Mellow yellow

One of the reasons we fly airplanes is to go fast. I confess: Theres a certain satisfaction from pushing the airplane hard, up into its yellow arc. If youre lucky, you fly something capable of such speeds without much effort, even in the summer or at high altitude, when the air is thinner and indicated airspeeds lower.

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 cant I get something back from it all?

But the piston-engine airplane airspeed indicators 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 Mans Land, an area filled with unknowns, marked “Here Be Dragons.” Whats the real significance of the yellow arc? Whats going on within it and under what circumstances is it okay to be there?

Anyone whos gone past their first solo should know the airspeed indicators 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 Kershners book, The Advanced Pilots 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 arcs bottom range begins at the airplanes 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 ASIs markings reference indicated speeds; earlier aircraft are marked in calibrated speeds.)

Fine. But whats 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, its 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).

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. Thats 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 wouldnt dramatically increase or decrease pitch when flying above the airplanes maneuvering speed (VA; see the sidebar on the opposite page), you dont want to encounter such a gust at a relatively high speed.

Loads imposed on the airplane and the structures 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 weve 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 airplanes ability to withstand gusts diminishes. Eventually, as we accelerate our hypothetical airplane into the yellow arc and beyond, well 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 thats 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.