V-Speeds: A Complete Reference

V-speeds are the reference airspeeds that define an aeroplane's limits and the way it is flown. Each is written as a capital V with a subscript that identifies it: VS for a stall speed, VR for rotation, VFE for the flap limit, VMO for the maximum operating speed. Some are fixed for the type and painted on the airspeed indicator. Others change with mass, configuration and conditions and are calculated for every flight, then set as bugs on the speed scale.

The definitions come from the certification codes. CS-Definitions and CS-25 in Europe, and 14 CFR Part 1 and Part 25 in the United States, define the large-aeroplane speeds in essentially the same wording, so their symbols and factors are the same in EASA and FAA material; light aeroplanes are certified under CS-23 and Part 23. Operating rules differ more, and where EASA and FAA usage differs this article says so. This article is an index: it defines each speed, gives the figures most often examined, and points to the articles that explain them in depth.

How V-speeds are defined

Most V-speeds fall into one of three groups:

• Design speeds, chosen by the manufacturer to size the structure, such as VA, VC and VD. They are equivalent airspeeds, because equivalent airspeed measures the dynamic pressure that loads the airframe.
• Limits, which the pilot must respect and which are shown on the airspeed indicator or in the flight manual, such as VNE, VMO, VFE and VLE.
• Operating speeds, calculated for the mass and conditions of each flight, such as V1, VR, V2 and VREF. The flight manual schedules the take-off speeds as calibrated airspeeds.

Many operating speeds are defined as multiples of a stall speed or a minimum control speed. The V2 of a twin-engined jet, for example, must be at least 1.13 times the reference stall speed and 1.10 times VMCA, so the margins built into one speed depend on how the others are set. FAA texts often write the zero of VS0 as a letter O, VSO; the meaning is the same.

Stall and reference stall speeds

A stall is set by angle of attack, not speed, so every stall speed is quoted for a mass, a configuration and a load factor (see stall). Stall speed varies with the square root of mass and with the square root of load factor: a 60° banked level turn, at 2 g, raises it by about 41 per cent.

Large aeroplanes certified since Amendment 25-108 of Part 25, and under CS-25, use VSR, a 1 g stall speed, instead of the lower minimum speed seen during a stall entry. That is why the factors changed: V2 went from 1.2 VS to 1.13 VSR, and the landing reference speed from 1.3 VS0 to 1.23 VSR0. Where a stick pusher is fitted, VSR may not be less than 2 kt or 2 per cent, whichever is greater, above the pusher speed. Stall warning must begin at least 5 kt or 5 per cent of calibrated airspeed, whichever is greater, above the speed at which the stall is identified.

Climb speeds: VX, VY and V2

VX, the best angle of climb speed, gives the greatest height for the distance flown; it is the speed of maximum excess thrust and the one for clearing an obstacle. VY, the best rate of climb speed, gives the greatest height in a given time; it is the speed of maximum excess power and is higher than VX. The two converge with altitude and meet at the absolute ceiling (see climb performance and VX and VY).

Light twins add one-engine-inoperative versions: VXSE, the best angle, and VYSE, the best rate of climb or least rate of descent, marked by a blue radial line on the airspeed indicator.

V2, the take-off safety speed, is the one-engine-inoperative climb speed of a large aeroplane, reached by 35 ft above the take-off surface and held to the acceleration altitude. It may not be less than 1.13 VSR for most jets and for two- and three-engined turboprops, 1.08 VSR for turboprops with more than three engines and for jets with provisions to reduce the one-engine-inoperative stall speed significantly, and 1.10 VMCA. At the end of the take-off path, in the en-route configuration, the aeroplane reaches VFTO, the final take-off speed, which must be at least 1.18 VSR. Airbus types add green dot, the engine-out operating speed in the clean configuration, at the best lift-to-drag ratio, which also gives the maximum climb gradient.

For EASA performance Class B aeroplanes the take-off safety speed at 50 ft must be at least 1.2 VS1 and, on multi-engine aeroplanes, at least 1.1 VMC.

Take-off speeds: VEF, V1, VR, VLOF and VMU

The speeds fall in a fixed order: VMCG ≤ VEF < V1 ≤ VR < VLOF, with V2 reached by 35 ft. V1 is not a moment of thought but a deadline for action, and the stop it protects must fit in the accelerate-stop distance available. A tyre speed limit, expressed as a ground speed, can also cap the lift-off speed on hot, high or tailwind departures. How the speeds are set, balanced field length and rotation technique are in take-off speeds: V1, VR and V2.

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Light aeroplanes have simpler rules. Under CS-23 up to Amendment 4, the basis of most light aeroplanes in service, a single-engine aeroplane's VR may not be less than VS1; a multi-engine one needs at least 1.05 VMC and 1.1 VS1.

Control speeds: VMCG, VMCA and VMCL

The minimum control speeds are handling limits for a multi-engine aeroplane whose critical engine fails. Below them the rudder, and at low speed the ailerons, cannot balance full thrust on one side (see minimum control speeds).

• VMCG, on the ground: the lowest calibrated airspeed on the take-off run at which control can be kept after a sudden failure of the critical engine, using the rudder alone without nosewheel steering, with no more than 150 lbf of pedal force and no more than 30 ft of lateral deviation from the centreline.
• VMCA, in the air: the lowest speed at which straight flight can be held with the critical engine inoperative, using no more than 5° of bank towards the live engine. It is demonstrated with take-off thrust, the most unfavourable centre of gravity, take-off flap, gear retracted and the dead engine's propeller windmilling unless it feathers automatically. On a light twin it is the red radial line near the bottom of the airspeed indicator.
• VMCL, in the landing configuration: as VMCA but with go-around thrust, plus the ability to roll through 20° away from the dead engine within 5 seconds.

The A320 gives typical values: VMCG 111 kt in CONF 1+F at sea level, VMCA 110 kt and VMCL 113 kt. VSSE, the safe single-engine speed, now called the safe, intentional one-engine-inoperative speed by the FAA, is the lowest speed at which a light twin's engine should be shut down deliberately in training. For EASA Class B multi-engine aeroplanes, VMC may not exceed 1.2 VS1.

Structural and configuration speeds

The logic behind them is set out in structural and operating speed limits. Two points are examined again and again. VA protects against one full, abrupt input on one axis, not against reversals or several axes at once, and it is lower at light weight: 20 per cent below maximum weight it is about 10 per cent lower. VMO is an indicated airspeed limit for dynamic pressure, while MMO guards against compressibility at altitude, taking over above the crossover altitude; the A320's figures are 350 kt and M 0.82.

Configuration limits are type figures. The A320's VFE runs from 230 kt in CONF 1 to 177 kt in CONF FULL; its VLE is 280 kt or M 0.67, and its VLO is 250 kt or M 0.60 for extension and 220 kt or M 0.54 for retraction. VLO is often the lower gear speed because the gear and doors in transit are unlocked and exposed to the airflow.

Approach and landing speeds: VREF, VAPP and VLS

VREF, the reference landing speed, is the speed at 50 ft over the threshold on which the certified landing distance is based. For CS-25 and Part 25 aeroplanes it is at least 1.23 VSR0 and not less than VMCL; for EASA Class B aeroplanes, at least 1.3 VS0. It varies with the square root of mass, so 10 per cent less mass lowers it by about 5 per cent, and more flap lowers it further. Speed carried beyond VREF costs runway: the FAA estimates that 10 per cent excess speed at the threshold adds about 20 per cent to the landing distance.

VAPP, the final approach speed, adds margins for wind and gusts. Boeing adds wind additives to VREF. The A320 starts from VLS, the lowest selectable speed, adding the higher of 5 kt and one third of the tower headwind component, with the wind correction limited to 15 kt. VLS itself is 1.13 VS during take-off, 1.23 VS after one step of flap retraction and 1.28 VS clean (see A320 characteristic speeds).

A separate speed sets the approach category. VAT, the indicated airspeed at the threshold, is 1.3 VS0 or 1.23 VS1G, whichever is higher, in the landing configuration at maximum certificated landing mass; the FAA (14 CFR 97.3) uses VREF, or 1.3 VSO where no VREF is specified, at maximum certificated landing weight.

The go-around has speeds of its own: the one-engine-inoperative approach climb is flown at no more than 1.4 VSR in the approach configuration, and the all-engines landing climb at VREF (see approach speeds and go-around climb requirements).

Colour arcs and speed tapes

On a light aeroplane the airspeed indicator carries the fixed speeds as colours:

• the white arc, the flap operating range, from VS0 to VFE;
• the green arc, the normal operating range, from VS1 to VNO;
• the yellow arc, the caution range for smooth air only, from VNO to VNE;
• the red radial line at VNE;
• on light twins, a red radial line for VMCA and a blue one for VYSE.

Stall speeds are shown by the lower ends of the arcs, never by a radial line. VA, VX, VY, VLO and VLE are not marked; the pilot takes them from the flight manual (see airspeed indicator and Machmeter).

Jet transports show limits that move. A round-dial indicator uses the red and white barber pole for VMO/MMO, which moves down the scale once the Mach limit takes over. A speed tape draws a red and black bar for the maximum allowable speed: on the 737 the lowest of VMO/MMO, the gear placard speed and the flap placard speed. At the low end the 737 shows amber minimum manoeuvre speed bars; the A320 shows VLS, the angle of attack protection speeds VαPROT and VαMAX and, when the protections are lost, VSW. The calculated speeds of the day, V1, VR and V2 for take-off and REF for landing, appear as bugs beside the tape.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.