Ground Controlled Approaches (GCA)

by on October 6, 2008

I have to admit that I have just started listening/watching audio and video podcasts related to aviation. That could be because I recently bought an iPhone and can download them to the phone and then listen/watch during the long bus rides to and from the layover hotels on my trips. I have just finished listening to five recent podcasts from Uncontrolled Airspace and thought I’d comment on one of them.

I can’t remember which of the episodes it was, but the conversation at some point was directed toward the GCA approach. That’s one of those abbreviations that has taken on a life of it’s own and, when unraveled, results in your saying Ground Controlled Approach approach. Anyway, I came away from the podcast with a little confusion about the subject and thought I’d try to clear it up with this post.

A GCA is an instrument approach in which the actions of the pilot flying the airplane are directed by a ground radar controller, similar to the directions given by Air Traffic Control when giving headings and altitudes in the vicinity of an airport. However, the job of the GCA controller begins where the approach controller’s job ends.  Just consider the GCA to be a verbal VOR or ILS approach. Instead of being told to maintain a heading and altitude until established on a segment of the approach and then being cleared for that approach, the controller will at the same point in the process turn control over to the ‘final controller’ and the pilot will receive verbal instructions to arrive at the runway threshold. There are two types of ground controlled approaches.

ASR Approach

The ASR approach, so-called because it uses Airport Surveillance Radar to determine the aircraft position, is the non-precision GCA approach. The ASR approach controller will provide verbal positioning instructions of the same type that would be received if the pilot were using aircraft instruments to fly a localizer-only approach (LOC). It will be up to the pilot to provide his/her own vertical guidance. The controller will provide aircraft position information relative to the runway centerline and will give heading changes to correct to the centerline. The pilot will be given a warning when approaching the ‘begin descent’ point and then will be told to begin the descent to the published Minimum Descent Altitude (MDA). Prior to the descent point the controller will provide missed approach instructions and instructions concerning what to do in the event of lost communications. If requested, the controller will provide recommended altitudes on final so that the aircraft will arrive at the MDA at/prior to the Missed Approach Point (MAP). The controller will announce arrival at the MAP and direct the pilot to take over visually or to execute the missed approach instructions that have been provided.

PAR Approach

The PAR Approach, using Precision Approach Radar, is, as the name implies, the precision ground controlled approach. The approach controller will provide the pilot with both lateral and vertical deviation information and appropriate corrections to attain an ‘on course, ‘on glidepath’ position. The controller’s radar screen will give two views of the aircraft position, one will provide a bird’s eye view showing an extension of the runway centerline that provides lateral guidance. The other view will be from the side and will have a line depicting the equivalent of an ILS glideslope and will show the aircraft position in relation to the glideslope. As with the ASR approach, a warning will be given concerning approaching the descent point and when to begin descent as well as a recitation of the missed approach and lost communications procedures. It will be up to the pilot to determine the initial rate of descent to use, but once begun, the progress of the descent in relation to the perfect glidepath will be provided. Verbal instructions will be given down to and often past the approach’s Decision Height (DH), the point on an precision approach when the decision must be made to either land or execute the missed approach procedure/instructions.

The podcast mentioned that the availability of these instrument approaches at a given airport may be determined by looking at the Airport/Facility Directory (A/FD). I have to admit that this seemed like the most logical place to find the information, however, an examination of the digital A/FD did not reveal any pertinent information. I did, however, find the following paragraph in the FAA Instrument Procedures Handbook:

Information about radar approaches is published in tabular form in the front of the TPP booklet. PAR, ASR, and circling approach information including runway, DA, DH, or MDA, height above airport (HAA), HAT, ceiling, and visibility criteria are outlined and listed by specific airport.

Which can easily lead to the question; What is the TPP booklet? A little more research into FAA acronyms provides the answer that it refers to the Terminal Procedures Publication, the 24 volumes published by the FAA that contain all of the instrument procedures available at a given airport.

The podcast also made reference to the controller’s instructions to “start turn”, “stop turn” and the use of standard or half-standard rate turns. These instructions are not used in the standard ASR or PAR approach. They are given in the event of an aircraft equipment malfunction that effects the aircraft’s heading indications and are referred to as a “no-gyro” approach. Since the pilot cannot read a valid heading on his/her instruments, the controller directs heading changes using start turn and stop turn commands and specifies using standard rate turns during vectors to final and half-standard turns while in the final approach segment.

I hope this has provided a little more information about the ground controlled approach. Once a common practice, especially at military facilities, the use of ASR and PAR approaches has become extremely rare. The next time you’re planning on flying for instrument proficiency, why not check that TPP booklet of yours, find a nearby facility that can provide a GCA and give it a try. If a facility does provide the service, you can bet that the controllers will be more than happy to have you practice so that they can renew their own proficiency.

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