Limitations and Flight Envelope: V-Speeds, Structural and Aerodynamic Limits, Wake Turbulence and Windshear
This domain tests the boundaries the aeroplane must stay inside - aerodynamic, structural and operational - and the hazards that can push it out: wake turbulence and windshear. Items ask what a V-speed means, when it changes with weight, and how to avoid or escape the hazard.
V-speeds
- Vs is the stall speed, Vr the rotation speed, Vx the best angle and Vy the best rate of climb, Va the design manoeuvring speed and Vb the design gust speed.
- Vno is the maximum structural cruising speed, Vne the never-exceed speed, Vfe/Vle/Vlo the configuration limits, and Vmca/Vmcg the engine-out limits.
- Va falls with weight: Va at weight W = Va(max) × √(W/Wmax). At or below Va, full control deflection is limited by the stall rather than the structure.
- Vne and Vno are IAS limits; Mmo is a Mach limit; either may be reached first at altitude.
Structural and aerodynamic limits
- Typical limit load factors are +3.8/−1.52 for normal category, +4.4/−1.76 for utility and +2.5/−1.0 for transport; ultimate is 1.5 × limit.
- The V-n diagram combines the stall boundary (aerodynamic), the load-factor boundary (structural) and the speed boundary.
Wake turbulence and windshear
- Wingtip vortices are counter-rotating, sink, drift with the wind, and are strongest when the generating aircraft is heavy, slow and clean.
- Avoid them by rotating before the preceding aircraft's rotation point and by landing above and upwind of its flight path; ICAO time and distance separations apply.
- Windshear is a sudden change of wind speed or direction. On approach, a decreasing headwind first improves and then degrades performance; recovery is full power, pitch towards the go-around attitude or stick shaker, and do not chase airspeed.
Worked example
Va is 120 kt at 3400 kg. At 2800 kg it becomes 120 × √(2800/3400) = 120 × 0.907 = 108.9 kt, about 109 kt. At the lower weight the wing stalls at a lower speed, so full deflection is safe at a lower speed; flying at the original 120 kt would exceed the design load in a full-deflection manoeuvre.
Traps
- Va is not constant with weight; it decreases as weight decreases.
- Wake vortices sink below the generator's flight path and drift with the wind; they do not stay at the same altitude.
- In windshear, lowering the nose to regain airspeed can fly the aircraft into the ground.
- Vne is an IAS limit, not a TAS limit.
- Exceeding a limit load factor is a structural event; exceeding CLmax is an aerodynamic one, and both appear on the V-n diagram.