Flying DME Arcs.

Firstly let me say that I have no experence in flying 747s the following is based on my own light aircraft experience and what I have read in books. I believe that the basic principles will be the same although the actual techniques could be different.

DME arcs are flown as a series of segments attempting to keep the aircraft within +/- 1nm of the arc distance. This may be my first mistake, I guess that with LNAV the 747 could stay on the arc by continually changing the aircraft heading. This however is not a problem for PS 1.2 users as the arcs that are constructed using HH's approach codes can only be done using segments. It's also worth noting that in flight if the crew needs to construct an arc they can only use the segment technique.

To enter and exit a DME arc you have to anticipate the turn. If you wait until the arc distance to turn, you will overshoot. The lead distance depends on the arcraft speed, about 2-3nm seems OK for 210kts. Another cavet, I have no idea of the correct speed profile for the 747. I mentioned 210 kts as it seems a good compromise speed. You don't want to spend forever on the arc ( they can involve a lot of track miles), conversely you need to have time to slow the aircraft for the final approach.

 On transitioning from the arc to final approach, if you don't turn until you are established on final you will overshoot. Many approach plates have published lead radials, these are the points on the arc where you start your turn to intercept final. If nothing is published you must work something out yourself. I was taught to divide the arc distance into 30 to get how many degrees before final track you should start your turn. These results however seem be a bit tight for a 747 so you'll have to play around to get an answer you're happy with.

Assuming you are tracking the aid that the arc is based on, then the number of degrees you turn to join the arc depends whether you are inside or outside of the arc. If you are inside (tracking outbound from the aid) then you turn 100 degrees. If you are outside turn 80 degrees.

Another point to consider is waypoint types. Arcs from STARs or Instrument Approaches are created by the software companys (for PS 12 it is us designers).These waypoints are flyover waypoints (that is the aircraft will fly over the waypoint before turning to the next one) or flyby waypoints (the aircraft will turn towards the next waypoint before it gets to the current waypoint). PS 12 designers can only create flyover waypoints. In flight the crew (PS also)can only create flyby waypoints. Why am I telling you all of this ? You need to know whether your first waypoint on the arc is flyby or flyover. If it is flyby great, the aircraft will lead the turn onto the arc. If it isn't you will overshoot. So you will need to either manually turn the aircraft early or replace the flyover waypoint with a flyby one (in the LEGS page).

Waypoint naming. I think this is correct. DME arc waypoints are named with the letter D then three numbers representing the radial the waypoint is on and finally a letter representing the DME distance (A=1DME, B=2DME etc.). So a waupoint D305J would be a DME arc waypoint on the 305 radial at 10 DME.

Now that we have turned onto the arc how do we stay there ? In my real flying I break the leg into 10 degree segments, but for the 747 15 degrees seems OK. This is good because in designing arcs in PS 12 15 degrees gives a nice looking arc.If you are using LNAV it is easy. Once you have got the 747 onto the arc it will happily track around the arc following the segments.

If you want to fly the arc without LNAV read on.

1. Make sure that you have the VOR that the arc is based on dialed up. I use both VOR 1 and 2. I use one to track around the arc and the other I leave set up with the final approach track. 
2. Antcipate the turn onto the arc by about 2-3nm. Turn either 80 or 100 degrees depending on where the aircraft is.
3. On the NAV RAD page enter a CRS that is 15 degrees ahead of the aircraft.
4. Hold your heading until you reach the selected radial, then turn the aircraft another 15 degrees.
5. Now enter a CRS that is another 15 degrees away.
6. Hold your heading until you reach the selected radial, then turn the aircraft another 15 degrees.
7. Keep repeating this process until you reach the lead radial and then select a heading to intercept final approach track.
8. Remember you are trying to keep the aircraft within +/- 1nm of the arc. If you are starting to go wide make the next turn 20 degrees. If you are cutting in, make it only 10 degrees.

As an illustration you might like to try the following. I have enclosed three freeze files DMEarc1, DMEarc2and DMEarc3. Make your own speed,flap and gear changes as desired.
 
DMEarc1:

Just replace the first arc waypoint D006L with a flyby one using the place/ bearing/ distance format - HB006/22. Enter it in L1, delete D006L and make D350L the next waypoint. Re-enter 5000A on RHS. (HB is 112.7) Execute, and let LNAV do the rest!

DMEarc2:

You are inbound to Hobart on the 006R at 80DME. ATC clear you via the Hobart 22DME arc for the 12ILS. The arc is not in your database.
In this example you have enough time to construct the arc. A look at the charts tells you that the arc starts on the  006R and finishes on the LLZ with a lead radial of HB 305. Altitude on the arc is not below 5000'.

Create a waypoint at HB006/22 and enter it into line 2L on the LEGS page. Now line select it to L1 and execute. Since it is created in the air it is a flyby waypoint so that you know the aircraft will turn early to follow the arc.
Now create a series of waypoints 15 degrees apart to describe the arc and enter them in order into the LEGS page. I find it helps it you keep the numbers easy so I'd make the next HB350/22 and then HB335/22 etc. finishing with HB305/22.The arc turns you onto final prior to TTR so make that the next waypoint, deleting GS-12. Add 5000A to the RHS of all the arc waypoints. Execute, and let LNAV do the rest.

DMEarc3:
 
You are inbound to Hobart on the 006R at 30DME. ATC clear you via the Hobart 22DME arc for the 12ILS. The arc is not in your database.
In this example you do not have enough time to construct the arc. A look at the charts tells you that the arc starts on the  006R and finishes on the LLZ with a lead radial of HB 305. Altitude on the arc is not below 5000'. Note there is no wind in this example

I have done a bit of the preparation already in this example. The NAV RAD page is open, the HB VOR is selected and the first arc radial and final approach radial are entered (Another technique might be to enter the lead radial instead.). Since I am inbound to the aid I know I have to do an 80 degree turn onto the arc. I have already entered 266 into the heading box.

1. At 24DME press the HDG knob so that the aircraft commences a turn onto the arc.
The chart tells me that the arc will put me on the LLZ outside of TTR so line selecte it to L1 on the legs page.Execute. Now return to the NAV RAD page.

2. Monitor the distance from HB when the aircraft is on the 266 heading and approaching the dashed 350 radial line (actual aircraft radial is shown on the NAV RAD page). If the aircraft is within  +/- 1DME of the arc all is OK. Otherwise adjust your next heading change by +/- 5 degrees.

3. On the 350R alter heading left by 15 degrees (+/- 5 degrees if needed).To keep the numbers neat I'd use 250 as my heading. Alter the course on the NAV RAD page by 15 degrees (335).

4. Repeat steps 2 and 3 until the last radial you select is the lead radial (305).

5. As you reach the lead radial either reselct LNAV or select an intercept heading for the LLZ (about 150 should do). Carry out the ILS approach.

You can try this example with different winds to so how the aircraft is effected.


Hope all of the above is of some use and there are not too many errors (both typo and technical). Pleaes let me know of any changes that I need to make.


Peter Franklin
pjfrank@gil.com.au

24th January 1998

