These pages are a collection of the ideas and impressions of the
Cardinal pilots who frequent this site. This information is anecdotal
and informal, and may not be completely accurate.
As always, the Cessna operations and flight manual, and the advice
of a certified flight instructor, should be your primary source of
information regarding the safe operation of your aircraft.
Contents:
Planning
Expected descent speeds
Power Setting
Planning
The detail that sets the Cardinal apart from most aircraft
in descent is in the planning. It is very easy to start down
too late and find yourself running near redline on the
airspeed and doing circles while you try to come down.
The trick to managing descent is to make use of the
energy and the low drag of the airframe to give you more
distance for your fuel.
If you fly it like a 172, drive to the airport then start
down, you'll be circling a long time while you lose altitude.
The primary choice in my experience is on how quickly you
are willing to descend. Few passengers like a rapid descent,
and you won't get your best performance with one either.
It is important to consider your passengers as you pick
your desired descent speed. Some inner ears are less
tolerant of rapid descents, and sometime a cold will
change one's tolerance.
If someone does have a cold (or a toothache.. they are
closely connected) you should really be prepared to
level off or even climb a little if a problem crops
up.
Based on my past experience I shoot for a 300 fpm descent.
333 FPM would be 3 minutes per thousand feet. So I decide
how many thousands of feet I need to descend, multiply by
three and start down that many minutes out.
The nice thing about this method is that it compensates
for any head or tailwinds you might have... that's rolled
up in the determination of minutes to destination.
There are three tricks: first, I use the airport elevation for
my final altitude, on the theory that I want to be at
pattern altitude a little ways out.
Second, remember that your minutes out calculation does not
enjoy the benefit of your increased speed in descent.
Finally, whatever winds you are in will change as you come
down hill, which will change your profile slightly.
So this is a start at the proper profile, but there is still
enough variability to impress yourself when you make it come
out perfect.
For example, if you are cruising at 14,500 with a 20
knot tailwind and landing at 850 feet, you need to start
down 42 minutes out, that's about 120 nm from your
destination.
Dave Patterson reports that he used a factor of 4 in his
calculations, because he likes to descend at 500 FPM. On
reflection, it appears that he's using 120 knots in his
descent and calculating the number of miles out to start
down.. 2 miles per minute, 2 minutes per thousand. Using
this method you'd have to adjust for winds.
Typical speeds in descent
I don't have much data in this section, but I hope
that over time we can observe what kind of TAS we
get with different descent rates.
My gut feel is that a 100 FPM descent will give me
at least 10 knots of cruise increase. If I have
a bunch of smooth air below me as I approach my
destination I will often start drifting down on
the theory that I'm getting maximum efficiency.
Any other owners have experience in this area?
Power Setting for descent
The key questions for descent power setting are two
fold: engine heat and turbulence.
Some will tell you that a substantial or sudden power
reduction from cruise will cool the cylinders too
quickly. That may be true, but experts I've read say
that they have never seen a cylinder crack from being
too cool.
On the other hand, an overhaul shop told me that about 75% of Lycoming 360 cylinders they get for overhaul are cracked,
and something is doing that.
Conventional wisdom is that you should reduce manifold
pressure one inch per minute to avoid cooling shock. Personally
I only do that on very cold days.
Turbulence is a more significant factor for power setting.
I usually set my FPM descent, then adjust the power to give
me the cruise speed I need for the air I'm in.
If it's smooth and likely to be smooth to the ground (due
to being night, early morning or over water) I will let it
run right up to redline and enjoy the groundspeed.
In turbulence I'll run at the top of the green. That takes
a fair amount of power reduction.
On a really bumpy day I'll reduce power to maneuvering
speed and hang on. My wife hates it when I tighten my
seat belt before descent.. she considers it foreshadowing.
Once you're in the pattern no matter what setting you
used for descent you'll need to pull it a bit more. I
drop the first notch of flaps as I pass through 130
knots.
As the speed continues down, and I fly level in the
patter, I drop the gear at my self imposed 100 knot
airspeed. I figure I'll have to replace the parts
that hang out in the breeze soon enough.
At this point the descent is over, and it's time for
a landing. Pick that up on this
page if you wish.
Copyright Keith Peterson 1999
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