That is how i used to do it, but the pulling 4" off turns out to be less of a
workload, which makes a difference in IFR conditions especially in the terminal
environment.

The difference is that using power to control the descent, you get a primary
instrument to maintain the descent (MP/RPM), where controlling it with trim you
have to rely on secondary instruments (VSI). Using power to control it, you set
the power to the target setting, wait for the descent to become established, cross
check the rate on the VSI, and then if necessary tweak the power a fraction of an
inch. On mine a 4" reduction gets me a 500 fpm descent regardless of the
configuration (flaps, trim). Once established, maintaining a constant rate
descent is just a matter of moving the throttle to keep the MP pegged (it will
creep up as you descend into denser air). As long as the MP is constant, the rate
of descent is too. Note that the trim sets the airspeed, and in this case you
maintain a constant airspeed throughout the maneuver. Since the power is reduced,
there is no harm in leaving the mixture at the leaned for cruise setting until you
increase power at the end of the descent (in fact it helps to keep the engine
warm). I have only one lever to move throughout my descent, one mixture
adjustment as I level off, and one gauge to look at to immediately see the result
of my moving that lever.

If you use the trim to control your descent, you have no indicator to directly
show the control setting, instead you have to rely on the VSI and the AI for
secondary info. Additionally, you need to adjust the trim as you pick up speed in
the descent to keep the descent rate up. You'll pick up speed both because of the
change in the trim, as well as from the gradual increase in power as you descend
into denser air. Since there is no primary instrument to use in maintaining the
trim, you have to rely on the secondary instruments to adjust the trim based on
the end result. This is a feedback loop and is prone to getting you into a
chasing the needle situation. This method also requires you to monitor and
continuously adjust the mixture because at full power you will need mixture
enrichment as you descend into lower altitudes to keep from running in an
overleaned high power configuration, which is bad for the engine. Also, the
amount of trim required to achieve a given descent rate varies considerably with
airspeed, so you have multiple settings to memorize to fit all the flight
configurations you use. You also need to keep close tabs on the airspeed unless
your airplane is a brick. I have a Cherokee Six, which is by no means a slippery
airplane. Using your technique will put me into the yellow arc in a descent from
cruise, which can be a problem if the air down there is bumpy as it often is.

Newps wrote:

If you don't have a slippery plane why pull the power at all for
descents? In my 182 I close the cowl flaps if they aren't already and
simply roll in two swipes of nose down trim. Leave power as is, which
for me is almost always top if the green throttle and prop or the max it
will give me. Bingo, 500 fpm down. Don't even get into the yellow.

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

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