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  Maneuvering Speed VA

Gulfstream G450

I get a lot of questions about this from the Gulfstream community, so let me explain. We Gulfstream pilots have that number, 206 knots, drilled into our heads by well meaning simulator instructors and less than knowledgeable Gulfstream test pilots saying if we get to VA we can maneuver to our heart's content without fear of stalling or overstressing the airplane. I have been hearing that in the Gulfstream world since 1991.

Well let me be gentle about this: 206 knots is a lie. How is it that you can have full control deflection and not stall or overstress a GIII and a G650 at the exact same speed? And you can do this at any weight or altitude? Yes, the published numbers are identical. And no, 206 knots almost never works for maneuvering.


 

Published Gulfstream Numbers

The trouble with a single published VA is that the true number varies with airplane configuration, weight, and altitude. "Nonsense!" many say, "there is no mention of that in the AFM." Well, take a look at this:

GIII:

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Figure: GIII VA, from C-20B-1, page 5-2.

GIV:

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Figure: GIV VA, from GIV Airplane Flight Manual, §01-03-60, ¶2.b.

GV:

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Figure: GV VA, from GV Airplane Flight Manual, §1-03-40

G450:

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Figure: G450 VA, from G450 Airplane Flight Manual, §1-03-40, ¶2.

G550:

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Figure: G550 VA, from G550 Airplane Flight Manual, §1-03-40.

G650:

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Figure: G650 VA, from G650 Airplane Flight Manual, §1-03-40, ¶2.

How could they all have the exact same speed in knots that permits full maneuvering? The answer is they do but each airplane achieves that number in remarkably different conditions. A better description of VA would be in the form of a chart.

An Example from Bombardier

Global 6000:

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Figure: Global 6000 VA, from Bombardier Global 6000 Flight Crew Operating Manual, Volume 1, §02-06-3, Figure 02-06-2.

A method of conveying maneuvering speed that is more pure, from the view point of an aeronautical engineer, would be to provide a chart like this one. But as a pilot this thing is useless; when you need to know what your maneuvering speed is you clearly don't have the time for this. It is clear that the Gulfstream method is easier, but wildly inaccurate. But it is equally clear that the Bombardier method is of no use to a pilot.

Regulatory

The published VA in your flight manual is a regulatory requirement from 14 CFR 25. It stipulates that the aircraft flaps are retracted but allows the manufacturer to choose a weight and altitude with no further conditions:

[14 CFR 25.335 Design Airspeeds] The selected design airspeeds are equivalent airspeeds (EAS). Estimated values of VS0 and VS1 must be conservative.

(c) Design maneuvering speed VA. For VA, the following apply:
(1) VA may not be less than VS1 √n where—
(i) n is the limit positive maneuvering load factor at VC ; and
(ii) VS1 is the stalling speed with flaps retracted.
(2) VA and VS must be evaluated at the design weight and altitude under consideration.
(3) VA need not be more than VC or the speed at which the positive CN max curve intersects the positive maneuver load factor line, whichever is less.

What it Really Means

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Figure: V-G Diagram, from Dole, pg. 193

[Dole, pg. 193-194] An interesting point on the V-G diagram is the intersection of the aerodynamic limit line and the structural limit line. The aircraft's speed at this point is called the maneuver speed, commonly called the corner speed. At any speed below this speed the aircraft cannot be overstressed. It will stall before before the limit load factor is reached. Above this speed, however, the aircraft can exceed the limit load factor before it stalls. At the maneuver airspeed the aircraft's limit load factor will be reached at the lowest possible speed.

From this explanation it becomes clear what matters to a pilot: VA is the speed below which the aircraft cannot be overstressed and above which the aircraft cannot be stalled. Of course there is an exception to the "cannot be overstressed" idea: rudder reversal.

More about this: American Airlines 587.

Note that 14 CFR 25 does not specify an altitude or weight for Va, but clearly the speed at which the aircraft will stall changes with both of these factors. While the regulation does specify a clean configuration, clearly you will also have different VAs for various gear and flap settings.

If you would like to know more about maneuvering speed and how it is computed, see: Operating Flight Strength.

Gulfstream Example

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Gulfstream does not give you anything more than this when it comes to maneuvering speed: VA = 206 knots.

With a little work, see Operating Flight Strength for a walk-through on how to do this, you can find VA for any weight, altitude, and flap configuration. The chart shown here is for a G450 at 70,000 lbs, 3,000' pressure altitude, clean configuration.

So this is one case, of many, where a G450 does have a VA of 206 knots. But the VA for varying weights, altitudes, and configurations are quite different:

Gulfstream G450 Maneuvering Speed (Sea Level)
GearFlapsWeight VA
Down 39 50,000 lbs139 knots
Up 20 74,600 lbs187 knots
Up Up 70,000 lbs206 knots

So all we have proven, so far, is that the real VA can only be 206 knots for a specific set of circumstances. We still have work to do.

G450 / G550 / GV Rule of Thumb

So how you going to keep all this straight? An interesting point about the V-G Diagram is that corner where cornering speed, VA, lies. It is at the intersection of stall speed and the G-limit. Using a lot of math, explained in Operating Flight Strength, we can see that:

VS(corner)=VS(1G)|G|

Where VS(corner) is the stall speed at VA for the conditions of the chart, VS(1G) is the stall speed for 1 G flight at those same conditions, and G is the G-limit for those conditions.

In the G450 we only have two positive G-limits to worry about. With the flaps up it is +2.5 G and with any flaps at all it is +2.0 G. Since VS(corner) = VA, we find that: VA(flaps up)=VS(1G)2.5 =VS(1G)1.58

and

VA(flaps down)=VS(1G)2 =VS(1G)1.41

The fact VA tracks with VS helps us tremendously, but having multiples of 1.41 and 1.58 are less than easy to deal with. We do have a Gulfstream recommendation to always keep our speed at least 1.5 times greater than VS when in turbulent air. See: G450 Turbulence Penetration. I think shooting for 1.5 times VS gets you very close to VA under most conditions.

We don't have a ready display of VS(1G) in the cockpit and you can't very well be looking up charts if you find the aircraft upset by wake turbulence or have another reason to wonder about maneuvering speed. But you do have this:

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Photo: Display Controller With VREF, from Eddie's aircraft.

The display controller gives you an accurate VREF based on the flap handle position all the way to 15,000' and from there all the way to 45,000' it appears to be very close. See: G450 VREF / Display Controller for more about this.

Remember that in these airplanes VREF = (1.3) VS(1G). So the number in front of you is 30% higher than stall speed. We've decided our VA is about 1.5 VS, so we need a relationship between VA and VREF:

VRA = VREF ( 1.5 1.3 ) = VREF ( 1.15 )

Rule of Thumb: Maneuvering speed in a G450 / G550 / GV is 15% above the display controller VREF.

I always leave the display controller on the FLT REF page. If, for any reason, I ever need to know what VA is, I know I just need to add 15% to the VREF and I've got it.

More?

References

14 CFR 25, Title 14: Aeronautics and Space, Federal Aviation Administration, Department of Transportation

Bombardier Global 6000 Flight Crew Operating Manual, Revision 2, Dec 21, 2011

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013.

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G550 Airplane Flight Manual, Revision 27, July 17, 2008

Gulfstream G650 Airplane Flight Manual, Revision 4, July 17, 2013

Gulfstream GIV Airplane Flight Manual, Revision 30, 11 October 2002

Gulfstream GV Airplane Flight Manual, Revision 30, 13 May 2008

Technical Order 1C-20B-1, C-20B Flight Manual, USAF Series, 1 November 2002

Revision: 20140614
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