Lasers and aviation safety

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Under certain conditions, laser light or other bright lights (spotlights, searchlights) directed at aircraft can be a hazard. The most likely scenario is when a bright visible laser light causes distraction or temporary flash blindness to a pilot, during a critical phase of flight such as landing or takeoff. It is far less likely, though still possible, that a visible or invisible beam could cause permanent harm to a pilot's eyes. And, although laser weapons are under development by the military, these are so specialized, expensive and controlled that it is essentially impossible for non-military lasers to cause structural damage to an aircraft.

Any aviation hazard from bright light can be minimized or eliminated in two ways. First, users on the ground can exercise caution, to prevent or minimize any laser or other bright light being directed in airspace and especially towards aircraft. Second, pilots should have awareness of laser/aviation hazards and knowledge of basic recovery procedures in case of laser or bright light exposure.

[edit] Lasers and bright lights

Although this article concentrates on lasers, it should be noted that other bright directional lights such as searchlights and spotlights can have the same dazzling/distracting/flashblinding effects. Searchlight/spotlight operators should take the same basic precautions as laser users. Similarly, pilots and safety officials should keep in mind that a reported "laser" incident may be caused by a non-laser bright light.

In a 1996 demonstration involving a helicopter exposed to laser light and a searchlight, produced for the television show "American Journal", both sources overwhelmed the video camera. The producers were unable to tell which was which, and in fact on the air, they identified the searchlight footage as being from lasers.

[edit] Lasers in airspace

Laser guide star used at Lick Observatory.

There are many valid reasons that lasers are aimed into airspace. Lasers are used in industry and research, such as in atmospheric remote sensing, and as "guide stars" in adaptive optics astronomy. Lasers and searchlights are used in entertainment; for example, in outdoor shows such as the nightly IllumiNations show at Walt Disney World's EPCOT Center. Laser pointers are used by the general public; sometimes they will be accidentally or deliberately aimed at or near aircraft. (Of course, no unauthorized person should deliberately aim any type of laser at or near an aircraft.)

Lasers are even used, or proposed for use, with aircraft. Pilots straying into unauthorized airspace over Washington, D.C. can be warned to turn back by shining eye-safe low-power red and green lasers at them.[1] At least one system has been tested that would use lasers on final approach to help line up the pilot on the proper guideslope. NASA has tested a Helicopter Airborne Laser Positioning System.[2]

Because of these varied uses, is not practical to ban lasers from airspace. This would unduly restrict legitimate uses, it would not prevent accidental illumination incidents, and it would not stop someone who deliberately, out of malice or ignorance, targeted aircraft. For this reason, practical laser/aviation safety is based on informed users and informed pilots.

[edit] Primary hazards of lasers and bright lights

There are some subjects which laser/aviation safety experts agree pose no real hazard. These include passenger exposure to laser light, pilot distraction during cruising or other non-critical phases of flight, and laser damage to the aircraft.

The main concerns of safety experts are almost exclusively focused on laser and bright light effects on pilots, especially when they are in a critical phase of flight: takeoff, approach, landing, and emergency maneuvers.[3]

There are four primary areas of concern. The first three are "visual effects" that temporarily distract or block pilots' vision. (For lasers, these effects are only of concern when the laser emits visible light.)

FAA flight simulator showing distraction where the light does not obscure vision but can distract the pilot. Light level 0.5 microwatts/cm²; for example, a legal 5mW laser pointer at 3,700 feet (1,130 m).

FAA flight simulator showing veiling glare where it is hard to see through the light to the background scene. Light level 5.0 microwatts/cm²; for example, a legal 5mW laser pointer at 1,200 feet (365 m).

Simulation of temporary flash blindness where the image takes from a few seconds to a few minutes to fade away, depending on how much light entered the eye. Light level 50 microwatts/cm²; for example, a legal 5mW laser pointer at 350 feet (107 m).

• Distraction and startle. An unexpected laser or bright light could distract the pilot during a a nighttime landing or takeoff. A pilot might not know what was happening at first. They may be worried that a brighter light or other threat would be coming.

• Glare and disruption. As the light brightness increases, it starts to interfere with vision. Veiling glare would make it difficult to see out the windscreen. Night vision starts to deteriorate.

• Temporary flash blindness. This works exactly like a bright camera flash: there is no injury, but night vision is temporarily knocked out. There may be afterimages -- again, exactly like a bright camera flash leaving temporary spots.

(Note: The photos at right flash because most incidents are of flashes and not of steady illumination. In accidental illuminations there may be just one or a few flashes. Even in deliberate illuminations, it is hard to hand-hold a laser on a moving target, so there will be a series of longer flashes. With helicopters at close range, it is possible to have a more-or-less continuous light.)

The three visual effects above are the primary concern for aviation experts. This is because they could happen with lower-powered lasers that are commonly available. The fourth concern, eye damage, is much less likely. It would take specialized equipment not readily available to the general public.

• Eye damage. Though it is unlikely, high power visible or invisible (infrared, ultraviolet) laser light could cause permanent eye injury. The injury could be relatively minor, such as spots only detectable by medical exam or on the periphery of vision. At higher power levels, the spots may be in the central vision -- in the same area where the original light was viewed. (This is why one should not look directly straight into the sun or any bright light. Instead, look away or far off to the side.). And most unlikely of all is injury causing a complete and permanent loss of vision. To do this requires very specialized equipment and a desire to deliberately target aircraft. Someone wanting to do this could find far less expensive, and much easier ways to attain their goals. (It should be noted that claims of permanent injuries are not proof of injury. Some laser experts are skeptical of some reported pilot injuries, as the injuries would have required different types or powers of lasers than those reported.)

It is extremely unlikely that any of the four elements above would cause loss of the aircraft, especially if the pilots react properly and work as a team.

[edit] Analyzing the hazard

The exact hazard in a specific situation depends on a number of factors.

[edit] Laser/bright light factors

• The power of the laser or bright light. The more light emitted, the brighter and more hazardous it will be.
• The beam divergence. A low-divergence "tight" beam will be a hazard at greater distances than one which spreads out rapidly.
• Visibility (wavelength) of the beam. An infrared or ultraviolet laser beam does not present any visual effect risk to pilots, as they cannot see it. However, at high powers it can present an eye damage risk. In some cases, this hazard may be greater since a pilot would not know they were being illuminated.
• Color of the beam (for visible wavelengths). A dark-adapted eye is most sensitive to blue-green light. Thus, a green beam is much more visible at longer distances than a red or blue beam of equivalent power. To give a specific example, a 10-watt continuous-wave YAG laser at 532 nanometers (green), appears brighter to the eye than an 18-watt continuous-wave argon-ion laser (10 watts of 514nm (green-blue) plus 8 watts at 488 (blue)).
• Pulsed/continuous nature of the beam. Some laser beams emit their energy in pulses. A pulsed laser presents a greater eye damage risk than a continuous laser of equal (average) power. This is because the power is packed into shorter pulses.

[edit] Operational factors

• Beam movement. If the beam is moving around such as in a laser show, it covers a greater area of the sky and thus has a greater chance to illuminate an aircraft. However, if it did scan across a cockpit, in general the exposure duration would be shorter. (A more precise analysis would look at the relative motion of the beam and aircraft.)
• Location of the beam relative to airports. The beam must avoid airspace around airports and busy air routes. The FAA has established safety zones around airports, which are described in the "Regulation" section below. It is possible to use beams within the zones, if the beam power is below the FAA limit for the zone.
• Projector and laser stability. To avoid accidents, the laser projector must be secured with relation to termination points and beam blocks. If a projector slips, or safety software fails, the beam could enter unsafe areas of airspace.

[edit] Situational factors

• Day vs. night. Almost all concern is over nighttime illumination. The three visual effects listed above (distraction, glare and flash blindness) are minimized during the day since the eye is not dark adapted, and since visible lasers are not often used outdoors in daytime.
• Motion and speed of the aircraft. A slow aircraft is at greater risk than a fast one (relative to travel across the viewer's line of sight). Helicopters are at greatest risk because they can hover, presenting a relatively stationary target.
• Distance to the aircraft. A low-flying aircraft is at greater risk. Again, helicopters are vulnerable due to their close ground proximity.
• Direction relative to the aircraft and cockpit. A beam aimed directly at an incoming aircraft gives the greatest risk to pilots. One aimed across the aircraft's travel gives less risk, partially because the light enters through the side windows, and partially because it is harder to keep the beam aimed exactly at the cockpit area. A beam aimed straight up gives the least risk, although it is still possible for the beam to illuminate the cockpit during a banking turn.

[edit] Pilot/aircrew factors

• Flight phase. The risk is greatest when the exposure comes during a time of high workload: takeoffs, critical or emergency maneuvers, and landings.
• Pilot awareness and response. Ideally, pilots will be aware of laser and bright light hazards, and will know how to recover in case of an incident. Conversely, a pilot can make the situation worse if he or she overreacts, stares at the light to try to locate its source, or takes immediate unnecessary evasive maneuvers.

[edit] Accidental vs. deliberate exposure

Laser users must take appropriate precautions to avoid accidents. (Some steps are outlined below in the section "Reducing the hazard".) In most cases, an accidental exposure is likely to be one or a few brief flashes, as the aircraft moves through a stationary beam, or as a hand-held beam sweeps over the cockpit.

There have been cases of deliberate intent, where someone through ignorance or malice deliberately aimed a laser at an aircraft. However, it is not easy to hand-hold a laser on a relatively distant target. (To demonstrate this, aim a laser pointer at a retroreflective street sign a block or two away.) It is even harder to hand-hold a laser on a moving target. As a result, a deliberate exposure is usually not a constant dazzling light but is more likely to be a series of longer flashes, as the perpetrator tries to keep the beam on the aircraft.

Whether an accidental or deliberate exposure, any pilot seeing a flash should avoid looking in the direction of the light, since it may be quickly followed by additional flashes.

[edit] Example laser safety calculations

Graphic illustrating how laser pointer hazards are most serious when the laser is close to the aircraft

The graphic at right shows many important laser/aviation safety concepts. For example, it shows that the areas of most concern -- eye damage, flash blindness and glare -- occur relatively close to the aircraft. The distraction risk covers the longest hazard distance, but fortunately also presents the least concern. The photos in the graphic also give an idea of what the visual effect looks like to the pilot, at various distances.

Note that while the distances given are exact ("52 feet", "262 feet"), the laser's brightness is in fact falling off slowly. It is not as if at 51 feet the laser is an eye hazard and at 53 feet it is eye safe. Effects diminish with distance.

Also, the weaker effects are part of any stronger effect. Even if a laser does not cause eye damage at 25 feet, it can still cause flash blindness, glare and of course be a distraction.

For any given laser, the relative distances shown here may change. For example, an invisible (infrared) laser can be an eye hazard for hundreds of feet, but presents no flash blindness, glare or distraction hazard. Because of this, each laser must be analyzed individually.

To give another example, here are calculations of a more powerful laser -- the type that might be used in an outdoor laser show. A 6-watt green (532 nm) laser with a 1.1 milliradian beam divergence is an eye hazard to about 1,600 feet (488 meters), can cause flash blindness to about 8,200 feet (1.5 mi/2.5 km), causes veiling glare to about 36,800 feet (7 mi/11.2 km), and is a distraction to about 368,000 feet (70 mi/112 km).

[edit] Reducing the hazard

Successful laser/aviation safety requires effort both on the ground, from laser and bright light sources, and in the air, from pilots. While ground-based laser hazards should be reduced as much as possible, there is always the chance of accidental (or deliberate) exposure. In such a case, the pilot should not panic, should avoid looking at or near the beam, and should continue to "fly the plane".

[edit] Laser/bright light hazard reduction

• Use termination and beam stops to prevent laser light from being directed into protected airspace. Terminated beams are those stopped by buildings, dense trees and other impermeable surfaces. Targets such as bounce mirrors should have beam stop barriers around them so that if the laser misses the mirror, it does not go off into airspace. Care must be taken that the laser projector cannot be misaligned without an operator and/or spotters noticing and stopping the show.

• Use outdoors only during "Low ceiling" weather to increase scattering of stray laser light. This may require weatherproofing of equipment. During the winter months, this may allow for more high temperature experimentation, as the low temperature of the air provides a cheap heat sink.

• Adjust beam divergence and/or output power to meet the appropriate irradiance distance. In other words, make the beam wider and/or less powerful, so it does not exceed the laser power for a particular safety zone (FAA safety zones are described in the "Regulation" section below).

• Direct beams away from areas with many aircraft, such as airports and flight paths.

• Use airspace observers (spotters), who can shut down the beam if they spot an aircraft. This topic is a complex one, which depends on observer abilities, distance to planes, aircraft visibility, communications to ensure shutdown occurs, etc. At Walt Disney World, EPCOT's nightly IllumiNations laser show relies partially on laser spotters to keep the show safe and legal.

• Use automated detection/avoidance systems, which shut down the laser or move the beam in case a plane is detected. These are complex, must be proven to work, and must be reviewed and not objected to (e.g., approved) by the FAA.

[edit] Regulatory and other hazard reductions

• Restrict the sale or use of laser devices. The Congressional Research Service notes that this was done in the United Kingdom with certain laser pointers, but in the U.S. this could "pose significant challenges because these devices are widely available at low cost and are used in a variety of applications such as laser pointers, laser levels and laser gun sights." [4]
• Amend existing laws, or enact new ones, to try to discourage irresponsible laser use. One U.S. federal effort in this direction is the "Securing Airplane Cockpits Against Lasers Act of 2005", discussed in the "History" section below.
• Educate the public in the safe use of laser pointers and other bright light sources (e.g., high-powered hunting-type hand-held spotlights).

[edit] Pilot/aircrew hazard reduction

• Read NOTAMs so that pilots are aware of potential laser or bright lights in use along their planned flight path.
• Know about laser hazards and defensive measures. Ideally, pilots would get formal training in how to "recognize and recover" from an illumination incident. Failing that, individual pilots can read more about the potential hazards in online and other resources.

The following three pilot/aircrew recommendations are under study. They may work in theory and in some situations such as military operations. However, they may not be suitable, practical or recommended for widespread use.

• Use laser safety goggles. This is a complex subject due to the wide variety of laser wavelengths/colors that may need to be defended against. If all wavelengths are protected, the goggles essentially are opaque. There are other issues as well, such as the discomfort of wearing goggles continually, and their potential interference with nighttime vision and cockpit indicators. It may be a benefit to have goggles available which can be donned by at least one pilot (in a multi-pilot aircraft) if a persistent, deliberate illumination occurs.
• Use active "smart" goggles which can detect laser light and then activate a blocking/dimming process.
• Use glare shields that can be pulled down over a windscreen to reduce all incoming light.

[edit] For pilots: What to do in a bright light exposure

• Look away. If exposed to a bright light flash, do not look at it -- at least, not directly. When two pilots are available, communicate so both do not look out the window. One can cautiously look outside to determine the nature and source of the light, while the other keeps his or her eyes on the instruments.
• Do not panic. A bright light exposure can be surprising but is recoverable. Continue to "fly the plane".
• Report the incident immediately. Follow the procedures in FAA Advisory Circular 70-2[5], Section 5 "Air Crew Reporting Procedures" so other aircraft in the vicinity can be warned, and so the perpetrators can possibly be caught.
• Do not rub the eye. Some reports of claimed eye injury may be due to vigorous rubbing of the eye after the incident.
• If warranted, have an eye exam soon after landing. An ophthalmologist is preferred, especially one with retinal and laser exposure experience.

[edit] Regulation and control

The U.S. FAA Laser Free Zone extends horizontally 2 nautical miles (3,700 m) from the centerline of all runways (two dark lines in this diagram) with additional 3 nautical mile (5,560 m) extensions at each end of a runway. Vertically, the LFZ extends to 2,000 feet (610 m) above ground level.

The U.S. FAA Critical Flight Zone extends horizontally 10 nautical miles (18.5 km) around the airport, and extends vertically to 10,000 feet (3,050 m) above ground level. The optional Sensitive Flight Zone is designated around special airspace needing bright-light protection.

In the United States, laser airspace guidelines can be found in Federal Aviation Administration Order 7400.2 (Revision "F" as of August 2006), Part 6, Chapter 29, "Outdoor Laser Operations".[6] Bright light airspace guidelines are in Chapter 30, "High Intensity Light Operations".[7]

In the United Kingdom, CAP 736 is the "Guide for the Operation of Lasers, Searchlights and Fireworks in United Kingdom Airspace." [8]

For all laser users, the ANSI Z136.6 document gives guidance for the safe use of outdoor lasers. [9] While this document is copyrighted by ANSI and is relatively costly, a flavor of its recommendations can be seen in NASA's Use Policy for Outdoor Lasers. [10]

[edit] Airspace zones

The U.S. FAA has established airspace zones around airports:

• The Laser Free Zone extends immediately around and above runways, as depicted at right. Light irradiance within the zone must be less than 50 nanowatts/cm².
• The Critical Flight Zone covers 10 nautical miles around the airport; the light limit is 5 microwatts/cm².
• The optional Sensitive Flight Zone is designated by the FAA, military or other aviation authorities where light intensity must be less than 100 microwatts/cm². This might be done for example around a busy flight path or where military operations are taking place.

In the UK, restrictions are in place in a zone that includes a circle 3 nautical miles (5.5 km) in radius around an aerodrome (airport) plus extensions off each end of each runway. The runway zones are rectangles 20 nautical miles (37 km) in total length and 1000 meters (3280 feet) wide, centered about each runway.

[edit] Reporting

In the U.S., those persons operating outdoor lasers are requested to file reports at least 30 days in advance, detailing their laser power(s). They must reference their operation location with respect to local airports and show that the laser light will not exceed limits within the Sensitive, Critical and Laser Free zones. FAA Advisory Circular 70-1[11] "Outdoor Laser Operations" contains two forms plus instructions. One form is a "Notice of Proposed Laser Operations", the other is a "Laser Configuration Worksheet" which is filled out for each laser or each different laser configuration. The FAA will review the report, and will either send a letter of objection or will send a letter of non-objection. Note that the FAA does not "approve" or "disapprove" as this implies a higher level of regulatory authority.

(U.S. laser light show users have a slightly different regulatory process. Any use of lasers in a show or display requires pre-approval from the FDA Center for Devices and Radiological Health. This is required both for the laser equipment, and separately for the show itself (site, audience configuration, beam effects, etc.). As part of the CDRH's show approval ("variance") process, the CDRH will require a letter of non-objection from the FAA. Without this, the show cannot legally proceed.)

In the U.S., laser activity in a given area is communicated to pilots before their flight via a NOTAM. Pilots exposed to a laser or bright light during flight should follow Advisory Circular 70-2[12] "Reporting of Laser Illumination of Aircraft".

UK laser operators should report outdoor laser, searchlight or firework operations at least 28 days in advance, using the Notification Form found in annex A of the CAP 736 document. [13]

[edit] Regulatory and standards development

A key group inside the U.S. working on laser/aviation safety is the SAE G10-T Laser Safety Hazards Subcommittee. It is comprised of laser safety experts and researchers, pilots and other interested parties representing military, commercial and private aviation, and laser users. Their recommendations have formed the basis of the FAA laser and bright light regulations and forms, as well as standards adopted in other countries and by the ICAO.

The ANSI Z136.6 standard is the "American National Standard for Safe Use of Lasers Outdoors". The Z136.6 committee has worked closely with SAE G10-T and others, to develop recommended safety procedures for outdoor laser use.

[edit] History

Up until the early 1990s, laser and bright light aviation incidents were sporadic. In the U.S., NASA's Aviation Safety Reporting System showed only one or two incidents per year. The SAE G10-T began meeting around 1993 as the number of incidents grew. Almost all of the incidents were known or suspected to be due to outdoor laser displays. Almost all of the concern was over potential eye damage; at the time visual effects were felt to be a minor consequence.

In late 1995, a number of illumination incidents occurred in Las Vegas due to new outdoor laser displays. Although the displays had been approved by the FDA as eye-safe for their airport proximity, no one had realized that the glare/distraction hazard would adversely affect pilots. In December 1995 the FDA issued an emergency order shutting down the Las Vegas shows.

Within the SAE G10-T, there was some consideration about cutting back or banning laser shows. However, it became apparent that there were a large number of non-entertainment laser users as well. The focus shifted to control of known laser users, whether shows or industry/research. New policies and procedures were developed, such as the FAA 7200 Chapter 29, and Advisory Circular 70-1. Although incidents continued to occur (from January 1996 to July 1999, the FAA's Western-Pacific Region identified more than 150 incidents in which low-flying aircraft were illuminated by lasers)[14], the situation seemed under control.

Then in late 2004 and early 2005, came a significant increase in reported incidents linked to laser pointers. The wave of incidents may have been triggered in part by "copycats" who read press accounts of laser pointer incidents. In one case, David Banach of New Jersey was charged under federal Patriot Act anti-terrorism laws, after he allegedly shone a laser pointer at aircraft.[15] Because there was no federal law specifically banning deliberate laser illumination of aircraft, Congressman Ric Keller introduced H.R. 1400, the "Securing Airplane Cockpits Against Lasers Act of 2005."[16]

As of September 2006, the wave of laser pointer incidents has died down, and H.R. 1400 is awaiting final passage.[17]

[edit] See also

Laser safety

[edit] References

"Laser Pointers: Their Potential Affects[sic] on Vision and Aviation Safety", Van B. Nakagawara, DOT/FAA/AM-01/7, April 2001.[18]

August 2003 FAA study: "The Effects of Laser Illumination on Operational and Visual Performance of Pilots Conducting Terminal Operations", DOT/FAA/AM-03/12.[19]

June 2004 FAA follow-up study: "The Effects of Laser Illumination on Operational and Visual Performance of Pilots During Final Approach", DOT/FAA/AM-04/9. [20]

Congressional Research Service Report for Congress RS22033, "Lasers Aimed at Aircraft Cockpits: Background and Possible Options To Address the Threat to Aviation Safety and Security" by Bart Elias.[21]

NASA's "Use Policy for Outdoor Lasers" [22]

A sampling of NASA ASRS laser incident reports can be done by searching for the term "laser".[23]