Fuel burn reduces weight, so the manoeuvring-speed value appropriate for departure may be too high later in the flight. The relationship is easier to remember by looking at the load-factor diagram than by memorising a placard.
What the speed means
Design manoeuvring speed, Va, is a structural design reference for specified control-input conditions. It is not permission to move every control to its stop. Repeated reversals, simultaneous inputs in different axes, and other loading conditions can still damage an aircraft below Va. Use the operating limitations and turbulence procedures for the particular aircraft.
In a simplified positive-load V–n diagram, the stall boundary rises with the square of airspeed. Its intersection with the positive limit-load line gives the familiar manoeuvring-speed illustration: beyond that intersection, the wing can generate a load above the limit before reaching its maximum lift coefficient.
Why weight changes the intersection
Load factor is lift divided by weight. At the same speed, configuration and maximum lift coefficient, a lighter aircraft reaches a greater load factor. The stall boundary therefore meets the same structural limit at a lower speed.
For an otherwise unchanged aircraft, the usual study approximation is:
Va at new weight ≈ reference Va × √(new weight / reference weight)
This follows from the lift equation: keeping the limiting load factor and maximum lift coefficient constant makes speed squared proportional to weight. Check the manufacturer's table before using an approximation operationally.
Worked example
Suppose the reference value is 99 kt at 2,550 lb and the new weight is 2,200 lb.
- Divide the weights: 2,200 / 2,550 ≈ 0.863.
- Take the square root: √0.863 ≈ 0.929.
- Multiply the reference speed: 99 × 0.929 ≈ 92 kt.
The weight fell by about 14%, while the estimated speed fell by about 7%. Reducing speed by the full weight percentage would apply the relationship incorrectly.
Check your understanding
- If weight becomes 81% of the reference, the estimated speed becomes 90% of its reference.
- A lower Va does not mean the structure became weaker as fuel burned. It means the same aerodynamic force produces a larger load factor on the lighter aircraft.
- Va is not a substitute for a published rough-air speed or for other aircraft limits. Identify the limit relevant to the manoeuvre before choosing a speed.
Further reading
FAA Safety Briefing, May/June 2015: understanding V-speeds and FAA Glider Flying Handbook, flight instruments and limitations. The calculation above is an illustrative study exercise.