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Southport, Manitoba, Canada
Steve Pomroy is a professional flight instructor and aviation writer. He has been teaching since 1995 and holds an Airline Transport Pilot License, Class 1 Instructor and Aerobatic Instructor Ratings, military QFI, and an undergraduate degree in Mechanical Engineering. He's written and published three flight training books through his company, SkyWriters Publishing, and has several other books under development. Steve currently teaches RCAF pilot candidates on their Primary Flight Training course.

Monday, October 25, 2010

Tubulence Penetration

Several posts ago, I wrote a 2-part piece about Maneuvering Speed—Va in V-speed notation. One of the common misunderstandings about maneuvering speed is that it is a turbulence penetration speed—or, according to some, a maximum turbulence penetration speed—not to be exceeded in gusty or turbulent conditions. As discussed in the posts, Va is used to define the minimum allowable structural strength of the control surfaces. The requirements set for Va make no reference whatsoever to turbulence.

We know, of course, that we do often fly in turbulence, and that turbulence subjects the airframe to structural loads. We also know (or can figure out) that these structural loads will vary with airspeed—they’ll be higher at higher airspeeds and lower at lower airspeeds. So it might be useful to know if there are V-speeds related to turbulence loading, what these speeds are, and what criteria is used to determine these speeds.

As it turns out, there are several speeds at which there is a (direct or indirect) required structural tolerance for turbulence (and Va is not one of them!). Two of these speeds are always published in the flight manual—and marked on the ASI—for normal and utility category aircraft: Normal Operating Speed (Vno), and Never Exceed Speed (Vne). Also related to turbulence, but not always published, are Vb (Gust Penetration Speed), Vc (Design Cruise Speed) and Vd (Design Dive Speed).

The required turbulence tolerance of the airframe is directly defined at three of these speeds, with progressively lower requirements at the progressively higher speeds of Vb, Vc, and Vd. The turbulence requirements for Vno and Vne are indirect since these speeds are defined in terms of other speeds (Vc and Vd, respectively).

Gust Penetration Speed, or Vb, is required to be determined and published for commuter category aircraft, but is usually unpublished for normal and utility category aircraft. Vb is the maximum speed at which the aircraft structure can withstand a 66 foot per second (fps) gust perpendicular to the flight path. How much is 66 fps? It’s about 39 knots, and that’s a helluva gust!

At first glance, 39 knots may seem pretty extreme, but isn’t really when you consider the nature of turbulence. In turbulence, we can encounter two opposing gusts one immediately after another. If these gusts are 20 knots each, the net effect as we transition from one to the other is a 40 knot change. So turbulence that corresponds to a 20 knot gust factor should have us slowing down to (or below) our Gust Penetration Speed, Vb.

Design Cruise Speed, or Vc, is the maximum speed at which the aircraft structure can withstand a 50 fps gust perpendicular to the flight path. At Vc, stronger gusts will overstress the airframe. Above Vc, a 50 fps gust will overstress the airframe. How much is 50 fps? About 29 knots. Using similar reasoning as above, a double gust of 15 knots each can bring us just beyond this limit. So if we’re operating in conditions where the gust factor is at, near, or above 15 knots, we should keep the airspeed below Vc.

How do we do that if Vc isn’t published? Easy peasy! Normal Operating Speed, or Vno, is based on Vc. Vno is usually equal to Vc, but under some conditions it can be lower than Vc. So using Vno as our turbulence limit will be correct, or, in some cases, will err on the side of caution. We can see Vno on the ASI: the top of the green and bottom of the yellow arcs.

It’s generally said that Vno should only be exceeded in very smooth air and even then only with caution. This is fair comment, if for no other reason than we like to have a comfortable margin of safety when it comes to structural failure. Further, even in smooth air, there can be turbulence that we don’t “see” until we’re in it. However, the certification standard does not require perfectly smooth air above Vno. As stated above, the turbulence allowed at Vno is 50 fps (almost 30 knots) perpendicular to the flight path. As we move above Vno, the airframe’s tolerance for gusts is reduced progressively until the lowest requirement (25 fps, or almost 15 knots) is met at Vd. Vd, in turn, is closely related to Vne.

Vne, as all pilots know, is the NEVER EXCEED speed. We never fly at speeds above this for fear of ripping the wings off—quite literally—and then flying much faster and in the manner of a lawn dart, followed by an unpleasant encounter with the ground. This is not hyperbole. Vne is never to be exceeded for several very good reasons (flutter, static divergence, control reversal), which will probably be the subject of a future post. For the moment, let’s have a quick look at the turbulence tolerance required at Vne.

Vne is defined as being no higher than 0.9 times Vd. Vd is where all of the nasty things noted above might happen and the airframe may rip itself apart. The 10% margin is there to allow for manufacturing tolerances and aging of the aircraft, so don’t fool yourself into thinking you can exceed Vne by 10%. The turbulence requirement is applied at Vd, and as such, we can "theoretically" take 10% more gust velocity at Vne. But why push our luck?

At Vd (and therefore at Vne for all practical purposes), the airframe must be able to withstand a gust of 25 fps perpendicular to the flight path. This is almost 15 knots (14.8 knots if you’re picky), which really isn’t very much at all when you account for the double-opposing-gust possibility discussed earlier. Flying in gusty conditions where the gust factor is over 7.5 knots is commonplace. In fact, a gust factor lower than 7.5 knots would barely be noticed in many aircraft bigger than a light 2-seater.

So if we plan on flying at speeds approaching Vne, we definitely want smooth air. This is especially true since Vne does NOT include a tolerance for momentary overspeeding due to airspeed variations in gusts (some speeds, such as the flap limit speed, Vfe, do in fact allow for such momentary overspeeding due to gusts). It’s possible (in truth, not entirely likely, but possible) that during flight at Vne, a gust can introduce catastrophic flutter and prematurely end our career. This alone is a good reason for applying a margin to Vne and avoiding turbulence when flying at very high speeds.

Ok, that’s all well and good, but what about Va? Well, as stated earlier, the standard for Va makes no reference whatsoever to turbulence or gusts. However, there is an argument that in extreme turbulence we may need to use full control authority (read “deflection”) in order to retain control of the aircraft. This is a valid point. However, I hope never to experience it first hand! The best way to deal with this problem is to avoid it. Check your weather, avoid forecast and reported severe turbulence, avoid thunderstorms, and avoid flying near ridges in strong winds. If you have stormscope or weather RADAR, use it. If there are turbulence PIREP’s, pay attention to them. Turbulence that requires regular use of full control authority likely goes beyond any turbulence defined by the certification standards—and beyond the ability of most pilots to maintain positive control for any extended period.

One final note with regard to turbulence PIREP’s. An aircraft’s sensitivity to turbulence is based almost entirely on it’s wing loading (almost because dynamic stability will also play a role). Higher wing loading results in a smoother ride. Lower wing loading results in a wilder ride. So if you’re reading (or listening to) a turbulence PIREP from a heavier aircraft with higher wing loading, interpret accordingly!

So there you have it. A few more tips on V-speeds, where they come from, and how they apply to flight operations.

Happy Flying!

8 comments:

Tracy C said...

Great description of Vno, Vc and Vb. These are rarely well understood. Thanks Steve.

Alex said...

Very informative and well written. Clears up some things.

On comment:
Here, though, are some question (of the "I'm curious" type):
You wrote in your article "An aircraft’s sensitivity to turbulence is based almost entirely on it’s wing loading (almost because dynamic stability will also play a role). Higher wing loading results in a smoother ride. Lower wing loading results in a wilder ride."

Seems to me that although lower wing loading (such as flying loaded well below MTOW) does indeed get you bounced around more in turbulance, on the other hand it means when you hit that turbulance, or do full deflection of the controls, there is less stress on the aircraft, and that you would have MORE tolerance and as a practical matter could fly in turbulance at higher speed s than the Vno or Va might indicate (as they're set IIRR based on flying at MTOW) with less risk of structural failures than fully loaded... albeit as you say with a wilder ride.

So while loading the plane to the limit MIGHT seem like a good strategy for a smoother ride in turbulance (as someone might infer from the quote above) it cuts down your safety margin.

Caveat: I don't claim to have any particular aeronautical engineering training or credentials.


Alex

Steve Pomroy said...

Hi Alex.

This is a commonly held misconception. It's probably so popular because it's half right.

At lower weights, the wings are indeed subjected to lower loads and stresses at a given load factor. So at first glance it may seem that we should be able to withstand higher g at lower weights. However, this applies to the wings and ignores the rest of the aircraft.

Take, for example, the engine and the engine mounts. If the aircraft is certified to 3.8 g's, the engine mounts are stressed accordingly. Carrying less payload and reducing the load on the wings does nothing to relieve the engine mounts. So the increased g that the wings can take at lower weights can still damage or destroy the engine mounts.

A structural failure in teh engine mounts is just as catastrophic as a wing failure. The change in aerodynamics and the shift in CofG when the engine departs our company will ruin our day.

Bottom line: Published load factor limits don't change as weight is reduced, unless the manufacturer tells us so (the transition from normal to utility category on some training aircraft is one example of where they may tell us so).

Cheers,
Steve

Grigorios Portokalakis said...
This comment has been removed by the author.
Gregory P said...

Great articles.
If the aircraft doesnt have a published Vb, what speed we should aim for? Vo? Especially when taking into account the relatively small margin (for a light aircraft) to low speed stall. Meaning that, ok, it would be a great benefit to slow down to avoid excess loads, but, we need to fly also! :D

(also I looked your articles about Va, which I found them very enlightening...)
An ATPL student

Anonymous said...

Vb can be calculated for any weight for any aircraft if it is not specified in the manual. The formula is
stall speed x square root of design load factor.
The stall speed is a function of the weight so use the stall speed at the weight you are flying. This implies that at lighter weights, your stall speed will be lower and so will be the Vb.

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MyFlightForecast.com said...

I like this blog, and in this blog there is some information about the speed, If you want to fly at a speed Vne then you must need a smooth air. From Turbulence Tracker you can know all information related to turbulence for a flight.

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