A runway number times 10 is the approximate magnetic heading of an aircraft using that end. Runway 15 is about 150 degrees magnetic, and Runway 18 is about 180 degrees magnetic. The number is the magnetic heading rounded to the nearest 10 degrees, with the trailing zero dropped.
The rule
Departing Runway 15, the airplane climbs out on about 150 degrees magnetic. The wrong readings are consistent and worth memorizing as wrong. 015 degrees reads the painted number as if it were already a three-digit heading. 510 degrees is not a heading at all. 105 degrees transposes the digits of 150. For Runway 18, 1,800 degrees inflates the multiplication, and 360 degrees is the opposite direction on the compass rose, not this end.
A closed runway is marked with a yellow X. When a large yellow X is painted at each end of one runway, that runway is closed. The airport is not automatically closed. Another runway may still be in use. The crew must still watch for manned traffic, yield under §107.37, and avoid interfering with airport operations under §107.43. The X is not a displaced threshold, and it does not reserve the pavement for helicopters. A displaced threshold is a different marking: the pavement before it may be used for taxi, takeoff, and landing rollout, while landing touchdown is at or beyond the threshold. Do not trade the X for that picture.
Which end is in use still depends on the wind. The sock and the segmented circle are on windsock and traffic pattern. How the five legs wrap the number you just decoded is on airport traffic pattern.
How to apply it
When you hear a runway in a traffic call, multiply before you look up. Runway 15 means traffic aligned near 150 degrees, so you know which way the airplane is pointed on final and which way it will climb if it is departing that end. Runway 18 means about 180 degrees, south on a magnetic compass that has no local variation applied yet. The number is magnetic, not true. You do not need a made-up radio phrase to use it. You need the direction so your aircraft is not on that final.
If you see yellow X marks on one strip and bare numbers on the parallel strip, treat only the marked strip as closed. Scan the open runway and its pattern. A roof job beside the field does not become unsupervised because one pavement is out of service. If both ends of every runway carry an X, the airport picture has changed, but one pair of X marks is not that picture. The airport data block still tells you field elevation and the frequencies. It does not overrule a closed-runway marking you can see on the ground.
Runway 15, Runway 18, and one closed strip
The diagram shows Runway 15. An airplane departing that end climbs on about 150 degrees magnetic. You keep the small unmanned aircraft off that extended centerline. The same airport's crosswind runway is Runway 18, about 180 degrees, not 1,800 and not 360. If the wind favors 18, the arrivals are aligned south, and the departures climb south.
On the pre-coordinated roof inspection next door, both ends of one of the two runways carry a large yellow X. Conclude that the marked runway is closed and that the other runway may still be active. Keep scanning. Yield if an airplane uses the open runway. Do not read the X as a displaced threshold and expect touchdowns farther down the closed pavement. Do not relax the scan because "the airport is closed."
The trap
The Runway 15 item lists 015 degrees, which forgets to multiply, and 510 degrees, which is not a direction on the heading indicator. The answer is 150 degrees magnetic. The Runway 18 item lists 1,800 degrees and 360 degrees. The answer is 180 degrees, because the runway number times 10 is the approximate magnetic heading.
The marking item says the entire airport is closed so the crew may relax, or that the X is a displaced threshold and airplanes will touch down farther along that runway. A yellow X closes that runway. Other runways may remain active. Yielding and staying out of the pattern are unchanged.
Decode a few numbers on the free practice test. The course pairs the multiplication with the closed-runway marking so a yellow X is not mistaken for a quiet field.