Aerodromes - Physical Characteristics — Page 385, Lesson 525

Let's pick up with rapid exit taxiways — these are the fast turn-off lanes you'll see at busy airports, and they're a real operational tool. A rapid exit taxiway, commonly called a "fast turn-off lane," is provided where traffic density is high. Its whole purpose is to let an aeroplane leave the duty runway at a speed higher than would be permitted for a right-angle turn onto a normal taxiway. Think about it — a normal 90-degree turn forces you to slow down a lot; a fast turn-off lane is angled so you can keep more speed while exiting. They're located along the runways and are designed and constructed to cater for specific turn-off speeds. For code 3 or 4 runways, that's 93 km/h, which is 50 knots, in wet conditions. For code 1 and 2 runways, it's 65 km/h, which is 35 knots, also in wet conditions. Note that the operator will specify the maximum speed for dry runway and taxiway operations — so the wet-condition figure is the design standard, but the operator sets the dry limit. Now, the geometry matters. The taxiway must include a straight section after the turn-off curve. That straight section lets an exiting aircraft come to a full stop clear of the intersecting taxiway — you don't want to stop while still blocking the runway or the next taxiway. The intersecting angle with the runway should not be greater than 45°, not less than 25°, and preferably 30°. So there's a design window, with 30° as the ideal. Now let's move to minimum requirements for taxiways generally. As I mentioned earlier, width is the most important factor in taxiway design and construction. The table specifies minimum clearances for the outermost main wheels when the nose wheel is on the centre line of the taxiway. Two terms you need to nail down. Wheel base refers to the distance from the front of the nose wheel to the trailing edge of the main gear. Wheel to edge clearance refers to the distance from the outer edge of the main gear tyres to the defined edge of the taxiway. Here's the clearance table by code letter. Code A: 1.5 metres. Code B: 2.25 metres. Code C: 3 metres if the taxiway is intended to be used by aeroplanes with a wheel base less than 18 metres; otherwise 4.5 metres. Code D: 4.5 metres. Code E: 4.5 metres. Code F: 4.5 metres. So you see, for the larger codes — D, E, F — the clearance is a flat 4.5 metres. For code C, the wheel base of 18 metres is the deciding factor. Now, the shoulders of taxiways used by turbine aeroplanes must be prepared to prevent erosion by jet blast and the ingestion of surface material into the jet engines. That's a critical safety point — loose material sucked into a jet engine is a serious hazard. Taxiway strips, similar to runway strips, are provided primarily to delineate the area to be cleared of objects which may be obstacles. So it's a cleared zone around the taxiway. Now for the width of taxiways themselves. The straight portion of a taxiway should have a width of not less than the table specifies. Code A: 7.5 metres. Code B: 10.5 metres. Code C: 15 metres if the taxiway is intended to be used by aeroplanes with a wheel base less than 18 metres; otherwise 18 metres. Code D: 18 metres if the taxiway is intended to be used by aeroplanes with an outer main gear span of less than 9 metres; otherwise 23 metres. Code E: 23 metres. Code F: 25 metres. So notice the pattern — for code C, the wheel base of 18 metres is the deciding factor again. For code D, it's the outer main gear span of 9 metres that decides between 18 and 23 metres. Let me pull these together. For code D, E, F, the wheel-to-taxiway-edge clearance is 4.5 metres. For code D, the width is 18 metres if the aircraft has an outer main gear span less than 9 metres, otherwise 23 metres — and D, E, F all share that 23-metre figure. For code C, the width is 15 metres if the aircraft has a wheel base less than 18 metres, otherwise 18 metres. And for rapid exit taxiways, the maximum speed is determined by the operator, but not more than 50 knots in wet conditions for code 3 or 4. That figure shows you the wheel-to-edge clearance concept visually — the distance from the outer edge of the main gear tyres to the defined edge of the taxiway. So the key relationships to remember: the clearance tables depend on code letter, and for codes C and D, the specific aircraft dimensions — wheel base and outer main gear span — decide which value applies.