Aerodromes - Physical Characteristics — Page 381, Lesson 517

Let’s start with the idea of an instrument runway, because it sets up everything else. An instrument runway is one to which instrument departure and approach procedures are applied. That means the runway is served by procedures that let aircraft operate without relying on visual reference to the ground. For departing traffic, the instrument departure procedure is defined in the form of a SID — that’s a Standard Instrument Departure. It’s a published route that aircraft follow after take-off to get from the runway into the en-route structure safely. For arriving traffic, instrument runways are sub-divided into two categories. First, precision runways, which use ILS, MLS, or PAR approaches. Let me expand those: ILS is the Instrument Landing System, MLS is the Microwave Landing System, and PAR is Precision Approach Radar. These give both lateral and vertical guidance. Second, non-precision runways, which use VOR, NDB, SRA, or ILS in azimuth only approaches. VOR is VHF Omni-directional Range, NDB is Non-Directional Beacon, SRA is Surveillance Radar Approach, and ILS in azimuth only means you’re using the localiser for lateral guidance but not the glideslope for vertical guidance. So the key contrast is: precision gives you both horizontal and vertical guidance, non-precision gives you lateral guidance only. Now, a take-off runway. This is a runway that can be used only for take-offs. That’s usually because terrain prevents an instrument approach, or precludes a missed approach — in other words, you can’t safely bring an aircraft back in for a landing or execute a go-around. A take-off runway is usually only in one direction, with the reciprocal not being used. So you only ever depart from it in one direction. Next, the location of the runway threshold. The threshold is the beginning of the runway that’s available for landing. It should normally be located at the extremity of the runway, unless operational considerations justify another location. If the threshold is displaced from the beginning of the paved strip, its location is shown by a transverse white stripe across the runway surface, and arrows leading to the position of the threshold. Reasons for displacing a threshold include unserviceable runway conditions, RESA — that’s Runway End Safety Area — radio altimeter operating area, glide path angle, obstacle clearance, and so on. So the threshold can be moved back from the physical start of the pavement for safety or operational reasons. Now the length of runways. The actual length of a runway should be adequate to meet the operational requirements of the aeroplanes for which the runway is intended, and should be not less than the longest length calculated to correct for local conditions — elevation, temperature, runway slope, humidity, and surface characteristics. There’s no requirement to cater for the worst-case aeroplane at critical mass. Where a secondary runway is constructed, the length criteria are applied to obtain a usability factor of 95%. Runway length is reported in metres, but the fixed distance markers — the distance-to-go signs — along the edge of runways are in thousands of feet, yet they’re defined in terms of 300 metres, starting 300 metres from the threshold. And for a concrete strip to accommodate instrument approaches from either end, it must be at least 1800 metres between thresholds. Finally, the width of runways. The width should not be less than that required for the aeroplanes using the aerodrome. During construction this is easily achieved, but with the introduction of big aeroplanes like the A380, previously acceptable runways may be rendered inadequate. The primary factors in deciding width are wing span and outer main gear wheel span. The width is specified in terms of the aerodrome reference code, which combines a code number and a code letter. For code 1, letter A or B, the width is 18 metres; letter C is 23 metres. For code 2, A and B are 23 metres, C is 30 metres. For code 3, A, B, and C are all 30 metres, D is 45 metres. For code 4, C, D, and E are 45 metres, and F is 60 metres. There’s a note: for a precision runway, the width is 30 metres where the code number is 1 or 2. So the higher the code number and letter, the wider the runway needs to be to accommodate larger aircraft. That’s the full picture — instrument runways and their precision versus non-precision split, take-off runways, threshold location and displacement, runway length criteria with the 1800-metre rule, and the width table tied to the aerodrome reference code.