Despite the fact that there are now defined international standards for AGL services, and meeting these is becoming increasingly mandatory, the reality is that most airports still struggle to consistently maintain their installation within the defined serviceability levels, despite having some form of maintenance programme in place.

The extent of 'non-compliance' was initially highlighted by an extensive research project into the performance of certain AGL systems on operational runways. This was undertaken by TMS Photometrics (a division of UK based Tailor Made Systems) for the UK Civil Aviation Authority (CAA), with the support of the Defence Estates (UK MOD), Defence Evaluation Research Agency (DERA) and UK aerodromes. The problems have been subsequently confirmed by TMS Photometrics' experiences of working with Airports worldwide.

As to why there should be such variation in performance, and difficulties in meeting requirements, the same research, found two clear causes. The first is the inadequacies of the traditional monitoring and maintenance practices that are employed by many aerodromes. These practices include the visual observation of lamp failures, spot readings with a light meter, measurement of the primary series circuit current, and the application of 'Block Change' and 'Spot Failure' replacement maintenance strategies. The fact is that these rather crude practices are based on the invalid assumption of uniform deterioration, and completely fail to take account of all the unpredictable factors (from non-uniform lamp aging and circuit current variations to highly variable dirt build up) that contribute to significant variations in the performance of a light fitting under service conditions.

Assessments also revealed an even more fundamental issue, which is that AGL performance can often be severely limited by the individual components of that system and their installation. Subsequent testing has repeatedly found new and refurbished runway lighting systems that actually perform around or even below the minimum standard, immediately after completion, either due to components on or below specification or poor installation practice. From this low starting level, it is then not surprising that it becomes difficult and expensive to maintain the systems to the standard required ( see - case study on Memphis International Airport).

The issues were clearly highlighted in an extended investigation, by TMS Photometrics, of one particular runway lighting service. This involved regular photometric measurements over a period of 6 months. The first photometric measurement was taken after the aerodrome had a complete refurbishment / block change of all light fittings; a routine procedure undertaken every two years as a standard part of the maintenance programme. The in-field measurements clearly demonstrated that even directly after this block change, serviceability was below the required level, with approximately 20% of the individual light fittings remaining unserviceable. Subsequent measurements showed that performance then deteriorated over time, and after 6 months over 50% of light fittings were unserviceable. After a major cleaning exercise , the overall performance then only improved slightly, bringing the serviceability level up to 65%. All these levels of serviceability were significantly below required standards.

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