Manoeuvring speed Va

Manoeuvring Speed (Va)

Va is the maximum speed at which a full, abrupt control deflection in a single axis cannot overload the aircraft. Above Va, a manoeuvre can cause structural damage.

Definition (CS-23, FAR 25)

Va = √(n_max) · Vs

with:

• n_max = maximum load factor (limit load factor, +3.8 g for Normal Category per CS-23)
• Vs = stall speed in the relevant configuration

Example C172 (Normal Category, n_max = +3.8 g):

• Vs (clean) = 50 KIAS
• Va = √3.8 × 50 = 97 KIAS (POH gives 99 KIAS — values can vary slightly by build)

How it works

Below Va (safe region)

On full control deflection:

• α rises quickly → CL rises → lift rises.
• When α reaches α_stall, aircraft stalls.
• Stall caps the lift → n stays < n_max → no structural damage.

Above Va (critical region)

On full control deflection:

• α rises quickly → CL rises → lift drastically grows (V² · CL).
• Stall does not occur (speed too high → α_stall not reached).
• Lift = n · W can exceed n_max → structural damage possible.

V-n diagram (flight envelope)

V-n diagram shows the limits:

n
+3.8|----+--------+    ← n_max (structural limit)
    | / +
    |/  |
+1.0|--*------+--+    ← cruise (1g)
    |        |  |
    |        |  |
 0  |--------*--*----→ V
    |     Vs   Va  Vno  Vne
    |        |
-1.5|--------+        ← n_min

• Left boundary: by stall (CL_max).
• Right boundary: Vne (max structurally tolerable at n=1).
• Upper boundary: n_max (structural limit).
• Lower boundary: n_min (negative structural limit, typ. −1.5 g for Normal Category).

Va values and load factor

Va depends on weight

Since Vs depends on weight (Vs ∝ √W) and n_max is constant:

Va (at weight W) = Va_MTOM × √(W / W_MTOM)

Example C172:

• MTOM 1043 kg: Va = 99 KIAS.
• 800 kg: Va = 99 × √(800/1043) ≈ 86 KIAS.

→ Lighter load: Va drops — important for Cessna 152 / 172 flights with fewer pilot+pax.

Practical significance

In turbulence

In strong turbulence: reduce speed to Va or below.

• Permitted: structural damage from full-strength gusts excluded.
• POH gives Va in the limits section.

On sudden manoeuvres

• Emergency avoidance (e.g. bird strike): no problems below Va.
• Aerobatic: only below Va (except in aerobatic-certified aircraft).

In wind shear

• When wind shear is expected: maintain approach speed below Va (usually already met).

Limitations

Important: Va protects only for single-axis full deflection. Combined manoeuvres across multiple axes can still overload — classic example: rudder reversal manoeuvre in A300 accident 2001 American Airlines 587 (vertical fin lost).

Source: NTSB AAR-04/04 American Airlines Flight 587, A300, Belle Harbor NY, 12 Nov 2001.

Other key V-speeds

| Vne | never exceed speed | | Vno | normal operating speed — max for normal operation, turbulence below | | Vfe | flap extended speed | | Vle | landing-gear extended speed | | Vlo | landing-gear operating speed | | Vy | best rate of climb | | Vx | best angle of climb | | Vs | stall speed clean | | Vs0 | stall speed landing config (full flaps) | | Vbg | best glide speed | | Va | manoeuvring speed (this lesson) |

Worked example

Cessna 172: in moderate turbulence cruise speed 110 KIAS. Pilot wonders: above Va?

• POH Va (MTOM): 99 KIAS.
• Current load: about 950 kg of 1043 MTOM.
• Va (950 kg) = 99 × √(950/1043) = 99 × 0.955 = 94 KIAS.
• 110 KIAS > 94 KIAS → pilot should reduce to 90 KIAS for the duration of turbulence.

Training recommendation

• Pre-flight: compute Va for current load or conservatively use Va(MTOM).
• In turbulence: speed at 90 % Va as buffer.
• Aerobatic exercises: exclusively in aerobatic-certified aircraft, observe Va.

CS-23 requirements

CS-23 Subpart C (structure) requires:

• Demonstration at Va: no damage on full control deflection.
• Vertical and lateral gust limits at Va.
• Vne and Vne-limit tests.