The Lowdown On Go-Downs

As usual, the Canaries were shining in the sun, the Bahamas of Europe, only 100 miles off the coast of the Western Sahara, Africa. It was almost embarrassing to admit I was tired in such a comfortable airplane, even if the day had stretched to nearly 10 hours of flying.

I had picked up a passenger in Bangor, Maine, the day before---John Thorson, an A&P mechanic and former charter pilot. Thorson was definitely the old pro; he was so comfortable in an airplane that he had slept for most of the last 2,000 nm. He woke a few minutes before Canaries control approved my descent into Tenerife.

Cleared down to 5,000 feet, I eased the nose over and reduced power slightly to keep speed in check. Passing through 10,000 feet, I started pushing the mixtures forward on the normally aspirated Continentals. That brought Thorson to life.

"Why are you advancing the mixtures?" he asked.

"Well, John, I'm fattening up the mixture for lower altitude," I answered.

"Should be going the other way," Thorson mumbled quietly, as if he was embarrassed that he might offend the captain.

I questioned his suggestion, and he said, "Look, you've got the power back to about 55%, so you can't detonate the engines with any reasonable mixture setting. Why not lean the mixture slightly, rather than richen it to help keep the cylinders warm during the descent and avoid shock cooling?"

That was a bit of mechanical wisdom I had never considered, but it made perfect sense. Like most pilots, I was in the habit of advancing the mixtures rather than retarding them as I descended.

Just another bit of aviation knowledge to file away, this on one of the least studied aspects of flying. Perhaps sadly, too many pilots pay too little attention to the process of losing altitude. As Rodney (Dangerfield) would have put it, descents can't get no respect.

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.

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.

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.

Those who fly behind a turbo and like to cruise high in thin air can have special problems. Controllers may assume you're pressurized and assign a descent rate that's not realistic for an uninflatable airplane. For that reason, you might want to specify your aircraft type in your call sign. If you have a turbo out front and you're flying at 17,500 feet, you might want to call yourself "Saratoga" rather than simply "Piper," or "Skylane" rather than "Cessna." That way, controllers who are pilots won't automatically assume that you're flying a pressurized Malibu or Centurion just because you're operating at the highest VFR altitude. That's a sure tip-off that you can't comfortably accept high descent rates.

For those who are instructed to go down and slow down simultaneously (a trick some diabolical controllers seem to enjoy), the best method is to deploy speed brakes if you have them. These are operable all the way to redline and don't put undue aerodynamic stress on the airplane. Second choice is approach flaps, if the limit speed is high enough.

Extending the landing gear is the final option, as most retractables have limit speeds that are well below flap extension velocities. These usually are set to avoid structural overloads on gear doors. (One exception is that some airplanes demand low gear-in-transit speeds then, once the wheels are down and locked, allow you to take the airplane practically to redline if you're stupid enough to go there.)

Whatever you do, don't extend cowl flaps as a drag device during a descent. You'll be increasing cooling in the descent automatically. Shock cooling is a proven factor in premature engine failure, as different metals inside the engine expand and contract at different rates. The more consistent you can keep cylinder head temps, the better.

It's important to remember on the way downhill that mid-air threats are still a risk, above, below and in all horizontal quadrants. The tendency is to look straight ahead to spot what you're about to overrun. A better policy is to continue a normal scan just as you would at cruise. The airspace obviously becomes more congested at low altitude. Most general aviation traffic is concentrated below 6,000 feet AGL, especially around busy airports.

Other major airports allow general aviation to fly directly overhead. At KLAX, gateway to the huge Los Angeles Basin, there's a VFR corridor between 2,500 and 5,000 feet that runs northwest/southeast above the four runways. The corridor is as wide as the runways are long, and it allows transitions from the south basin to the north basin and vice versa, with no chance of conflicts with an airliner.

Intelligent descents 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. Just be sure to tell everyone to Valsalva during the descent, and watch the weird looks people give you.