Descent techniques

russfarris · Post by **russfarris** » Sun Feb 20, 2005 4:07 am

Related to the long-range discussion, but I've broken it out for future searches.

Descending from cruise altitude - for years, in smooth air I maintain cruise power and let the airspeed increase while coming down at 3-500 FPM. But recently I've seen some discussion on the Cessna Pilot Association board that claims this will burn more fuel overall; i.e. the decrease in flight time will not reduce the total fuel burn as effeciently as reducing power and making a constant airspeed descent. Now, with a jet airplane, this prodedure would be correct but I'm not so sure in a fixed gear single like the 170 you could measure the difference.

So I'm throwing this one out there as well. What do you guys think? Russ Farris

zero.one.victor · Post by **zero.one.victor** » Sun Feb 20, 2005 5:41 am

How much of your flying time is spent descending from altitude? I think the fuel saved or burned in one descent method versus another is pretty negligible.

Eric

blueldr · Post by **blueldr** » Sun Feb 20, 2005 6:42 am

I;ve never really thought about fuel burn in the descent. I usually just maintaim cruise speed and throttle back to a 3-500
ft. rate using time to go vs. altitude to lose to find the miles out to start the descent. It doesen't seem to me to be a significant matter in my type of airplane considering the time involved. A descent of 15 or 20 minutes at a slightly reduced power setting isn't going to amount to much on the average.

lowNslow · Post by **lowNslow** » Sun Feb 20, 2005 5:07 pm

Russ, are you saying you don't touch the throttle at all for descent? It seems if you were running at 2500rpm for cruise and you started a 500fpm descent the rpm would increase to close to redline. I generally do as you suggest, but keep the rpm the same which requires a power reduction.

buchanan · Post by **buchanan** » Sun Feb 20, 2005 5:52 pm

If I remember correctly, parasite drag squares as speed doubles so the faster you go; even down-hill, the more drag you have and the more drag you have the more fuel you burn.

Buck

Post by **GAHorn** » Sun Feb 20, 2005 10:43 pm

Russ, I believe that for maximum range a reduction in power commensurate with altitude is appropriate for the 170 for exactly the same reasons as it is with the jet.....difference being that we can better keep our recip engines leaned. (Lower power with a tailwind, higher power with a headwind, etc.)
Aerodynamics for Naval Aviators addresses this issue I believe. (Got my copy loaned out just now.)