Gremline Flight Safety Report: Turbulence and Design Manoeuvring Speed (Va)

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the gremline digest — caution! turbulence ahead

Caution! Turbulence Ahead

 Most pilots will have heard of an airframe limitation known as Va or Design Manoeuvring Speed. This is an Indicated Airspeed above which one cannot apply full control deflection without risking structural damage to the airframe. Many aircraft have a green arc on the airspeed indicator, and pilots are probably aware that the fast end of this arc is Vno (Normal Operating Speed). Perhaps not all PPLs are aware just where Va occurs on the green arc and of the significance of Va and its safety implications. A further limitation, Vra, the Rough Air speed, may be specified for some aircraft.

 

What practical implications have Va, Vno and Vra got that make it important to understand their derivation and why we need to keep these limitations in mind? There you are, cruising along at best range speed when the aircraft begins to jump about a bit because of turbulence. This turbulence may be caused by strong wind over undulating ground, by thermal activity below cloud, by entering unstable cloud or by any other cause you care to mention.
      What action should you take as commander of the aircraft? Put your coffee cup aside, having first drained it; tighten your lap and shoulder straps; warn your passenger(s) to do likewise; check for loose articles around the cockpit, get both hands on the wheel and prepare to be shaken a little. You have not the slightest intention of applying full deflection of any of the control surfaces so you are perfectly safe to maintain the ASI needle quivering gently at the fast end of the green arc, right?
WRONG!

 

 

Design Manoeuvring Speed (Va)
The purpose of the Va and Vra limitations is to prevent overstressing of the airframe by application of excessive g, either intentionally by the pilot or
accidentally by flying through turbulent air. Instead of expressing these limitations as so many positive g and equipping every general aviation aircraft with an accelerometer, the limitations are expressed as indicated airspeeds. An accelerometer only indicates a past event whereas observation of the Va and Vra limitations can protect your aircraft from future events. These limitations enhance safety for flight through turbulence even if you are not applying any control deflection.
      I will try to explain why these limitations are necessary, how the actual number for Va is arrived at and what are the dangers of ignoring the airspeed limitations while flying through turbulence.
      Airframes are built to withstand specific amounts of g, depending upon their designed and authorised use. Large transport aircraft may have a positive g limitation as low as +2.5g; normal category general aviation aircraft may have a positive limit of 3.75g while aerobatic category aircraft are probably approved to as much as +7g. On some modern high agility combat aircraft the positive g limit may be in double figures. The numbers quoted above are ball park, for the purposes of this discussion, and should not be assumed to apply to your particular aircraft. By the way, what is the g limitation on your aircraft?
      What do these limitations actually mean? Well, I suppose a simplistic view would be that if you don’t apply more than the specified amount of g to your particular aircraft it should stay in one piece while in the air.
      There is (or should be!) a safety margin designed into airframes to allow for anyone who is either stupid enough or unlucky enough to exceed the laid down g limitations. This safety factor may be around 1.2 which would, THEORETICALLY, allow you one application of +3g to an aircraft with a +2.5g limit before the wings took up a permanent increased dihedral angle, or there was a terminally crunching sound as the wingtips met above the fuselage. This safety factor must be paid for in extra weight, so the designer has to perform a balancing act between performance and robustness. If your GA aircraft has a positive g limitation of 3.75g then you would be most unwise to exceed that limit on the assumption that your aircraft had a built-in safety factor. It would be equally stupid to try your hand at aerobatics in an aircraft that was neither stressed nor approved and cleared for aerobatics.
      How can we know how much g is being applied by us or by the bumpy air outside the cockpit? We can’t without an accelerometer, and an accelerometer has a considerable lag so does not give a good indication of instantaneous g, which is what interests those flying through turbulence. We can, however, arrive at another figure by a roundabout route which, if observed, will give the same protection as a g limit. This figure is Va, the
Design Manoeuvring Speed.

 


Let’s go back a step or two and think about why we NEED a design speed limitation. It is probably fairly obvious that if you roar across the airfield 10 feet up at Vne and instantly apply full up elevator it is quite likely that the wings will fold up and your impromptu display will come to a spectacular and unplanned end. Perhaps it is not quite so obvious that it is also possible to exceed the g limitation for your aircraft simply by flying through turbulent air at too high an indicated airspeed. How much airspeed is too much?
      We all know that when our aircraft is flying just a tad above the stalling speed (Vs) we cannot apply any more than +1g without inducing a stall. Let’s assume that the stalling angle of attack, or alpha, for our aircraft is +15. Let’s further assume that when you are cruising at range speed (or faster) the angle of attack on your wings is of the order of +3 and we have +1g applied to the airframe. What happens to the angle of attack when we fly into an updraft? The vertical vector of the rising air causes an immediate increase in the angle of attack that causes an equally immediate increase in the lift being produced by the wing. There is also an immediate increase in the positive g being applied to the airframe. Every aircraft pilot and passenger has experienced these ‘bumps’ that force down into the seat for a short period. That’s the increase in positive g that you are feeling.
      If the vertical gust is strong enough in relation to your horizontal speed then it is possible that the angle of attack on the aerofoil will reach the stalling angle and the wing will stall. Glider pilots are probably more aware of this fact than the average PPL. Power pilots would probably find it odd to be taught to INCREASE their airspeed when rejoining the circuit, as one does in a glider. To approach at a few knots above the stall in a low inertia, low wing-loading sailplane (or ultralight) is to invite an unplanned stall short of touchdown should you encounter even a slight updraft.
      All pilots know that the stalling speed increases as the amount of positive g applied increases. That’s why, given hairy enough arms and enough heave on the pole, you can stall at any indicated airspeed in a steep turn or during aerobatics. Looked at another way, the faster you are going the more g you can apply before you stall. There is a very simple formula that allows us to calculate the relationship any airspeed and the instantaneously applied g that will cause the wing to stall.
     
The formula is: g=(Vi/Vs)
where Vi is our present IAS and Vs is the +1g stalling speed for our aircraft in its present configuration.
      Playing with a few numbers will show the significance of that little formula and the importance of Va, the
design manoeuvring speed. Let’s assume that our GA aircraft has a normal stalling speed of 65KIAS and we are flying along at 150KIAS. Our formula shows that we can apply an instantaneous g load of +5.325 before the wing will stall. In contrast, if we assume that a F16 Falcon fighter has a 1g stalling speed of 130KIAS and is batting along at 600KIAS, then, theoretically, it can pull an instantaneous +21.3g before stalling. A sailplane that stalls at 30KIAS and is flying at 90KIAS can pull an instantaneous +9.00g before the wing will stall. The significance of the word ‘instantaneous’ is the fact that as soon as positive g is applied the drag will rapidly increase causing a rapid reduction in IAS. That’s why the F16 pilot would find that, even using maximum afterburner power, he could only SUSTAIN a maximum of, say, +9g. A computer programme would slap his wrist if he tried to ‘snatch’ more at the risk of folding the wings.

 


The numbers in the foregoing paragraph have more than a theoretical interest when we return to the turbulent air scenario. The F16, thundering along at 600kt, is extremely unlikely to meet a vertical gust of sufficient speed to cause a critically significant increase in the angle of attack or to apply enough g to overstress the airframe. On the other hand, a sailplane at 90KIAS could quite possibly encounter a gust in cloud or in mountain wave conditions that could get near the airframe limit. Equally, our GA aircraft could encounter an instantaneous load of +5g that could easily bend the airframe even if it didn’t break it.
      This is not all purely theoretical stuff. My very beautiful Cessna 310 was found to have a crack in the main spar and was relegated to become an engineering training airframe, being beyond economic repair. The C310 cruised at 170KIAS and stalled clean at 75KIAS so, at 170kt, a gust could apply 5.15g before the wing stalled. Hitting severe (mountain wave?) turbulence while bumbling up the airway from Brecon to Wallasey in medium level cloud with your brain in neutral is not a smart thing to do.
      So, how can GA pilots avoid the risk, however slight, of bending their pride and joy by an encounter with turbulence?
By reducing to Va (or Vra if this is specified for your aircraft) whenever significant turbulence is encountered or expected. You can always expect turbulence during cloud penetration so it is wise to limit your descent speed to a maximum of the Design Manoeuvring Speed. Let’s look back at a few of the numbers in the previous paragraphs and see how flying at (or below) Va will help our aircraft cope with turbulence.
      If we transpose the original little formula we can make it tell us that Va=Vs√g where Va is the design manoeuvring speed, Vs is the 1g stall speed in that configuration and g is the normal acceleration. Using the assumed numbers for our GA aircraft with a design g limitation of +3.75 and a Vs of 65KIAS this new formula produces a figure of 125.87196kt for Va. Taking 126 knots as ‘near enough for Government work’ and checking back through the first formula, we find that 126 knots gives us g loading due to a vertical gust of +3.7576331 – or 3.75. This means that if we limit our airspeed to 126kt then, no matter how severe a vertical gust we encounter, our aircraft wing will stall before we exceed the airframe limitation of +3.75g. Looking back at the sailplane example previously mentioned, we see that with an elastic limit of +5.5g and an applied safety factor of 1.2 this aircraft should not have more than +4.58g applied without the risk of deforming the airframe. Our derived formula gives a figure of 68.5kt for the sailplane Va.
      Having waded through all that stuff, I hope the safety message is clear. The Va limitation specified in your Pilots Manual has real importance whenever you encounter,
or can reasonably expect to encounter, significant turbulence. If you are flying at, or below, Va when a vertical upwards gust strikes the aircraft then the g loading will remain within limits. Also, because you are operating below Va you will be able to counteract any upset caused by turbulence by application of full control deflection, if this is required.

 

 

Airspeed Indicators
It is worth pointing out that there are many different types of airspeed indicators fitted to GA aircraft and there may be different ASIs, with different markings, fitted to the same model aircraft. Some ASIs will be calibrated in mph while others will show knots. The markings on the ASIs will also vary. Some may have a green arc with the fast end of the arc ending at Vno, or even Vne. Others will have a green arc ending at Va, with a yellow arc continuing to a red line at Vne. It is essential to know exactly what the markings on YOUR ASI mean. What about the blue lines? Take a few minutes to correlate the markings on your ASI to the various figures give in the Pilots Manual, then you will know where the needle should be any circumstance. Taking a Cessna 310Q as an example (because I happen to have the Owners’ Manual handy), the airspeed limitations are quoted as:

 

Maximum Structural Cruising Speed Level Flight or Climb      210 MPH

Maximum Speed

Flaps Extended 15      180 MPH

Flaps Extended 15 - 35      160 MPH

Gear Extended      160 MPH

Maximum Manoeuvring Speed      170 MPH

 

The Airspeed Indicator markings in this aircraft were as follows:

 

Never Exceed (glide or dive, smooth air)        257 MPH (red line)

Caution Range      210 – 257 MPH (yellow arc)

Normal Operating Range      86-210 MPH (green arc)

Flap Operating Range (0 - 35)        73.5 – 160 MPH (white arc)

Minimum Control Speed        86 MPH (red radial line)

Best Single-engine Rate of Climb      117 MPH (blue radial line)

 

All of these speeds are given in MPH. The Cessna 310Rs that I flew had their ASIs calibrated in knots and had slightly different limitations, so it is necessary to be sure that you are thinking about the aircraft you are flying to-day, not about the similar one you are used to flying most of the time.

 

Gusting on Approach
Having considered the effects of vertical gusts on an aircraft operating towards the fast end of the envelope, it is worth a moment to think about the effects of gusts at the slow end, like when flying an approach.
      Your Pilots Manual will specify the correct approach speed. It may list several speeds for different weights and for different flap settings, but for simplicity, we will assume that correct approach speed is 72kt. That means that your 1g stalling speed in that configuration is 60kt. (The approach speed is almost always set at 1.2Vs). If you are trying to perform a short landing onto a restricted length strip you should still fly the approach at 72kt and not be tempted to try to drag the aircraft in at a lower airspeed (and higher power?). Let’s see why. If you decide to fly the approach at 65kt instead of 72kt then you are already at a higher angle of attack and closer to the stalling angle. Just how close it is impossible to say without tedious calculations. Suppose you meet an updraft on the approach. A I knot vertical gust will produce an instantaneous one degree extra angle of attack. A 5 knot vertical gust will instantly add five degrees to the angle of attack, so your aircraft will certainly stall although the ASI appears to be giving you a 5 knot margin above the stalling speed.
      If you and your passengers are lucky enough to walk away from the wreckage you are then faced with the task of persuading the accident investigators that ‘the wind shear snakes’ got you and the accident had nothing whatsoever to do with your error of skill.

 

 

Summary
Flying into turbulence in a light aircraft at cruising speed should make you immediately consider reducing your airspeed to below Va. Why not look it up for your aircraft and remember it? Flying at Va when necessary will not only reduce wear and tear on the airframe but will also make it safer and more comfortable for everyone on board. Fly the CORRECT speed on the approach and never be suckered into trying to fly the approach slowly in an attempt to shorten the landing run, then we won’t have to read about yet another pilot trying to blame the ‘wind shear snake’ which is reputed to lurk on runway approaches waiting to grab innocent and blameless pilots.

 

 

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