The Height-Velocity diagram or H/V curve is a graph

Graphics

Graphics are visual presentations on some surface, such as a wall, canvas, computer screen, paper, or stone to brand, inform, illustrate, or entertain. Examples are photographs, drawings, Line Art, graphs, diagrams, typography, numbers, symbols, geometric designs, maps, engineering drawings,or...

 charting the safe/unsafe flight profiles relevant to a specific helicopter

Helicopter

A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...

. As operation outside the safe area of the chart can be fatal

Death

Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

 in the event of a power or transmission failure it is sometimes referred to as the dead man's curve or Coffin Corner

Coffin corner (aviation)

The coffin corner is the altitude at or near which a fast fixed-wing aircraft's stall speed is equal to the critical Mach number, at a given gross weight and G-force loading. At this altitude the airplane becomes nearly impossible to keep in stable flight...

 by helicopter pilots

Aviator

An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...

.

In the simplest explanation, the H/V curve is a diagram

Diagram

A diagram is a two-dimensional geometric symbolic representation of information according to some visualization technique. Sometimes, the technique uses a three-dimensional visualization which is then projected onto the two-dimensional surface...

 where the shaded areas should be avoided, as the average pilot may be unable to complete an autorotation

Autorotation (helicopter)

Autorotation is the state of flight where the main rotor system of a helicopter is being turned by the action of air moving up through the rotor rather than engine power driving the rotor...

 landing without damage. The H/V curve will usually contain a take-off profile, where the diagram can be traversed from 0 height and 0 speed to cruise, without entering the shaded areas or with minimum exposure to shaded areas.

The portion in the upper left of this diagram demonstrates a flight profile which will likely not allow the pilot to successfully complete an autorotation, primarily due to having insufficient airspeed to enter an autorotative profile in time to avoid a crash. The shaded area at the lower left is not portrayed in H/V curves for multiengine helicopters capable of safely hovering and flying with a single engine failure. The shaded area on the lower right is dangerous due to the airspeed and proximity to the ground resulting in dramatically reduced reaction time for the pilot in the case of mechanical failure, or other in-flight emergencies

Emergency

An emergency is a situation that poses an immediate risk to health, life, property or environment. Most emergencies require urgent intervention to prevent a worsening of the situation, although in some situations, mitigation may not be possible and agencies may only be able to offer palliative...

.

The following examples further illustrate the relevance of the H/V curve to a single engine helicopter.

At low height

Height

Height is the measurement of vertical distance, but has two meanings in common use. It can either indicate how "tall" something is, or how "high up" it is. For example "The height of the building is 50 m" or "The height of the airplane is 10,000 m"...

s with low airspeed

Airspeed

Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....

, such as a hover taxi, the pilot can simply cushion the landing

Landing

thumb|A [[Mute Swan]] alighting. Note the ruffled feathers on top of the wings indicate that the swan is flying at the [[Stall |stall]]ing speed...

 with collective, converting rotational inertia

Angular momentum

In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

 to lift

Lift (force)

A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

. The aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 is in a safe part of the H/V curve. At the extreme end of the scale (say a three-foot hover taxi at walking pace) even a complete failure to recognise the power loss resulting in an un-cushioned landing would probably be survivable.

As the airspeed increases without an increase in height, there comes a point where the pilot's reaction time would be insufficient to react with a flare in time to prevent a high-speed, and thus probably fatal, ground impact. Even small increases in height give the pilot much greater time to react, thus the bottom right part of the H/V curve is usually a shallow gradient

Gradient

In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....

. If above ideal autorotation speed, the pilot's instinct is usually to flare to convert speed to height and increase rotor RPM through coning; which also immediately gets them out of the dead man's curve.

Conversely, an increase in height without a corresponding increase in airspeed will put the aircraft above a survivable un-cushioned impact height, and eventually above a height where rotor inertia can be converted to sufficient lift to enable a survivable landing. This occurs abruptly with airspeeds much below the ideal autorotative speed (typically 40-80 knots). The pilot must have enough time

Time

Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

 to accelerate

Acceleration

In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...

to autorotation speed in order to be able to successfully autorotate; this directly relates to a requirement for height. Above a certain height the pilot can achieve autorotation speed even from a 0-knot start, thus putting "high hovers" outside the curve.

The typical safe takeoff profile will involve initiation of forward flight from a low hover, only gaining height as airspeed approaches a safe autorotative speed but keeping above the lower right area of the H/V curve.