How VOR Approaches Work

By Boldmethod

08/06/2022

VOR approaches are one of the most widely used non-precision approaches in the US. Here's how they work.

First, How Exactly Does A VOR Work?

The frequency range for a VHF Omni Directional Range Radio (VOR) is between 108.0 MHz and 117.95 MHz. The VOR sends out one stationary master signal, and one rotating variable signal. These are also called "reference" and "variable" phases.

An aircraft's VOR antenna, which is usually located on the tail, picks up this signal and transfers it to the receiver in the cockpit. The aircraft's VOR receiver compares the difference between the VOR's variable and reference phase, and determines the aircraft's bearing from the station. This bearing is the radial that the aircraft is currently on.

It's A Non-Precision Approach

Every VOR approach is non-precision, meaning there is no vertical guidance signal from the VOR. On a VOR approach, you need to manage your descent and altitude manually.

The final approach course is charted based on a radial FROM or TO a nearby VOR. After flying a procedure turn or receiving vectors to final, you'll usually find yourself stepping down along a series of fixes to minimum published altitudes on an approach chart. Along the final segment of the approach, you'll reach your MDA (Minimum Descent Altitude).

Once you've reached the MDA, you'll (typically) fly at that altitude until you can see the runway environment. Once you see the runway, you can continue your descent to land. And if you don't see the runway, you'll fly to the missed approach point, begin your climb, and fly the missed approach procedure.

"In flat terrain with no obstacles, VORs can provide MDAs as low as 250 feet above the runway" (FAA). However, most of the time, you'll find MDAs quite a bit higher than this. VOR approaches are often times coupled to DME, or Distance Measuring Equipment, to give you reliable distances to use along the final approach course.

So how do you manage your descent along the stepdown fixes of an approach?

The FAA recommends that you fly non-precision approaches using the Continuous Descent On Final Approach (CDFA) method. 

On-Field vs Off-Field VOR Approaches

Some VOR approaches are based on VOR NAVAIDs located at the airport. Just like an ILS, as you approach the VOR, the signal will get more sensitive as you fly closer to the station.

The opposite is true for off-field VOR approaches, where the VOR might be located quite a few miles away from the airport you're flying into. The FAF for many VOR approaches might even be the VOR itself.

In the example below, the VOR/DME RWY 13 Approach into Crookston is based on the GFK VOR (the same VOR used for approaches into KGFK, over 20 miles away). FAA TERPS criteria use nearby terrain, obstacles, and VOR sensitivity to analyze the safe area surrounding the final VOR approach course.

Arcing Approaches

Unlike just about any other approach, the VOR/DME into Runway 15 at KMTN is one big DME arc. Each fix along the approach is a DME radial, and the final approach course is constantly curving. According to the FAA...

"The criteria for an arc final approach segment associated with a VOR/DME approach is based on the arc being beyond 7 NM and no farther than 30 NM from the VOR, and depends on the angle of convergence between the runway centerline and the tangent of the arc. Obstacle clearance in the primary area, which is considered the area 4 NM on either side of the arc centerline, is guaranteed by at least 500 feet."

There Might Not Be A FAF, Or Even An Associated Runway

Some VOR approaches don't have a designated FAF (Final Approach Fix). This is typical when physical equipment (like a marker beacon or VOR station) is not in-place to designate a FAF for non-DME equipped airplanes. As long as you're within the minimum distance from the station, you can descend all the way to MDA when you're established inbound on the final approach course when there's no FAF published.

You also might find airports with VOR approaches that are circling-only approaches. They'll be published as "VOR-A" or "VOR/DME-B", for instance. Once the airport is in-sight on these approaches, you'll perform a circle-to-land to a suitable runway.

When Was The Last Time You Flew A VOR Approach?