Plane & Pilot
Tuesday, September 3, 2013

The Lowdown On Go-Downs

Descents should be more than simply throwaway methods of losing altitude

Some airliners have enough lights to resemble the alien spacecraft in Close Encounters of The Third Kind, and they also run their lights in the daytime, as well as at night. Perhaps surprisingly, airliners have another advantage over general aviation: superior glide ratios. A typical Bonanza/Mooney/Centurion will score a glide of 10 to one, 10 feet forward for one foot of altitude lost. Airliners benefit from glide coefficients of 15 to one or higher, especially the jumbo Boeing 747—it scores 18 to one.

In cruise configuration, airliners are remarkably efficient machines. That's one reason you'll note those dramatic power reductions when you're riding on Airbus or Boeing as it begins descent from the flight levels. The power rarely comes all the way back to flight idle, but you can't help but notice that the airplane has definitely started down.

If you live near a major-airline airport, you may notice the string of big jets on the profile descent, each following the other by about five miles. More than coincidentally, ATC tries to sequence aircraft on roughly a three-degree glideslope. That dovetails nicely with a typical ILS approach when conditions are inclement.

Some pilots of piston aircraft like to leave power pumping at 75% for the letdown to recover some of the speed lost in climbing, but that doesn't always work. Turbine aircraft that fly tall in thin air may benefit from strong tailwinds that are reduced or disappear completely at lower altitudes. Even pilots of piston equipment need to consider that in determining an appropriate descent point.

Pilots who regularly operate in positive airspace above 18,000 feet can expect more professional treatment from ATC, and the controller will expect no less from them. You may have limited options in your choice of descent rate and altitude when ATC needs to juggle airplanes in the middle-altitude segment.
Intelligent decents aren't really that challenging for pilots who are simply awake. There's no great mystery to bringing off a smooth, professional letdown with minimum discomfort for your passengers.
Under the best circumstances, ATC may lead you by the spinner right onto the ILS. In the worst case, you may be held 20 miles from your destination and given no further clearance time. Once, during thunderstorm season off Brisbane, Australia, Brisbane Control had me orbit out over the Great Barrier Reef in a Grand Caravan for an extra hour, descending a few thousand feet in irregular intervals while they sorted out problems with too much traffic. Obviously, any calculations of descent rate becomes moot in such situations.

If you're flying into a controlled airport, you may need to aim for specific ground entry checkpoints that could frustrate your calculations. Additionally, if you're flying IFR, you'll be subject to ATC's guidance and, as mentioned above, that may have nothing to do with an efficient descent. ATC will most often ask for a faster descent rate than you'd use under VFR conditions. It's unusual to be allowed to descend all the way to the airport at pilot's discretion.

Other factors may dictate additional variations. If it's summer and you're descending to a hot destination, you may want to stay higher longer to avoid the thermal chop and keep your passengers as comfortable as possible, then dirty up the airplane and drop down at a fairly rapid descent rate when you're in close. If there's an inversion working where temperatures are actually hotter aloft than on the ground, you may once again elect to descend in steps, a slow rate initially and a faster letdown in close.

Similarly, if you suspect icing in the clouds below, you may want to wait as long as possible, then descend with a rapid vertical rate and slow airspeed to minimize ice accretion.


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