How do airports determine when they should change their runway numbers?

Question

Runway numbers in most of the world are based on the runway's orientation relative to magnetic north. For example a runway with a magnetic heading of 135° to 144° will be numbered 14, and one with a magnetic heading of 145° to 154° will be numbered 15.
Runway numbers are occasionally changed due to changes in the Earth's magnetic field.

At my home field Runway 14/32 is oriented 145.7°/325.7°, per the FAA's airport diagram, and it has been this way for some time (I first recall noticing it over a year ago).
My understanding is that this runway should, at some point, be renumbered 15/33.

How does the FAA determine when to request that an airport change its runway numbers?
Is the decision to update the runway designation made by the FAA, or by the airport's management?

Answer 1

Implementing changes to runway numbers is quite complicated and requires a lot of different parties to get involved. There is a ton of related information in FAA Order 8260.19E and it has the details.

Simple Version

According to an article by the NBAA called "How Changes in Magnetic North Are Impacting Airports":

“Adjustments to runways like this and to navigational aids are ongoing,” according the Kathleen Bergen, a spokeswoman for the FAA Southern Region. “Every five years, the FAA reevaluates shifts in the pole—its magnetic variation—and makes changes to runways and flight procedures as needed,” she said.

Medium Version

NOAA, NOS, and NGS (see below) provide the magnetic variation information, and publish it every five years. They work with a bunch of different offices, and different branches of the military as they make adjustments. When a determination is made that a runway number should be changed, they coordinate with several other groups (such as AeroNav Products, OSG-FPT, the applicable Airportt Traffic Service Area Office, and the Airports Division), to choose the actual number used, based on "careful consideration and evaluation of a number of factors" and then to make the actual changes in charting, runway markings and signs, etc.

They are supposed to change it when it is more than three degrees off from what it is should to be. AC 150/5340-1L - Standards for Airport Markings contains the official runway designator rules (paraphrased): If the actual magnetic course of the runway ends in 5, it can use either number, otherwise you round to the closest runway number (there are some variations for special cases like parallel runways and such, but this covers most runways). So in your example, 145 is still valid as runway 14, and they wouldn't need to change it until it exceeds 148 deg. In your case, the annual rate of change for Farmingdale airport (as of 2010) is 0.0 deg East, so I don't see that happening for awhile.

Long Version - Quotes from FAA Order 8260.19E (above)

Section 5. Implementing Epoch Year Magnetic Variation (MV)

a. Background. The National Oceanic and Atmospheric Administration (NOAA), National Ocean Service (NOS), and the National Geodetic Survey (NGS), for all areas of the United States and its territories for application to navigation charts and maps, is the source for magnetic variation (MV) information and tools for establishing magnetic variation. Changing values for MV are tabulated and published on a 5-year epoch basis; e.g., 00, 05, 10, 15, 20, etc. In order to assist in stabilizing the National Airspace System (NAS), a fixed value of MV is assigned to each navigational aid and airport as the Magnetic Variation of Record. This value is applied to true directions to obtain the magnetic values for radials, courses, bearings, and headings published in instrument flight procedures. Periodic updating of the MV assigned to navigation facilities is required to maintain reasonable proximity of alignment with the earth’s ever-changing magnetic field. ...

b. Participating Offices. Management and control of Epoch Year MV values require action by the following offices:
(1) AeroNav Products.
(2) Military Organizations.
(3) National Flight Data Center (NFDC).
(4) Western, Central, and Eastern Technical Operations.
(5) Western, Central, and Eastern OSG-FPTs.
(6) Regional Airports Divisions.

...

f. Regional Airports Division/Airports District Office (ADO). Coordinate with the applicable OSG-FPT prior to establishing or revising runway designator numbers for an airport having one or more instrument approach or departure procedures, to determine the appropriate MV to be applied to the runway true bearing. Determination of the runway designator number should be a matter of joint agreement with AeroNav Products, and be accomplished sufficiently in advance to allow for procedural amendments. Take appropriate NOTAM action if repainting of an affected runway has not been accomplished on the required date.

...

2-18. Guidelines. The identification and selection of navigational aids or airports as candidates for revision of MV of Record require careful consideration and evaluation of a number of factors - as the impact of MV changes can be considerable. The applicable Air Traffic Service Area Office may have to initiate or revise published air traffic procedures; the Technical Operations Service (AJW-0) is directly involved in facility rotations and requires proper coordination. The Airports Division, or appropriate military authority, may have to arrange for repainting of runway designator numbers [see paragraph 8-58e(2)(e)].

a. MV versus Epoch Year Value. When the difference between the MV of Record and the nearest future Epoch Year value of any navigational aid or airport is 3 degrees or more, the MV of Record must be changed to the nearest future Epoch Year value.

Answer 2

There is no real need to change the runway identifier just because the variation changed. Sometimes this is done but you'll find enough places aroud the world where there is up to a 10 degree offset (maybe even more) between the numbers on the runway and it's magnetic orientation. At some biger airports with more then three parallel runways this is even done by purpose because if you already have xx left, center and right you can't call the fourth "right right" but what you do is you name them xx left and right and xx+1 left and right see for exampel LAX.

Now lets think about the use of the runway numbers. I only need them to x-check if I'm on the right runway prior to departure, to have a reference while approaching an airfield and to quickliy calculate the x-wind - my math isn't so good that five degrees would make a difference ;) and the other things are for sure important but also work out with some offset (I have never seen intersecting runways with only 10 degrees difference in orientation where a confusion would be possible)

There is no source where I found this but in my eyes it's very dependent on the importance of the airfield if they change it when there are more then 6 degrees offset or not.