MIL-STD-411F
5.1.3.2 Legend. Except when NVIS compatibility is required, the warning legend, when energized, shall be opaque on a translucent background as shown on figure 1. When NVIS compatibility is required, the legend, when energized, shall be translucent on an opaque background. The character height of the legend shall subtend a viewing angle of not less than
20 minutes of arc when measured from the aft most point of the design eye line (see 3.7). The character proportions shall be in accordance with MIL-M-18012. The minimum horizontal separation between two legends shall be two times the width of the widest character. The minimum vertical separation between two legends shall be two character heights. The minimum vertical separation between two lines in the same legend shall be one-half the height of a character.
5.1.3.3 Luminance. The luminance of light signals shall be not less than 150 fL at the point of entry of the light source when the signals are operated at rated voltage. Not less than two lamps, operated in parallel, shall be employed for each incandescent and light emitting diode type legend warning light. When installed in the flight aircrew station and unless otherwise specified by the acquiring activity (see 6.3.1), lights shall automatically be dimmed to 15 ± 3 fL when the pilot's primary interior light control is "ON". The design shall preclude inadvertent dimming during daylight conditions. When installed in other aircrew stations, the warning lights shall automatically be dimmed to 15 ± 3 fL when the operators primary light control is "ON". Except when NVIS are being used within a crew station, the dimmed warning lights shall automatically reset to full bright when any of the following conditions exist.
a. Main aircraft power is turned off.
b. Pilot's primary interior light control is off. c. High intensity lights are energized.
When an NVIS-compatible lighting mode is selected, the dimmed warning lights shall not reset to full bright when any of the above conditions exist, but shall remain at their NVIS compatible level. When an NVIS compatible lighting mode is being used within a crew station, the lighting circuit logic shall preclude the system from obtaining a luminance level which will degrade external vision or NVIS performance.
5.1.3.4 Presentation. Warning signal presentation in the cockpit shall be steady "ON". An exception to the steady "ON" presentation is the Wheels Warning Legend which must attract the aircrew's attention during a critical mission phase (see 5.1.3.6 and 5.1.3.7). When installed in other aircrew stations, the warning signal presentation may be flashed as specified in 4.3.5.
5.1.3.5 Master warning signal reset. The master warning signal shall have a push-to-reset or voice reset capability which deenergizes the master warning signal while the applicable legend
light remains "ON".
5.1.3.6 Wheels warning legend (VSTOL aircraft). A "WHEELS" legend indicator, when specified by the acquiring activity (see 6.3.1), shall be installed and connected so that it flashes whenever the following conditions exist simultaneously:
a. The aircraft is in a landing configuration, as specified by the acquiring activity (see 6.3.1),
and,
b. The landing gear is not down and locked.
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