McCready Default on Flight Computers
To continue the discussion about flight computers, I have found a number
of recent instances that suggest some do not understand how final
glides work and what are appropriate safety margins. Beyond education, a
solution is to adjust the defaults in flight computers.
Modern sailplanes can glide a long way as they are really efficient
flying machines.A high performance glider descends at around 1.5 mph
while cutting through the sky at about 60 mph. The manual would suggest
that from a mile in the sky, the glider would be able to glide 40 miles.
As the setting is usually set to MC 0, this what the default setting in
the flight computer assumes, corrected for wind speed and direction.
However, reality cannot be all the more different. Since the default
does not necessarily have a built altitude margin, it assumes that the
pilot will arrive with no altitude to spare. With very limited
exceptions, this is very unsafe as it gives no time to prepare for a
safe approach or a landing. Furthermore, with little or no margin on
arrival, it takes very little sinking air to prevent the glider from
arriving. This puts the glider in a “coffin corner” wherein the pilot
finds himself below 300ft above the ground, unable to make the airfield,
having to make a very hasty and dangerous landing in a field. A number
of pilots have been injured or killed in such situations. The result is
that we must expect to arrive with a reasonably good margin, usually
1000ft above airfield.
The second concept is that how safe the glide angle is depends upon the
volatility of the air. What goes up must come down. The more of the air
is going up, in order for the entire atmosphere to conserve mass, a
similar amount of air must come down. (This disregards pressure systems
influencing subsidence, which could also influence the net amount of
movement in the air.) As John Cochrane points out in his well-written article about McCready theory as it applies to safety glides,
“How bad is [the] glide in sink? The answer is terrible. You fly through
steady 1 knot sink, your speed director will tell you to speed up to 65
mph. You will achieve a L/D of 27. Yes 27. You fly through steady 2
knots sink, your speed direction will say to speed up to 73 and you will
acheive a 19:1 glide.”
Bearing in mind this an ASW27, a 15 meter class racing sailplane. Higher
performing gliders respond even more sensitively to sinking air and
lower performing gliders less so. In any case, one can tell that the
performance drops very dramatically when there is any sink. As the day
becomes more and more convective and volatile, the odds of being able to
find air that is neutral or positive over a long stretch on final glide
become more and more slim. The solution is to have a steeper glider
built in as a safety margin, which can be achieved by putting in a
higher MC setting.
From a behavioral perspective, the problem is that some novice pilots do
not understand these fundamental concepts and are at high risk of
making a marginal final glide. As these pilots begin to fly and venture
further and further from the airport, they may find themselves at a
point where they feel they must make a low approach back to the airport.
Usually these experiences are very stressful as a pilot with less
experience is also one that has probably not landed out before. When a
pilot has only landed at home base, there is an inherent complacency
built into the pilot’s head that, “I always made it back, so I will now
too.” Lastly, since the flight computer says he can make it, that only
reinforces the belief that he can. All of this points to that pilots who
understands final glides least is least likely to adjust the defaults
and fly a glide that has an appropriate safety margin.
There is no question that pilots should be educated about final glides
and MC theory through the soaring community and mentors within their
clubs. However, by also adjusting the default parameters built into the
flight computers, it would be possible to greatly increase the safety
of those who are ignorant of the risks they are taking.
Now the question is what would be an appropriate default setting? On one
hand, it is possible to set a default that is extremely conservative.
However, this would make the setting less meaningful. On the other hand,
by setting a default too aggressively, which is the case now, it could
lead to pilots unknowingly taking large risks. The balance is in the
middle.
I think that the most appropriate default setting is MC 4, 1000ft
arrival height. I believe that this setting would be an acceptable glide
for most thermal soaring locations. My guess is that it would be
possible to make 95% of final glides with this configuration, if not
more. Secondly, it would be a meaningful measure of a reasonably safe
glide margin for most pilots who are starting to explore the outer
boundaries of gliding distance from their home field. This is not to say
that a MC 4 final glide will always be sufficient for all sets of
conditions. Pilots must use good judgment and must be aware of the risks
associated with final glides and how to assess those decisions. But at a
minimum, pilots who are the most ignorant and at most risk would be
anchored to a set of numbers that have a much greater probability of
resulting in a safe final glide. In the spirit of choice architecture
wherein the programmer must decide what the default must be, choosing
the parameter that a higher probability of working is the most ethical
and safest choice.
Soaring Economist 