Bruce Webb H/V Diagram video series, Feb.-Mar., 2022

In a four part video series published in February and March 2022, Bruce Webb, Director of Aviation Education and Community Outreach, Airbus Helicopters, examines the Height / Velocity (H/V) Diagram, reviews the operational realities of an autorotation and offers practical recommendations on performing a successful autorotation.

Episode 1

Episode 1 discusses the development of the actual curve and highlights several differences between certification requirements and operational realities:

Key points from Episode 1

• The H/V envelope is the combination of height and airspeed from which a safe landing in case of an engine failure cannot be guaranteed because of the overall aircraft energy balance.
• The H/V diagram in the Rotorcraft Flight Manual depicts the boundary of the height and speed envelope where actual successful landings, after engine failure, have been demonstrated during certification test flights.
• Within the H/V envelope (the red zone), a successful landing, in case of an engine failure, has NOT been demonstrated and CANNOT been guaranteed.
• This video series focuses on single engine, normal category helicopters, as they represent the majority of helicopters in service.
• The H/V curve is defined under specific conditions (14 CFR Part 27): landings at up to 7000 ft Density Altitude (DA), maximum Internal (without external loads) take-off Gross Weight (IGW), landing surface smooth and hard, calm winds, normal pilot skill, and no permanent deformation of aircraft, including the landing gear.
• All H/V diagrams must include a recommended takeoff corridor for pilots using normal skill to remain clear of the avoid area (the red zone) by a minimum of 5 knots.
• There is no recommended landing corridor: as the H/V curve only represents two flight conditions: level flight or take off, the reciprocal of the takeoff corridor does not represent a recommended landing corridor.
• During landing, with all other parameters remaining equal, you are in a better position to enter an autorotation because you already have the collective lowered and are in a descent. That will moderate the initial RPM droop.

Episode 2

Episode 2 explores possible differences between an operational environment and the conditions required for certification:

Key points from Episode 2

• Certification tests are designed to be conservative. However some of the test conditions may not represent your operational environment. For instance, there may not be a curve for your specific altitude and weight combination (the diagrams are formed using 7000 ft DA and maximum IGW).
• If you are at lower DA and lighter, the avoid area will be smaller. But if you are at a higher height than 7000 ft DA, the avoid area will be larger.
• Operational conditions may differ from the certification requirements used to establish the H/V curve, as they may not allow for smooth and hard landing surface or calm winds. What if you have to perform an autorotational with a crosswind or worst, a tailwind? What about pilot skills: can we really remove exceptional skills from test pilots? What about alertness: during the testing, the test pilots know precisely when the engine will fail and have been mentally prepared for each test point, whereas you will be surprised if not about the engine failure, by the availability of suitable landing location.

Episode 3

Episode 3 examines the operational realities of an autorotation:

Key points from Episode 3

• Certification test pilots would not be surprised by the power loss, in fact they induce it. This is an advantage that you won't have in the real world.
• There is another advantage for certification test pilots: they gradually build up to the more difficult points on the curve. You will not have the same luxury. In operations, engine failure will occur with no consideration for preparedness  or flight profile. And you will not be given the opportunity to practice a few times in advance: the first one will be the one that counts!
• During certification testing, landings were performed to a smooth hard surface, as opposed to what will be be available to you.
• During testing the helicopter may touch down at any speed, and slide any distance, so long as there is no permanent damage to the aircraft. But in an actual autorotation, touching down at a speed much above walking speed may be unacceptable.
• Or you may need to land on water or be flying at night.
• All possible combinations of height, speed, wind, weight, temperature, surface, composition, illumination, and pilot alertness cannot be tested.
• Within the H/V envelope, successful autorotational descents have NOT been demonstrated. Some conditions may be more advantageous: for instance performing an autorotation with half mass, while flying over and airport, with 20 knots of headwind. But alternatively, some conditions can be worst: for instance flying powerline patrol near gross mass at 100 FT AGL over a soft field with 10 knots of tailwind.

Episode 4

Episode 4 completes the H/V Diagram series and offers some practical recommendations on performing a successful autorotation:

Key points from Episode 4

• Flying a helicopter requires fantastic hand-eye coordination, dexterity, mental gymnastic, situational awareness and a bit of intuition. It is not easy but that is why we love it.
• What practical recommendations would be useful to execute a successful autorotation?
• First know your aircraft. You should be capable of quickly adopting the correct attitude to initially obtain the approximate airspeed for the autorotation. The RFM provides the recommended airspeed, usually very close to Vy (speed for best rate of climb). You should know the pitch attitude necessary to obtain and maintain that speed. But Vy decreases with altitude. Therefore recommended airspeed for autorotation also decreases with altitude.
• Second, rotor RPM is fundamental and absolutely critical: it MUST be maintained within the specified range. Learn to manage RPM with your ears and confirm with your eyes.
• Third, fly as high as your work and condition allow: altitude is your friend! Having time to select the best available landing option is a much better option.
• Finally, train as often as possible. While there is no better way to train the landing portion of the autorotation that actually flying the helicopter, simulators are excellent ways to practice the other aspects.
• Timely recognition of the emergency and proper entry into the autorotation are key to a successful outcome, an area which may need improvement and where training efforts should focus.
• Use all the training tools available: if you do not have access to a simulator, practice the recognition and glide segments in the helicopter as often as necessary, but always within gliding distance from a suitable landing area.
• A good entry is critical and sets the scene for a good touchdown.
• Practice doesn't make perfect but it certainly improves the odds.

Bruce Webb's YouTube channel

You will find more instructive rotorcraft safety videos in Bruce Webb - YouTube.