Flying the coupled approach

FLYING THE COUPLED APPROACH

Last revised: 14 February, 2008

Purpose of this tutorial

To give the computer flight simmer a general idea of the methods and practices utilized in flying an approach to a landing using the aircraft's autopilot. In aviation-speak, a "coupled approach" is one that utilizes the aircraft's autopilot to navigate the aircraft in the approach phase right down to the approach minimums, and, in some cases, on to the runway itself.

Background

It may have just been a long day, and you're tired from your journey. Or, your operation may require it, many airlines, real and virtual, do. Or, you're just beginning your training and are not comfortable with your hand-flying skills yet. Whatever your reason, using the autopilot to fly the approach can have its advantages.

However, there is a role change involved when you engage the autopilot to fly the aircraft's heading and navigational modes. You, the pilot, become to a degree a "systems manager". Whereas before you were controlling the inputs to the aircraft, now you must monitor the autopilot to be certain that it is doing it's job properly.

It's easy enough to sit back and "let George do it", but if you do not maintain your situational awareness it is possible to end up in the land of total disorientation, rather than at your destination airport.

This tutorial is not intended to instruct you on how to fly a IFR (Instrument Flight Rules) approach, using all of the proper procedures. Instead, it is intended to give you sufficient information on procedures and techniques using the autopilot, so that you, the flight sim pilot, can transfer them to your "virtual world" and make your computer flight simulator time that much more enjoyable.

Aircraft equipment required

First off, we know that you have an autopilot. As far as the instruments go, that's kind of up to you and the aircraft that you are operating. In this short tutorial we'll cover the two basic types of approaches, precision and non-precision. A precision approach is one that has lateral (left/right) and vertical (up/down) guidance. A non-precision approach is one that has the lateral guidance only.

Precision approach

Although there are other types of approaches that have vertical guidance, the ILS is the most ubiquitous, and is the one we will discuss .

The normal scenario is for Approach to vector (give you headings to fly) your aircraft until you intercept the localizer, then for you to track the localizer inbound toward the airport until intercepting the glideslope, then tracking the localizer and the glideslope down to approach minimums.

This is what we are looking at:

So there you are, flying along with good old George, the autopilot, in the "Navigation" and "Altitude Hold" modes, when Center tells you to "contact Approach on 124.75". You call Approach with "Good afternoon Approach, Red Baron 123 is with you at 7,000, Delta". "Delta" is the current ATIS (Automatic Terminal Arrival Information Service.) "Roger Red Baron 123, maintain 7,000, fly heading 040, vectors for the ILS runway 29 Left".

You reach up and set your "heading bug" to 040 degrees, and select the "Heading" mode on the autopilot. The aircraft remains in the "Altitude Hold" mode.

Now you can go about setting your radios. Number One, the one that drives the display right in front of you, should be set to the ILS frequency, in this case 109.3. The Course Selector on your HSI or VOR head should be set to the ILS "magnetic heading" (in this case 296 degrees) as shown on the approach plate, or in the approach word description. In most cases the runway and the magnetic heading do not match. The runway may be "29L", but the actual magnetic heading may be 296 degrees. If you are unable to obtain the correct magnetic heading for the runway, set in the runway heading itself, in this case 290 degrees.

If you really want to set things up properly, and your aircraft is so equipped, you can do the following:

Set the ADF to the LOM (Locator Outer Marker) frequency, and place it in the "ADF mode". This will help you to maintain orientation, as the ADF needle will be pointing at the Outer Marker during your vectors from Approach. When flying an approach on instruments, anything that you as the pilot can do to help your orientation is of benefit.

If the weather is right down towards minimums, and you anticipate the possibility of a Missed Approach (that's when you get down to minimums and cannot see the runway or it's "environment"), you can set your Number Two radio to the Missed Approach configuration, VOR frequency, course, etc.

Approach calls. "Red Baron 123, descend to and maintain 5,000". You acknowledge with a "Red Baron 123 is out of 7,000 for 5,000". Depending on your installed autopilot, you either dial in 5,000 in the Altitude Hold window, or roll the knurled knob on the autopilot out of the "Altitude Hold" detent and allow the aircraft to pitch down until it reaches 5,000 feet, at which time you roll the wheel back into the "Hold" detent. In order to avoid giving two sets of instructions each time, we'll use the "Altitude Hold Window" in these examples.

Approach calls. "Red Baron 123, intercept the localizer on this heading and track it inbound, descend to and maintain 3,000." "Roger, Red Baron 123 is out of 5 for 3, intercept the localizer and track it inbound." Dial in 3,000 in the Altitude Hold and start your descent to 3,000. Depending on your autopilot, you do one of two things. On some models you can press the "Heading" mode and the "Nav" mode at the same time. Both annunciator lights will illuminate. The aircraft will maintain the set heading in the "Heading" mode until the localizer is intercepted. At that time the autopilot will switch out of the "Heading" mode and go into the "Nav" mode automatically. Or, in other models, you remain in the "Heading" mode until you see the ILS needle on the HSI start to center. At that time switch to the "Nav" mode for ILS tracking.

Your aircraft is now in the "Nav" mode, tracking the ILS inbound to the airport, level at 3,000 feet. Now that you are no longer in the "Heading" mode, this is a good time to reach up and set the heading bug to the Missed Approach heading. It never hurts to be prepared. If good old George the autopilot is doing his job, the localizer needle is centered right in the middle of the gauge. The Boys down in Approach Control are looking at their scopes saying "Man, this guy is good! He's flying that localizer dead nuts right down the middle!"

Then Old Charley strolls by and says "Nah, he's flying a coupled approach." Well, they'll never know for certain anyway.

Approach calls. "Red Baron 123, 5 miles from the Marker, you're cleared the ILS runway 29 Left approach, contact the Tower at the Marker inbound." "Red Baron 123 is cleared the approach, Tower at the Marker. Good day!" At this time, you should have your approach power set, and your first notch of flaps down. You watch as the glideslope slowly leaves it's peg at the top of the gauge and descends towards the center of the HSI. As it reaches the middle, depending on your autopilot, you will either watch as the autopilot automatically switches out of "Altitude Hold" into the "Glideslope" mode, or you will have to press the "Glideslope" button on the autopilot to make it track the glideslope downward.

Shortly after glideslope intercept, the marker beacon goes off with a "Dah, Dah, Dah" and the ADF needle swings around to point at the tail of the aircraft. You have just crossed over the Outer Marker. Gear down, adjust flaps and power as necessary. Contact the tower and report the marker inbound. You monitor your aircraft until you reach the Descision Height (usually 200 feet above the end of the runway). Now it's decision time. Can you see the runway, or it's "environs" ? Environs include the runway lights, the REIL (Runway End Identifier Lights- those bright strobe lights aimed to flash at you, the pilot, during periods of low visibility), perhaps you see the white distance to go markers on the runway. All of these count in the "environs" category. If you can see any of these things, disengage the autopilot and land.

If you cannot, it's time for the Missed Approach. Add climb power, set the Altitude Hold to the published missed approach altitude, press the "Heading" button (you already set the Missed Approach heading, didn't you?), retract the gear after a positive rate of climb is established, remove flaps in increments so that the aircraft does not settle downwards, make the "Red baron 123 missed approach" radio call and fly the published Missed Approach procedure.

Non-precision approach

In this type of approach there is no vertical guidance, I.E. no glideslope. This could be an ADF approach, a localizer approach, a VOR approach etc. For our example, we'll use a VOR approach, more specifically, a VOR/DME approach utilizing a VOR with Distance Measuring Equiipment. In this case, let's say that the runway is once again 29 Left, and the VOR is located off of the field. The 292 degree radial of the VOR crosses the end of the runway at exactly 11.2 miles from the station.

This is what we are looking at:

You're flying along in your trusty Red Baron, on autopilot, when Center calls. "Red Baron 123, fly heading 140, maintain 7,000, contact Approach on 123.75". "140 on the heading, maintain 7,000. Good Day!" You reach over to the heading bug and set it to 140 degrees. If you are in the "Nav" mode, switch to the "Heading" mode. Now that you are being vectored by Approach, set your radios up for your approach. The VOR frequency is 115.7, the small "d" by the VOR name shows that it has DME (Distance Measuring Equipment). In fact, you will be shooting a VOR/DME approach, one that requires the DME to be in operation. If it is not, or if your equipment has failed, you may not shoot this particular approach. (This isn't totally correct. If Approach is willing, you can tell them that your DME has failed, and request that they "call" the appropriate DME points on the approach for you. Depending on their radar coverage and work load, they may do this for you. This is perfectly acceptable and legal.) Once the radio is tuned to the right frequency, set your Course Selector in the HSI or on the VOR head to the final approach course, 292 degrees.

Pop Quiz: If your aircraft is as shown on the diagram above, and you tune in the Beavr VOR, and your Course Selector is set to 292 degrees, what does the "TO/FROM" indicator show? What about the CDI (Course Deviation Indicator)? Will it be to the left, or to the right on your HSI (Horizontal Situation Indicator)?

If this question totally throws you, you may want to read the tutorial on "Understanding VOR's". Spatial orientation is extremely important when flying, especially so when you're in the "soup" getting ready to commence an approach.

Back to the Pop Quiz. With the situation shown in the diagram, the "TO/FROM" indicator will show "FROM". How about the CDI? The needle will be pegged to the far left side of the HSI.

(Oh, and why do I keep spelling the "Beavr" VORTAC that way? In the U.S. the FAA started converting all intersections to five-letter designations some time ago. Some VOR's were changed too. Hence, the "Beavr" vs. Beaver. Call it a small homage to Ward and June.)

Having an excellent spatial relationship with your aircraft and the airspace that it operates in, you realize that Approach is going to "widen you out" to the East before "turning you back in" for the approach. If you were to maintain your present Southbound heading it would require too tight of a turn to establish your aircraft on the approach radial to the airport.

This is a good time to review the approach procedure, and set up your Number two radio for the Missed Approach if the weather appears questionable.

Approach calls. "Red Baron 123, turn farther left, heading 120, descend to and maintain 4,000." "Out of 7 for 4, 120, Red Baron 123." You reach up and set the Heading Bug to 120 degrees, and set the altitude hold to 4,000 feet. Leveling at 4,000, Approach calls. "Red Baron 123, turn right heading 270 degrees." "Right to 270, Red Baron 123." You reach up and set the Heading Bug to 270 degrees.

Approach calls. (Incessant, aren't they?) "Red Baron 123, intercept the inbound radial for the runway 29 Left approach on this heading, descend to an maintain 3,000." "Out of 4 for 3, intercept the radial, Red Baron 123." Once again, depending on you type of autopilot, you will either press both the "Heading" mode button and the "Nav." mode buttons for an automatic transfer from heading to navigation when the radial is intercepted, or wait until the CDI swings into the center of the HSI and then press the "Nav." button on the autopilot.

The CDI centers. You are on the reciprocal radial of 292 degrees from the Beavr VOR, the 112 degree radial, level at 3,000 feet. The "TO/FROM" arrow is pointing "TO" , and the CDI is centered in the HSI. The autopilot is in the "Nav." mode, and the Altitude Hold is at 3,000 feet. The DME readout shows 5 miles to the Beavr VOR. It's time to get your pre-landing check list completed as much as possible for this stage of the game. Power should be set at your approach settings. This should put the good old Red Baron in the flap operating range on the airspeed indicator. Hold off on the flaps for the time being though, no sense in dragging this out and eating up that expensive avgas. The DME now shows 2 miles to the Beavr VOR. Might as well drop that first notch of flaps at this point.

Approach calls. "Red Baron 123, I show you 2 miles from the Beavr VOR, you're cleared the VOR/DME approach to the Oxford Airport, contact the Oxford Tower at Beavr inbound." "Cleared the approach, Tower at Beavr, Red Baron 123." Now the CDI is really sensitive, as the radial you are navigating "closes" on the VOR. The autopilot tries to keep up, and finally quits searching, "knowing" that it will be able to track the radial after you cross this "zone of confusion" where all of the radials compress into such a small area. The "TO/FROM" arrow flips at this point from "TO" to "FROM".

You have just crossed over the Beavr VOR. Reach over and set your "Altitude Hold" to the "Minimum Descent Altitude", let's call it 1,000 feet for this example. Gear down, mixtures rich, check power settings, flaps set as necessary, check list complete, call the Tower.

Remember, fly the airplane first, communicate second.

"Oxford Tower, Red Baron 123 is at Beavr inbound". "Roger Red Baron 123. Not in sight, cleared to land 29 Left." "Cleared to land, 29 Left, Red Baron 123."

Note that for this approach the end of the runway is at 11.2 DME from the Beavr VOR. Let's say that the MAP (Missed Approach Point) is at the 10.0 DME point, in effect about one mile of visibility is required for the approach. You want to be certain that your aircraft is at the MDA of 1,000 feet before you get to the MAP of 10.0 miles from the Beavr VOR. This assures that you will have the maximum opportunity to "look around" for the runway environment. If you descend so as to reach the MDA and the MAP at the same time, you will only have a split second to look and make your decision as to "Yes, there's the airport, I can land." or "Nothing out there, going Missed Approach." Don't worry about descending too fast, this is all taken into account when the FAA (in the U.S.) lays out the approach.

Back to the cockpit of your Red Baron. You are level at 1,000 feet. The DME reads 9.0 miles. Gear and flaps are down and set. You're looking out the window, through the murk, trying to find the airport. There it is! You disengage the autopilot, maneuver as necessary to line up with the runway, and land your trusty bird in a perfect squeaker.

Or, you don't see it. The DME clicks down to 10.0 miles and there is nothing but the view from the inside of a bottle of milk. "Oxford Tower, Red Baron 123, Missed Approach." "Roger Red Baron 123, Missed Approach, contact Approach on 123.75." (You did set that frequency on Comm. 2 just in case, didn't you?) You contact Approach. "Approach, Red Baron 123 is with you, missed approach at Oxford."

Unless you have advised Approach of your alternate intentions before, this is the guaranteed reply that you will receive: "Roger 123, what are your intentions?" You had best have an answer. Never shoot an approach where the outcome is in doubt without having a plan "B". Grinding around in the soup on a Missed Approach, reconfiguring your airplane from the approach mode to the climb mode, is not the time to try to figure out what you are going to do next. If it appears that a "miss" is likely, advise Approach at some time prior to shooting the approach that "If Oxford is below minimums, and we go missed approach, we would like vectors over to Big City Airport for the ILS 30 approach." Now that's a plan "B". When flying airplanes, always, always, have a plan "B". Sometimes, depending on the circumstances, you may want to extend even farther down the alphabet.

What happens if you start to go Missed Approach, and then see the airport down below you? Well, unless you have stumbled into a very large clear area that allows you to maneuver the aircraft in a normal fashion, at normal pattern altitudes (not too likely) you must continue on your Missed Approach. The "Circle To Land" minimums for an instrument approach are much higher than those for the Straight In approach that you just shot.

You do not want to be flying low to the ground, circling around trying to get back to the approach end of the runway. People stall airplanes and hit the ground doing that kind of thing, it will ruin your entire day. And, just in case you do pull it off and make it on to the runway, the friendly fellows in the Tower will ask you to call them on the telephone, and will start filling out forms for a violation . Most likely you will lose your ticket.

Don't do that.

Conclusion

I hope that this has been helpful to you in understanding how the autopilot can be used in approach and landing situations. If there are any errors, or if I didn't explain something to your satisfaction, please contact me.

Have fun flying.

Correction made 4/4/07: Corrected typo that had Red Baron 123 referred to as "Beavr 123." Thanks to Willie van Niekerk for pointing this out.

Addition made 2/14/2008. Notatiion made regards contacting tower at the marker inbound, and declaration of missed approach. Thanks to Phil Covington for pointing this out.

This tutorial is available on a CD

This tutorial, along with additional content, is available on a CD. Click here for more information.

Hal Stoen

revised, 26 August, 2000

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