This lesson covers the numbers an aerodrome publishes about itself: the two-part reference code that matches facilities to aeroplanes, the reference point that fixes the aerodrome on the map, the pavement-strength system that decides whether your aircraft may park on it, the four declared distances every performance calculation begins from, and the surface-condition reporting that tells a crew what today's runway will actually do to their braking.
The aerodrome reference code
The intent of the aerodrome reference code is to provide a simple method for interrelating the numerous specifications concerning the characteristics of aerodromes, so as to provide a series of aerodrome facilities suitable for the aeroplanes intended to operate there: one code, and a designer can read off the widths, separations and clearances the whole aerodrome must offer. It is not meant to determine runway length or pavement strength requirements, and it does not limit or regulate the operation of any aircraft. Its two elements divide the work:
Element 1 — a code NUMBER, based on the aeroplane reference field length: 1 for less than 800 m; 2 for 800 m up to but not including 1,200 m; 3 for 1,200 m up to but not including 1,800 m; 4 for 1,800 m and over. The number is selected from the highest reference field length of the aeroplanes the runway is intended for, a performance-derived scale.
Element 2 — a code LETTER, based on the aeroplane wingspan: A up to 15 m; B 15 to 24 m; C 24 to 36 m; D 36 to 52 m; E 52 to 65 m; F 65 to 80 m (each band up to but not including its upper figure), a size-derived scale that drives taxiway widths, separations and clearances.
The aerodrome reference point
Every aerodrome has one official position: the aerodrome reference point (ARP), located near the initial or planned geometric centre of the aerodrome, and it shall normally remain where first established, even as the aerodrome grows around it, because charts, airspace boundaries and bearings have been hung on it for decades. It is measured and reported to the AIS in degrees, minutes and seconds.
Pavement strength: ACN and PCN
Whether a pavement can carry an aircraft is answered by comparing two numbers. The aircraft classification number (ACN) expresses the relative effect of an aircraft on a pavement, for a specified standard subgrade category; the pavement classification number (PCN) expresses the bearing strength of the pavement for unrestricted operations. Their mutual dependence is the tested clause: an aircraft with an ACN equal to or less than the published PCN can use the pavement without restriction; overload operations beyond it only under conditions the authority regulates. The PCN is published for pavements intended for aircraft of apron (ramp) mass greater than 5,700 kg, as a five-part code: the PCN value; the pavement type for ACN-PCN determination (R rigid, F flexible); the subgrade strength category (A high to D ultra-low); the maximum allowable tyre pressure category (W unlimited, X high, Y medium, Z low) or value; and the evaluation method (T technical, U by aircraft experience). ICAO has since replaced this system with the ACR-PCR method (aircraft and pavement classification RATING), with the same comparison logic on a finer scale, but the syllabus, and many AIPs in transition, still speak ACN-PCN.
For the light end of aviation the code collapses: the bearing strength of a pavement intended for aircraft of apron mass equal to or less than 5,700 kg is reported as the maximum allowable aircraft mass and the maximum allowable tyre pressure, two plain numbers instead of the classification machinery.
The four declared distances
Annex 14 provides guidance on the calculation of the declared distances — the four numbers, published for each runway direction, from which every take-off and landing performance calculation starts:
TORA — take-off run available: the length of runway declared available and suitable for the ground run of an aeroplane taking off.
TODA — take-off distance available: the TORA plus the length of the clearway, if provided (ground run plus the obstacle-protected air beyond it).
ASDA — accelerate-stop distance available: the TORA plus the length of the stopway, if provided (the pavement available to stop on after an abandoned take-off).
LDA — landing distance available: the length of runway declared available and suitable for the ground run of an aeroplane landing, starting at the threshold, which is why a displaced threshold shortens the LDA and nothing else.