Runways, Strips and Taxiways

The last lesson published the numbers; this one walks the ground they describe. It covers where a threshold belongs, the protective geography around every runway (the strip, the runway-end safety area, the clearway and the stopway), and then the taxiway system: why rapid exits are built, why taxiways grow wider in curves, and where the various holding positions and bays that regulate surface traffic must stand.

The runway and its threshold

A threshold, the beginning of the portion of runway usable for landing, should normally be located at the extremity of the runway, unless operational considerations justify another choice. The classic justification is an obstacle beneath the approach: displacing the threshold down the runway lifts the approach path clear, at the price of landing distance, which is why the previous lesson's LDA begins at the threshold, wherever it stands. A displacement can also be temporary: works at the runway end, or an unserviceable portion, move the threshold until the surface is whole again.

Where a runway is associated with a stopway or clearway, the general consideration is economic and performance-driven: they let an aerodrome offer more accelerate-stop or take-off distance without building more runway. A stopway should be capable of supporting an aeroplane abandoning its take-off without inducing structural damage, while a clearway need only be free of obstacles that would endanger the initial climb: one is pavement you can stop on, the other is protected space you climb through, and the declared distances of the previous lesson (ASDA and TODA) are exactly how their existence reaches the crew.

The protective geography

Four defined areas ring the runway, and their definitions are pure exam currency:

Runway strip — a defined area including the runway and stopway, if provided, intended to reduce the risk of damage to aircraft running off a runway, and to protect aircraft flying over it during take-off or landing operations. Two intents, and the exam asks for both: the strip catches the excursion and clears the overflight.
Runway-end safety area (RESA) — an area symmetrical about the extended runway centre line and adjacent to the end of the strip, primarily intended to reduce the risk of damage to an aeroplane undershooting or overrunning the runway. Note the anchor: the RESA begins where the strip ends, not at the runway's own end.
Clearway — a defined rectangular area on the ground or water, under the control of the appropriate entity, selected or prepared as a suitable area over which an aeroplane may make a portion of its initial climb to a specified height. It need not be pavement (water qualifies), but it is under the appropriate entity's control, kept free of obstacles that would endanger an aeroplane climbing through it.
Stopway — a defined rectangular area on the ground at the end of the take-off run available, prepared as a suitable area in which an aircraft can be stopped in the case of an abandoned take-off.

Taxiways: exits, curves and holding

A landing aeroplane on the runway blocks it for everyone else, so busy aerodromes build rapid-exit taxiways: taxiways connected to the runway at an acute angle and designed to allow a landing aeroplane to turn off at higher speed than other exit taxiways permit. The reason is minimising runway occupancy time, which is what runway capacity is made of. The design requirements follow from the speed: the shallow intersection angle (not greater than 45 degrees nor less than 25, preferably 30), a turn-off curve engineered for the higher exit speed, and a straight portion after the turn long enough for the aeroplane to come to a full stop clear of any intersecting taxiway.

In a taxiway curve the cockpit follows the centre line while the main wheels cut the corner, so taxiways are widened in curves (extra fillet pavement on the inside of the turn) so that the wheel-to-pavement-edge clearance is not reduced below the specified minimum when the cockpit remains over the centre line. The bigger the aeroplane and the tighter the curve, the more the main gear tracks inside the nose, and the more fillet the corner needs.

The holding infrastructure completes the system, each element with its prescribed place:

Holding bays — defined areas where aircraft can be held or bypassed, to facilitate efficient surface movement; they should be provided when traffic density is medium or heavy: they let a later aircraft with an earlier slot pass, and give a piston aeroplane somewhere to run up without blocking the queue.

Runway-holding positions — designated positions protecting a runway, an obstacle limitation surface, or an ILS/MLS critical or sensitive area, at which taxiing aircraft and vehicles stop and hold unless otherwise authorised by the tower. They are established on the taxiway at every taxiway/runway intersection; and on a runway itself, where that runway forms part of a standard taxi-route and crosses another runway: the holding position sits on the taxi-route runway, protecting the runway it is about to cross.

Road-holding positions — the vehicle counterpart: established on every road at its intersection with a runway, the line at which drivers hold short.

Intermediate holding positions — established on a taxiway at any point other than a runway-holding position where it is desirable to define a specific holding limit: typically where two taxiways cross, so one stream can be stopped clear of the other.

The runway and its threshold

The protective geography

Check you understood

Taxiways: exits, curves and holding

What the exam asks