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Date: Thu, 12 Nov 1998 23:31:49 -0600
From: Snowbird <snbird@ibm.net>
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Subject: Re: Estimating distance
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Mike Regish wrote:
 
> Is there any magic formula for estimating distance form an airport 
> when you are making your call before entering the pattern? I'm talking > in the 3 to 5 (maybe up to 10) mile range when you want to let the 
> airport you're approaching know you're coming. Is it just a matter 
> of experience or is there a way, other than by known landmarks, that 
> it can be fairly accurately judged. I'd hate to call in to a tower and > say I'm 5 miles out and really be 10.

> Any tricks to it or will it just come with time?

Judging distance to an object while you're in the air is
tricky.  I'm not much good at looking out the window and
just telling how far away something is, but there are a couple 
tricks.  It partly depends upon what you're doing.

Are you flying a cross-country to a planned stop?  I like to
pick a landmark which is at a good call-in distance.  If none,
you probably have your ground speed pretty well worked out by 
the time you're approaching your destination, so you can figure 
out when you're 10 miles away (or 5 or whatever) from your ETA
and call in then; it should be accurate within less than a mile.

If you're just out flying, landmarks are really a good thing.  I
calibrate my fingers (know length of my thumb etc in nm) so I
can quickly guesstimate distance from the landmark I'm near to
the airport.  I think controllers appreciate use of charted VFR
reporting points because then they can feel a little more comfortable
that you are where you think you are. 

Another tip is, use the flight vis.  If you're dealing with flight vis
of 4-5 miles in haze (esp. if you've checked this out with landmarks
previously) and you can just see the airport, then it's a good bet
you're just about at the limit of the flight vis. or 4-5 miles away.
If it's hazy and you can see the airport, chances are you aren't
10 miles away.

If you can see the airport, use the runway.  You know the length,
guesstimate how many of the things would fit between you and it.
A 4000 ft runway is 3/4 of a mile.

The more advanced trick is "fun with trig".   For a real explanation
of this, you want to read Stick and Rudder or John Denker's online
book (http://www.monmouth.com/~jsd/fly/how/htm/landing.html). A
couple pictures are really worth more words than I could type, but
the idea is this.  You can use known ratios of your thumb or palm
to arm to tell you the descent angle you'd need to reach an object.
For example, if you hold your arm straight out with the the tip
of your thumb on the horizen, what's on the ground at the bottom of the
first joint is about a 4 degree glidepath.  If you hold your arm
straight out with your palm bent towards you, index finger parallel 
to the horizen and just touching it, what's on the ground at the bottom
of your palm is about a 8-9 degree glidepath.

8 degrees is about a 1 in 7 ratio.  If you're 5000 ft up, what's
at the bottom of your palm is 7 miles away.  4 degrees is about a
1 in 3.5 ratio.  If you're 5000 ft up, what's at the first joint
of your thumb is about 3 1/2 miles away.  You can play around a
bit with trig and the altitudes you usually fly to work out some
useful guesstimates, then play with testing how well they work.

One place where this is useful is judging instantly whether or not 
one could glide to a given spot (you skip the "how far away am I?"
part; most lightplanes can do a 1 in 7 glide ratio even with a bit
of a wind so if it's under the bottom of your palm you can get there
with room to manuever).

Hope this helps,
Snowbird

