PIREP turbulence

Design Maneuvering Speed (Va) and Turbulence Penetration Airspeed (design speed for maximum gust intensity, Vb) are both defined in CFR 25.335. For those with a background in Aeronautical Engineering, the formulae presented in those paragraphs may be an adequate explanation of the two terms and their differences. For the rest of us a more practical explanation may be appropriate.

Background

Part 25 of the CFRs outlines the design criteria for Transport category aircraft. It requires that manufacturers publish quite a list of airspeeds for these aircraft—Va, Vb, Vc, Vd, etc.—and defines how these speeds may be determined.

Manufacturers of general aviation aircraft below 12,500 lbs. maximum gross weight are not required to publish most of these speeds and specifically are not required to publish Vb for their aircraft. There are statements in the various POHs and training materials that may lead an aviator to the assumption that Va is the Turbulence Penetration Airspeed: “When turbulent air is encountered or expected reduce speed to maneuvering speed.”

PIREP turbulence

A rough ride is ahead: what speed should you fly?

Design Maneuvering Speed (Va) is the speed where the limiting positive load factor will produce a stall in the clean configuration, or the speed below which the plane will stall before it overstresses. In the world of fighters it is also referred to as “Corner Velocity,” the minimum speed at which maximum Gs can be pulled. It is the speed for the quickest, tightest turn. This is a purely aerodynamic speed that varies directly with aircraft weight.

Vb, Design Speed for Maximum Gust Intensity, is less specific. The CFRs define its upper and lower limits but leave it up to the manufacturer to specify the recommended speed for its aircraft within that range. Vb may not be higher than the speed at which maximum gust will cause the plane to exceed Va, nor lower than the speed which a similar maximum loss of speed will cause it to fall below Vs1 (clean stall speed). The intensity of the gust is defined as 66 fps (4,000 fpm) for Transport category aircraft, but is undefined for others.

Applying this same approach to light aircraft, it should be clear that when expecting/encountering turbulence, that pilots should fly a speed that is slower than Va by at least the value of the maximum gust—airspeed gain—they expect to encounter, and higher than Vs1 by the same value for potential airspeed loss. From personal experience flying in mountainous terrain for over 45 years, the effects of the turbulence (jolts) to the aircraft and passengers are progressively dampened with reduction in speed. Va is simply too fast!

Recommended procedures

A simple rule of thumb would be to split the difference and fly a speed that is approximately half way between Vs1 and Va. However, a slightly faster speed will help improve controllability in very rough conditions. Since airspeed will be varying considerably in turbulence, it is not important that the pilot try to maintain an exact speed, but rather work to maintain near a level attitude and not exceed either the Va or Vs1 limits.

The B-737 Flight Manual of a major airline offers the following excellent guidance, which is also applicable to light aircraft: “The two major concerns when encountering turbulence are minimizing structural loads imposed on the aircraft and avoiding extreme, unrecoverable attitudes.”

O.C. Hope
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10 replies
  1. William Campbell
    William Campbell says:

    Okay, understand the concept for transport category. But what of the vast numbers of GA who have a Va of less than 120kts. When you are already “slow” what is the value of Vb to these airplanes. Seems landing is preferable to the crawl which Vb would suggest.

    Reply
  2. Ron Blum
    Ron Blum says:

    Interesting interpretation of the regulations. The only speed that is not defined for small aircraft (14CFR23) is Vb. 14 CFR 23 and 14 CFR 25 vary significantly in the high speed areas. Vb is simply a conservative airspeed below Va. At or below Va, the airplane will stall prior to structural failure. In addition, full aileron input will also not cause a structural failure OR damage the flight controls system. There are safety factors of at least 50% on all structural numbers.

    There are pros and cons of reducing speeds below Va. Because the condition is a vertical gust, the slower the airplane is flying, the larger the angle of attack change will be due to the gust, and therefore, the higher the probability of stalling. On the other hand, the faster the airplane is flying, the higher the loads will be.

    The most important part to remember is that Va decreases with weight. Va marked on the airspeed indicator (top of the green/bottom of the yellow) is at gross weight. If one is lighter than gross weight, Va is also lower.

    Good article. We can all learn.

    Reply
  3. Mandy
    Mandy says:

    The calculations are adequate. My question is very simple, why would I allow myself to deal with such violent flying conditions?

    Reply
  4. William Johnson
    William Johnson says:

    FAA definitions for airspeed indicator markings.

    White arc—commonly referred to as the flap operating range since its lower limit represents the full flap stall speed and its upper limit provides the maximum flap speed. Approaches and landings are usually flown at speeds within the white arc.

    Lower limit of white arc (VS0)—the stalling speed or the minimum steady flight speed in the landing configuration. In small aircraft, this is the power-off stall speed at the maximum landing weight in the landing configuration (gear and flaps down).

    Upper limit of the white arc (VFE)—the maximum speed with the flaps extended.
    Green arc—the normal operating range of the aircraft. Most flying occurs within this range.

    Lower limit of green arc (VS1)—the stalling speed or the minimum steady flight speed obtained in a specified configuration. For most aircraft, this is the power-off stall speed at the maximum takeoff weight in the clean configuration (gear up, if retractable, and flaps up).

    Upper limit of green arc (VN0)—the maximum structural cruising speed. Do not exceed this speed except in smooth air.

    Yellow arc—caution range. Fly within this range only in smooth air and then only with caution.

    Red line (VNE)—never exceed speed. Operating above this speed is prohibited since it may result in damage or structural failure.

    Note: Va is not shown on the airspeed indicator.

    Reply
  5. Ron Blum
    Ron Blum says:

    Thanks to John K. for pointing out my error of top of the green/bottom of the yellow, which is not Va. It is my practice to slow to Va in any turbulence more than minor (Flight Test definitions, not spill your coffee definitions).

    Va, at any weights under maximum gross weight, is there to protect the rest of the airplane (not the wing).

    Reply
  6. Steve Ford
    Steve Ford says:

    Interesting article and worthy of consideration if caught in this environmental condition. The best tool in the toolbox still remains the threat and error management (TEM) on the ground based on the forecasts. Either stay on the ground or re-route if possible. In the air faced with a radar picture of that intensity it is still in the TEM box…………avoid!

    Reply
  7. Thomas P. Turner
    Thomas P. Turner says:

    Excellent summation of the differences between Va and Vb, and the vital need to select a target indicated airspeed such that the maximum speed encountered in gusts does not exceed Va.

    A point rarely touched upon in articles on this topic: Va is the indicated speed at which the wing stalls just as it reaches the design load. In a Normal Category airplane this is 3.8G, in a Utility Category airplane this is 4.4G. Following the recommendation to slow to turbulent air penetration speed (or a target speed where in gusts indicated airspeed just reaches Va adjusted for weight) means the aircraft (and its occupants) will be repeatedly subjected to 3.8G or 4.4G and even repeated accelerated stalls from this maximum G load–a wild ride indeed!

    Reply
  8. George Futas
    George Futas says:

    Interesting and informative article, and equally interesting comments.
    No sensible pilot intentionally flies a GA aircraft into thunderstorms or the severe rotor downdrafts that occur on the lee side of mountains under strong wind conditions. However, when flying over mountains, or desert, even in clear air, one can encounter strong vertical drafts without warning. It’s happen to me, and it’s an attention getter. That’s when Vb becomes very important. Look at a typical V-G diagram and we see that the structural damage due to negative G’s occurs at a lower speed than Va with positive G’s. I’ve found multiple articles that compute Vb at vertical gusts of 3000 fpm, and Vb computes close to Vy. By slowing down to Vy, it saves the airframe and even if the wing stalls, it’s easy to recover at best climb rate.

    Reply

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