The PAPI (Precision Approach Path Indicator) system is a simple and efficient, positive visual aid to the pilot on final approach . The system consists of a line of four units at 90º to the runway centerline which are typically spaced at 9 metres apart. Units are identical, producing a beam of light with the upper portion white and the lower red. A pilot sees the four individual lights in a combination of red and white depending on his vertical position. Typically  the elevation setting of each unit vary by 20 minutes of arc, the nominal 3º glideslope angles being midway between the centreline between the centre pair. The system is easily set for other glideslope angles. Using a PAPI system it is possible to follow the glideslope to less than 300 metres from touchdown (approx. 15 metres height).

The PAPI may have either two or three projectors. The two projector PAPI meets both ICAO and FAA performance specifications. The three projector PAPI is used to comply with specific local regulations and military applications. The three projector PAPI maintains its performance, exceeding international specifications even if one lamp fails.

The PAPI with its modular design is unique. Interchangeable pre-aligned projectors can be easily fitted onto a variety of bases for both fixed and portable applications.         

• Proven Design - Over fifteen thousand projectors installed around the world over thirty countries for both civil and military use. 
• Interchangeable projectors - Individual projectors are fully interchangeable, simplifying maintenance and spares management. 
• Field Maintenance - With no need for realignment, lamps and filters can be replaced and optics cleaned. A defective projector can be quickly replaced with a spare and then serviced in the ideal conditions of the workshop. 
• Stable Reference -  Projectors are mounted on a rigid aluminium casting which has integral clinometer reference pads. This casting is the optical base that is adjusted for cross level and elevation. Subsequent replacement of pre-aligned projectors does not require base readjustment, the unit retaining its correct settings.
• Compact Size -  Large enough to provide optical precision but small enough to minimise the effect of jet blast. Easy storage and handling.
• Instant Projector Change - Quick release fasteners are used. No need for any special tools or realignment.
• Corrosion Resistance - All projectors are manufactured from anodised light alloy. All fasteners are stainless steel.