Dead Man’s Curve
Helicopters are the only aircraft with a chart in the flight manual unofficially known as Dead Man’s Curve. Fixed-wing airplanes don’t have one.
One of the misconceptions about helicopters is that in the case of a total power loss, they glide. They don’t, they autorotate.
Some fixed-wing airplanes have charts that show how the plane will glide at various airspeeds from a specific altitude. Helicopters don’t have one. If one must do an autorotation, you are looking at your landing spot between your feet. For more on autorotations, see the post – “Helicopters Don’t Glide, They Autorotate” – elsewhere in From the HAC’s Seat.
Officially, the chart pilots nicknamed Dead Man’s Curve is officially known as the Height Velocity Diagram (HVD). It is buried in every helicopter’s Pilot Operating Handbook, Air Force Dash-One, U.S. Navy/Marine Corps NATOPS Manual.
Since I began flying helicopters in 1969, the Height Velocity Diagram has evolved in that it has more data and is easier to read. However, the message is the same – pilots flying helicopters should stay out of the shaded areas on the chart. In Navy and Marine Corps NATOPS Manuals, they are noted with two words – Avoid Area.
Why is the chart necessary? In each helicopter’s flight envelope, there are flight regimes in which if something ugly happens, such as a sudden shift in the wind that reduces the lift that the rotors are generating, or an engine failure, or a hydraulic failure, the likelihood of a crash is high.
Accompanying this post is a copy of one of the HVDs from the NATOPS Manual for a Navy SH-3A/G, HH-3A and UH-3A helicopter dated 1 December 1975 with changes as late as June 1978. You’ll note that it is for single-engine operations. The logic is that if you are in the “Avoid Area” and an engine fails, you’ve got a big problem.
Yes, I know, the chart is 47+ years old, but the format for all the HVD charts I’ve seen over the years hasn’t changed. What has is the amount of shading and the data which tells one a lot about the helicopter’s flight envelope.
When you look at the chart from the SH-3A/G, HH-3A, and UH-3A NATOPS Manual you can see the shaded section in the lower right where one shouldn’t fly the helicopter at those altitudes very fast. On the left, there is an area when one should avoid flying slow.
The problem is that the left side is where helicopters spend much of their time operating, i.e., hovering at 80 feet over trees while a survivor is being winched up puts one in the heart of the shaded area on the left
This chart tells me if I am flying at 50 feet on a standard day below 17 knots true airspeed, and have an engine failure, unless I exhibit extraordinary skill and enjoy a lot of luck, I will not be able to fly out of this condition. In other words, I will probably land or worse, crash.
For the record, a standard day is 590 F with a barometric pressure of 29.92 and dry air. If you’re an experienced pilot, you probably can count on your fingers and toes the number of days you took off on a Standard Day.
Why is HVD chart significant? Because the ability of a helicopter to hover or fly slow often puts one right into the heart of the left side of the HVD where it says “Avoid Area.” Here are some examples.
U.S. aircraft carrier decks are usually between 80 and 90 feet above the water. When landing on the forward part of the angled deck – the most common place one plunks a helicopter down on a Nimitz-class carrier – one slows to a creep to match the ship’s speed at about 100 feet over the water and at least a rotor blade and a half’s distance from the ship.
The pilot flying the helicopter “creeps” alongside the ship until the helicopter is slightly aft of the landing spot before sliding over the deck and landing with no forward airspeed. Until one is over the deck, you are in Dead Man’s Curve once you enter the “creep” phase of the approach. This is one technique.
Or, if one is hovering at 40 feet with a sonar dome a hundred or more feet below the ocean’s surface listening for a submarine, the helicopter is just below the Avoid Area of Dead Man’s Curve. This explains why if an engine fails, you may ditch. If you have burned off much of your fuel, or it is really cold, or a host of other factors that makes the air (and the amount of lift) different than on a standard day, you may be able to fly out of the hover.
Here’s how. As the engine starts to wind down, guillotine the sonar cable, dump the nose to reach 10 – 15 knots, and get into translational lift and ground-effect so you don’t hit the water. If you are lucky and good, you’ll be able to stay about 10 feet out of the water in ground effect. Translational lift occurs when the rotor blades act like a solid disk and generate 10 – 15% more lift. Ground effect is a cushion caused by the compression of the air but dissipates as one accelerates.
Or, if you are in a sightseeing helicopter on Maui in Waimea Canyon and the pilot is hovering several hundred feet off the bottom of the canyon, 100 feet from the rocks. While the view from the cabin may be spectacular, the pilot who does this puts the helicopter so far into Dead Man’s Curve, the chances of the pilot being able to fly it out if he has an engine failure are nil.
Or, hovering over a house 100 feet in the air for 10 – 15 minutes while your aircrewmen hoists up all members of a family whose home has been flooded. Your passengers might now include pets.
Or, while lifting an air conditioning condenser from the ground and then lowering it on the top of a 10-story building.
Or, in an EMS helicopter landing on the helipad on the roof of the eighth floor of a hospital with an injured accident victim in the cabin. During the last 1/8th of a mile, the helicopter is in the Avoid Area of the HVD.
Or, flying a helicopter carrying a TV crew hovering or flying very slowly around a newsworthy event. Depending on how slow or high off the ground, the helicopter is in the shaded area of the HVD.
One could keep listing the flying evolutions in which the helicopter must be flown either well inside or on the periphery of the Avoid Areas in the HVD to complete the mission.
To be fair, the performance of modern helicopters is better than those designed decades ago. Nonetheless, they all have their own Height Velocity Diagrams and the men and women flying helicopters accept the risks of flying in the “Avoid Areas.” Unfortunately, emergencies when flying inside Dead Man’s Curve can be, well, fatal.