Descending at a constant speed, rather than high speed |
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I don't think high speed descents are a good idea, both because
of the stress they can place on the airframe and because its important to
maintain flight plan speed both for traffic and regulatory reasons (remember:
IFR pilots are supposed to advise ATC of a change in airspeed?). Just like
APS's engine management techniques, I believe that if you're good to your
airframe it will be good to you: in other words, stay well below the yellow arc
and the airplane won't complain at an inopportune moment.
Here's the technique I use in order to get a constant speed descent and avoid a
high speed descent. First if ATC instructs more than a 1,000 foot descent, I
always come down from cruising altitude at a three degree angle (which is the
standard descent rate for the big boys). To do this, take the ground speed,
divide by two and add a zero: that gives you the descent rate in feet per
minute. Second, to avoid picking up speed in the descent, cut the power back to
19-20 inches MP, which in my SR22 conveniently happens to be just before the
detent. Third, leave the fuel mixture alone until leveling off at or below
4,000 feet, at which point I increase fuel flow to 15.5 gph, which will give me
a 160 KIAS upon leveling off with 2600 rpm at those lower altitudes.
Here's an example: I'm cruising at 11,000 feet with a nice tailwind: going 200
knots ground speed and 140 KIAS. A three degree descent rate (in fpm) can be
computed by dividing 200 by 2 (100) and adding a zero: 1,000 fpm. I set the VS
bug for that descent rate, hit ALT and VS, and pull the power lever back just
below the detent (no need to even look at the gauge: its all by feel, just like
the Big Mixture Pull). Unless the winds change dramatically during the descent,
the GS and IAS will stay fairily constant with the cruising altitude. Within
200-400 feet of level-off altitude, I increase the power lever back to cruise
power (e.g., 2600 rpm).
When ATC instructs a descent of 1,000 feet or less, its not worth playing
around with the power lever and the mixture setting, so in those situations I
automatically use a 400 fpm descent rate. This will lead to a roughly 5 knot
increase in IAS, which is no big deal in terms of ATC or stress on the
airframe.
I rarely fly VFR, but in those instances where I need to compute the distance
from the airport at which to begin the descent, there's another easy math trick
(which is just as good as using VNAV on the Garmin): take the number of feet
(in thousands) that you need to descend and multiply by three. That is the
number of miles, at a three degree descent rate, that it will take to reach the
new, lower altitude. So if I'm cruising along at 11,500 feet and pattern
altitude is 1,500 feet, take the difference in thousands (10) and multiply by
3=30. So at the standard, three degree descent rate, it will take 30 miles to
descent from cruise altitude to pattern altitude. Incidentally, this technique
is also useful when briefing an IFR approach in a mountainous region, where some
of the interim step-downs are not at the standard descent rate, and some
pre-planning is required to descend some segments at a 3.2 to 3.5 degree rate.
I learned these math techniques from an article in IFR magazine about two years
ago, and then played with the power and mixture settings in the Cirrus to get
consistent results. I learned about the utility of these techniques in briefing
an IFR approach in a mountainous setting from the experience of flying into
airports like Charlottesville, VA, Grand Junction, CO and Ontario, CA, where
the STARs, approach plate and ATC instructions require a steeper descent once
past the mountains. I can now anticipate those situations and work out in
advance the descent rate that will be required for a stable approach.
I still get cracked fairings, but its not because of high speed descents.