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.
Text and Photographs © 2008 Gremline & Hill House
Publications, unless otherwise stated.
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