Tuesday, September 3, 2013
The Lowdown On Go-Downs
Descents should be more than simply throwaway methods of losing altitude
Sometimes, it seems that descents are the forgotten flight segment. What goes up…The question is how intelligently pilots will return their airplanes to Earth.
There are at least two classes of aircraft for which descents take on special meaning—or no meaning at all, depending upon your point of view. Jet fighters burn such huge amounts of fuel and offer such limited endurance that pilots need to minimize time spent climbing or descending, so it's no surprise that they do both expeditiously.
I flew the then-McDonnell Douglas F-15 Eagle 35 years ago, and my pilot insisted that most major altitude changes in fighters be very quick, typically on the order of at least 10,000 fpm, up or down. Unlike most general aviation airplanes, the F-15 offers near-total control of the vertical element, a good thing since it's easier to counter an air-to-air threat from high altitude. The Eagle can bust the Mach in a vertical climb with a light load. (Don't even bother to check. It's over 60,000 fpm.) For descents, the usual combat procedure is to roll inverted, extend the huge spoiler behind the cockpit to avoid building speed and go straight down at 30,000 fpm or more.
Dissimilarly, at the opposite end of the scale, true puddle-jumpers such as Champs, Chiefs, Cubs, T-crafts, Ercoupes and 140s, rarely fly much above 2,000 to 3,000 feet AGL, down where pilots can smell the roses (and the cows). Planning descents usually isn't much of a consideration for those aviators.
For the majority of piston drivers, descents should be more prescribed and deliberate, and there are probably dozens of methods of returning to Earth. The simplest solution is to choose your desired rate of descent, most often 500 to 1,000 fpm, divide that into the difference between pattern altitude and your cruise height, and that will dictate the startdown point in minutes—almost. (You'll obviously need to verify that there's nothing to hit in between.) Correlate that with speed, check GPS, DME or RNAV (remember those?), and you'll know how far out to start down.
Since you probably don't want to arrive at pattern altitude directly over the airport, add whatever standoff distance you'd like, usually five or seven miles, and that will move your descent point farther out. Alternatively, if traffic is landing from the opposite direction, you may want to overfly the airport for an approach back toward your destination.
Some pilots like to start down early so they can be established at low altitude 15 or more miles from the airport. The premise is that it's better to be below other traffic looking up than above it looking down. It's far easier to differentiate a bogey against a blue sky or white clouds than to spot them among the gray buildings, roads, parking lots and schoolyards of the ground.
To that end, pilots need to remember to turn on every light they can find when entering even moderately busy airspace. The modern generation of strobes and landing lights make any airplane super visible, as they're powerful lights with an average life that will outlast the TBO of two successive engines. That means you can simply turn lights on before takeoff and off after landing. I have a LoPresti Boom Beam installed in my Mooney that's rated for 5,000 hours, and I run it all the time on every flight, day or night. At 250 hours' use a year, it should last 20 years.
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