In a previous article we talked about Aviation Light Optics. In that article we talked about the Precision Approach Path Indicator (PAPI). Now we would like to go more in detail about that system and describe their design requirements.
PAPI系统是一组四个光源，可帮助飞行员在达阵期间纠正飞机。您可以在图1中看到Papi系统，因为柏油碎石左侧的四个红灯靠近地平线。Papi光源可以是白色或红色。如果飞机太低，Papi系统将显示四个红灯（如图1所示），如果飞机太高，则所有四个灯都是白色的。因此，红色和白光的结合可以帮助飞行员纠正其着陆途径。
Figure 1. PAPI lightning system. Figure fromhttps://solutions4ga.com/
The traditional PAPI optical systems use incandescent sources. The characteristics of each light unit are identical: each PAPI light consists of a light source, red filter and lenses. A high-intensity beam which shows two distinct colors is emitted from each unit – the lower part is red while the upper part is white. The color transition boundaries of the four units are fixed at different angles, creating the PAPI guidance beams. A simplify image is shown in Figure 2.
Figure 2. PAPI Optics schematics. mage from Canadian Transport Advisory Circular No AC 300-006
More modern designs use arrays of LEDs as a light source. There are two common configurations that are shown in Figure 3. The first one is similar to the design that we shown previously in figure 2. We have a single white light source, and a red filter that covers the upper half of the light emitted by our source. The second design uses two sets of arrays, one of white light and other one of red light, creating two different color sectors. In the enclosure there is a separation screen
FIgure 3. LED-based PAPI system. On the left, a single LED white source with a red filter. On the right two LED arrays one white, the other red, with a separation screen.Lužica的图像，štefan等。
引领-based PAPI optical designs present several advantages over the traditional incandescent light sources. Lower power consumption, longer life, very small degradation over the life span, better operation under extreme weather.
When an aircraft approaches the runway, the angle of its approach determines whether the light beams appear red or white to the pilot. If the aircraft is approaching on the correct glide path, the four light PAPI system will show two red lights and two white lights; too steep of an approach increases the number of white lights, while too shallow of an angle increases the number of red lights. This angular approach is shown in figure 4.
Following this guidance makes it easy for the pilot to correctly angle their aircraft and land in the designated part of the runway.
Figure 4. PAPI approach profile. AS the angle of approach changes the color display of the PAPI systems gets adapted. Image from Canadian Transport Advisory Circular No AC 302-009
