Wednesday, February 1, 2006
The Lowdown On Descents
There’s a right way and a wrong way to bring your airplane down
Controllers don’t differentiate between turbine and piston airplanes in the flight levels, and that means they sometimes may assume your airplane is pressurized and that high descent rates are no problem. You’ll often hear controllers ask you to increase your descent rate, but you’ll rarely be asked to slow your letdown. In 30 years of operating turbocharged, turboprop and pure jet aircraft in the flight levels, I’ve probably been asked to expedite my descent a hundred times, but I’ve rarely been asked to reduce the rate.
There’s a noticeable difference in descent planning from the flight levels and letting down from a mile or three MSL. For one thing, winds are less of a factor down low, so you’re likely to see a slight improvement in groundspeed at modest heights and exactly the opposite while descending from high altitude.
When operating at a comparatively low level and flying VFR in a typical 160-knot, single-engine retractable, calculating the vertical descent point can be a simple matter of dividing the altitude to be lost by your chosen descent rate, comparing that to your expected average descent airspeed and, assuming there’s no topography in the way, starting down at the appropriate time. For example, if you’re cruising at 9,500 feet and planning a 500 fpm descent to a pattern altitude of 2,000 feet, five miles from the airport, that’s 7,500 feet to lose (15 minutes’ worth at 500 fpm). If you realize a 20-knot cruise improvement (consistent with Vno limits, of course), you’ll want to start down about 50 miles out.
In the VFR world, most general-aviation pilots like to be down to pattern altitude at least five miles before the airport, so it’s necessary to add that to your distance calculation. This gives you time to stabilize and prepare for the approach, even if it’s just a standard 45 entry to a downwind, base and final. Another benefit of getting low early is that it’s far easier to spot traffic at the same altitude or even a little above you against a featureless blue sky than trying to pick it out of a mishmash of houses, freeways and shopping centers below.
As mentioned above, descents from the flight levels can often result in a slower groundspeed. If that seems contrary to common sense, remember that jets, turboprops and turbocharged piston airplanes are much more efficient in the thin air above 18,000 feet. Similarly, a fringe benefit of high-altitude flight can be tailwinds. For these reasons, pilots of turbine equipment prefer to fly high as long as possible. Controllers oblige with their own “keep ’em high” policy, typically assuming pilots will oblige with descent rates of at least 1,500 fpm.
When you’re flying VFR at 17,500 feet or less, there are a couple of methods you can use to figure your descents. One is to simply choose a descent rate (500 fpm in the example above) and back in to the descent point.
Another is to assume a given ratio of altitude to forward distance. The airlines like to assume a 3:1 ratio—three miles of horizontal travel for every 1,000 feet of altitude loss. More than coincidentally, that’s roughly a three-degree glide-slope and will dovetail nicely with a typical ILS approach.
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