Aerodromes - Physical Characteristics — Page 385, Lesson 521

Let’s pick up right where we left off in the physical characteristics of the aerodrome. We’ve already covered the runway strip and its recommended extensions. Now I want to walk you through the next three defined areas that sit at the ends of the runway, because each one has a very specific job and a very specific geometry you must know cold. First, the clearway. This is a defined rectangular area on the ground under the control of the appropriate authority. It’s selected or prepared as a suitable area over which an aeroplane may make a portion of its initial climb to a specified height. So think of it as an extension of the runway in the air, not on the ground — it’s there to give you room to climb out after you’ve left the runway surface. The origin of a clearway should be at the end of the take-off run available, which we abbreviate as TORA. That’s the key anchor point. Now the limits: the length of the clearway should not exceed half the length of TORA. And the width should extend laterally to a distance of at least 75 metres on each side of the extended centre line of the runway. So you’ve got a rectangle that starts at the end of TORA, stretches forward no more than half of TORA’s length, and spreads at least 75 metres either side of the centre line. Next, the stopway. This is also a defined rectangular area on the ground at the end of TORA, but its job is completely different. It’s prepared as a suitable area in which an aircraft can be stopped in the case of an abandoned or rejected take-off. So if you have to abort the take-off and you can’t stop on the runway itself, the stopway is the surface that’s built to take your weight and bring you to a halt safely. The critical geometric rule here is that the stopway is to have the same width as the runway with which it is associated. So unlike the clearway, which spreads wider than the runway, the stopway matches the runway width exactly. Now I want to move to something that’s directly tied to your instrument approach minima — the Radio Altimeter Operating Area, often just called the rad alt operating area. This should be established in the pre-threshold area of a precision approach runway. For CAT II and CAT III operations, the use of the radio altimeter is mandatory to determine the decision height, which we abbreviate DH. Let me give you the numbers. For CAT II, the minimum DH is 100 feet, at system minima. On a 300 feet per nautical mile glide path, that point would be at 600 metres, which is one-third of a nautical mile, from the aiming point. Now, usually the aiming point is 300 metres down the runway beyond the threshold. So if the DH point is 600 metres from the aiming point, and the aiming point is 300 metres past the threshold, then the DH point ends up being 300 metres before the threshold. That’s the geometry you need to hold in your head. For CAT III, the DH is less than 50 feet. And here’s a useful consequence: because the CAT III DH is lower, a rad alt operating area that’s suitable for CAT II would also be suitable for CAT III. The area itself should extend before the threshold for a distance of at least 300 metres. Laterally, it should extend on each side of the extended centre line of the runway to a distance of 60 metres. There’s one exception: when special circumstances so warrant, that distance may be reduced to no less than 30 metres, but only if an aeronautical study indicates that such a reduction would not affect the safety of operations of aircraft. So 60 metres is the standard, 30 metres is the absolute floor, and only with a study backing it. Finally, the surface of the rad alt operating area should be level, with no undulations of more than 7%. That’s a tight tolerance, because the radio altimeter is measuring your height above the ground, and any bump in that surface corrupts your DH callout. Now let’s shift from the runway ends to the taxiways. I want to start with the introduction to this whole topic. A major limitation to the use of an aerodrome is the capability of the taxiways to accommodate different sizes of aeroplanes. Clearly, a narrow taxiway cannot cope with a large aeroplane. But width is not the only consideration. The strength of a taxiway needs to be equal to that of the runway. And here’s a subtle point that catches many people: the surface of taxiways is more vulnerable to damage than a runway, because of constant loading and unloading, turning, and stopping. So the pavement gets worked much harder in a concentrated way. Taxiways may include turns, especially close to runways. And taxiways may cross, join, intersect, and require signs and markings to — well, the text cuts off there, but you can already see the theme: taxiway geometry and marking is a whole discipline of its own, and we’ll build on that next. Let me make sure you’ve got the three runway-end areas straight, because they’re easy to confuse. The clearway is for your initial climb, it starts at the end of TORA, its length is capped at half of TORA, and it’s at least 75 metres wide each side of the centre line. The stopway is for stopping on a rejected take-off, it also sits at the end of TORA, and its width equals the runway width. And the rad alt operating area is for your CAT II/III decision height, it sits before the threshold, extends at least 300 metres forward, 60 metres laterally each side — reducible to 30 with a study — and its surface must be level within 7% undulation. Those are the numbers you’ll be tested on, and more importantly, the ones that keep your approach and take-off safe.