Airplanes are nothing more than marvelous translators of energy. They take the energy released by gasoline through explosions, then convert it—through propellers and airfoils and other gadgets governed by the laws of physics—into the basic ingredients of flight: altitude and airspeed. Everything else is based upon those two factors. But one of the magical factors of flight is that all the energy that was required (within certain limitations) to get the airplane up to an altitude and flying at a given speed is still there—if the engine quits, much of the energy is still available to the pilot, if he knows what to do with it.
When an airplane is off the ground, it has energy that exists solely because of its altitude. That’s the airplane’s potential energy. If a rock was released from altitude, it would come straight down. But your airplane has wings, which are designed to harness the energy released by the very act of falling (assuming that the nose is angled down so air flows over the wings). And they will generate lift so the aircraft can stay up longer, able to glide a given distance from the point at which the engine stopped. The radius of the circle that defines the gliding distance is part of what is called its energy footprint. The higher the airplane is, the farther it can glide, simply because it has more energy at that altitude. In other words, an airplane’s energy footprint gets bigger as it gains altitude.
The other half of the energy footprint equation is based on speed, and this changes the shape of the footprint and forces decisions on us at critical moments. For example, suppose we lose an engine on takeoff and the airplane has plenty of speed but not much altitude. In other cases altitude gives a circular pattern or footprint, but here the speed clearly changes the shape of that pattern, bulging it in the direction the airplane was heading when the engine quit. The relationship between the speed and the altitude define an area, a footprint, that the airplane can reach from that altitude and speed. When the airplane is taking off, the size and shape of that footprint changes with every second we’re in the air, because we’re gaining both speed and altitude.
When the airplane is on the climbout, initially the options of where we can go are slim to none: The energy from altitude is too low to allow turns, so the pilot will have to rely on the energy derived entirely from speed to reach a favorable spot nearly straight ahead; from here the energy footprint resembles a straight line.
As the airplane gains altitude, the potential energy that comes with the greater height gives more options. The airplane can actually be turned right or left, and the higher the airplane climbs, the wider the energy footprint becomes and the more choices there are. As the speed and the altitude increase, the footprint initially extends in front of the airplane only. But, as the altitude becomes a bigger portion of the equation, the ability to turn right or left increases until it’s possible to turn the airplane completely around. The footprint actually extends behind the airplane. The altitude required to do this will vary from airplane to airplane and with the airplane’s configuration and loading.
As the speed increases, say after leveling out in cruise flight, speed can dominate the shape of the footprint area that can be reached from a given altitude. This is especially true if the airplane is a fast machine at moderate altitudes. In theory, if an airplane is fast enough and low enough, the option to turn completely around may not be there—all options will be in front of the wings. For most general aviation airplanes, however, that’s not the case because they aren’t that fast compared to the altitudes they usually fly. The height gives options that the speed may not.
OK, this is all the theoretical stuff. How does it work in real-life situations such as takeoffs? As the airplane leaves the runway, the energy is bottled almost 100 percent as speed. It’s an arrow of energy pointing straight ahead. As the speed builds, regardless of altitude, it provides some energy that in theory could be used for a slight turn. Depending on the situation the airplane probably shouldn’t be turned (you might be able to avoid an obstacle at the end of the runway).
As the altitude builds, more options are available for turns because the energy from altitude is combined with the energy from speed. Until an airplane has a fair amount of altitude—at least 300 to 500 feet—turns of any kind should be avoided (depending on the airplane type, terrain, and obstacles). When an engine quits, you have your hands full changing tanks, hitting the boost pump, and trying to pick a clear spot ahead. You should avoid introducing any kind of maneuver that might steal speed or altitude or, more important, add distraction at a time when your brain is already maxed out.
As the altitude builds, the ability to turn left or right to pick better fields (or avoid really bad ones) becomes very real. However, at this point don’t even think about turning around and trying for the runway behind you. Yes, in some conditions (lots of altitude, lots of headwind that will push you back to the runway, an airplane that glides well in that configuration) a 180-degree turn is possible—but how many of us know exactly how much altitude that takes in our airplane? And how many can make that kind of a power-off, descending turn and not lose excess altitude because of sloppy technique?
For as long as the FAA has been keeping records, it has been clearly shown that attempted turns back to the runway seldom work out; in fact, they can easily lead to a stall/spin accident. For this maneuver to succeed, conditions have to be nearly perfect. The pilot must able to analyze those conditions correctly, then make a maximum-efficiency turn, power off, in an airplane that is descending toward ground that is uncomfortably close; know for a fact that he can make the runway; and do all of this while his brain is awash in adrenaline.
In the unlikely event that the engine abandons us on takeoff, conserve energy and use every bit of it to orchestrate a successful and fairly normal approach to the best landing area available. And “conserving energy” means sticking to proper speeds like glue and keeping the ball perfectly centered. Ideally, we want to fly any kind of approach to an emergency field at the pilot’s operating handbook approach speed because we know a bunch of factory engineers and test pilots have determined exactly what speed will maximize the energy efficiency and give the maximum range from a given altitude. Anything above or below that speed wastes altitude. But there may be situations where we have too much energy in the form of altitude, and we want to get down sooner to reach a field that’s closer. In that situation the most common urge would be to drop the nose and get rid of altitude.
The problem is that whatever altitude we lose in a dive usually returns as energy in the form of speed. Then we arrive over the edge of the field with too much speed and can’t get the airplane to stop flying. An open field does no good if we’re so fast that we float past it. Speed has to be controlled. Note that this is going to vary with the airplane involved; in a few aircraft with full flaps, for example, the drag is so high that the nose can be lowered briefly and the airspeed won’t increase during a moderate altitude loss. Others instantaneously build up speed if the nose is lowered.
A more consistent way to get rid of altitude (potential energy) and maintain approach speed is to slip the airplane, consistent with what the POH says about slips with flaps in that particular airplane.
One of the most important things we can do when flying an airplane is to visualize the energy footprint it possesses at any given moment. If we have an innate understanding of the energy involved and the relationship between altitude and speed and how one can be converted into the other, we’ll be able to make the right decisions at all times in flight, whether it’s an emergency situation or not.
Related Articles
