonding is another important aspect of this installation. Fiberglass housings are often very adaptable to numerous installations but an inherent property of fiberglass is that it is not a good conductor of electrical current. Appropriate top coatings and bonding straps if not included could leave the aircraft with a good view but significant noise in the radios.

Maintenance and operational concerns

Most photographers know the importance of keeping the camera lens clean and free of foreign objects. In the case of infrared imaging, dirt on the lens is not usually a significant factor. With most systems in production today the camera lens has a factory-applied coating which can be removed or damaged during frequent cleaning. Most manufacturers recommend that a solution consisting of only a mild soap and water be used when cleaning is necessary. Never use abrasive cleaner or cleaning pads on a lens.

Susceptibility to ice formation may be another concern. Most equipment available today comes with self-contained heaters. When the system is activated, power is delivered automatically to the electrical elements and the viewing window temperature is maintained at around 70 F. In fact on cool days the lens of the sensor may be warm to the touch. If the heaters fail and the aircraft is in an icing environment, infrared energy will be absorbed by the ice and the effectiveness of the system will be degraded. The indication within the flight deck will often be a progressively fading image on the display. In this case the flight crew should turn off the enhanced vision system and the sensor unit should be sent to an authorized repair station for analysis.

The material used to construct the thermal eye is also a concern. In most cases the sensor is sealed from the factory and may be filled with an inert gas such as nitrogen to eliminate moisture. In the event the case is opened the gas charge will be lost. Germanium is one of the common photo optical materials used in the construction of the sensor.

If the lens is ever broken, use extreme caution when handling broken Germanium pieces or dust. It is recommended to wear gloves and a mask during any cleanup operation. Although Germanium does possess excellent properties regarding infrared radiation detection it is also susceptible to damage when it is exposed to direct sunlight for prolonged periods. There is a sensor currently in use that is manufactured from barium strontium titanate and is immune to heat and sun damage.

An electrically actuated lens cover may be employed to provide protection to the eye when the system is not in use. A recommended practice is to not operate the system when the aircraft is parked and pointed directly at the sun, and with the system selected off the lens cover should automatically close to provide protection against damage. Technology is in place to provide ongoing calibration to a sensor that has been exposed to direct sunlight. In the event of uncorrectable damage the only solution is to return the unit to the manufacturer for repair and detailed calibration.

As this type of system does work based on variations of temperature there are factors that may impact the system’s image quality. Some of the variables include the size of the target, background temperature, visible moisture, and even relative humidity. If the video image appears clear on a cool morning and a bit washed out on a hot and humid summer afternoon, there may not be a malfunction with the system.

As was stated earlier thermal imaging systems most often are made up of a cockpit-mounted ON/OFF switch as well as a video monitor and elsewhere in the airframe is a power supply and the sensor. Repairs are for the most part limited to sending out Line Replaceable Units (LRU).

By the way, for those of you wondering, no animals were hurt or killed during the writing of this piece. And about the shot that rang out, there was no gun involved. Someone in the hangar made a mistake troubleshooting an engine fire extinguisher. Sorry for the adrenaline rush.

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