What is a pavement temperature sensor?

It measures the temperature of the runway or taxiway surface, and often of the pavement a few centimetres below it. Some units also report the surface condition, such as dry, wet, icy or treated with chemicals. It is a different instrument from the air temperature probe, because the pavement and the air can be at quite different temperatures.

A contact sensor is a probe set flush in the pavement, sealed in, with its cable in a duct. It measures the pavement it sits in. An infrared sensor is mounted on a pole beside the pavement and looks at a spot on the surface without touching it, so it needs no pavement cutting but needs a clear line of sight and a clean lens. Many aerodromes use embedded probes, sometimes with an infrared unit as a cross-check.

On clear, calm nights the pavement loses heat by radiation and can fall below 0 degrees Celsius while the air a metre above it is still above freezing. Frost or ice can then form on the surface even when the air temperature looks safe. The pavement temperature, together with the dew point from the weather station, tells winter operations teams whether frost or ice is likely.

In the runway and taxiway pavement at representative positions, often at the touchdown zone, the mid-point and the stop end, and on key taxiways. The aerodrome and the meteorological authority decide the positions. Probes are placed where they represent the pavement but where wheel loads and snow clearing will not damage them, and they must sit flush with the surface.

It shares the pavement with the in-pavement lights, base cans and ducts, so the installation is coordinated with the lighting. It is not part of the series circuit, and its cable must be kept separate and recorded like any other airfield cable. Ice and frost on the surface also affect the lights themselves, which is one reason the readings are useful to the lighting maintenance team.

Typical faults are a probe that is not flush or has a cracked seal, water in the cable or connector, damage during resurfacing or snow clearing, a dirty or iced infrared lens, calibration drift and a wrong position that does not represent the pavement. A probe left standing above the surface is also a hazard to aircraft tyres and a damage risk, so it needs the same care as an inset light.

Work in the pavement needs the aerodrome's permit-to-work and aircraft-movement controls, and usually a runway or taxiway closure. Before cutting or drilling, locate and mark the AGL cables and ducts so they are not damaged. Because the readings affect winter operations decisions, tell the airside operations team and the meteorological authority before taking a sensor out of service.