الوصف

As the integral part of airport approach light system, the Precision Approach Path Indicator indicates for the pilot whether the approach angle is within the proper scope. 
The PAPI system consists of four-light units, which locate at the side of the runway adjacent to the origin of the glide path.
The nominal glide slop angle is midway between the angular settings of the central pair of the four units.
If the aircraft approach is too high, an increased number of white light indicators will be seen. If the aircraft approach is too low, the pilot will see an increased number of red light indicators.
Compliance 
The PAIPI-640-H unit is in compliance with standards:
ICAO: Annex 14 volume ;
FAA: AC 150/5325-28
CNS: GB/T 7256-2005; 
GB/T 3777-2008;
GB/T 7000.1-2007; 
GB 2423.17-93;
GB 3979-2008.
Main Advantages
Using precision angle sensor to detect the tiny change of the inclination angle of each unit (precision 0.6 minute), and the real-time data is showed on the LED digital tube display at the back of each indicator. When the angle deviate more than the setting value (normally not exceeding 15 minutes), the system will shut off itself, and alert the control center.  
The special optical strengthened filters have high transmission factor and good thermal resistance, and with no discoloration.
Structure design ensures very good water-tightness and protection against corrosion, and ensures the electronic components not affected by the heat from the lamps. The structure has gotten application patent (Pat. No:ZL201320196674.4)
The unit is mounted in standard on three legs, each leg has a unitized structure, which comprises three mental components for adjusting the angle. The structure has gotten application patent (Pat. No:ZL201320196153.9)
¬ Easy sit adjustment by using a clinometer.
¬ Red to white transition is accurate with value not exceeding 3 minutes.
¬ Excellent frangibility without sacrifice of any stability.
¬ Easy maintenance: replacement of main elements does not require either unit adjustment or any special tools.
¬ Front glass protecting lenses against sad, wind, and engine blast.
¬ Very easy access to all components by removing the cover.