See and Avoid

For many years the FAA has promoted a “See and Avoid” solution to prevent mid-air collisions. To have a mid-air collision there must be zero relative bearing change between the objects (aircraft, birds, missiles, torpedoes, etc.) prior to the collision. Since change of relative bearing is not a cue, something other must cause the eye to notice a threat. It can be as simple as the threat is just very large, or it could be a bright light, preferably flashing.

After World War II, the Air Force conducted an extensive investigation which resulted in a conclusion that to protect against a collision between two aircraft traveling toward each other at 250 knots each in daylight conditions would require flashing strobe lights of 3,500 ecp (equivalent candle power). Less powerful lights can be used for slower aircraft.

At least as early as 1954, Air Line Pilots Association (ALPA) members requested bright white strobe lights on aircraft for daytime visual avoidance under the “see and avoid” concept. The one operating environment where this concept is amazingly effective is under nighttime visual conditions during which there are practically no midair collisions or reported near misses. The obvious dominant factor is the conspicuity of aircraft under these conditions.

At the FAA Airworthiness Review in 1974 and the Operations Review of 1975 (both were the first and only bi-annual reviews), ALPA proposed bright white strobes on all aircraft, citing the Air Force study and other available information. The Aircraft Owners and Pilots Association (AOPA) vigorously opposed this unless white strobes could be used to replace the required red anti-collision lights; they achieved that goal. However, the dinky white strobe lights used at night are ineffective in the daytime. An objection to adequate daytime strobes was that they were too bright at night and distracting in IFR conditions, which is true.

ALPA received no support from industry except an Army representative who stated that the Army was experiencing a significant number of midair collisions during helicopter training until they installed the bright white strobes, after which time there had been no midair collisions, but the lights had to be turned off at night and in instrument conditions. Proposals that standards for aircraft lights require that the lights can be used during all conditions effectively rule out adequate daylight collision avoidance lights.

In 1970, Mohawk Airlines reported that several aircraft on which it had installed bright strobes had significantly fewer bird strikes.

However, current FAA criteria for anti-collision lights (FAR 25.1401) which require that they not detract from the conspicuity of position lights limit the maximum ecp so as to be ineffective during daylight conditions. Red and green position lights are an anachronism from slow moving sailing ships for use at night, mainly to establish right of way. They are ineffective for aircraft; in fact so ineffective that the red anti-collision lights were mandated. It is illogical that these lights limit the effectiveness of daytime anti-collision lights. After many years of experience involving daylight mid air collisions the FAA and the NTSB continue to ignore the obvious. The FAA needs to require a separate standard for daylight collision avoidance lights. Considering past history, this will not likely happen unless Congress directs it.

There is a great volume of information on this subject. One reference of note is: Greenlee, Paul H., High Intensity Flashing Lights & Collision Avoidance, Air Force–Industry Conference, Mid Air Collision Prevention, 1976. Strobe light manufacturers are very aware of the above information and can probably provide the original Air Force study.

August, 2010