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APP200.TXT
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1995-06-15
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I have had a number of responses from users both on CompuServe and
Internet that are using Approach Control v2.0, and I am taking the time
to write this Technical Note to help answer some of the questions that
have been coming in, as well as discuss some of the ways you can increase
the flexibility of the program.
Remember, if you have any comments or suggestions, I can be contacted
at the addresses given in the README.1ST file in the APP200 archive:
mechalas@gn.ecn.purdue.edu (Internet)
71673,3041 (CIS)
--------------
- The .VOC Files -
The decision to refer to the aircraft as "Flight Simulator" was, to be
honest, made for lack of a better option. In real ATC communications,
you are typically referred to by the type of aircraft you are flying,
followed by your four-digit ident number. For example, "Twin Cessna
4854" or "Learjet 4981". Since there is no way of predictin what kind
of aircraft is going to be flown, I didn't want to assign such names in
the program, and stuck with "Flight SImulator" which is pretty universal.
Those of you with ATP will note that subLOGIC did roughly the same
thing, and to be honest, I couldn't think of a better approach to the
problem than theirs. I am, however, open to suggestions, so if you have
a better name than mine, I'd love to hear them. To be honest, I don't
like being referred to as "Flight SImulator" either. :) :)
- Airport/Approach Control Names with Multiple Names -
Some airports, such as Fort Wayne and Ohio State, have tower or
approach controls that require two words in their name. In the current
version of APP, due to the I/O routines I used, if you try to enter
a two-part name, the space character is interpreted as the termination
of the string. So, instead of reading in "Fort Wayne", the program will
read in "Fort".
One way to circumvent this problem is to use an underscore, and enter
the names as Fort_Wayne or whatever. Then, using a text editor, you can
do a global search/replace in the .AAF file, and change "Fort_Wayne" to
"Fort Wayne".
Any kind of search/replace would work, of course...this is just one
example.
I plan of fixing this little quirk in the next release.
- Modifying the Traffic Pattern -
The idea of being able to change the traffic pattern is probably the
most flexible aspect of the Approach Control program. The default
settings in the .APP files are set up for a pattern that is aimed at
jet-aircraft, but by modifying these values you can re-size the pattern,
as well as alter it's shape, to accomplish any of the following:
* "Tighten" the pattern to allow for slow aircraft
* Roughly approximate a VFR pattern (not perfect, but it will work)
* Modify the flight pattern in case the default vectors have you
flying through mountains or other objects :)
Before we can alter these settings, though, we need a little basic
understanding of how the approach "engine" works:
Your flight pattern is defined as three "fixes", a Final Approach Fix,
and the flight vectors in between them. The FAF is not defined directly,
but rather is indirectly defined by the location of fix number "1", the
fix closest to the runway/approach centerline. The default pattern is
set up *roughly* as follows (as an example for runway 9):
FAF ------------ 9======27 ( <-Runway )
/
/
/
1
\
\
\
\ <- Flight path
\
\
2 ------------------------ 3
This is hard to draw in ASCII, so bear with me. :) The numbers
1 through 3 denote the fixes that you are vectored to, in order. Note
that, depending on your approach angle, ATC may vector you straight to
fix number 2 or 1. If you look at the sample OHARE.APP file, you will
see that the default distances to the fixes are defined as,
87.5322 <-- distance to fix #1
132.4499 <-- distance to fix #2
171.4833 <-- distance to fix #3
Note that all units are in FS Units. 1 NM = 7.2 FS Units (approx.),
so these distances become 12.2 NM, 18.4 NM, and 23.8 NM respectively.
The numbers given below the distances in the .APP file give the default
angles between the respective fixes and the runway/approach centerline.
In OHARE.APP, we see the defaults are:
17.7435 <-- angle between fix #1 and runway heading
45.2737 <-- angle between fix #2 and runway heading
88.6013 <-- angle between fix #3 and runway heading
All angles are given in degrees. Most of you will probably recogize
this as a polar coordinate system, with the origin centered at the
airport, and the "zero" angle being the runway/approach heading. By
changing these values, you can alter the size and shape of the traffic
pattern to fit your needs. It is definitely a good idea to draw your
traffic pattern up on a sheet of graph paper, then calculate the distances
and angle offsets that way (it's how I designed my pattern, which exlpains
the weird numbers/decimal points).
NOTE: The same pattern will be applied to all runways! The only way
to alter the pattern for each runway individually is to go into the .AAF
file and edit the fixes yourself...a tedious but workable process. It
would be too bulky and too complicated to arrange for the program to
create a different traffic pattern for each runway, but if I get enough
calls for it, I'll consider adding the option in a future release.
ANOTHER NOTE: The patterns are "mirrored" about the runway/approach
centerline. So, in the above example, somone approach from the "north"
would go through a mirror image of the drawn traffic pattern.
The FAF is placed so that you intersect the localizer/approach path
at a 40 degree angle (real life ATC uses a 30 degree angle, but I didn't
want any tight turns...this provides for a wider margin of error in the
ATC "engine"). You can modify its distance from the runway by changing
the distance to fix #1.
DO NOT PLACE FIX #1 ON THE RUNWAY/APPROACH CENTERLINE! The program
won't "break" if you do this, but it will bend quite a bit. :) The
adventure generator is assuming that the FAF and fix #1 are different
points, so if you put them in the same place, you may get some strange
results.
Below is an example I posted to CompuServe that gives a rough VFR
traffic pattern (or the closest approximation to one that APP is capable
of). Note that I chose arbitrary distances....those of you with more
realistic goals in mind would want to lay out a more accurate setup than
this one. This example is for demonstration purposes more than for
accurate modeling.
Changing the angles and distances as follows,
52.4167 ( <-- these are the distances to fixes 1 through 3 )
66.3804
61.0942
15.9454 ( <-- these are the angles between fixes 1 through 3
40.6013 and the runway/approach centerline )
135.0
gives the following traffic pattern:
FAF------ 9========27 ( <- runway )
/
1
|
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2-------------------------------3
This gives a 13 NM downwind, 4 NM base with a vector to final, and
a 5 NM final.
Sharp turns will lead to your being re-vectored during mid-turn,
since ATC decides that you are supposed to end up at a certain point.
It calculates the heading you need to fly from your current position,
and as you are turning, that heading may change, leading to a new
flight vector. This is a characteristic of the ATC "engine", and isn't
likely to be changed any time soon as I haven't really come up with a
better system that gives reliable and consistent results.
- Closing Comments -
I hope you are enjoying the program. :) I have had a lot of fun
both creating it and flying the adventures that it builds, and I am
always looking for ways to improve what I have done. Remember to send
me any comments or suggestions that you may have, no matter how trivial.
I can't always promise a speedy reply (emailing me at Internet is
probably the best way to get a hold of me quickly), but I'll do my best
to keep up.
Happy flying!