Many pilots of fixed-wing aircraft sometimes feel envious of their fellow airmen who fly helicopters, because they're capable of taking off and landing almost anywhere. In the rotary-wing world, operations within cramped quarters, known as confined area operations, are comparable to the fixed-wing pilot's short-field takeoffs and landings. The ability to get in and out of tight spaces is, of course, one of a helicopter's most notable assets. But as with anything involving the third dimension, there are limits. Those unfamiliar with helicopter operations think of heliports atop buildings, oil platforms, and hospital complexes. That often leads the uninitiated to imagine parking lots and small clearings in wooded areas, along with dreams of commuting to and from one's own suburban backyard. In fact, most pilots who never had the opportunity to take off and land vertically usually think that controlling a cyclic and collective automatically endows one with extraordinary powers. Well, theoretically it could--but practically speaking, in the world of small piston helicopter operations, it doesn't.
Yes, it's theoretically possible to land even a small piston helicopter in a suburban backyard or on a suburban street. I've done it twice (after notifying my neighbors, the city government, and local police). But helicopter operations into any clearing smaller than a football field require significant preparation, practice, and training. And when you're talking about an area the size of a tennis court, then you're definitely introducing a much higher level of risk. Some of you have heard it, I'm sure. It usually goes something like: "Runways? Who needs runways? You see that little clearing down there? Why, if I wanted to, I could set us down in there, with room to spare." Maybe that's an exaggeration, but the message usually comes through loud and clear: Rotary-wing flight equals unrestricted maneuverability.
Anyone who has taken more than a few helicopter flight lessons already knows there's more to this picture. Like a Hollywood legend, this idea that helicopters can go anywhere somehow lives on. When I first took helicopter lessons, every invoice from my flight school had this printed note at the bottom saying--I'm quoting it exactly--"Learn to fly straight up! See Todd, and learn to fly helicopters!" Well, maybe in a multi-million-dollar, twin-turbine, corporate trophy helicopter with a high-inertia rotor system, but in a light piston helicopter? You'd better not! Take off straight up in the winter, into a 30-knot headwind, maybe (still at some risk), but on a nice calm spring day? You'd be asking for trouble.
Although helicopters are incredibly maneuverable, that maneuverability has a price. For starters, there's this thing called the height-velocity diagram, which is also known as the dead man's curve.
If you lose your engine while flying a helicopter, you don't automatically become a glider pilot, as you would flying an airplane. You must immediately perform an autorotation. Basically, your fan needs to become a windmill. Above a certain height--which increases as your airspeed decreases--you can theoretically milk the rotational energy in your rotor blades just enough (assuming your timing is right) to cancel your precipitous plunge earthward and touch down ever so gently. For smaller training helicopters with little or no forward airspeed, that curve tops out hundreds of feet above ground. By the way, when I say precipitous, we're talking up to 2,000 feet per minute, in a zero-airspeed autorotation.
A helicopter in flight has several possible means of withdrawing energy from this windmill. First, there's the angular momentum of the rotor blades themselves. (According to Jeff Saneman, a helicopter flight instructor at Advanced Helicopter Concepts in Frederick Maryland, if a Robinson R22 pilot did nothing after a power failure, that could decay to uselessly low levels in a little over one second.) Second, there's some amount of kinetic energy available from whatever horizontal motion the helicopter may possess. Finally there's the crucial potential energy component, based on your altitude at any given moment; it takes time to "wind up the windmill" in an unpowered descent. (It's also vitally important for the pilot to immediately lower the collective pitch of the main rotor for this to be possible. See "Give It a Whirl," May 2004 AOPA Flight Training.)
When doing what is usually of necessity a steep approach to a landing within a confined area, forward speed is negligible. The inertia of the rotor blades in a small piston helicopter is so low, that account would be drastically overdrawn before you could get near the ground. What's left is altitude. That's the reason for the dead man's curve; the lower you are, the greater your airspeed must be (up to a point) to have enough energy in reserve to milk from the rotor blades so that you alight gently after a power failure. Your timing and "feel" for this must be precise.
If you've ever seen an air ambulance or medevac helicopter coming in for a landing near a hospital's emergency entrance, did it buzz straight in unexpectedly and do the aerial equivalent of an adjutant's halt right over the landing pad? That kind of hustle has its place in a combat zone, but chances are that even if someone is grievously injured back there, that pilot is going to be doing several things in an orderly sequence.
In fact, any time a helicopter pilot lands off-airport, he or she should treat it like a confined-area landing. A medevac helicopter taking seemingly forever to arrive at the helipad is actually doing two things: making a "high reconnaissance," and then a "low" one. What's the motivation? I'll give you a good one: wire strikes. They're one of the leading killers in helicopter flying. Wires, as well as poles, are very hard to spot from a few hundred feet up. That's important to remember if you have to select a forced-landing site in a fixed-wing aircraft, as well.
Another good reason is that small clearings, especially on windy days, are another form of "sucker hole" for chopper pilots, just as a sudden shear and headwind loss on short final can take a fixed-wing pilot by surprise. Once you descend below a line of obstacles, you may lose your headwind and can drop below effective translational lift, or ETL, into a wind shadow, at which point one can find oneself descending into one's own downwash (the so-called vortex ring state), after which a power recovery is impossible. (ETL is the point during any departure, or deceleration during an arrival, when airspeed is at 10 or 15 kt and the efficiency of the rotor blades changes. During an arrival, as less air presents itself to the rotor system within that general speed range, there's a decided drop in efficiency and generation of lift.) In addition, buildings and trees can contribute to low-level turbulence when wind exceeds about 10 kt, and gusts greatly increase the difficulty in making proper control inputs.
A complication is the fact that helicopters frequently operate away from airports, where people aren't as aware of safety concerns. At lower speeds a sudden gust of wind can cause the main rotor blades to flap down, which can be particularly dangerous for an inattentive passenger or onlooker standing nearby.)
Despite the natural tendency of the fuselage to weathervane in a slipstream of air, most helicopters are fighting a continual battle to overcome torque from the main rotor. A hovering helicopter in the low-altitude/low-airspeed flight regime isn't anchored by its gear, and a phenomenon called loss of tail rotor effectiveness--or LTE--can occur. The basic culprit is differing rates of turn for a given tail rotor pedal position under varying wind conditions, and uncommanded rapid yaw rates. The result can be loss of aircraft control.
Descending into one's own downwash is unhealthy (and potentially unrecoverable) simply because at extreme descent rates, the angle of attack on the rotor blades can exceed a critical value. Let's just note that the body does not recover well from spinal compression injury. As Jeff Saneman explains, generally it can be avoided by never letting the airspeed get below ETL--for the R22 typically 30 kt is used--until the rate of descent is well under a rate that wouldn't cause one to descend into one's own downwash. (For the R22, we use 300 fpm as a maximum descent rate at low airspeeds.)
Another manifestation of this occurs on downwind approaches. An airplane pilot might accept a five-knot tailwind for an up-slope landing, but a light helicopter, especially if a steep approach is involved, shouldn't try it. And finally, approaches into confined areas are often done at steeper angles (like 15 degrees, and sometimes much more, like 45 degrees); they can involve operations on the inside of that height-velocity curve, especially in the last few dozen feet. It's not a comfortable place to hang out for very long.
During a high recon, typically done at 500 feet agl, you would be checking for landing zone size, shape, obstacles, and wind. Generally, you want to try to visualize the tradeoffs you might make to minimize turbulence, and maximize the longest dimension of the site versus the headwind you might have to partially sacrifice. You'll be starting to plan what options you have for a forced landing if you have an engine failure on approach or upon departure. You make tentative plans on how you're going to get in, and how you're going to get out. You get your prelanding checks done, and then go in for the low recon.
The low recon is like a low approach followed by a go-around. You look more closely at landing zone slope, surface condition, possible obstacles (again), and double-check low-level winds. You pick a reference point near the middle length of the touchdown area, to avoid both downdrafts from the upwind boundary and too steep of an approach over the downwind boundary. You also want to make certain that the ground looks firm and even enough to allow a safe landing. Then you circle around, like a normal pattern, and come in again from 500 feet and perhaps a half-mile out. The idea is to keep as much normality as you can in any unfamiliar situation. Unless the landing zone is dusty or snow-covered, this is where you cautiously lower the collective and land.
One low-level consideration airplane pilots would not think about is being sucked back into the ground! Even though helicopters can land almost anywhere, we pay close attention to where we do land. If the ground is soggy or the tarmac is hot, both present the possibility of something called dynamic rollover on departure if one skid picks up, but Mother Earth doesn't want to let go of the other one. (The high center of gravity of a helicopter lifting off generally makes this a hazard when operating from a sloped surface or under the influence of a significant crosswind, but a "stuck skid" can result in the same thing.) Another reason for caution during confined area operations is that, paradoxically, the closer to the ground you are in a helicopter, the more aware you should be of density altitude. Airplanes down low can just add power and go around, but helicopters are power-limited; the difference between power available and power required is much less.
Getting out again as safely as possible usually involves a maximum performance takeoff, which is the rotary-wing version of a short-field takeoff. When departing a confined area, there isn't the vortex ring issue, and because you'll usually encounter a greater headwind once you're clear of obstacles (as opposed to the sudden loss of a headwind), there's no sudden loss of ETL. However, the first things a helicopter pilot must do are establish a hover (at a reasonable distance downwind of any obstacles) and determine if the amount of excess power available is acceptable. Then he or she will return to the surface and, similar to a fixed-wing pilot who taxis to the very end of the runway, holds the brakes, and adds full power, the helicopter pilot will smoothly add maximum power and climb out at a fairly low forward airspeed somewhat above ETL.
Until the obstacles are cleared, this, too, can easily involve operation inside the height-velocity curve.
Jeff Pardo is an aviation writer in Maryland with a commercial pilot certificate for airplanes, and instrument, helicopter, and glider ratings. He has logged about 1,300 hours since 1989. An Angel Flight mission pilot, Pardo has also flown for the Civil Air Patrol.
Want to know more? Learn about helicopters and what it takes to get the rating at AOPA Flight Training Online.
Related Articles
