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Kodiak,
Alaska, DME arc approach, ILS DME Rwy 25 approachBy Andrew Herd
Standard basic avionics fit plus VOR indicator, DME and optionally an RMI
The first part of this tutorial is written for Steve Small's Cessna 182 RG flight model, although the Microsoft default Cessna 182 RG could be used. The second part is written for an aircraft with an RMI - I suggest the Microsoft King Air, but if you use this aircraft I would recommend using Steve Small's .air file if you want the aircraft to fly anything like the real thing. I appreciate that the King Air is not available in FS2000 standard, but if you are adept at panel programming you could add the freeware RMI from Dai Griffiths to a panel of your choice.
The Kodiak scenery by Dennis Waggoner for FS2000 will add to the experience.
Kodiakarc.zip - extract all these files into your \FS2000\pilots folder
This is a precision ILS DME approach flown via a DME arc. The tutorial assumes that you understand the basics of VOR navigation and can fly an ILS approach.
Please make sure you have Indicated Airspeed (IAS) set. If you have True Airspeed set, the instructions which follow will make no sense at all. Also, ensure that you do not have gyro drift checked under the realism settings, unless you are proficient at making corrections for this. Do not use the GPS on pain of death. If you do not have appropriate approach plates, click on the image at the top of this page to view a Final Approach plate courtesy of Georges Lorsche.
Situation 1: Precision approach via a DME arc ILS DME-1 Rwy 25 using VOR indicator - select situation Kodiak182arc
To
the non-initiated, flying DME arcs is in the same ballpark as practising alchemy.
Sometimes you will see a flight simmer tracing one of these graceful lines across
an approach plate, but seldom does anyone ever fly them because they look so
tricky. The truth is that flying a DME arc is no more difficult than making
a good cup of coffee, and the only problem is that you can't do both at the
same time, or at least, not unless your aircraft is equipped with a Flight Management
Computer (FMC).
Apart from being invented to test your and my patience, DME arcs exist in order to shepherd aircraft from the en-route structure to the approach course without a particular need to overfly an on-field VOR. As you can imagine, this makes arcs a popular method of approach to heavily built-up areas, or where high ground needs to be avoided, but they can be found on all sort of approaches. The one compensation there is for having to fly around the arc is that by definition you don't have to fly a procedure turn, as the arc should line you up to fly straight down the ILS.
DME arcs are usually depicted with their IAFs hanging in the middle of nowhere on approach plates, but this is an illusion - the IAF on an arc procedure is almost without exception located within the en-route structure or else there would be no way to fly there. I have no low-level en-route charts for Alaska (if someone wants to update me here, be my guest), but my guess is that the IAF on the Kodiak arc lies along the V439 airway.
The situation places you in the Cessna, 7.5 nm outbound from the ODK VOR at 3200 feet. ODK is the navaid on which the DME arc is based (before you ask, RWO is not a possibility, because it is an NDB. You can't fly a DME arc based on an NDB. Why not? They don't have DME facilities). There is light cloud and no wind. The radios are tuned, with the exception of the ILS at Kodiak. If you are flying using the DreamFleet RG panel, you should tune the spare NAV1 frequency to 110.9, if you are using the Microsoft plane, tune NAV2 to this frequency as a reminder. Trim the aircraft and prepare to make a turn to intercept the arc when the DME reads around 9.2.
While flying a DME arc armed only with a VOR indicator isn't easy, the rules for doing it aren't too tough to follow, assuming you use the right method and you start with the heading indicator synched to the compass - this is a big trap if you have the gyro drift box checked in Flight Simulator. This is one approach where you may wish to use the autopilot to get around instead of flying it manually, I leave it up to you. One school of thought says you fly the approach like this:
1. set your OBS to a radial 20 degrees ahead of your current position
2. set the heading bug to the bearing of the radial you are currently on, plus 100 degrees
3. when the CDI (the needle on the VOR indicator) centers, repeat the process
To
put this into English, we are flying out on the 335 radial from ODK, so to use
the method above, you would need to set your heading bug to 075 degrees (i.e.
335, our current radial, plus 100 degrees), twist the OBS to 355, then fly on
a heading of 075 until the CDI centers, then twist the OBS to 015 (355+20) degrees,
set the heading to 095 (355+100) and fly that heading and so on. The trouble
with this method is that all the calculation involved saps your will to live,
and don't forget that you have to watch the DME and altitude as well. Sooner
or later you run the risk of adding the wrong numbers together. The other problem
is that if you are within 15 miles of the VOR, you may need to add more than
100 degrees to your current radial each time to stay within the arc; perhaps
as much as 125 degrees, and it gets tough staying in command of the figures.
If you love math, you have a USB port in your forehead, and one of your relatives
is a Cray supercomputer, this is the method for you.
The "easy way" is to begin with the same method, setting the OBS to the radial you are on plus say 15 degrees (at 10 nm we are quite close to the VOR and shorter arcs work better) setting the heading bug to 075 degrees (i.e. 335, our current radial, plus 100 degrees) as before and making the turn. But this time, when the CDI needle centers, twist the OBS on another 15 degrees, and add 15 degrees to your heading - you are way ahead of me, I am sure. The advantage of this method is that you only have one number to add and there is much less room for error. Not only that, but corrections are easier to figure out, too. If the DME increases to 10.5 nm, you alter course by 25 degrees, rather than 15, if it goes down to 9.5 nm, alter course by only 10 degrees and so on - all you have to do is remember to keep twisting the OBS around 15 degrees every time the CDI centers.
If
you look carefully at the plate, this arc is slightly complicated by the fact
that you need to maintain 3000 feet until you cross the 360 radial from ODK,
so maintain your present altitude for now. Maintain a heading of 075, and watch
the CDI slowly swing to the centerline. While you do this, you need to watch
your VSI, the DME, and altitude. If you have any spare capacity, you may like
to spin some plates at the same time.
By the third time you have tried to get this arc right, you will appreciate why it is generally not recommended to fly them with a VOR indicator - there is just too much to do. In rough weather, a DME arc in the Cessna is an interesting exercise, as you can imagine.
If you look at the screen shot, you will see what the panel looks like when you goof up and begin to stray outside the arc - alert readers will already have spotted the heading bug, which I have set to 070 instead of 075 in all three of the illustrations above - which is why the DME reads 10.1. I have altered course to 080 to make up a little bit of ground.
Once you have crossed the 360 radial - remember you work this out from the VOR1 gauge, not from your heading - you can descend to 1600 feet. Reduce the RPM to 2000 and manifold pressure to 18 inches or less and start a gentle descent, watching the airspeed doesn't build up too much. Remember to keep watching the CDI and your heading, and keep twisting the OBS every time the needle centers and altering course to follow it.
If
you pause the sim and take another look at the plate, you will see that the
line of the DME arc ends in an arrow just after it crosses the 068 radial from
ODK (on the Jeppesen plate the arrow is turned in somewhat to give you a hint).
This is the point where you begin the turn to intercept the ILS; so the last
twist of the OBS should be to 068.
When the CDI centers on 068, it is time to say goodbye to the arc. You need to twist the OBS to 249 (in Flight Simulator, 250), and tune the ILS frequency. In the DreamFleet 182RG, you just swap frequencies on NAV1 if you tuned them earlier. Once you have the ILS dialled in, the needles should spring to life. If you adjust your heading to around 205, you should intercept the localiser gently enough to capture it without any problems. Once you have the glideslope, configure for landing and you should be set assuming you can make the minimums.
One other point - the approach plate states that the ILS is unusable within D2.0 of IADQ. This isn't a problem in Flight Simulator, but if you want to go the whole nine yards, you might bear it in mind. Now lets do it with an RMI.
Situation 2: Precision approach via a DME arc ILS DME-1 Rwy 25 using an RMI. This situation requires the King Air, or another aircraft fitted with an RMI. Select situation Kodiakkingairarc
The
Microsoft King Air is equipped with an Radio Magnetic Indicator, a device that
might have been specially invented to fly DME arcs. Essentially, the RMI card
remains aligned with magnetic north, so that the card appears to rotate as the
aircraft heading changes. There are two ADF needles on an RMI - in this case,
a yellow one which can be tuned with the ADF, and a green one that tunes with
the NAV2 radio. On a real aircraft RMIs are a bit more flexible than this, but
we only need the green needle for now. The other difference between the King
Air and the 182 is that the Beech has an HSI - I am assuming you are familiar
with this instrument. If not, please see my
glass cockpit tutorial.
The King Air is much faster than the 182, and Microsoft hasn't helped by giving their FS2000 turboprop model an idle that is too high. The result is that it is tough slowing the bird down. If you use Steve Small's .air file it takes some of the game out of its tail, but it is quite a tricky aircraft to handle and in level flight it never seems very happy under 160 knots, which is a significant constraint. The trouble is that to get the most out of this exercise, we need to go round as slowly as we can. I suggest reducing speed to about 140 knots, which is close to final approach speed - so I recommend cutting back to 1250 RPM, 50% torque, dropping some flap and maybe throwing out an anchor if you have one. I have included a table on King Air settings if you need to look anything up.
I
have placed the aircraft much nearer to the ODK VOR than the Cessna scenario
in order to give you time to get it settled down. Check that both NAV radios
are set to 117.1 - the RMI needle should be pointing downwards. Check your alignment
or uncheck the gyro drift setting under the realism settings, because we need
all the cards pointing the same way. If you aren't familiar with the King Air
you might like to use the autopilot to give you time to think.
Maintain a heading of 335 and an altitude of 3000 feet until you get to 9.0 DME, then turn to 075 degrees. Watch your altitude and the DME readout on the HSI. When you straighten up, you should be at DME 10.0 or less and the green needle should be almost horizontal - ignore the yellow needle. Fly this heading until the green is pointing ten degrees below the horizontal, at the first large white division on the card.
At this point, your next alteration of course depends on how fast you are going and what the DME is saying. In the Cessna, we could get away with 15 degree alterations of heading, but in the King Air, 20-30 degrees is necessary on this rather tight radius if you aren't to trespass outside the magic circle. So if the DME reads, say 10.3, when the green needle reaches 10 degrees below the horizontal, alter course about 25 degrees to starboard. If you get it exactly right, as you roll out of the turn, the RMI needle should be pointing about 5 - 10 degrees above the horizontal, and if you keep on your heading, the needle will fall below the horizontal, until it gets to 10 degrees under, and then you repeat the process.
This
is great, isn't it? The first time I used an RMI, I nearly flew around a complete
circle before I woke up. The RMI is a truly wonderful instrument, and when you
get used to it you can more or less fly the arc using continuous correction,
but we have to get down sometime, don't we?
The next problem is this: how do we work out where we are on the arc? We have to descend after the 360 radial, remember? On the Cessna, it was obvious which radial we were on, because all we had to do was read the OBS setting, but the RMI, for all its subtleties, doesn't tell us which radial we are on. The solution is to twist the course selector on the HSI to 360 and watch the needle. In the screen shot at right, the King Air has just passed over the radial and we can descend. Good job we remembered, huh? The next radial that makes a difference is 068, which is our cue to turn in to the localiser. It might be a good idea to turn the HSI course setting to 068 now.
Keep on flying within 10.0 DME by adding twenties and twenty-fives until the HSI needle centers. Then you need to twist the course on the HSI to 250 and tune NAV1 into 110.9, so you can track the localiser.
The
screen shot on the left shows the situation as you begin to approach the end
of the DME arc. You have just completed another segment and the RMI needle is
ten degrees forward of the horizontal, with DME reading 10.2, which will fall
to 10.0 if the aircraft maintains its current heading. It is time to descend
to 1600. The King Air should pass the 068 radial in another couple of "bites"
of the arc, and it is time to think of getting ready to intercept the localiser.
One of the few things that you can get really wrong if you get round this far on an arc is to intercept the localiser at too great an angle. If you do this, you will fly right through the beam before you can catch it, and all that hard work on the arc will be forgotten as you fishtail desperately around trying to get back on course - so aim for an intercept of less than sixty degrees and you should be all set.
If you aren't happy with flying a glideslope, please read my ILS DME approach tutorial.
Andrew Herd
