That, in fact, is one of the primary benefits of speed brakes. They provide aerodynamic braking when you need it and have essentially no effect when you don’t. They’re an ideal advantage for those slam-dunk, go-down/slow-down directives that some nonpilot controllers love to command. Speed brakes double parasitic drag instantly and provide easy glidepath control.
Pilots who operate turbine equipment are very familiar with speed brakes and refer to them as “the boards.” Virtually all jets include speed brakes as standard equipment, and many turboprops also feature aero braking to help slow things down.
Turbines need glideslope help by the nature of their operation. They must operate far up in the flight levels to realize any semblance of efficiency. A Malibu JetPROP, for instance, burns about 34 gph at max cruise up at 25,000 to 26,000 feet, its normal cruise height, and delivers around 260 knots. Descend to 18,000 feet, however, and fuel burn remains 34 gph while speed in the thicker sky drops to about 230 knots, somewhat less efficient. For that reason, turboprop and jet drivers like to stay as high as possible for as long as possible.
Flying higher means more altitude to lose during the approach, however. Turbine aircraft, airline or corporate, are nearly always pressurized, so descent rates of 2,000 fpm or more are reasonable and common without causing passengers any discomfort. Trouble is, most turbine equipment is so aerodynamically clean that simply pushing the nose over, even after reducing thrust to idle, may sometimes drive airspeed through the barber pole, better known in piston parlance as the redline. Jets are clean machines, often sporting glide ratios of 18 to one compared to nine or 10 to one for most piston aircraft.
Speed brakes or spoilers are the answer. They hold speed in check while allowing the pilot to maintain a manageable descent rate. They can be deployed right up to the aircraft redline without undo stress on the surrounding wing structure. Despite the dramatic effect they have on ride and attitude, they place little stress on the wing spar.
Spoilers have an extra benefit for piston airplanes. In addition to artificial speed reduction, they can be especially helpful in reducing shock cooling in some private and corporate piston models. There’s still some debate about the negative effects of shock cooling, but most engine shops agree that quick cooldowns of big-bore engines are a bad idea. Different metals expand and contract at different rates, and the more consistent you can keep temperatures inside the engine, the better.
Joe Polizzotto, manufacturer of the world’s most popular GA engine analyzers, J.P. Instruments’ EDM series (www.jpinstruments.com
), says shock cooling is a definite factor in premature failure and reduced TBO on engines over 250 hp. His instruments have an electronic page that indicates shock cooling. “Lycoming issued a recommendation a while back suggesting no more than 50 degrees per minute of cooldown,” says Polizzotto, “and any abrupt increase in airspeed or reduction in power could easily generate much more than that.”
With or without speed brakes, an occasional operating practice in big twins, such as Commanders, Cessna 421s, Dukes and Chieftains, is to actually lean the mixture slightly during letdowns to run the engines hotter and compensate for the cooling of descent.
The beauty of speed brakes for piston airplanes is that they allow you to descend at cruise speed without reducing power or increasing the flow of cooling air through the cowling. Speed brakes allow you to leave the throttle(s) alone during the initial letdown, then reduce power gradually to maintain a reasonable descent profile. Chop and drop approaches aren’t necessary when you have the speed brake option.
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