Estimated reading time 12 minutes, 42 seconds.
The unique attributes of vertical flight have many amazing benefits, but they also come with some downsides. Fixed-wing airplanes have a “coffin corner” close to the limits of their flight envelope at high altitude; helicopters have “avoid” areas close to the ground, where a combination of height and velocity can render them vulnerable in the event of a power loss. This combination of height (H) and velocity (V) is graphically depicted in the rotorcraft flight manual (RFM) in the HV diagram. A more ominous epithet for the HV diagram — but one commonly used in the field — is “dead man’s curve.”
If an engine quits in flight, potential and kinetic energy are exchanged until a new steady-state is reached. If a pilot has enough height and speed, they can manipulate the controls to arrive in a position to execute a final flare and recovery. But at low heights and low speeds, this energy exchange becomes increasingly difficult; somewhat analogous to base jumping from too low a height — you need to be high enough for your parachute to deploy, fill with air, and slow your descent. This need for height and/or speed gives rise to a shaded “avoid” area in the HV diagram.
The HV diagram appears in almost every helicopter flight manual (the Sea King is an exception). Here’s a definition of the HV diagram, taken from the medium-twin Leonardo AW139:
“The Height-Velocity diagram defines, in the event of a single engine failure during takeoff, landing or other operation near the surface, a combination of airspeed and height above ground from which a safe single engine landing on a smooth, level and hard surface cannot be assured.”
Put another way, it defines the height and airspeed combination that will allow for a safe landing if an engine fails.
There may be minor differences in the technical language of performance, but the essence remains the same — certain combinations of H and V should be avoided unless operationally required. It is not meant to completely preclude operation within the “avoid” zone; rather, it informs crew about their risk of “exposure” within it, so that the time they spend there is minimized.
The determination of HV diagram is done analytically. The analytical model is validated by flight tests that cover engine failure, “lever delay” tests, and engine-off landings (EOL). Such tests are inherently risky and undertaken by qualified experimental test pilots and flight test engineers.
Engine failure at low heights
One of the skills required to become a helicopter pilot is the handling of a “power loss” situation, brought about either by an engine failure, or any situation requiring the engine(s) to be deliberately shut down in flight.
The recovery from engine failure on a single-engine helicopter, or transition from all engines operative (AEO) flight to one engine inoperative (OEI) flight on a twin/multi-engine helicopter, requires the pilot to take specific actions. This will entail lowering the collective (with attendant height loss) to preserve rotor rpm (Nr) under most conditions. The Nr decay rate is directly proportional to rotor torque at the instant of failure, and inversely proportional to the moment of inertia of the rotor system. Within a certain combination of H-V, safe, repeatable recovery from engine failure may not be guaranteed. The locus of such points defines the HV diagram.
HV diagrams usually have a set of graphs or shaded areas — one addressing the low speed section, and other dealing with high speed (due to the risks of kinetic energy dissipating with high-speed ground contact). The point of inflection (where the low-speed trace curves back towards the Y-axis) is called “knee speed,” defined by a combination of minimum safe height (critical height) and forward velocity (critical velocity), below which is a range of heights that should be avoided.
Operating inside the HV diagram’s low-speed “avoid” section implies inadequate height to safely force land or reject takeoff (RTO), or execute “fly away” for multi-engine helicopters. Operating inside the other “avoid” section may entail a high-speed touchdown, or even a “zoom climb” of the aircraft out of “gate” conditions during the recovery process.
The Sikorsky SH-60 RFM notes that, “delay in recognition of the single engine failure or excessive maneuvering to reach a suitable landing area reduces the probability of a safe touchdown.”
Single-engine helicopters operate within Performance Class Three, which means that a “forced landing” must be performed in case of an engine failure at any point in the flight profile. At low heights, losing an engine can be hazardous for two reasons: If the vertical descent method for recovery is employed, there may be insufficient kinetic energy available in the rotors to cushion the vertical speed below the limiting sink rate of the landing gear. If the conventional flare and recovery method is attempted, there may be inadequate height to reach a suitable speed at which flare becomes effective.
On light singles, expect to find no more than a single HV diagram in the RFM, since no “fly away” is possible. Additional systems or “box conditions” for things like environmental control systems (ECS), anti-icing, or selectable rotor rpm (Nr) are also not standard on light singles.
Rotor inertia may smooth the glide somewhat to provide requisite lever delay time (LDT) or intervention delay time (IDT) — the pilot’s reaction time to recognize the condition and start lowering collective. During the design of a helicopter, if a low-inertia rotor system is not found capable of providing the regulatory IDT (typically two seconds), tip-weights or some other feature may have to be incorporated to improve rotor inertia.
The HV diagram for multi-engine aircraft
Complete power loss on a multi-engine rotorcraft is very rare, but may be caused due to fuel starvation or contamination, mishandling of engine controls, battle damage, or a (common) driveshaft failure in some designs. It is more practical to focus on OEI at hover.
Anti-submarine helicopters that operate dipping sonar spend extended time inside the “avoid” area. Hence, a judicious height has to be chosen to mitigate the risks while ensuring optimum conditions for the sonar to do its work. The Indian Navy’s Westland Sea King Mk. 42B hovers at 50 feet (15 meters), while the Russian coaxial Kamov Ka-28 chooses to hover at about 75 feet (25 meters).
The obvious difference from a single-engine helicopter is that only half the power required is lost. Another engine steps up to make good some of the power deficit after OEI. At hover or low speeds, a partial power descent may be chosen over autorotation. Unlike single-engine helicopters, a safe RTO or fly away may even be possible under certain combinations of H and V. The vertical descent method may not be employable beyond a certain height, due to the danger of the vortex ring condition that can develop when the rate of descent is about half the induced velocity at hover.
The other notable difference with twin-engine rotorcraft is the “contingency” power rating that can be invoked during recovery from a single engine failure. This is done by resetting the “live” engine governor to operate at a higher temperature limit, thereby providing a higher power output. This explains the host of HV diagrams in twin-engine RFMs, catering to different weight-altitude-temperature (WAT) conditions, contingency power ratings (there could be more than one), use of engine air bleed, anti-icing, wind conditions, selectable rotor RPM, and so on.
Check if the published HV diagram for your twin-engine helicopter is applicable for an OEI condition or total power loss. It is usually the former. Simulation and math modelling can be employed for the generation of HV data, but actual flight tests of total power loss on medium and heavy twin-engine helicopters are rarely done for safety reasons.
Multi-engine helicopters today, especially in commercial applications, are generally certified to Category A specifications, with scheduled performance information and profiles included in the RFM. Such helicopters may therefore operate under Performance Class One or Two. For a Cat A certified helicopter, operating within Cat A regulated takeoff/landing charts and flying to stipulated profiles, there is no HV restriction.
HV diagram in other emergencies
Certain emergencies in helicopters, such as loss of tail rotor (TR) or tail drive, may call for an intentional power-off autorotation landing. Could the HV diagram be applicable?
Complete loss of directional control at hover and low speeds is a decidedly tough combination. Extreme aircraft attitudes, yaw rates and cross-coupled responses may develop within seconds. With the experience and skill level of an average pilot, to which the HV diagram is pegged, it is difficult to estimate the outcome, given that no real tests can be performed.
Analytical models or flight testing for the HV diagram does not take into account rotorcraft response or pilot response time for catastrophic tail rotor failures at hover or low speeds, which are often accompanied by large and rapid changes in aircraft attitude. Large sideslip angles can also develop, increasing pressure error and degrading instrument indications.
The fly-away charts would not be applicable, either. Though useful as a rough guide, the HV diagram for single engine failure when read across for directional control failure (partial or full), in my view, would hide many variables, some of which can give a misleading sense of safety.
Adam Lowes, chief rotary-wing flight test instructor at the International Test Pilots School (ITPS) in Canada, one of the world’s leading civilian test pilot schools, agrees with this.
“HV testing specifically allows for forward speed on landing and for the landing gear to absorb some of the landing stresses,” he said. “And the only reason you can have forward speed is because you have directional control, so the flight test data may not be directly applicable to a tail rotor failure.”
While it may be human to fly and divine to hover, it is certainly not divine to lose an engine at hover or low speeds — especially if it’s the only one you have. So the next time you are planning a hover out of ground effect (HOGE) for rotor track and balance measurements, fixing marker balls on power lines, hooking-up an underslung load, or hovering at any critical combination of H and V, remember to first calculate where your task puts you. Prolonged exposure to the “avoid” area of the HV diagram increases your chances of an unhappy ending if you were to lose an engine.
Plan well, and fly safe. Happy landings!