Aerodromes - Physical Characteristics — Page 385, Lesson 528

Let's pick up with taxiway curves, then move into holding bays and runway holding positions. 19.52 Taxiway Curves. I want you to think about what happens when an aeroplane turns off a straight taxiway onto a curve. The nose gear follows the centreline, but the main wheels track inside the curve — they cut the corner. So the whole point of designing a taxiway curve is to make sure the main wheels still keep their required clearance from the edge of the pavement. The rule here is simple: changes in direction of taxiways should be as few and as small as possible. Every curve you add is a place where an aeroplane has to slow down and manoeuvre, so you want to minimise both the number of bends and how sharp they are. The radii of the curves — that's the radius of the turn itself — should be compatible with the manoeuvring capability and the normal taxiing speeds of the aeroplanes for which the taxiway is intended. In other words, the curve has to be gentle enough that the aircraft type using that taxiway can actually negotiate it at its normal taxi speed without the main wheels running off the edge. And the way they achieve that wheel clearance is by widening the taxiway through the curve — that's the example shown in Figure 19.10, taxiway curves. The pavement gets wider on the inside of the turn so the main wheels keep their specified clearance from the edge. 19.53 Holding Bays and Runway Holding Positions. Now let's talk about what happens when traffic has to wait. Inevitably, at a busy aerodrome, aeroplanes will be required to wait their turn for take-off. It's common to see a stream of traffic waiting along the taxiway for take-off, but at an international airport the queues for take-off can exceed the capability of the taxiway to accommodate all the aircraft waiting. So the taxiway simply can't hold everyone. In that situation — or where environmental considerations exist — holding bays are provided adjacent to runway entrances, and their purpose is to leave the taxiways clear. So instead of blocking the taxiway with a queue, aircraft pull into a holding bay next to the runway entrance, and the taxiway stays free for other traffic to move. Now, at some major aerodromes with particularly high traffic levels or intense peaks in traffic density, there's a special facility called a 'sin bin'. That's the term used in the notes. A sin bin is established to take aeroplanes out of the stream if something has gone wrong. So if an aircraft has a problem — a technical fault, a delay, something that means it can't take off right away — it gets pulled out of the active stream into the sin bin so it doesn't block everyone else. That's the purpose: to remove the problem aircraft from the flow. Now, regardless of the position of holding bays, the entrance to the runway will be protected by a defined mandatory holding point set back from the edge of the runway to accommodate all the traffic using the runway. So there's always a mandatory holding point — a defined position where aircraft must stop — and it's set back from the runway edge far enough to accommodate all the traffic using that runway. That's the protection for the runway entrance. Now here's a critical point. A runway holding position must be established on a taxiway if the location or alignment of the taxiway is such that a taxiing aircraft or vehicle can infringe an obstacle limitation surface or interfere with the operation of radio navigation aids — specifically ILS or MLS. ILS is the Instrument Landing System, MLS is the Microwave Landing System. So even if the taxiway doesn't connect directly to the runway, if a taxiing aircraft or vehicle could poke up into an obstacle limitation surface — that's the protected airspace around the runway — or could interfere with the ILS or MLS signals, then you must have a runway holding position there. The method of indicating a runway holding position is covered later in the notes, under Signs and Markings — so we'll get to that. And finally, the distances. A runway holding position, or positions, must be established at the distances in the table on page 384 at two specific locations. First, at an intersection of a taxiway with a runway. Second, at an intersection of a runway with another runway — but only when the former runway is part of a standard taxi-route. So if one runway is being used as a taxiway for traffic going to another runway, then at that intersection you also need a runway holding position, set at the distances in that table on page 384. So to tie it together: holding bays keep the taxiway clear when queues get long, sin bins pull problem aircraft out of the stream, and the mandatory holding point — set back from the runway edge — protects the runway entrance. And the holding position distances are specified in the table on page 384, which we'll look at next.