Field trials have generally confirmed the ineffectiveness of strobes in daylight. The following U.S. military trials are outlined in a US Air Force report (Schmidlapp 1977).
1. In 1958 the USAF Air Training Command conducted flight tests to compare strobe anticollision lights with rotating beacons. It was concluded that in daylight conditions, no lighting system could be expected to prevent collisions.
2. Further tests in 1958 at the U.S. Air Force’s Wright-Patterson Base again found that strobe lights were ineffective in daylight.
3. A major U.S. Army study was conducted in 1970 in which observers on a hilltop were required to sight approaching helicopters equipped either with strobes of 1800, 2300 or 3300 effective candela or a standard red rotating beacon. It was found that none of the lights were effective against a background of daytime sky, however strobes were helpful when the aircraft was viewed against the ground.
4. U.S. Air Force tests in 1976 found extremely poor performance of strobe lights on aircraft. In all cases, the aircraft was sighted before the strobe. In addition, it was found that after two years service on aircraft, strobe lights were about half as intense as expected.
5. Extensive trials in 1977 by the US Air Force Aeronautical Systems Division used strobes fitted on a tower and observers at various distances and viewing angles. The results indicated that in daylight, even a strobe of 36000 candelas was not particularly conspicuous. However, strobes were more visible when the background illumination was less than 30 candelas per square metre, equivalent to a very dark day.
FAA studies have also concluded that there is no support for the use of strobes in daylight. A 1989 FAA study of the effectiveness of see-and-avoid concluded that ‘Aircraft colours or lights play no significant role in first directing a pilot’s attention to the other aircraft during daytime’ (Graham 1989).
An earlier FAA study considered that there was ‘little hope that lights can be made bright enough to be of any practical value in daylight’ (Rowland and Silver 1972).
A major FAA review of the aircraft exterior lighting literature concluded that ‘During daytime, the brightest practical light is less conspicuous than the aircraft, unless there is low luminescence of background ...’ (Burnstein and Fisher 1977).
In conclusion, while strobes are not likely to be helpful against bright sky backgrounds, they may make aircraft more visible against terrain or in conditions of low light.
