Aerodromes - Visual Aids, Markings and Signs — Page 411, Lesson 564

We're starting a new topic now: markers. These are the physical objects you see on an aerodrome, and I want to make sure you understand exactly what they are and how they differ from markings and lights. Let's begin with the formal definition. A marker is an object which is displayed above ground level in order to indicate an obstacle or delineate a boundary. So, a marker is a physical, three-dimensional object. It's not painted on the ground like a marking, and it's not an active light source. It sits above the surface. Markers are used where lights are not provided, or where lighting or ground markings do not serve the purpose. So they fill a gap. For example, edge markers may be used to indicate the extremity of a taxiway where snow has fallen. If snow covers the painted lines, you need something physical that sticks up. They can also mark areas of bad ground on a grass aerodrome, so you know where you can't safely roll. Now, a critical design constraint: their height shall be sufficiently low to preserve clearance for propellers and for the engine pods of jet aircraft. Engine pods are the nacelles, the housings that contain the jet engines, which hang low under the wing. A marker that's too tall could strike a propeller or an engine pod. So they must be low enough to avoid that. At Oxford, the refuelling area is marked by reflective green markers. That's a specific example, but note the colour: green, and they're reflective. Now let's look at the specific types. First, unpaved runway edge markers. These should be provided when the extent of an unpaved runway is not clearly indicated by the appearance of its surface compared with that of the surrounding ground. So if the runway surface doesn't visually stand out from the grass or dirt around it, you need markers to define its edges. Where runway lights are provided, the markers should be incorporated in the light fixtures. So they're built into the light units. Where there are no lights, markers of flat rectangular or conical shape should be placed so as to delimit the runway clearly. Two shapes are allowed. The flat rectangular markers should have a minimum size of 1 metre by 3 metres, and they should be placed with their long dimension parallel to the runway centre line. So the 3-metre side runs along the runway direction. The conical markers should have a height not exceeding 50 centimetres. Next, taxiway edge markers. These should be provided on a taxiway where centre line or edge lights, or taxiway centre line markers, are not provided. So they're a substitute for those other aids. The markers should be installed at least at the same locations as would the taxiway edge lights had they been used. In other words, wherever you would have put an edge light, you put a marker instead. Two more requirements here. Markers are required to be reflective blue. And taxiway edge markers shall be frangible. Frangible means designed to break easily on impact, so if an aircraft accidentally hits one, it breaks away rather than damaging the aircraft or causing a serious hazard. Finally, we have taxiway centre line markers, which should be provided on a... and that's where the excerpt cuts off. So we'll pick that up next. Let me just recap the key points you need to hold onto. A marker is a physical object above ground level, used to indicate an obstacle or delineate a boundary, when lights or markings won't do the job. They must be low enough to clear propellers and jet engine pods. Unpaved runway edge markers come in two forms: flat rectangular, minimum 1 by 3 metres, long side parallel to the centre line; or conical, no more than 50 centimetres high. Taxiway edge markers are reflective blue, frangible, and placed where edge lights would go. And remember the Oxford example: reflective green markers for the refuelling area.