Mellow yellow: as enticing as it may be even in smooth air, flight in the airspeed indicator's yellow arc risks structural failure and more.
Especially in the winter, when the air is denser, many airplanes of even modest performance are capable of cruising in their yellow arc without much fuss. And when letting down from altitude, I have been known to leave the power alone and let speed build in the descent. After all, I spent a lot of valuable time and fuel getting that high; why can't I get something back from it all?
But the piston-engine airplane airspeed indicator's yellow arc is there for a reason (turbine-powered airplanes--even when converted from piston engines, have a red never-exceed line where the yellow arc would begin). Its bottom range starts where the normal operating range--denoted by the green arc--ends. At its upper range is the red line, beyond which only test pilots wearing parachutes should venture.
Between the two extremes is something of a No Man's Land, an area filled with unknowns, marked "Here Be Dragons." What's the real significance of the yellow arc? What's going on within it and under what circumstances is it okay to be there?
Anyone who's gone past their first solo should know the airspeed indicator's yellow arc denotes the "caution range"--operations within the yellow arc should be conducted in smooth air only and then with great caution. According to the late Bill Kershner's book, The Advanced Pilot's Flight Manual, "Strong vertical gusts could damage the airplane in this speed range; therefore it is best to refrain from flying in [it] when encountering turbulence of any intensity."
We should add abrupt control inputs to turbulence among things to be avoided within the yellow. This is because the airplane rarely knows the difference between the two, or cares: The effects on the airframe produced through greater g-loading are the same.
As we also should know, the yellow arc's bottom range begins at the airplane's maximum structural cruising speed--Vno--while its upper limit is the never-exceed speed, Vne. (If your airplane was manufactured as a 1976 or later model, the ASI's markings reference indicated speeds; earlier aircraft are marked in calibrated speeds.)
Fine. But what's a "strong vertical gust?" For FAA certification purposes, there are basically two gusts, one of 15 feet per second and another moving at 30 fps. Again, for certification, it's presumed these gusts are sudden, or "sharp-edged," and do not gradually build over time. Most light airplanes--those certified in either the Normal or Utility categories--must be able to withstand the FAA-standard 30-fps vertical gust at VNO (see sidebar above).
FUSSING ABOUT GUSTS
Why all the fuss about gusts? And what effect can they have on your airplane?
When the wing encounters a positive vertical gust, the effect is to increase the angle of attack and generate greater lift. This, in turn, increases load factor and the weight being supported by the airframe. That's pretty much the same as pulling pitch to increase the angle of attack, which also loads up the airplane. So, for the same reasons you wouldn't dramatically increase or decrease pitch when flying above the airplane's maneuvering speed (VA; see the sidebar on the opposite page), you don't want to encounter such a gust at a relatively high speed.
Loads imposed on the airplane and the structure's ability to withstand gusts are linear. In other words, encountering a 30-fps gust when flying at 100 knots has roughly the same effect as a 15-fps gust at 200 knots, all other things being equal. As we've discussed, airframe certification rules mean the airplane can handle a 30-fps gust at VNO, the top of the green arc. Above that speed, as reflected in the gust envelope diagram on page 21, the airplane's ability to withstand gusts diminishes. Eventually, as we accelerate our hypothetical airplane into the yellow arc and beyond, we'll get to the point where a 15-fps gust will break something. That point on the gust envelope is well past VNO, approaching VNE.
Kershner maintains 30-fps gusts easily can be found in the vicinity of thunderstorms; 45-fps gusts are likely closer in. Much higher gusts are the norm inside a convective cell, and 100-fps can be considered routine for a developed storm. Keep in mind that 20 knots equals roughly 34 fps. In other words, a typical March crosswind is greater than the maximum gust our airplanes are designed to withstand at the top of the green.
Clearly, no one in their right mind fools around in such weather without a good reason or a bad vector, and--as we have seen--slowing down to VA or lower is the way to go. But that's a bit far afield from our discussion of the yellow arc. So staying away from a thunderstorm when we want to push up the power and/or tuck the nose is a good policy.
WHERE'S THE "SMOOTH" AIR?
Everything you've read about operations in the yellow arc mentions "smooth air." Yet, the atmosphere is dynamic and rarely absolutely motionless. Can we ever use the yellow arc? If so, when? And what is the likelihood of finding a 15- or 30-fps gust in normal operations?
Unless we're just unlucky, relatively smooth air usually isn't that hard to find if one climbs high enough. Once it gets to, say, the low teens, the average normally aspirated piston airplane probably can't come close to the yellow arc unless pointed sharply downhill. And that's where the temptation for many of us comes in.
Coming downhill from our lofty, smooth perch, it's easy to leave the power up and watch the groundspeed numbers tick up. It's generally a long way down from our cruising altitude and the temptation to let the speed build into the yellow is too great. (It's also a hoot when ATC asks us to slow down or gives us a delaying vector to ensure adequate spacing because we're eating up the turboprop or "near Lear" preceding us to the airport.)
And that's okay in smooth air, at least as long as we don't get too close to the red line--figure the middle of the yellow arc is about as fast as we want to go. The problem is, while the air behind us and beside us is smooth, the air in front of us might not be. This is especially true the lower we descend and the closer we get to any underlying clouds or surface features. And a windy day makes it even less likely we'll find smooth air as we descend from cruising altitude.
And that's the problem with flying in the yellow arc: It might be smooth now, but what will the next mile bring? There's no way to tell, even if you're following someone reporting smooth air. Slowing to the top of the green protects us against that 30-fps gust. The airframe you save may be your own.
Remember this from your primary training? When a g-loading diagram is overlaid with predicted gusts, it becomes a gust envelope. It shows the various airspeeds with which we are concerned, including maneuvering speed (Va), as well as the green and yellow arcs, plus the red line. The dashed white lines represent 15- and 30-fps gusts, and their impact on the airplane's structural loading. The idea is to operate at a speed where a representative gust will not result in structural damage or failure.
Airspeed Indicator Markings
Depending on your airplane's date of manufacture, the V-speeds required will be marked either as an indicated airspeed (IAS) or as calibrated (CAS). The practical difference is non-existent: Just fly the airplane within its limitations.
If your ASI is marked in CAS and you are really concerned how that translates to IAS, your AFM/POH should have a correction chart showing the pitot-static system's built-in error. Use it to convert IAS to CAS. In any event, the differene should be no more than three percent, by regulation
Meanwhile, here are the airspeeds you'll find on your ASI. See the sidebar on the opposite page for some of the important airspeeds you won't find marked on the ASI.
Stalling speed or minimum steady flight speed in the landing configuration. It's the bottom of white arc.
Stalling or minimum steady flight speed in a specified configuration, usually "clean" (gear and flaps retracted). Displayed as bottom of green arc.
Maximum speed with wing flaps fully extended. A slightly higher speed may be available for partial (approach) flaps, but it won't be marked on the ASI. This is the top of white arc.
Normal operating speed, or the maximum structural cruising speed. This is displayed as the top of the green arc and is the maximum speed to be used in turbulent conditions.
Never-exceed speed. This is the red line and top of yellow arc.
In addition, on a light multi-engine aircraft, VYSE (best single-engine rate of climb) is indicated by a blue line; VMC (minimum controllable airspeed with the critical engine inoperative) is indicated by a red line near the bottom of the green arc.
Airspeeds You Should Know
Not all the airspeeds we need to know for everyday flight operations are marked on the airspeed indicator. Since many of them vary with weight, manufacturers use placards to ensure we have the speeds we need readily at hand or publish them in the POH. To fly the airplane at its peak performance, we might want to compute some speeds and develop a weight-based chart, especially if we're trying to achieve maximum range or endurance. Regardless, you should know these speeds by heart:
Design maneuvering speed is also the stalling speed at the airframe's maximum legal g force. As such, it is the maximum speed at which abrupt, full deflection of the controls will not cause the aircraft to exceed its load limit. Maneuvering speed is limited by aircraft structural characteristics and varies with weight.
Maximum landing gear extended speed, as its name implies, is the maximum speed at which the aircraft may be flown with the landing gear extended. VLE may be the same as VLO (see below), but it is never lower.
Maximum landing gear operating speed. The maximum speed at which the landing gear may be raised or lowered. This speed may be the same as VLE, but is usually lower.
The best angle of climb provides the best altitude gain per unit of horizontal distance. It is usually used to clear obstacles--the proverbial 50-foot tree, for example--during takeoff. It increases slightly with altitude.
The best rate of climb provides the best altitude gain per unit of time. This speed typically reduces slightly as altitude is gained. At some point, VX and VY become the same, at or near the airplane's service ceiling.
As might be expected, the best endurance speed is the speed providing the greatest time aloft for fuel consumed. It can come in handy when waiting for improved weather or a clear runway.
If your airplane's single engine fails, the best power-off glide speed is the one at which you want to fly your airplane. It achieves the maximum lift-to-drag ratio and the greatest gliding distance available is the result.
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|Author:||Burnside, Joseph E.|
|Date:||Mar 1, 2007|
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