The 20 Lux Problem: Apron Floodlighting Compliance at Regional Airports

When CASA remade the Manual of Standards Part 139 in 2019, most of the attention went to runways and taxiways. One of the changes that has caused the most practical difficulty since then sits quietly in Chapter 9: the apron floodlighting requirements.

For many regional airports, the effect was blunt. Some aprons that previously needed an average of 5 lux now need 20 lux. That is not a refinement; it is a fourfold increase, and at many regional aerodromes the site geometry makes it genuinely difficult to achieve.

What changed

Under the previous MOS 139 (section 9.16), apron floodlighting worked on a two-tier system based on aircraft size. Aprons used by large aircraft (larger than code 3C) needed an average horizontal illuminance of 20 lux at parking positions, with a uniformity ratio not exceeding 4 to 1, plus an average vertical illuminance of 20 lux at 2 metres height in the relevant parking direction. Aprons used by smaller aircraft (code 3C aircraft and smaller) needed 5 lux on the same basis.

The updated MOS 139 (sections 9.113 to 9.116) restructured this into a three-tier system. The headline changes:

• Illuminance requirements at RPT parking positions are now set by Table 9.116(3) according to the aerodrome reference code letter for the parking position. For many regional RPT aprons catering to code C aircraft, this moved the requirement from 5 lux to 20 lux.
• Vertical illuminance is measured at 2 metres above the apron along the aircraft centreline, from the nose to the rearmost passenger or cargo door of the intended aircraft (section 9.116(3)(a)(ii)).
• Non-RPT aprons, such as general aviation, aeromedical and similar operations, have a lower standard with horizontal illuminance and uniformity requirements only (section 9.116(3)(b)).
• Floodlighting must be located outside the separation distances on the apron (section 9.115(1)), which constrains where masts can physically go.
• Other apron areas need 50 percent of the minimum average illuminance applying to the associated parking positions (section 9.116(3)(c)).

On paper, this aligns Australian practice more closely with international guidance for busier aerodromes. On the ground at a regional airport, two site realities collide with it.

Collision one: the OLS caps your mast height

Floodlighting design is fundamentally about getting enough well aimed light from an achievable mounting height. Quadrupling the illuminance target generally means taller masts, more masts, more luminaires, or a combination of these.

At regional airports, the apron often sits close to the runway. That places the apron, and any floodlight mast on it, under the transitional surface of the Obstacle Limitation Surfaces, which generally rises at a gradient of 1 in 7 from the edge of the runway strip.

The arithmetic is unforgiving. A mast position 70 metres beyond the strip edge is limited to roughly 10 metres of height under a 1 in 7 transitional surface, and the permissible height drops further as the apron gets closer to the strip. Delivering 20 lux with acceptable uniformity from mounting heights like that requires many closely spaced poles, each of which is itself an obstacle, a cost and an apron obstruction to manage.

Meanwhile section 9.115(1) pushes the masts outside the apron separation distances, and operational reality pushes them out of ground service equipment circulation areas. At many regional aprons, the set of positions where a mast is simultaneously outside the OLS problem, outside the separation distances and outside the GSE paths is small, and sometimes no such position exists.

Collision two: regional parking geometry

The 20 lux requirement was written with reference to the way larger airports operate: nose in parking, perpendicular to the terminal, with tugs performing pushback. That geometry suits floodlighting. The stands are parallel to each other, the aircraft centrelines all point the same way, and the vertical illuminance requirement can be met with masts along the terminal face aimed consistently down the stands.

Regional airports rarely work that way. Without tugs, parking positions are angled so aircraft can power in and power out under their own thrust. That angling has three consequences for floodlighting design:

• The vertical illuminance direction rotates with each stand. Light that arrives nicely along one aircraft centreline arrives obliquely on the neighbouring stand, so a mast layout that satisfies one parking direction can fail another.
• Angled stands spread across more apron frontage, stretching the area that must be lit to 20 lux with a uniformity ratio no worse than 4 to 1, from masts that the OLS has already made short.
• Aircraft turning on and off angled stands sweep their cockpit view across a wider arc, so mast positions and aiming that would be comfortable for nose in parking can put glare directly into a taxiing pilot’s eyes.

First: check what actually applies

The three-tier structure means the first question is not how to hit 20 lux, but whether the apron in question genuinely attracts it. The classification of the apron and its operations matters. A parking area used by general aviation or aeromedical operations rather than RPT services sits under a lower requirement with no vertical illuminance component. Getting this classification right, and documented, costs a fraction of any lighting upgrade it might prevent.

Second: make sure the non-compliance is real

A surprising share of apron lighting compliance problems are measurement problems. AS/NZS 3827.1 is explicit that measurements are only meaningful when the measurement grid matches the grid used in the design calculations. In practice, designs are frequently calculated on one grid, measured on another, with uncorrected supply voltage, unknown lamp depreciation and dirty fittings, and then declared non-compliant.

Before committing to capital works, it is worth having the measurements taken properly: a calibrated, cosine corrected light meter, a marked grid matching the design calculation points, vertical readings taken at 2 metres parallel to the aircraft centreline, and the supply voltage and maintenance state recorded. Sometimes the system complies once it is measured correctly and the fittings are cleaned and re-lamped. That is a much cheaper outcome than a redesign.

When the gap is real: the options

• Modern LED floodlights with asymmetric optics deliver far more well controlled light from low mounting heights than the fittings most existing installations were designed around. A like for like recalculation with current photometry is often the first move.
• More, shorter masts with deliberate aiming can achieve uniformity that fewer tall masts cannot, where the OLS rules the tall masts out anyway.
• Revisiting the parking layout is sometimes cheaper than fighting it: small rotations or relocations of stands can align vertical illuminance directions and reduce the lit area.
• The design maintenance factor, cleaning cycle and re-lamping strategy are part of the compliance position (the note to section 9.116(10) leaves the outage factor to the designer). A realistic factor with a documented maintenance regime beats an optimistic one that fails its first audit.
• Where the site genuinely cannot comply, the conversation with CASA should happen early, with a documented assessment of what is achievable and why.

The takeaway

The updated MOS 139 apron floodlighting requirements set a standard that regional airport geometry makes genuinely difficult: the OLS caps the masts, the separation distances and GSE areas constrain the positions, and angled power in, power out parking works against the nose in assumptions behind the vertical illuminance requirement.

The airports that handle it well follow the same sequence: confirm what the apron actually requires, verify the existing performance with measurements that would stand scrutiny, and only then design the upgrade, with the OLS, the parking geometry and the maintenance regime treated as design inputs from day one. Done in that order, the 20 lux problem is usually solvable. Done in reverse, it is usually expensive.