Robinson R44 Raven II helicopter hovering

Dead Man’s Curve: The Helicopter Height-Velocity Diagram Explained

by Connor Kerr | Sep 28, 2026 | Mondo dell'aviazione | 0 comments

A helicopter hovering 200 feet above a field looks like the safest thing in the sky. It is going nowhere, slowly. But if its single engine stops at that moment, the pilot is in one of the worst places a helicopter can be: too high to simply drop onto the skids, and too slow to glide down safely.

Pilots call that zone the dead man's curve. On paper it is the shaded part of a chart called the height-velocity diagram, printed in the flight manual of every helicopter. Understanding it explains why helicopters take off the way they do, and why the first moments of a flight get so much attention from instructors and accident investigators.

Informazioni rapide

Nome ufficiale: Height-velocity (H/V) diagram
Soprannome: The dead man’s curve
What it shows: Combinations of height and airspeed from which an average pilot can land safely after an engine failure
Danger zone 1: Low airspeed at moderate height: too high to cushion the landing, too slow to enter autorotation
Danger zone 2: High airspeed very close to the ground: too little reaction time
Key manoeuvre: Autorotation: the rotor keeps turning on the air rushing up through it
Ideal autorotation speed: Typically 40 to 80 knots, according to the FAA

How a Helicopter Glides Without an Engine

A helicopter with a dead engine is not a falling rock. A freewheeling clutch disconnects the engine from the main rotor, and if the pilot immediately lowers the collective, air flowing up through the descending rotor keeps the blades spinning. That is autorotation, and it is analogous to the glide of a fixed-wing aircraft. It is so central to safety that every single-engine helicopter must demonstrate it to be certified.

Near the ground, the pilot raises the nose and pulls collective in a flare, trading the rotor's stored energy and the helicopter's speed for a last moment of lift to cushion the touchdown. The famous extreme case is Jean Boulet, who set a helicopter altitude record of 12,440 metres in an Aérospatiale Lama in 1972; his engine flamed out in the cold at the top, and he autorotated all the way down to a safe landing.

Reading the Dead Man's Curve

Autorotation needs two things: enough airspeed to set up a stable glide, or enough height to build that airspeed, and enough time for the pilot to react. The height-velocity diagram maps where those conditions are not met.

Sample height-velocity diagram for a Robinson R44 II helicopter — The FAA’s sample height-velocity diagram for a Robinson R44 II. The hatched area on the left is the dead man’s curve; the strip along the bottom right is the low, fast danger zone; the dashed red line is the recommended take-off profile. Image: FAA Helicopter Flying Handbook (FAA-H-8083-21B), public domain

“In the simplest explanation, the H/V diagram is a diagram in which the shaded areas should be avoided, as the pilot may be unable to complete an autorotation landing without damage.”

Amministrazione federale dell'aviazione — Helicopter Flying Handbook (FAA-H-8083-21B), chapter 7

The large shaded area on the left covers low speeds from a few feet up to several hundred feet. Hovering or crawling forward there, a pilot whose engine quits has neither the height to accelerate to autorotation speed nor a height low enough to survive simply dropping. The FAA notes that above a certain height, even a zero-airspeed start leaves enough room to dive and build speed, which is why very high hovers sit outside the curve.

The strip along the bottom right is the opposite problem: fast and very low. An engine failure there leaves almost no time for the flare before the helicopter hits the ground at speed.

“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.”

Amministrazione federale dell'aviazione — Helicopter Flying Handbook (FAA-H-8083-21B), chapter 7

Why Helicopters Take Off Like That

Watch a well-flown helicopter depart from a field. It lifts to a low hover, then noses forward and accelerates along the ground before climbing. That is not style. The FAA describes the typical safe take-off profile as starting forward flight from a two to three foot hover and gaining height only as the aircraft accelerates towards a safe autorotation speed, keeping it out of the shaded areas.

The diagram also changes with conditions. It is valid only within the weight and density-altitude limits in the flight manual; on a hot day at a high airfield, or heavily loaded, the dangerous region grows. And helicopters are not forbidden from entering the curve. Crop spraying, film work, rescues and hoisting all happen there. The FAA's point is that the pilot should weigh the risk of the manoeuvre against its value.

OH-6A Cayuse student practising hovering autorotations in Vietnam — A student practises hovering autorotations in an OH-6A Cayuse at the US Army Vietnam Cayuse Transition School at Vung Tau. Photo: SP5 Dennis D. Connell, US Army / National Archives, public domain

Two Engines Change the Picture

The low, fast danger zone on the bottom right is not shown on H/V diagrams for multi-engine helicopters that can hover and fly safely with one engine failed, according to the FAA. That is one reason offshore, air ambulance and many military operators prefer twin-engine types. For the many single-engine helicopters used for tours, training and private flying, the dead man's curve remains part of every departure.

Real engine failures do happen, and pilots who have trained autorotations many times often bring the aircraft down intact. When they do not, the aircraft's height and speed at the moment of failure are usually one of the first things investigators look at.

Sources: FAA Helicopter Flying Handbook (FAA-H-8083-21B), chapters 7 and 11; Wikipedia (Autorotation); U.S. National Archives image records.

Domande frequenti

What is the dead man’s curve in a helicopter?

The dead man’s curve is the nickname for the shaded area of a helicopter’s height-velocity diagram. It marks combinations of height and airspeed from which a pilot may be unable to complete a safe autorotation landing if the engine fails, typically low speed at moderate height.

What does a height-velocity diagram show?

According to the FAA, the height-velocity diagram shows the combinations of airspeed and height above the ground that allow an average pilot to land safely after an engine failure. Shaded areas mark combinations to avoid, and many diagrams include a recommended take-off profile.

What is autorotation?

Autorotation is when a helicopter’s main rotor keeps turning without engine power, driven by air flowing up through the rotor as the helicopter descends. A freewheeling unit disconnects the engine, and the pilot uses the stored rotor energy for a flare to cushion the landing.

Why do helicopters accelerate forward before climbing on take-off?

Gaining airspeed close to the ground before climbing keeps the helicopter out of the dangerous parts of the height-velocity diagram. The FAA describes starting forward flight from a two to three foot hover and climbing as the aircraft approaches a safe autorotation speed.

Do twin-engine helicopters have a dead man’s curve?

Multi-engine helicopters that can hover and fly safely on one engine do not show the low, fast danger zone on their height-velocity diagrams, according to the FAA. Their exposure to engine failure near the ground is much lower than for single-engine helicopters.

What is the highest autorotation ever flown?

In 1972 Jean Boulet set a helicopter altitude record of 12,440 metres in an Aérospatiale SA 315B Lama. At that height the engine flamed out in the extreme cold and could not be restarted, and he autorotated down to a safe landing.