Learn how airport runways are numbered by magnetic heading, why opposite ends differ by 18, how parallel runways use L/C/R, and why numbers change.
In real-world aviation, airport runways are numbered from their approximate magnetic direction of travel, rounded to the nearest 10 degrees and with the final zero removed. A runway pointing about 270° is Runway 27. The opposite end points roughly 180° away, so the same strip is normally designated 09/27.
How is a runway number calculated?
A runway number is derived from the magnetic bearing of its centreline when viewed from the direction of approach.
- Take the centreline bearing: use the direction in which the aircraft will land or take off on that runway end.
- Round to the nearest 10 degrees: a bearing of 087° becomes 090°, while 274° becomes 270°.
- Remove the final zero: 090° becomes Runway 09 and 270° becomes Runway 27.
- Format it from 01 to 36: single digits receive a leading zero, and north is designated 36 rather than 00.
| Approximate magnetic bearing | Runway number | Direction |
|---|---|---|
| 006° | 01 | Roughly north |
| 087° | 09 | Roughly east |
| 183° | 18 | Roughly south |
| 274° | 27 | Roughly west |
| 356° | 36 | Roughly north |
Most runway designations use magnetic north. In designated regions where magnetic compasses are unreliable, particularly at very high latitudes, the aviation authority may authorise designations based on true bearings. The published airport chart identifies the runway actually in use.
What do Runway 09 and Runway 27 mean?
Runway 09 is used in the direction of approximately 090° magnetic, while Runway 27 uses the same pavement in the reciprocal direction of approximately 270°.
Reciprocal runway numbers normally differ by 18 because their directions differ by 180°. Add 18 to numbers from 01 through 18, or subtract 18 from numbers from 19 through 36: Runway 01 pairs with 19, 09 with 27, and 18 with 36.
The two designations identify runway ends, not two separate strips of pavement. They also say nothing about runway length, surface or landing capability. Our walkthrough for interpreting an airport diagram shows how these reciprocal labels appear on charts.
How are parallel runways numbered?
Parallel runways share the same basic number and use a letter to distinguish their position when viewed from the direction of approach.
- Two parallel runways: L and R for left and right, such as 09L and 09R.
- Three parallel runways: L, C and R for 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 therefore be split between 18L/18R and 19L/19R.
The letters reverse at the reciprocal end because left and right swap when the approach direction changes. The opposite end of Runway 09L is therefore Runway 27R; the opposite end of 09C remains 27C.
Why does the compass heading not exactly match the runway number?
A runway number represents a rounded magnetic direction, so it should not be expected to match every cockpit heading display exactly.
- Rounding: Runway 27 may have an actual centreline bearing several degrees either side of 270°.
- Different north references: a GPS or simulator display may show true track while the runway uses magnetic direction. Our guide to comparing cockpit north references explains this common mismatch.
- Crosswind correction: an aircraft may point into the wind on final while tracking along the runway centreline, so heading and ground track differ.
- Outdated simulator data: scenery textures, ATC data, airport charts and navigation databases can represent different dates following a real runway renumbering.
When simulator scenery and navigation data disagree, compare the runway’s location and approach course rather than assuming the old and new numbers represent different pavement. Updating the scenery and navigation data to compatible revisions is the proper fix.
Why do airport runway numbers change?
Runway numbers change because Earth’s magnetic field shifts while the physical runway remains fixed.
Once the magnetic bearing moves far enough to round to a different ten-degree value, the airport may renumber the runway. That requires changes to threshold numerals, holding-position signs, charts, procedures and navigation databases. Our explanation of runway markings and visual identifiers shows where the designation appears around the airfield.