Stress Relief

When the going gets tough, the tough get going. Unless theyre flying, that is.

Eventually you will en-counter the pilot's counterpart of a rough time - turbulent air. Unless the airplane you're flying sports armor and camouflage paint, you won't speed up - you'll slow down. Ah, but to what? To maneuvering speed, of course.

Maneuvering speed, or V_A, is another one of those numbers that isn't on your airspeed indicator because it varies with weight. You should see references to it on a placard, however, and modern aircraft have at least a recommended maneuvering speed (or perhaps rough air penetration speed) listed in the pilot�s operating handbook (POH).

Cars don't have specified slower driving speeds for traversing potholes, so why must we? It comes down to stresses on the airframe. Aside from the obvious motivation that even the most lackluster training airplane might equal a Lexus in value, there's the more compelling incentive that, unlike a broken axle on the road, a bent wing when flying can't be fixed with a call to the automobile club. We adhere to maneuvering speed because we were told that it's the speed up to which "full abrupt control deflection" can be made without exceeding the airplane's design load factor - any faster and the wing might produce lift that exceeds the design load limit.

Simply put, at or below maneuvering speed the aircraft will stall before the structure fails. (A wing can't impose airframe stresses when it's no longer developing lift, so you may lose altitude, but not your wings. I'd say that's a fair trade.) At lighter weights, you should slow down even more.

The path to wisdom always seems to have some thorns in it, though, and here's one irony. Reduced weight puts many an airplane in the more nimble Utility category, where limit load factors are higher, right? Yet such agility seems to invoke the stigma of fragility. After all, our maneuvering speed decreases at lighter weights. So why is it that you may perform certain maneuvers only at a lighter weight (with a possible promotion to an entirely more maneuverable and privileged Utility category), but you have to go slower in rough air, flying the same airplane at a lighter weight?

Although that may not sound fair at first, it actually is. Butterflies are maneuverable, but famously fragile. Bumblebees pack quite a punch, but they are somewhat lacking in grace. In a nutshell, the core concept behind such an aeronautical irony involves a similar and equally simple disparity, but it's resolved quickly and anticlimactically when we differentiate between wing loading and inertia of the fuselage. First, whether you refer to this speed limit as design maneuvering speed, turbulence penetration speed, or just rough air speed, they all exist to keep the airplane intact. Maneuvering speed isn't so much a force-related limit as it is an acceleration-related one: Less mass gets shoved more easily by the same force, and that's why we have to slow down.

Yes, with less weight, structures can assume a greater additional load before the same gravitational forces are imposed on the airframe. For example, a 2,400-pound airplane at 3.8 Gs results in approximately the same apparent weight as the same airplane lightened to 2,070 pounds but experiencing a force of 4.4 Gs. (This weight quotient roughly matches the reciprocal of the two categories' respective negative-G limits, as well.)

But reduced weight is like a lighter wing loading. Your aircraft becomes more like a kite; it takes less force to accelerate a lighter object at a rate that would invoke an airplane's design limit load. Yes, the aircraft has inertia, but the wings and tail must bear the brunt of the air loads. Yes, it takes less force to accelerate the fuselage (and wings), but the structure pays the price. With a reduction in weight of fuel and passengers, many structures such as the empennage and engine mounts still have the same mass and get accelerated, bent, and pushed all the harder.

As you know, aircraft lifting surfaces experience stress, whether it's from the air, such as convective, mechanical, or clear air turbulence - or you, pulling back abruptly on the yoke. One term engineers use to quantify stress is load factor, which is the ratio of the lift produced by the wings to the airplane's weight, measured in multiples of the Earth's normal gravitational acceleration, or Gs. We innocently invoke it in a turn - or by blundering into a thunderstorm. Airplanes can handle some imposed G-loads with a margin of safety (typically quantified by a sudden 30-fps vertical gust at maximum level flight speed and normal rated power) up to their maximum structural cruising speed, V_NO(where the green meets the yellow on the airspeed indicator).

Go any faster and the gust tolerance goes back down, with very little margin by the time the yellow meets the red at V_NE. Also, apart from reduced weight, it's always possible that we'll encounter some other type of turbulence (such as horizontal wind shear, which might just add 20 knots), and these are not included in the above formal tolerances.

Add to all of this the fact that if you're in a turn you're already putting maneuvering loads on the airframe, and you have many good reasons to stay well below maneuvering speed (and keep the wings level, if you inadvertently blunder into that convective rodeo ring).

Now, a little math: Lift improves - up to a point, known as C_Lmax - as the angle of attack increases. If you graph the maximum lift the wing can produce, expressed as a ratio of load factor vs. airspeed, this curve will intersect the 1-G line at stall speed, and the limit load factor line at maneuvering speed. Go any slower than this and the wings will stall before they will break, no matter what the angle of attack. At slower speeds, any "normal" updraft or rapid control deflection could increase the angle of attack up to C_Lmax but beyond that, the wing stalls. In turbulence, such stalls are only momentary (listen for the intermittent chirp of the stall warning horn) and don't require stall recovery techniques.

You don't want to fly so slowly that a stall will persist (especially at low altitude; say, while on approach), but you don't want to overstress the airframe, either. The happy medium is maneuvering speed, which decreases with the square root of the ratio of present weight to max gross weight. The "square root weight ratio" at minimum flying weight for most small airplanes might be as low as 0.75, so the safest turbulence penetration speed could easily be significantly less than the published maneuvering speed. And although this doesn't apply to most of us, for airplanes grossing more than two tons (actually it's 4,117 pounds) Federal Aviation Regulation 23.337 actually allows the positive limit load factor for Normal category airplanes to be less than 3.8.

To every pilot's favor is the fact that airplanes are designed with an ultimate load factor that's 50 percent more than the limit load factor. However, that's a corner of the flight envelope that you don't want to explore.

Jeff Pardo is an aviation writer in Maryland with a commercial private pilot certificate for airplanes, and instrument, helicopter, and glider ratings. He has logged about 1,100 hours in 12 years of flying. An AirLifeLine mission pilot, Pardo has also flown for the Civil Air Patrol.