GPS glide ratio calculations

Andy Blackburn wrote in message ...
Tom,
I took a look at your references -- by N#. The first
and third appear to be low passes - though in both
cases they were in the context of doing low-altitude
aerobatics, rather than traditional 'contest finish'
maneuvers, but I'll let them pass. The reports on the
second and fourth had no references to low, fast passes.
The BG-12 lost a wing at 2000' and the Blanik was just
high on approach. (Excerpts from the accident reports
attached).

Your formula for estimating accident rate by phase
of flight seems ok, but you have to be careful about
making conclusions from VERY low rates of occurence
(i.e. one every ten years). Also, I would disagree
with your estimate of 1,000 operations for
high-speed passes in the US in a year. The Arizona
Soaring Association weekend contest series generates
over 200 contest finishes in a summer - and that's
just one soaring site. Add all the other sites in the
US, plus sanctioned contests and you get a much bigger
number. Finally, you used 9 years for the time period,
but your examples spanned 19 years (1984 to present).
Given the low rate of occurence, I emphasize that calculating
a rate is likely to be misleading. The rates associated
with landing phase would seem to be higher, since there
we have several per year.

In any event, I remain unconvinced that making contest
finishes is inherently dangerous, particularly now
that the evidence in favor of the argument has been
presented.


Those two accidents were mostly caused by a badly botched attempt at a
hihg-speed low pass. The pilot of the BG-12 exceeded Vne; why did he
do that entering the pattern? The only ligit explanation, in my mind,
is that he was starting a low pass and didn't pay attention to
airspeed. The other one was very curious. They described a high pass,
yet he hit A/C on the ground! These two statements are completely
inconsistent:

WITNESSES REPORTED THAT THE ACFT APPEARED TO BE TOO
HIGH, TOO FAST, AND IN A CRAB AS IT APPROACHED THE
RWY. OVER THE RWY THE GLIDER WAS OBSERVED TO ROLL INTO
A RIGHT TURN AS THE NOSE CAME UP. SHORTLY THEREAFTER
THE RIGHT WING STRUCK 3 PARKED ACFT AND COLLIDED WITH
THE TERRAIN.

I concluded that the glider wasn't as high as the witnesses thought
(surprise!) and that he was doing a show off low pass for his
passenger (surprise again).

The accident rate calculation is illustrative, giving an order of
magnitude to the situation. You can adjust the calculation by
assigning an educated probability to the accident falling in this
category; it doesn't change things materially. If you compare this,
small, category to all fatal accidents it is totally out of proportion
by any measure.

Your attitude from the outset has been "show me!". I've been in this
sport a long time (over 20 years), and that's a red flag attitude in
my book. I'm sure you've seen pilots who over estimate their
capabilities, given your resume. I've grown more conservative in my
old age (old pilots vs bold pilots). I say to you, show me how it is
safe, considering ALL conditions, such as pilot capabilities and
surroundings (i.e. glider only A/P vs GA A/P). The bottom line is: is
it worth it? Are you going to assume an avoidable risk vs a quick
thrill? My message isn't to you guys (you're already to emotionally
invested in proving me wrong), its to low time pilots who are
wondering what's this all about. Do I really have to explain what is
wrong with showing off? Are you guys so thick skulled as to not see a
problem with that kind of attitude? If so I hope I don't see your name
on an NTSB report. I have said it befo I would rather have someone
****ed off at me for voicing my oppinion than to stand silent and
watch them kill themselves. Personally, given your background I am
surprised at your attitude. I will consider my comments a complete
success if I have thrown a note of caution into just a single pilot,
or pilot to be, that has followed this exchange.

BTW: Go for the glass!