Operating Flight Strength (2024)

The V_A maneuvering speed in your flight manual is just a number picked by your manufacturer to satisfy the requirements of 14 CFR 25.335 which only requires the speed cannot be less than the stalling speed of the aircraft with the flaps retracted and does not specify a weight or altitude. If you would like the maneuvering speed for any other configuration, weight, or altitude, you will need to construct a V-g diagram for those conditions.

I did precisely that when called upon to conduct combat operations in an Air Force Gulfstream III into Sarajevo back when there was a shooting war going on. (See At Least They Aren't Shooting at Us.) The AFM published V_A of 206 knots would have been the wrong speed to "yank and bank" under enemy fire on takeoff where the correct speed would have been 196 knots, or on arrival where the correct speed was 151 knots. A 55 knot error seemed significant enough, but how many people are using V_A in the peacetime world?

There is an increased emphasis on maneuvering aircraft in an Unusual Attitude situation and some are advocating that flying at V_A gives you a license to maneuver to your heart's content. It does not but studying a V-g diagram helps you understand your aircraft's operating envelope.

I shall construct three V-g diagrams and derive V_A for a Gulfstream G450 for these conditions as a demonstration for others wanting to do the same, and to provide the numbers for myself:

• Typical Landing at 1,000' Pressure Altitude (Gear Down, Flaps 39°, 50,000 lbs)
• Heavy Weight Takeoff at 1,000' Pressure Altitude (Gear Up, Flaps 20°, 74,600 lbs)
• Heavy Weight Climb at 3,000' Pressure Altitude (Gear Up, Flaps Up, 70,000 lbs)

I'll go through the steps in detail for the first example and then more quickly for those that follow.

Landing - stall charts

Using Gulfstream G450 performance charts, we see the flaps 39° stall speed at 50,000 lbs and 1,000 feet pressure altitude is 98 KCAS.

Landing - Derive Range of Stall Speeds for G-Envelope

Recall that

$$V_S=V_{\mathrm{S(1G)}}\sqrt{\mathrm{|G|}}$$

We can use that formula to derive a range of stall speeds for varying G loads:

We plot that on a graph of load factors on the vertical axis and speeds on the horizontal axis.

We can fit a curve to replace the dots. Because of the stall chart value was rounded to the nearest 1 knot, errors are magnified on this chart so the curve may not fit exactly. As will be clear a few steps later, the critical bit is the KCAS for the load factor limit.

Landing - add G-limits

We know from G450 limitations the flight load acceleration limits with Flaps 39° up to landing weight are 0 to 2.0 G. We can plot these as well.

Because the V-g Diagram should show the aircraft's envelope, we can add the limiting airspeed to the chart.

Landing - determine corner speed (V_A)

V_A maneuvering speed is where the maximum stall speed intersects with the maximum load factor. Graphically, it occurs in the top left corner, the reason some call this "corner speed.'

The G450 published V_A of 206 KCAS is much higher than the true V_A for a G450 in landing configuration at 50,000 lbs gross weight and 1,000 feet pressure altitude. Full control deflection at 206 knots in this situation could very well overstress the aircraft.

Takeoff - stall charts

Using Gulfstream performance charts, we see the flaps 20° stall speed at 74,600 lbs and 1,000 feet pressure altitude is 132 KCAS.

Takeoff - derive range of stall speeds for G-envelope

Recall that:

$$V_S=V_{\mathrm{S(1G)}}\sqrt{\mathrm{|G|}}$$

We can use that formula to derive a range of stall speeds for varying G loads:

Takeoff - add G-limits

We know from G450 limitations the flight load acceleration limits with Flaps 20° are 0 to 2.0 G.

Takeoff - Determine Corner Speed (V_A)

Just as we did for the landing example, we can plot the G-Limits, airspeed limits, and find the corner speed. In this case, V_A = 187 KCAS, still much lower than the published number of 206 KCAS.

Climb - stall charts

Using Gulfstream performance charts, we see the flaps 0° stall speed at 70,000 lbs and 3,000' pressure altitude is 130 KCAS.

Recall that

$$V_S=V_{\mathrm{S(1G)}}\sqrt{\mathrm{|G|}}$$

Because the load factor limit now includes negative G-loads, we must also plot the speeds at -1.0 G.

Climb - Add G-limits and determine corner speed

We know from G450 limitations the flight load acceleration limits with Flaps 0° are -1.0 to 2.5 G.

Now we finally see a V_A that equals the published number of 206 KCAS. This isn't the only one, there are many combinations of weight and pressure altitudes that will yield this number.

G450 V_A

As we can see, the published V_A is rarely right. Do you need to memorize an continuum of V_A's? No, not at all. Maneuvering speed is a certification issue. When controlling your aircraft, do so with a gentle hand and you can avoid overstressing or stalling without the benefit of this made up number. The next time a simulator instructor recommends you fly this speed and the maneuver as required, nod politely and ignore the suggestion.