Obstacle Marking and Aerodrome Services — Page 447, Lesson 597

Let’s start with emergency vehicles, because that’s where the text begins. On an aerodrome, you have vehicles that are permanently based there — the aerodrome ambulance, police or security vehicles, and the fire and rescue appliances. Now, these vehicles already have to meet the general marking and lighting requirements from paragraph 22.6, which we covered earlier. But on top of that, they must also carry blue flashing lights, and those lights are for one specific purpose: use whilst responding to an emergency. So the blue flashing light is the signal that the vehicle is on an emergency response. Now here’s a subtle but important distinction. What about vehicles that are not normally based on the aerodrome — for example, a civilian fire or rescue vehicle from a nearby town that gets called in to assist? Those vehicles are also required to show flashing blue lights. But there’s an extra condition: they must be escorted by vehicles with radio communication with ATC. So the civilian vehicle doesn’t just drive onto the aerodrome on its own; it needs an escort, and that escort vehicle must be in radio contact with Air Traffic Control. That way, ATC always knows where the responding vehicle is and can coordinate movement on the manoeuvring area. Now let’s move to the lighting of objects. The rule is simple in principle: the presence of objects which must be lit shall be indicated by low, medium, or high intensity obstacle lights, or a combination of such lights. So you have three categories of obstacle light, and you can mix them. And there’s a note you must remember: high-intensity obstacle lights are intended for day use as well as night use. That’s a key point — the high-intensity ones are bright enough to be seen in daylight, so they’re not just a night-time aid. Let’s take each category in turn, starting with low intensity lights. On fixed objects, low intensity obstacle lights shall be fixed red lights. The word “fixed” here means steady — not flashing. And the intensity must be sufficient to ensure conspicuity, which just means being clearly noticeable. The standard is that the light must be bright enough to stand out, considering the intensity of the adjacent lights and the general level of illumination against which they would normally be viewed. So it’s not an absolute brightness; it’s relative to the background lighting. Now, there’s a special case for objects with limited mobility, such as aerobridges — those are the passenger boarding bridges that move to meet the aircraft. On those, the low intensity obstacle lights shall be steady-red. So again, steady red, not flashing. And the same conspicuity requirement applies: the intensity must be sufficient considering adjacent lights and the general illumination level. But here’s the catch — sometimes low intensity lights just aren’t enough. Where the use of low intensity obstacle lights would be inadequate, or where an early special warning is required, then you should use medium or high intensity obstacle lights instead. So low intensity is the baseline, but if the situation demands more visibility or earlier warning, you step up. Now medium intensity lights. These shall be flashing red lights — note the contrast: low intensity is fixed red, medium is flashing red. But there’s an exception: when medium intensity lights are used in conjunction with high intensity obstacle lights, they shall be flashing white lights. So the colour depends on whether they’re paired with high intensity lights. The flash frequency for medium intensity lights shall be between 20 and 60 per minute. And there’s a coordination requirement: medium intensity obstacle lights located on an object should flash simultaneously — all the lights on that object flashing together, not in sequence. When should you use medium intensity? The text gives you two conditions. Medium intensity obstacle lights should be used, either alone or in combination with low intensity lights, where the object is an extensive one — that means a large, spread-out object — or where its height above the level of the surrounding ground is greater than 45 metres. So that 45-metre figure is your threshold: above that height, you’re looking at medium intensity. Finally, high intensity lights. These shall be flashing white lights. Like medium intensity, they should flash simultaneously at a rate between 40 and 60 per minute — note that’s a slightly narrower range than the medium’s 20 to 60. But here’s the interesting part: high intensity obstacle lights located on a tower should flash sequentially, not simultaneously. And the sequence is specific: first the middle light, second the top light, and last the bottom light. So on a tower, you get a wave of light travelling — middle, then top, then bottom — rather than all flashing at once. Let me show you what these look like in practice. shows the emergency vehicles with their blue flashing lights. illustrates the three methods of obstacle lighting — low, medium, and high intensity. And shows tall objects, which is where that 45-metre height rule and the tower sequencing come into play. So to tie it all together: low intensity is fixed red, for general conspicuity; medium is flashing red (or flashing white when paired with high intensity), flashing at 20 to 60 per minute, for extensive or tall objects over 45 metres; and high intensity is flashing white, flashing at 40 to 60 per minute, visible day and night, with that distinctive sequential pattern on towers. That’s the full picture of obstacle lighting as it stands in this part of the text.