January 1, 2011
By Barry Schiff
Turbofan airplanes are so aerodynamically clean that pilots can have difficulty slowing down and going down, especially at the same time. These aircraft do not have the drag created by windmilling propellers, which is why they typically have twice the glide ratios of piston aircraft. They do, however, have spoilers. Deploying them allows an aircraft to descend more steeply without an increase in airspeed. (On piston-powered airplanes, they enable a rapid descent and reduce the risk of shock cooling the engine.)
Typical spoilers consist of one or more rectangular plates lying flush with the upper surface of each wing. They are installed approximately parallel to the lateral axis of the airplane (depending on wing structure and geometry) and are hinged along their leading edges. When deployed, spoilers deflect up and against the relative wind, which interferes with the flow of air about the wing. This both spoils lift and adds drag. Spoilers usually are installed forward of the flaps. They are not placed ahead of the ailerons because this would interfere with roll control. The purpose of spoilers is to spoil lift, even though some drag is created in the process. The purpose of speed brakes, however, is to produce drag even though some lift might be sacrificed in the process. The difference between them is their location along the wing chord. When relatively far forward, they are most effective at dumping lift and are called spoilers. When relatively far aft, they are most effective at creating drag and are called speed brakes.
A pilot can perform a simple test to determine if his aircraft is equipped with spoilers or speed brakes. He needs only to note the effect of deploying the devices while maintaining a level attitude. If a significant sink rate develops and airspeed decay is minimal, they likely are spoilers; if the results are initially the other way around, they most likely are speed brakes. Pure speed brakes are sometimes installed on the fuselage of an airplane, such as the dive brakes on military jets. Extended landing gear, of course, also makes for a splendid speed brake.
Despite what they are called, however, spoilers and speed brakes are used in the same manner and for the same purpose: to steepen the descent profile without increasing airspeed. They also can be used to reduce airspeed by holding the sink rate in check. Many pilots prefer not to use spoilers during descent because spoilers often create a rumbling buffet that can be disconcerting to passengers (especially those seated in the rear). Another reason for not “popping the boards” is that this might be interpreted to mean that a pilot did not plan his descent properly and is using spoilers to correct for this.
There are, of course, several reasons for using spoilers, not the least of which are unexpected descent clearances from ATC. Another is the desire to descend rapidly through a band of reported turbulence without exceeding the turbulence-penetration speed. Because spoilers destroy some lift in their immediate vicinity, wing loading elsewhere on the wing necessarily increases. This has the effect of reducing gust-induced G loads, which softens the ride somewhat.
An important role for spoilers is to enhance an emergency descent necessitated by a loss of cabin pressure. Training for such a maneuver often involves teaching pilots to react rapidly to the simulated emergency. Almost without thinking, we are expected to quickly don an oxygen mask, retard the thrust levers, deploy the spoilers, lower the nose, and maintain an airspeed somewhat shy of the barber pole (redline) all the way down.
Pilots now are taught to first evaluate the emergency before reacting to a decompression with a steep dive. If the pressure loss is caused by structural failure, descending at high speed could worsen the damage. Although spoiler deployment is allowed in some aircraft with flaps extended, this ordinarily should be avoided. This is because of the hazards associated with high sink rates near the ground while in a high-drag, low-power configuration.
Another problem with using spoilers during an approach is that—on some aircraft—there is so little buffeting that a pilot can forget that the boards are deployed. Deployed spoilers have curiously little effect on stall speed and seldom affect stall quality. They do, of course, make it more difficult to recover from a stall with a minimum loss of altitude. Spoilers also are invaluable when deployed at touchdown. They obviously add drag to enhance aerodynamic slowing, but they also kill a great deal of wing lift (as much as 80 percent). This immediately places more aircraft weight on the wheels, which improves braking performance. Spoilers also reduce rolling distance during an aborted takeoff.
When landing some airplanes with an aft center of gravity, deploing spoilers and simultaneously applying reverse thrust can cause the nose to pitch up enough to cause a tail strike. This can be avoided by lowering the nose immediately after touchdown.
Although spoilers are used primarily to adjust the descent profile, they also are used on some aircraft for roll control because some ailerons lose effectiveness during high-speed flight.
A design advantage of using spoilers to supplement roll control is that this allows the use of smaller ailerons, which makes room for larger flaps. Roll-control spoilers are used differentially (on one wing at a time). Entering a right turn, for example, deploys the spoiler(s) on the right wing, and vice versa. Some airplanes have no ailerons and use “spoilerons” exclusively for roll control. The use of spoilers for roll control typically creates less adverse yaw than ailerons do.
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One of the chief benefits of flying a turbine-powered airplane is the pressurization it can afford. You may be cruising at FL350, but the airplane’s pressurization system makes sure that the cabin’s altitude is at a much more comfortable level—say, 6,000 or 7,000 feet. The cabin’s a lot warmer, too! Low cabin altitudes mean that passengers and crew receive an adequate supply of oxygenated air without the hassle of wearing oxygen masks. Fatigue is kept at bay, as well.
Most modern pressurized airplanes now use automated pressurization controls. You simply set the destination airport’s field elevation, and the system figures out the rest: when and at what rate to begin and taper off pressurization for climbs and descents, and arrival at the destination with a cabin that’s equal in pressure to the field elevation. In other words, the pressurization differential—the difference between the pressure inside and outside the cabin—is zero.
Manually operated systems require that pilots set cabin pressure for climb, cruise, and descent, as well as selecting a proper cabin rate of climb or descent. Whether the system is automatic or manual, the pressurization control panel will always include a pressurization dump valve. Dumping cabin pressure is serious business (that’s why the switch is usually guarded and/or safety-wired to prevent inadvertent activation) for several reasons. First of all, the pressurization differential drops to zero almost instantaneously.
Depending on your altitude, an emergency descent will be required to reach a lower, “breathable” altitude—usually 12,000 or 10,000 feet msl. In the meantime, pilots and passengers will have to don oxygen masks to prevent hypoxia. The sudden depressurization, followed by the steep banks and high descent rates involved in an emergency descent, can certainly be scary. Even so, there are rare times when you may need to depressurize the cabin in flight. For example, many emergency checklists advise dumping cabin pressure if smoke or fumes enter the cabin and can’t be eliminated.
Another situation that calls for dumping cabin pressure would be a failure of the pressurization controller that results in the cabin overpressurizing beyond its differential limits. A cracked windshield is another case in which internal pressures need to be quickly relieved by either selecting a lower cabin altitude on the pressurization control panel, descending so as to reduce the cabin pressure differential, or—as a last resort—dumping cabin pressure. Once cabin pressure is dumped, your troubles may not be over. Smoke or fumes may persist, even after they were initially sucked out of the cabin during the depressurization. That means fighting a fire while continuing to wear oxygen masks, and perhaps an emergency landing—quickly. Continuing the flight at a lower altitude means burning more fuel at a faster rate, so a diversion to an alternate airport may be necessary. Either way, your skill and decision-making are put to an extreme test. —TAH
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