LED Airfield Lighting: What Changes for Inspection

LED airfield lighting changes photometric inspection in four ways. An LED fades slowly and does not burn out, so each fixture’s output has to be measured. Its colour can shift. It makes little heat, so ice can stay on the lens. The 50 percent intensity limit applies to LED and incandescent lights alike.

How LED lights fail differently

An incandescent lamp reaches the end of its life with a complete failure. The FAA review of LEDs in airfield lighting (AM-22/08) says that LEDs instead tend to become slowly less bright until a failure. It concludes that a maintenance schedule must check the brightness of each fixture.

EASA gives the reason. An LED has no filament that breaks. Its light output decreases and its colour shifts over time, and the rate increases at higher operating temperatures. At some point the source falls outside the specification for its use. It may still be lit, but it is no longer useful. EASA concludes that the long life of an LED is no reason to “install and forget”, and that a system of preventive maintenance is necessary.

Fixtures with several light devices

Many fixtures use several LEDs to make one light. FAA Engineering Brief 67D requires such a fixture to meet its output specification while it is on and to turn off if more than 25 percent of the light devices fail. That is a design rule for new fixtures. It does not show how a fixture in service ages, so the output of each installed light still needs checking. The sources checked do not say how many devices a typical airfield fixture holds or how often a partial failure happens.

Why the eye is a weak check

A single dark unit stands out against its neighbours. A row that ages together has no brighter neighbour to give it away. The loss is gradual, so a night walk-around sees lit lights and cannot tell a light at full output from one at a lower output. This is the basis of the FAA advice to check the brightness of every fixture.

What the limit is and how it is measured

Annex 14, paragraph 10.5.1, deems a light unserviceable when its main beam average intensity is less than 50 per cent of the value in Appendix 2. EASA ADR.OPS.C.015(c) has the same 50 percent limit. Neither text sets a different limit for an LED light. Annex 14 Note 1 says these are maintenance performance objectives and do not define whether a lighting system is operationally out of service.

For a Category II or III precision approach runway, Annex 14 recommends in-field measurement of the intensity, beam spread and orientation of the lights (10.5.3 to 10.5.5), with a mobile measuring unit of sufficient accuracy. Paragraph 10.5.6 sets the frequency at no less than twice a year for in-pavement lights and once a year for other lights. The paragraph does not say that LED lights change these intervals. Photometric testing of the luminous intensity of each light is the check that finds a light that is lit but weak.

Colour needs a measurement as well

Chromaticity is the colour of a light as a pair of coordinates. FAA Engineering Brief 67D tests it with a spectroradiometer in steps of 2 nm or less and gives boundaries for aviation white LEDs. The FAA review notes two consequences. The allowed ranges are wide, so LEDs from different makers may differ. And the chromaticity of a set of lights can drift as lenses age and discolour. It adds that incandescent lights shift in chromaticity with brightness.

Colour is a serviceability matter. Under EASA ADR.OPS.C.015(c) a light that does not produce the correct colour beam is unserviceable, whatever its intensity. A colour error in a lit light is hard to judge by eye.

Heat, ice and snow

An LED does not produce the infrared heat of an incandescent lamp. Annex 14 Note 2 to 10.5 and EASA GM2 ADR.OPS.C.015(b) say the same. Operators who count on that heat to melt ice and snow need to evaluate a modified maintenance schedule or fixtures with heating elements. EASA notes that the low temperature can help, because drifting snow does not melt and stick to the lens. It adds that an LED PAPI is likely to need some form of lens heater to avoid condensation and icing. EB 67D allows an optional arctic kit.

The same lack of heat matters for pilots who use enhanced flight vision systems. EASA requires the operator to report the LED parts of the lighting system to the AIS.

Contamination on the lens of an in-pavement fixture also reduces performance (EASA). Monitoring is electrical. EB 67D 2.7 requires monitored fixtures to fail open, and the sources checked do not describe optical output monitoring. A dirty lens and a faded LED both show up as low output, and only a measurement tells them apart from a good light.

Incandescent and LED compared

What a drone survey adds

A drone survey does not set the intensity limit. It finds the lights that need a closer look. TarmacView flies airfield lighting inspection between aircraft movements. Each unit is reported lit or dark, with its colour against its position, its output relative to its neighbours and to the commanded dimming step, and an RTK-surveyed position. The runway edge light inspection works this way.

That is a relative result. A drone survey does not report absolute candela. TarmacView does not yet have a calibrated camera, and a verdict against the 50 percent limit needs a calibrated photometer. A repeated survey does show which lights fall behind their neighbours from one inspection to the next. The photometric measurement guide sets out how to anchor readings to a calibrated instrument. See also when runway lights drift out of tolerance .

For the life rating of an LED, see the glossary page on LED lifespan .

Conclusion

An LED airfield light does not announce its end. It fades and its colour can move while it stays lit, so inspection moves from looking at the lights to measuring each one. The limits of Annex 14 and EASA stay the same.