Learn how runway numbers use rounded magnetic headings, why north is 36 not 00, how reciprocal and parallel runways work, and why numbers change.
In Aviation & Real-World Flying, runway numbers come from the centreline’s direction, normally magnetic, rounded to the nearest 10 degrees with the final zero removed. Thus 087° becomes Runway 09 and its reciprocal end is Runway 27. Physical alignment is chosen principally for prevailing wind, then terrain, obstacles, airspace, land and noise.
How is a runway number calculated?
A runway number is calculated from the centreline bearing in the direction an aircraft travels after crossing that runway end’s threshold.
- Find the centreline bearing. Measure clockwise from the applicable north reference, normally magnetic north, while looking along the landing or take-off direction.
- Round to the nearest 10 degrees. A bearing of 087° rounds to 090°, while 274° rounds to 270°.
- Remove the final zero. The rounded values become 09 and 27. A result of 360° becomes 36, never 00.
- Calculate the reciprocal end. For an ordinary runway pair, add 18 to designators 01 through 18 or subtract 18 from 19 through 36. This represents the 180° change in direction.
| Centreline bearing | Runway designation | Opposite bearing and designation |
|---|---|---|
| 006° | 01 | 186°, Runway 19 |
| 087° | 09 | 267°, Runway 27 |
| 184° | 18 | 004°, Runway 36 |
| 274° | 27 | 094°, Runway 09 |
| 359° | 36 | 179°, Runway 18 |
Many countries write single-digit designations with a leading zero, such as 04 or 09. United States pavement markings commonly omit it, so Runway 9 and Runway 09 describe the same approximate direction under different presentation conventions. We treat the applicable chart and local phraseology as authoritative.
What do Runway 09 and Runway 27 mean?
Runway 09 carries an aircraft approximately east along the pavement, while Runway 27 uses the same strip in the opposite, approximately westbound direction.
The designations identify two runway ends, not two separate strips. An aircraft using 09 approaches its threshold from the west; one using 27 approaches from the east. The number is painted near each threshold and appears on signs and charts alongside the other threshold, centreline, aiming-point and touchdown-zone markings.
Is a northbound runway 00 or 36?
A northbound runway is Runway 36, never Runway 00.
Runway 36 represents a direction near 360° or 000° and carries traffic approximately north. Its reciprocal end is Runway 18, used in the approximate direction of 180° or south. Valid numeric runway designators run from 01 to 36, even when the measured northbound bearing is exactly 000°.
Do runway directions use magnetic north or true north?
Runway designations normally use magnetic north, although an aviation authority may prescribe true bearings in designated high-latitude regions where magnetic references are unreliable.
This can make a runway appear misaligned with a geographic map, a true-heading display or a METAR wind direction, which is normally referenced to true north. Aircraft instruments and simulator displays may use either reference; our explanation of how true and magnetic headings differ helps identify which value is being shown.
Why does the compass heading not exactly match the runway number?
A runway number is a rounded directional identifier, not an exact centreline, aircraft heading or instrument-approach course.
For an ordinary designation, the initial rounding difference is about five degrees or less, so Runway 27 could have a centreline bearing of 274°. Magnetic drift before a scheduled renumbering and adjacent-number allocation at airports with many parallel runways can produce a larger difference.
Crosswind correction also matters: an aircraft may crab into wind while its ground track remains on the centreline. An ILS or other published approach course may differ slightly as well, particularly where the localizer is offset. Use the published course and matching simulator navigation data rather than setting 270° automatically for every Runway 27 approach.
How are parallel runways numbered?
Parallel runways use letters to distinguish pavements with the same or nearly the same numeric designation.
- Two parallel runways: L and R mean left and right when viewed on approach, such as 09L and 09R.
- Three parallel runways: L, C and R mean left, centre and right.
- Four or more parallel runways: authorities may divide them between adjacent number groups even when their physical headings are almost identical. Four broadly southbound runways might be split between 18L/18R and 19L/19R.
Left and right reverse at the reciprocal end because the viewing direction changes. Runway 09L therefore normally pairs with 27R, 09R with 27L, and 09C with 27C. Keeping L and R unchanged at both ends is a common error.
How are physical and active runway directions chosen?
Airport planners determine the physical alignment, while wind, ATC and operating procedures determine which end is active for a particular take-off or landing.
Physical runway alignment
A runway is usually aligned to provide useful headwind coverage under the area’s prevailing wind patterns.
Planners must also account for terrain, obstacles, available land, drainage, surrounding development, controlled airspace, nearby airports, noise-sensitive areas, terminal access and required capacity. The runway number is assigned after the alignment is established; selecting a number does not dictate where the pavement is built.
Active runway direction
The active direction is normally selected to minimise tailwind and excessive crosswind while accommodating traffic flow, runway condition, available approaches and local restrictions.
Runway 27 carries aircraft approximately west, so it is often favoured when the wind comes from the west. Runway 09 may suit an easterly wind. The key distinction is that wind direction identifies where the wind comes from, while the runway number identifies where the aircraft travels; our guide to headwind, tailwind and crosswind effects during take-off and landing explains the operational consequences.
Terrain, noise procedures or instrument-approach availability may lead an airport to retain a preferred direction with a limited tailwind. ATC instructions, runway conditions and aircraft limits still govern the operation.
Which runway numbers are most common?
There is no single worldwide most common runway number because each airport’s alignment reflects local wind patterns and physical constraints.
The cardinal pairs 09/27 and 18/36 are especially easy to recognise because they represent approximate east–west and north–south alignments, but that does not make them universally dominant. Regions with prevailing south-westerly winds, for example, often favour diagonally aligned runway pairs.
A runway number says nothing about pavement length, surface, lighting or approach capability. Two airports can have identically numbered runways with entirely different operating characteristics.
Why do runway numbers change?
Runway numbers change when long-term movement of magnetic north makes the existing designation unsuitable for the runway’s measured magnetic bearing.
The pavement has not rotated. Once the bearing rounds to a different ten-degree value, the authority may schedule a renumbering. Our guide to magnetic variation and its effect on aviation references explains why the fixed runway and its magnetic designation gradually diverge.
Renumbering requires coordinated changes to threshold numerals, holding-position and taxiway signs, charts, instrument procedures, airport records and navigation databases. Airports therefore do not repaint a runway after every small magnetic shift.
Why does a flight simulator show two runway numbers?
A simulator mismatch usually means its scenery, navigation database and ATC data represent different airport revisions following a real renumbering.
The old and new numbers may still refer to the same pavement. When troubleshooting, we compare its location and published approach course rather than assuming that two separate runways exist. Updating the scenery and navigation data to compatible revisions is the proper fix where matching updates are available.
- For an instrument procedure, keep the chart, procedure and loaded navigation data on matching revisions; do not combine an old approach name with a newer course.
- For visual taxiing, follow the signs and markings rendered by that scenery, while expecting built-in ATC to use another designation if its airport data is newer.
- For real-world flying, use the applicable authorised publications, operational information and ATC clearance rather than treating simulator scenery as evidence.