Height/Velocity Diagram

A chart in every helicopter POH that maps combinations of altitude and airspeed against autorotative recovery. The shaded ("avoid") regions show flight regimes where, if the engine quits, you don't have enough altitude or airspeed to autorotate to a survivable touchdown. The H/V diagram is most relevant during takeoff and initial climb; the goal of a max-performance takeoff profile is to escape the shaded regions as quickly as possible.

Also called: H/V curve, dead man's curve, "the curve"

Reading the diagram

An H/V diagram has airspeed on the horizontal axis and height-above-surface on the vertical axis. Two shaded regions:

Upper shaded region (high altitude, low airspeed) — typically a roughly triangular zone covering ~10 to 400+ ft AGL at airspeeds below ~30–40 kt. From inside this region, an engine failure leaves you with too much altitude to land immediately and too little airspeed to fly out — the rotor RPM bleeds off before you can establish autorotation properly.
Lower shaded region (low altitude, high airspeed) — a smaller crescent close to the ground at high airspeeds. From inside this region, you have enough airspeed but not enough altitude to flare to a stop before ground contact.

Between the two regions is the safe corridor — combinations of altitude and airspeed where autorotation is feasible. Most cruise flight regimes are well inside the corridor.

Why the upper region is the dangerous one

The upper shaded region — the "dead man's curve" proper — is where most H/V-related accidents happen. The reason: many takeoff and approach profiles transit it briefly. If you depart vertically before transitioning forward (a confined-area technique sometimes used), you climb up through the shaded region.

From inside the upper region, autorotation is mechanically possible but extremely demanding: you must

Lower collective immediately (within seconds) to preserve rotor RPM.
Get the nose down to gain airspeed quickly.
Flare and touch down — all within the available altitude.

The H/V envelope assumes a 1-second pilot reaction time. In reality, surprise + recognition + decision often exceeds 1 second, especially at lower-time pilot levels. The shaded region is therefore conservative — but pilots routinely operate near or inside it.

The lower region — high speed, low altitude

The lower shaded region is the "scud running" risk. From low altitude at high speed, you have plenty of energy to autorotate, but no altitude to convert that energy into a flare maneuver. The autorotation entry would be a controlled crash into whatever is in front of you.

Operationally, this matters most for:

Low-level surveillance, photo, or pipeline patrol work.
Skip-style cruise close to obstacles.
Helicopter EMS, where low-and-fast often beats high-and-slow for response time.

The mitigation: maintain enough altitude to recover. Most operators set minimum altitudes for routine cruise (often 500 ft AGL or higher) precisely to stay clear of the lower shaded region.

The takeoff profile — how to escape the curve

A standard takeoff aims to escape the upper shaded region as quickly as possible. The classical profile:

Hover-check IGE.
Begin a small forward acceleration while still in IGE — building airspeed before climbing.
As airspeed approaches ETL, transition to a climb. By the time you've reached the back of the H/V curve in altitude, you should already have enough airspeed to be in the safe corridor.
Continue climb to cruise altitude.

Confined-area takeoffs and pinnacle departures may force you to climb vertically through the curve. That's a deliberate accepted risk — you're trading H/V exposure for clearing an obstacle. The mitigation is to spend as little time in the shaded region as possible: don't dawdle in a hover at 50 ft AGL while you sort yourself out.

What expands the curve

The H/V diagram in your POH represents one configuration: a specific gross weight, density altitude, and rotor RPM. Move away from those reference conditions and the shaded region grows:

Higher gross weight — more energy to dissipate, less rotor inertia margin → bigger curve.
Higher density altitude — less power, less rotor efficiency → bigger curve.
Lower rotor RPM — less stored kinetic energy in the rotor → bigger curve.
Tailwind — reduces effective airspeed margin during the takeoff transition.

For a high-DA, max-gross-weight, downwind takeoff, the H/V curve in your POH may not be valid at all. POH H/V data is usually published for one or two reference conditions; outside those, the conservative assumption is "the curve is bigger than the chart shows."