3:5 descent profile.

[dashtrash400]

Prior to Horizon, I eschewed so-called rules of thumb when it came to descent planning. I'd simply figure out how many miles per minute I was doing, and multiply that by each 500 feet of descent. IE 150 kts & 3000 ft to lose, start down (2.5 x 6 = 15 miles) early.

Now I deal with bigger numbers, though. 400 kts and 25,000 ft to lose, I get lost somewhere between subtracting 360 and carrying the 3. And VNAV, of course, is for wussies. So early on a captain reintroduced me to the 3:5 descent profile. You simply start down 3 miles out for every 1000 ft to lose, and descend at 5 times your groundspeed. In the above example, I'd start 75 miles out, and descend at 2000 fpm. This usually works out perfect.

Anybody else use this profile? What kind of aircraft do you use it for? Does it work well for you? I tried it in the Chieftain but it doesn't quite work out due to shock-cooling considerations (end up at rather low power settings towards the end of the descent, & I'd rather end up doing the majority of my descent there to keep my airspeed up)..

[Glenn Calvin]

We descend at flight-idle on the 737 at 2:1, 2.5:1 or 3:1, depending upon the model and the airspeed in the descent. At 280 knots indicated, in the -300/-500 the 2.5:1 rule seems to work better. In the -700, 3:1 seems to work best at speeds between 280 and 300 KIAS. We add 1 mile for each ten knots of airspeed we need to slow to make a restriction and also add 1 mile for each 10 knots of tailwind (subtract 1 mile for each ten knots headwind).

For instance, descent from 36,000 to a fix with a crossing restriction of 10,000 feet at 250 knots, 300 knot descent, 20 knots tailwind : (26,000 feet)/1000 = 26 x 3 = 78 + 5 + 2 = 85 miles to descend and descelerate to cross the fix at 10K and 250.

Clear as mud?

[dashtrash400]

Yeah Glenn, I guess my descent profile would actually be a 3:1, used just as you described. The 5:1 part is for determining what descent rate to use, based on groundspeed, since our initial descents are rather seldom at flight idle. So I call it a 3-5 descent profile.

[check6]

DashTrash,

I've been taught to use the 3 to 1 rule but the 5 times GS is new(I'll try it out on flightsim=). This would be in a Cheyenne. The descent rate is usually 2000fpm.