Picking Optimal Altitudes

"O. Sami Saydjari" wrote in message ...
When planning a flight against the wind, how to you pick the best
altitude when trying to minimize flight time?

If I read my performance charts correctly, my aircraft (Piper Turbo
Arrow III -- service ceiling 20,000 ft) seems to gain about 2 nts of
*true* airspeed for every 1000 feet you go up. Firstly, does that sound
about right to folks? I assume that this effect is from the decrease in
air friction at high altitudes (even though the prop also has less air
to "push on").

So, unless there is some other factor, I think this means that if the
winds increase at anything higher than 2 nts per 1000 feet, I am best
off staying at the Minimum Enroute Altitude. Is that right?

My experience thus far suggests that most of the time, the winds aloft
speed increase far faster then 2 nts per 1000 feet, so, in general, it
is unlikely that I will do much better than staying at the absolute
minimum altitude. Is that consistent with other folks' experience?

The 2 kts per 1000 feet seems about right. The speed increase is due
to the fact that the turbo allows the engine to maintain power output
at a much higher elevation than a normally aspirated engine. The
increased power output coupled with thinner air allows the plane to go
faster.

Generally speaking, the winds above 10k are out of the west, and can
be strong enough to negate the speed advantage of the turbo. On these
trips, it makes sense to fly lower. Going Eastbound, you will
generally want to fly high to take advantage of both the high TAS and
the large tailwinds.

There are exceptions however, so you need to look at the forecast and
PIREP'ed winds aloft as part of your flightplanning to decide whether
or not it makes sense to climb high.

However, you will find many times that having a turbo and oxygen are a
great advantage no matter what the winds aloft. Just last week, I was
on top of a rising cloud layer at 10,000 in my Cherokee, a smooth ride
and in the sun. (The clouds were ~9kft thick at my location.)
However, as I traveled North, I was to pass a weak cold front, and the
temps were already dropping. I considered climbing higher, but had
visions of my Cherokee struggling to outclimb iceladen clouds, and of
course the hypoxia issues as one continues climb... I ended up
descending and flew 2 hrs in bumpy, rainy IMC. Not much fun, but I
kept the OAT above zero and made it to my destination no problems.

If I had turbo and oxygen, I would have zipped up to 12,14,16,
whatever it took to clear the clouds, enjoyed a smooth ride home in
the sun, and an easy descent in the better weather at my destination.

-Nathan