Descending at a constant speed, rather than high speed



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.