Height/Velocity Diagram - Helicopter Emergencies and Hazards

The height/velocity diagram or H/V curve is a graph charting the safe/unsafe flight profiles relevant to a specific helicopter. As operation outside the safe area of the chart can be fatal in the event of a power or driveline failure, it is sometimes referred to as the dead man’s curve by helicopter pilots. By carefully studying the height/velocity diagram, a pilot is able to avoid the combinations of altitude and airspeed that may not allow sufficient time or altitude to enter a stabilized autorotative descent. A pilot may want to refer to this diagram during the remainder of the discussion on the height/velocity diagram. [Figure 1]

Figure 1. Height/velocity diagram

In the simplest explanation, the H/V curve is a diagram in which the shaded areas should be avoided, as the pilot may be unable to complete an autorotation landing without damage. The H/V curve usually contains a takeoff 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 that probably does not allow the pilot to complete an autorotation successfully, primarily due to having insufficient airspeed to enter an autorotative profile in time to avoid a crash. 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. This shaded area at the lower right is not portrayed in H/V curves for multi-engine helicopters capable of safely hovering and flying with a single engine failure.

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

At low heights with low airspeed, such as a hover taxi, the pilot can simply use the potential energy from the rotor system to cushion the landing with collective, converting rotational inertia to lift. The aircraft is in a safe part of the H/V curve. At the extreme end of the scale (e.g., a threefoot hover taxi at walking pace) even a complete failure to recognize the power loss resulting in an uncushioned landing would probably be survivable.

As the airspeed increases without an increase in height, there comes a point at which 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. Another thing to consider is the length of the tailboom and the response time of the helicopter flight controls at slow airspeeds and low altitudes. Even small increases in height give the pilot much greater time to react; therefore, the bottom right part of the H/V curve is usually a shallow gradient. If airspeed is 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 puts the aircraft above a survivable uncushioned 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 to accelerate to autorotation speed in order to autorotate successfully; 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 OGE hovers outside the curve.

The typical safe takeoff profile involves initiation of forward flight from a 2–3 feet landing gear height, only gaining altitude as the helicopter accelerates through translational lift and airspeed approaches a safe autorotative speed. At this point, some of the increased thrust available may be used to attain safe climb airspeed and will keep the helicopter out of the shaded or hatched areas of the H/V diagram. Although helicopters are not restricted from conducting maneuvers that will place them in the shaded area of the H/V chart, it is important for pilots to understand that operation in those shaded areas exposes pilot, aircraft, and passengers to a certain hazard should the engine or driveline malfunction. The pilot should always evaluate the risk of the maneuver versus the operational value.

The Effect of Weight Versus Density Altitude

The height/velocity diagram [Figure 1] depicts altitude and airspeed situations from which a successful autorotation can be made. The time required, and therefore, altitude necessary to attain a steady state autorotative descent, is dependent on the weight of the helicopter and the density altitude. For this reason, the H/V diagram is valid only when the helicopter is operated in accordance with the gross weight versus density altitude chart. If published, this chart is found in the RFM for the particular helicopter. [Figure 2] The gross weight versus density altitude chart is not intended to provide a restriction to gross weight, but to be an advisory of the autorotative capability of the helicopter during takeoff and climb. A pilot must realize, however, that at gross weights above those recommended by the gross weight versus density altitude chart, the values are unknown.

Figure 2. Gross weight versus density altitude

Assuming a density altitude of 5,500 feet, the height/velocity diagram in Figure 1 would be valid up to a gross weight of approximately 1,700 pounds. This is found by entering the graph in Figure 2 at a density altitude of 5,500 feet (point A), then moving horizontally to the solid line (point B). Moving vertically to the bottom of the graph (point C), with the existing density altitude, the maximum gross weight under which the height/velocity diagram is applicable is 1,700 pounds.

Charts and diagrams for helicopters set out in Title 14 of the Code of Federal Regulations (14 CFR) Part 27, Airworthiness Standards: Normal Category Rotorcraft, are advisory in nature and not regulatory. However, these charts do establish the safe parameters for operation. It is important to remember these guidelines establish the tested capabilities of the helicopter. Unless the pilot in command (PIC) is a certificated test pilot, operating a helicopter beyond its established capabilities can be considered careless and reckless operation, especially if this action results is death or injury.

Common Errors

1. Performing hovers higher than performed during training for hovering autorotations and practiced proficiency.

2. Excessively nose-low takeoffs. The forward landing gear would impact before the pilot could assume a landing attitude.

3. Adding too much power for takeoff.

4. Not maintaining landing gear aligned with takeoff path until transitioning to a crab heading to account for winds.