Aeronautical Ground Lighting (AGL) installations (typically covering runway centreline, touchdown zone, edge and taxi-way lights) support safe, efficient and regular aircraft operations by providing essential visual guidance to pilots and aerodrome personnel. They enable suitably equipped aircraft to operate with safety and regularity under all visibility conditions, and t he performance of lighting equipment is an important factor in maintaining Landing Success Rates. Also, while no one is suggesting that a poor AGL could be a prime cause of an accident, incidents have shown that the poor state of an AGL system can be a contributing factor.

It is therefore critical that AGL services operate effectively, and that the light sources are observable by pilots even when visibility is impaired by fog and cloud (i.e. there is sufficient light entering the eye for the observer to differentiate the light source from the background ambient light level).

The importance of AGL installations, and their effective operation, has been increasingly recognised over the past few years, and the result has been that all the major regulatory authorities have now established minimum operating criteria for these systems. Which aim to ensure that adequate visual cues are available to the observer at any position within the glide path for landing, or on the ground for taxiing.

The ICAO sets out minimum sets out the minimum performance criteria for aerodrome lighting services in Annex 14. This specifies the characteristics of each light type in terms of Isocandela diagrams, which define the required beam intensity and shape, and also the required setting angles and the colour of the emitted light. It also specifies performance level objectives for both the individual light fittings and the overall system serviceability. Essentially, the standard deems an individual light fitting to be ineffective or 'unserviceable' - and thus requiring maintenance - when its beam average intensity (brightness) is below a specified level (typically 50% of the beam intensity value specified in the standard) - and not, as is normally assumed, when the light source has failed completely. As regards operating standards and tolerance limits in terms of overall visual aid system 'serviceability', for many of the services in question, such as edge lights and centreline lights for CAT II and CAT III runways, the recommended limits are set as high as 95%. This means that no more than 5% of these light fittings can be unserviceable. Adjacent unserviceable lights are also not permitted.

The FAA (AC 150/5340-26A???) has also recently produced far clearer recommended guidelines for maint enance procedures relating to aining maintaining the light output of runway lighting systems within specified levels.

Finally, as well as setting out minimum operating and performance criteria, the regulatory authorities increasingly expect aerodrome operators to submit compliancy reports against the standards.

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