Dead Man’s CurveBy Jannie Matthysen
We all know dangerous curves. There’s that last little right-hander at the top of Sir Lowry’s Pass that you always seem to approach much too fast. Then there’s the curve-ball question in the last CPL Navigation exam. Let’s not forget about the rear-view of Olivia at the local video store. All dangerous curves indeed…
What on earth could then be so dangerous that it should be called the Dead Man’s Curve? Helicopter pilots are all familiar with this term, but very few truly understand the meaning behind the sensational phrase. This is, once again, one of the many aspects about the wonder of helicopter flight of which very little official information is available. The Dead Man’s Curve is more accurately known as the Height / Velocity Diagram, and is found in the Performance sections of helicopter Flight Manuals or Pilot Operating Handbooks.
In simple terms, the diagram depicts combinations of height and airspeed that should be avoided during helicopter operations. The information in this diagram is calculated by engineers and test pilots during the certification and test flight phases of a new helicopter’s development. It is an American FAA requirement that each helicopter certified in the USA must have an H/V diagram. Almost all other aviation authorities have similar requirements.
How exactly is this information useful to us? Most helicopter manuals contain only the graph as indicated in Diagram A. No supplemental information is available on how to interpret the graph, or how the information derived at, should be implemented during normal operations. This is where the pilot population is left to its own devices and many scary opinions surface. Note that nothing in this diagram is a limitation as such. The shaded areas indicate danger areas, which mean that you are allowed to operate within the shaded areas with due consideration to the additional risks assumed. Ironically, it is exactly these areas that make a helicopter unique. If we were to avoid these danger areas all the time, we could just as well be flying an aeroplane (shock and horror).
The Height Velocity Diagram recommends combinations of height and airspeed to avoid because the average pilot is unlikely to accomplish a safe autorotation under the demonstrated set of conditions following an engine failure. Diagram A is taken from the Performance Section of a Robinson R44 Raven II. It lists the demonstrated conditions as: Smooth, hard surface / wind calm. What is doesn’t say is that the procedure requires that an autorotation be achieved as directed in the Emergency Section of the R44 manual. In other words: achieve 70 KIAS and Rotor RPM at 100% when height allows, or the appropriate procedures as published at low level. This is usually a tall order under less than ideal conditions.
The Recommended Takeoff Profile shown in the example carefully guides us past the danger areas to a path where, in case of an engine failure, we average mortals should be able to put the helicopter down without any damage or injury provided that we land on a smooth, hard surface. Sounds simple enough, doesn’t it? When last did you achieve 100 ft AGL during climb at 60 KIAS in your Robinson R44 with nothing but a smooth, hard surface in front of you?
Diagram A: Robinson R44 Raven II H/V Diagram
The lower shaded area starting at 45 and 55 KIAS / 20 ft, is based on the pilot’s reaction time. We simply cannot react fast enough following an engine failure at 15 ft while flying at 100 KIAS, for instance. I cannot imagine why anyone would be breaching the boundaries of this area anyway.
The bigger shaded area is a bit more complex and requires a much better understanding of a particular helicopter’s performance capabilities. The area provides for two sets of conditions defined by altitude and aircraft operating weight. The worst combination would be a heavy helicopter, at high density altitude, hovering at 100 ft above ground, for instance. In this example following an engine failure, at least half the height would have been lost before the average pilot was even able to correctly identify the problem. The surface would shortly thereafter rise up to meet the aircraft at 3000 ft/min plus…
It does not often become this obvious how important airspeed is. Notice the steep gradient of the curve from 600 ft / 0 KIAS to 150 ft / 50 KIAS. Even 10 knots of headwind (airspeed) during a 600 ft hover, will create an additional safety margin of almost 100 ft. A safe autorotation requires two critical components: Rotor RPM and airspeed. While it is theoretically possible to autorotate without any airspeed, a loss of rotor RPM would be catastrophic. Apart from the fact that the rotors need to be turning at a sufficiently high speed to produce lift, the correct rotational velocity also makes the helicopter rotor disc rigid. Rotor blades slowed down sufficiently during flight, will produce insufficient lift, stall, and physically remove themselves from the helicopter in spectacular fashion, leaving the pilot with the best alternative of jumping. Not a recommended procedure, to say the least. Airspeed during autorotation is only required to provide some range or “glide distance”, and to produce an efficient flare that slows the helicopter down for a safe landing. This deceleration occurs in terms of forward speed and downward speed. Any operation within the shaded area makes the goal of ideal rotor RPM and airspeed virtually impossible. There is absolutely no argument that the old adage of speed and altitude as the biggest safety factors ring true once again.
Why do we then often see helicopters clearly operating inside the danger areas with pilots who seemingly have no regard for the inherent risks? Power-line maintenance, fire fighting, construction, and aerial filming are some examples of operations that require much time spent in the Dead Man’s Curve. Even our PPL training syllabus requires exposure to the Curve when we are taught Confined Area Operations. We often find it very difficult during normal operations to follow the Recommended Takeoff Profile. This profile is realistically very theoretical in its application as it requires acceleration in ground effect to 45 KIAS, followed by a 60 KIAS climb to a safe altitude – all while having a smooth, hard surface readily available, as demonstrated. This set of conditions invariably only exists at airports where helicopter takeoffs are executed from runways or taxiways. The rest of the time we are assuming additional risk. Simple.
The reality is that we apply common sense when breaching the boundaries of the graph. We know that we are exposing ourselves to additional risk, but is it justified in getting the job done? Many operations, as mentioned previously, rely on the helicopter’s unique low speed capabilities, often at low altitude. Even a normal takeoff away from an airfield requires a steeper climb gradient than the Recommended Takeoff Profile. Do we try and stick to the profile as directed and run the risk of flying into unseen obstacles, or do we opt for a steeper angle thereby reducing the danger of making contact with any unseen branches, wires, or fences? It should be a very easy decision. Statistics indicate many more accidents where pilot fly into things, than accidents following true engine failures.
Many pilots think that if you breach the boundaries of the Dead Man’s Curve, you will surely die! Breach all you like. Your life expectance only reduces when the engine quits. There are many examples of pilots saving themselves, their crew, passengers, or helicopters following a brush with the Dead Man’s Curve. This was sometimes accomplished through admirable skill and contemplation of the unexpected, but too often through nothing but blind luck. Our best options are to always look for better alternatives when flying inside the H/V Diagram and to continuously correctly justify why we are making the decisions that put us in that position. If we decide that there is no better and safer alternative, we had best be sure that we know the capabilities of our helicopter chosen for a specific job and that we are sufficiently trained, qualified, and experienced to handle an engine failure from inside the Dead Man’s Curve.