AIRBUS A319/A320/A321: FIXES AND DME ARCS

This document is specific to the PSS Airbus A319/A320/A321 MCDU.

Entering a Fix into the Airbus MCDU There are many occasions when it might be necessary for you to enter a fix into the Airbus MCDU because there is not a pre-programmed waypoint in the Airbus database. For example this could be when ATC gives you a specific instruction or when planning to fly a DME arc. If you don’t have a named waypoint in the database there are three ways to enter a fix:

Place/Bearing/Distance (PBD)

Place-Bearing/Place-Bearing (PBX)

Latitude/Longitude (LL) Each of these has its uses although the PBD is the easiest one to use and the one you are most likely to use. In order to enter these you will need to consult charts, and for the PBX and LL fixes it will be very helpful if you have en route charts or access to a flight planing program like FSNavigator. But I have included portions of the relevant charts within this guidance for ease of reference.

For this scenario we are on an approach to runway 25 at Belfast Aldergrove (EGAA) from Birmingham (EGBB). Unfortunately Aldergrove is not currently in the British Airways schedule, but it provides good practice for entering fixes and DME arcs.

Place/Bearing/Distance (PBD) Looking at the chart you will see that we need to make a turn to the right to start an arc 14 miles from BEL (D14.0 BEL), in other words two miles after the D16.0 BEL fix. But the D14 BEL fix is not in the flightplan or in the MCDU database so we need to enter it manually.

The way you it is exactly as the description describes: 1) the place then 2) the bearing from that place then 3) the distance from that place. In this case the place is BEL, the bearing is shown on the chart as 131 degrees from BEL, and the distance is 14 miles from BEL.

Call up the lateral revision page from the named waypoint before D14.BEL. You then enter the P/B/D fix into the scratchpad of the MCDU in the following format:

BEL/131/14

Then press the Next Waypoint LSK and confirm it by pressing the Insert LSK and the fix will be entered into the MCDU and will be shown as PBD01. As you enter more fixes the names will increment: PBD02..03..04 etc. If you delete a fix and re-enter it you will find that the numbering becomes out of sequence, PBD02 before PBD01 perhaps, but don’t worry about this. The airplane will not fly the fixes in numerical order but in the strict order they are in the MCDU. You might not always have the bearing conveniently shown on your chart. In this case I’m afraid it means reverting to your old PPL techniques of using a protractor and a pencil or estimating the bearing. Place-Bearing/Place-Bearing (PBX)

You are likely to use this method of entering a fix far less frequently than the Place/Bearing/Distance method. It is used to place a fix at the intersection of two airways and unless you have en route charts it can be a fiddly process. If you were in the Scouts and did map reading, you may remember this as triangulation. It is also

a method that was used during the second world war to determine a pilot’s location. There might be an DME associated with the intersection so you can enter that as a named waypoint, but if not you will need to enter it manually into the MCDU. When you have done this it gives you a fix named PBX01 (the X quite logically denoting the crossing of two airways).

place bearing distance

Let us suppose that on the way to Belfast we have been routed via the Wallasey VOR (WAL) and then receive the following ATC clearance: “BAW448 at Wallasey turn right heading 2 degrees and intercept A25/W2D to Isle of Man”.

You need to enter the Place-Bearing/Place-Bearing fix from the nearest VORs. In this case you would enter:

WAL-002/POL-289 You would then press ‘the Next Waypoint” LSK and then confirm that with the Insert LSK. After that you would enter IOM as the next waypoint from the PBXxx fix if it was not already in the flightplan.

Look at chart (from FSNavigator) here and you will see that there isn’t anything at the intersection of the A25 and W2D airways, so how can we enter this into the MCDU?

Latitude/Longitude (LL) This is rarely used as it can be difficult to work out exact latitude and longitude co-ordinates. But if you have appropriate charts or FSNavigator you can enter a specific latitude/longitude fix. Looking at

the chart on the previous the interception of the A25 and W2D airways is at coordinates of 53 53.16N/003 12.55W. To enter this as a fix you would enter it in the format:

5353.2N/312.6W Then press the ‘Next Waypoint’ LSK and the co-ordinates will be entered as a fix marked LL01. If you look at the lateral revision page from this LL01 fix you will see at the top of the page that the coordinates it actually puts in are very slightly different: on mine I had 53 15.19N/003 12.59W but I have not yet found a way of entering the coordinates more accurately than this, but the small difference should not cause you any problems.

Note: While writing this guidance leaflet I noticed that if you repeatedly put in several PBX or LL fixes and then delete them and do it again, after a while the database just adds the nearest VOR instead of what you are trying to enter. I do not know if this is a bug with the software but the only workaround I found was to shutdown the flight simulator and reload it. This is something for you to be aware of if you practice entering these fixes.

DME ARCS It is quite likely that at some stage you will need to fly a DME arc as you transition from en route to the final approach fix. As you know, DME stands for Distance Measuring Equipment and the way it works is that the aircraft sends an

interrogation signal to the ground based beacon which responds with a reply signal. The equipment on board the airplane measures the time difference between the two signals and calculates the straight line distance between the airplane and the beacon. A DME arc is a course that the airplane flies at a set distance from the beacon. Normally the margin of error is 1 mile which means you should not fly more than one mile either side of the arc at any time. Looking back at the Belfast Runway 25 Approach Chart we used earlier we entered the PBD fix at D14 BEL but we now need to enter the rest of the arc. Can we do this in one entry? No, we can’t. It would in theory be possible to fly from the D14 BEL point direct to the D12.9 IAG fix shown on the chart but this risks going outside the one mile tolerance of the arc. So the way to enter the DME into the MCDU is to enter a series of straight legs. If you look very closely at the arc itself on the chart you will see that it has been drawn this way rather than as a pure arc. At the end of each straight leg will be a heading change of between (usually but not always by any means) 10 and 30 degrees depending on how shallow the arc is. The more shallow the arc is the lower the amount of heading change required. The decision on the number of fixes you put into the arc rests with you: the more you put in the more closely the airplane will follow the exact arc; the fewer you put in the more you risk deviating from the arc. But you also have to bear in mind that a shallow arc will probably require fewer fixes than a fairly tight arc. 3 or 4 fixes is often sufficient. Some approach charts will give you sensible fixes already marked which speeds entering the data. You enter the DME Arc into the MCDU as a series of PBD fixes. So the entry to the arc is at BEL/131/14 and the last point on the DME arc is BEL/083/14 (it shows this fix as D12.9 IAG but IAG is the ILS DME which you cannot enter that here). So we will enter the arc as follows:

BEL/131/14 BEL/111/14 BEL/091/14 BEL/083/14

The fixes that this puts in are 5 miles apart until the last one which only 2 miles and then the localiser will be captured for runway 25. You can see this in the image below as well as the DME Arc as shown in the plan view on the ND.

Make sure you put in an altitude constraint of 4000 for each of the PBD fixes in the arc until the last one which should have an alt itude constraint of 2470 as these are the altitudes shown on the chart. You can test how closely you fly the DME arc by entering the BEL VOR 117.2 into the FREQ/VOR2 entry on the RAD NAV page of the MCDU (LSK 1R). Make sure that on the FCU you have the right hand switch under the range knob set to VOR so you can monitor the distance at the bottom right of the ND, although you can also monitor it on the standby DME 2 gauge. While you are in the arc the distance to the BEL VOR should be no less than 13 miles and no more than 15 miles (the one mile tolerance). While flying the arc to test this tutorial the distance between the PBD fixes kept at a steady 13.8 or 13.9 miles from BEL, so the number of fixes we put in was right. If you have any questions or comments about this tutorial, please let me know at the email address below. Rob Elliott Airbus Fleet Training Captain British Airways Virtual airbusfleet@speedbirdonline.co.uk www.speedbirdonline.co.uk/airbus.html Version 1: June 2003