Call it what you want; height-velocity curve, dead-man’s curve or even limiting height-speed envelope for those who like sophisticated phrases. The “dead-man’s curve” is probably a carryover from our fixed-wing brethren while the industry generally accepts the simple reference of “H/V curve”. The inside of the curve is the area from which it will be difficult or nearly impossible to make a safe landing following an engine failure if you are in the same conditions depicted with respect to airspeed and altitude.
The H/V (height-velocity) diagram is a staple in the helicopter arena but sadly is often misunderstood by student and instructors alike. So, let’s take a look at what it is and how it is developed.
What is it?
The Height-Velocity diagram (curve) is a chart showing various heights above the ground with a combination of a velocity (indicated airspeed) where successful autorotation and landing is or is not possible. This magical combination of numbers yields two major regions on the chart; the area above the knee and the area below the knee. These areas are what actually plot much of the “curve”. During initial helicopter certification, test pilots evaluate several characteristics of the helicopter that help determine the H/V curve. These factors include the helicopters initial response to a power loss, steady-state descent performance and power-off landing characteristics and capabilities.
Unknown to many, the development of the H/V curve and its associated number combinations is based on “pilot minimum skill level”. So, in a perfect world this means if the engine fails while I’m going this fast (KIAS) and at this height a pilot at a “minimum skill level” should be able to make a successful autorotation and hopefully some resemblance of a landing.
How do we define the pilot “minimum skill level?” That is a question I and many others can’t answer. Many will agree the current practical test standards are somewhat lacking and aren’t necessarily cultivating the “minimum skill level” necessary.
No cookies for me please
To delve into this quandary let us recap the typical sequence of an autorotation training exercise. The instructor has the student line the helicopter up with the runway so that the power-recovery phase of the autorotation will occur as closely to the runway numbers as possible. Sound familiar? You know what I’m talking about, the “3, 2, 1, roll-off power” etc.
It’s the same thing with a 180-degree autorotation where the student is taught where to “fail” the engine based on tailwind strength and land at the “spot” within the practical test standard. Is there really anything practical about it? Just what, if an engine failure occurs in the real world and the only spot you have is 600 feet directly below you? Could you get there? Safely? What about engine failures at night time with the same situation, the only place to go is directly below you or just out in front of you. If you remember anything from this article remember this, autorotations are like fingerprints in that no two are exactly the same.
How is it developed?
When test pilots are developing the H/V Curve for a specific helicopter the testing is done in very defined environments and conditions. The test is conducted at max gross weight for sea level and max OGE weight with the lesser of aircraft maximum altitude capacity or 7000 density altitude with 2 knots or less of wind, and with the landing portion being conducted to a smooth hard paved surface. Maybe this “worst case scenario” is what helps define the elusive “minimum pilot skill level” in the hopes that the average pilot will not often be found in these conditions.
Another tidbit of importance to mention is that when H/V curve testing is being done it is accomplished by a “throttle chop” and not an actual flameout. As best described by my friend Pete Gillies of Western Helicopters, “There is a world of difference between chopping the throttle to idle versus flaming out the engine.” In one of my favorite helicopters the engine actually develops 15-17 horsepower at idle. This minimal power can make the flare and pitch pull at the bottom so much easier than if the engine has gone away completely. The piston-powered helicopters on the other hand can deliver a more realistic prediction of what pilots may experience when they have a real engine failure. In turbine helicopters only the Lama, Alouette and Gazelle will provide similar results as the piston-powered because of their centrifugal clutches.
The development of the H/V curve also includes an “intervention time” that attempts to mimic the surprise factor encountered in real engine failures. This intervention is measured in terms of seconds (or lack thereof) depending on where you are located on the H/V curve.
The civil certification calls for a 1-second intervention at an area above the knee. So, in essence, during testing and certification the throttle is rolled to idle and a “thousand-one” count is initiated. The standards require the pilot’s hand to be on the cyclic and feet on pedals (sounds reasonable) but the pilot’s hand is not required to be on the collective. This standard applies to your basic helicopter certified under FAR Part 27 standards. Although it is assumed you won’t have your hand on the collective above the knee the standard requires that the power be set for level flight at any specified airspeed. Certification standards for areas below the knee have NO intervention factor as it is assumed you will be at full or takeoff power with all feet and hands on controls.
Why it all matters
So how do we tie all of this knowledge together? Some agree that the theory of entering and being in autorotation isn’t necessarily lacking as most can accomplish that phase safely, it is the more advanced autorotation fundamentals that are being left out of most syllabi, such as teaching how to get to a spot by varying airspeed (and rotor RPM) while in autorotation. It is what transpires between the time of the engine failure and the point at which time the pilot initiates the flare that can make all of the difference. Getting the collective down has been hammered into students heads from the beginning, and it is obviously required and critical; however, getting the cyclic back immediately is absolutely critical!
As a designated pilot examiner it is common to see applicants struggle with hitting their spot. Why? My subjective opinion is that they are taught to enter the autorotation at a specified location in order to hit that spot. They enter the autorotation, realize they are short or long and then the specified airspeed and rotor RPM they were taught is either getting critically low or astonishingly too high. Who is the blame? The student? The instructor? Or the system? After all, it is the practical test standards (PTS) that dictates that an applicant will “Establish proper aircraft trim and autorotation airspeed, ±5 knots.”
I am absolutely amazed to see students, pilots, and instructors alike think something bad is going to happen if their airspeed drops below the “recommend autorotation airspeed” during the initial entry or at some point below the high hover point and or above the “knee”. Where does this mindset come from? Obviously it comes from the PTS that dictate that an applicant will maintain best autorotation speed within the given tolerance. This is rather unfortunate considering airspeed indicators are typically not reliable in autorotation. In an engine failure or a practice autorotation approach from cruising level altitude (at or above the high hover point on the H/V curve) all I care about is getting upward airflow through the rotor disc to serve as the driving force and making sure my rotor RPM is where it needs to be. Initially I couldn’t care less about airspeed. The airspeed becomes critical prior to the initiation of the flare for one solid cardinal principle, which is that there is an airspeed below which the flare will be spectacularly non-effective. I’m not suggesting that one become a weekend warrior test pilot. Don’t experiment to find the lowest airspeed you need at the flare initiation. Just know you will need a practical amount of airspeed and this is something that can be learned with the proper instructor. Also keep in mind the design factors of a helicopter; unlike a fixed-wing aircraft helicopters are designed to land vertically with little forward horizontal momentum.