What is a PAPI foundation?
The footing under each PAPI unit — set out in one line, firm, level and true, carrying the anchor bolts, duct stub-up and earth connection, so the unit keeps the angle it was set to.
A PAPI unit is only as steady as what it stands on. The foundation is a concrete footing, normally one for each unit, carrying anchor bolts or a mounting plate, a duct stub-up for the cable and an earth connection. It is set out in one line, firm, level and true, because adjacent units are set only about 20 arcminutes apart and a small tilt or settlement can move where each one changes colour.
The unit stands on a frangible coupling above a low foundation top, so an aircraft that strays onto it breaks the unit away. A tilted or settled foundation spoils the glide-path cue in a way the pilot cannot see. Use a template for the bolts, cure the concrete fully before mounting, and record the as-built position. Check cable locations and hold the permit before excavating, and treat any cable as live until it has been isolated, locked out and proved dead.
- Set out. Mark each footing from the survey, in one straight line at the design spacing and offset from the runway edge.
- Excavate. Excavate to the design depth and size after cable locations are checked, and leave a firm, compacted, drained base.
- Prepare. Fit formwork, the duct stub-up and earth connection, and fix the anchor bolts in the manufacturer's template.
- Cast. Place and compact the concrete with no voids around the bolts or duct, and keep the top low and true.
- Level. Check the top and bolt pattern are level and true to the drawing tolerance before the concrete sets.
- Cure and record. Cure fully, check for cracks, record the as-built position and level, then mount the unit.
Frequently Asked Questions
Q1What is a PAPI foundation?
It is the footing that carries a PAPI unit: normally a concrete base for each unit, with anchor bolts or a mounting plate, a duct stub-up for the cable and an earth connection. It holds the unit in the right place, level and steady, with the frangible coupling above it.
Q2Why does a PAPI foundation have to be so stable?
Adjacent PAPI units are set only about 20 arcminutes apart in elevation, so a small tilt or settlement changes where each unit changes from white to red and distorts the pilot's cue. The foundation has to stay put for the life of the installation.
Q3Why is a PAPI unit mounted on a frangible coupling?
A PAPI stands close to the runway, so the unit is mounted to break away if an aircraft strays onto it. The foundation top is kept low and the frangible point is above it, as the manufacturer and design specify.
Q4What is cast into or fixed to the foundation?
The anchor bolts or mounting plate, set from a template; a duct or sleeve stub-up so the cable can reach the unit; and an earth connection. The exact set follows the manufacturer's drawing for the PAPI type.
Q5How level does the foundation need to be?
To the tolerance on the manufacturer's drawing, with the top and bolt pattern level and true. The foundation gives a sound, level base; the unit's elevation angle is set afterwards using the unit's own adjustment, not by the concrete.
Q6What is checked before the unit is mounted?
That the foundation is in the surveyed position and in line with the others, the top and bolts are level and true, the duct and earth are in place, the concrete has cured with no cracks or voids, and the backfill is firm and drained. Cable locations are checked and the work permit issued before any excavation.
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