My wife was considering the purchase of a handkerchief for my last birthday present. Unfortunately she didn't know my nose size. She kids me about this because my nose size keeps changing. I keep running into solid objects with it.
Running into a door at 2 mph is a very uncomfortable experience. It is not, however, a deadly one. But imagine hitting something solid in your airplane at 100 mph. If a 2 mph collision can swell a nose, a 100 mph collision can eliminate one. Pilots sometimes face similar consequences when an engine failure occurs over inhospitable terrain.
In the event of an engine failure, you typically have several emergency landing options to choose from. Roads and fields are just a few of the opportunities you have to make your own impromptu airport. But suppose a reasonable landing surface isn't available? Suppose you have only trees, rocks or structures to choose from? How might you handle an emergency landing under these conditions? As you're about to discover, there is more science than superstition to this process.
High speed collisions require the dissipation of enormous kinetic energy (energy associated with motion). This energy varies with the square of the airplane's speed. Double the speed of impact and you quadruple the amount of energy involved in the crash. Or, think of it this way. Crashing an airplane at 85 knots is twice as hazardous as crashing one at 60 knots (Figure 1). The fact is that speed kills. Knowing how to dissipate energy in a crash is vital to your survival when forced to land in unfavorable terrain.
Let's explore three simple but very important rules regarding how to crash-land an airplane under these conditions. We're concerned about dissipating the kinetic energy and minimizing bodily damage to you and your passengers. The three rules are shown in Figure 2:
Rule number 1 is self explanatory. In an emergency landing situation, groundspeed determines the amount of energy involved in the impact. You want to do everything possible to touch down at the slowest possible groundspeed.
Even a small wind can make a big difference in the survivability of a crash landing. For instance, a 12.5 knot wind makes the difference between a 60 knot crash and an 85 knot crash. If I'm approaching at 72.5 knots indicated airspeed and have a 12.5 knot tailwind, my touchdown groundspeed is 85 knots (72.5 + 12.5 = 85). Make a turn into the wind and that 72.5 knot indicated airspeed becomes a 60 knot groundspeed (72.5 - 12.5 = 60). Comparing touchdown speeds of 85 knots to 60 knots, you can see that landing into the wind (instead of landing with it) allows you to reduce the impact energy by 50%. (See Figure 1 again.)
If there was ever a good time to use the groundspeed function of a GPS, it's when you're descending over inhospitable terrain with a failed engine. If you're going to plant the airplane in rough terrain, you might as well do it as slow as possible. Assuming you have enough time to do this and assuming your GPS has a reasonably quick refresh rate, you can make a 360 over the chosen landing site and monitor groundspeed readings on the way down. The heading with the lowest groundspeed should factor heavily into your choice for the final landing direction. Yes, yes, yes, I know, I know. We're talking theory here, but it can be done (and I'd do it, if possible!).
Additionally, keep in mind that surface winds normally shift about 30 degrees to the left from their direction at 2,000 feet above the ground (Figure 3). Therefore, if the wind is from 300 degrees at a few thousand feet above the surface, expect it to blow from approximately 270 degrees at the surface. The plain truth here is that it's worth going to all that trouble to know the wind direction when it's not obviously apparent. Even a few knots can make a very big difference.
Applying flaps also allows you to reduce the airplane's impact speed. Some flaps have more of an aerodynamic effect than others. Cessna's Fowler flaps are very effective in reducing the airplane's stall speed. On the other hand, plain type flaps don't affect the stall speed as much. Nevertheless, it's probably worth deploying them anyway.
One consideration to be wary about with flaps concerns how they affect the airplane's cabin. Figure 4 shows the impact of a Cessna between two trees with flaps deployed. Notice how the flaps penetrate the passenger seating section of the cabin. In this situation, if I elected to use flaps and expected to have the wings absorb the impact, I'd consider having my passengers assume the typical airline crash position. You know the one I mean. It's where you bend over and touch your chest to your legs and grab your ankles. This may prevent passenger injury from flap intrusion into the cockpit. I have a feeling that this position is also very effective for helping digest airline food, which may explain why the flight attendant is always trying to get you to look at the emergency information card after boarding. Humm?
Examine your airplane and take into account flap size, flap position and possible flap movement if the airplane's wings were used to absorb the energy of impact. If you deem full flaps to be a danger to rear seat passengers, you can use partial flaps instead of full flaps during an emergency landing. You're the pilot of your airplane. Decide for yourself what's best for the conditions under which you fly.
Rule number two requires you to arrive at the ground under control. Never, under any circumstance, lose control of your airplane during an emergency landing. For instance, some pilots think that it's better to stall the airplane onto the tops of trees (assuming this is their only choice, of course) rather than fly the airplane directly into the tree tops at slightly above the stall speed. I'd opt for controlled flight into the trees any day. Here's why.
General aviation airplanes are designed to withstand force from the forward direction and from underneath the airplane. If the airplane stalls and rolls inverted (begins to spin) during a tree top landing, the impact energy may be applied to the top of the airplane as you descend through the trees. There's very little structural protection provided by the upper portion of the cockpit of most airplanes. Additionally, seat belts don't work very well in restraining loads applied in the upward direction. Anyone who's ever hit their head on the roof during turbulence knows what I mean. Remember, all bets are off if you can't maintain control of your airplane during an emergency crash landing.
Rule number three requires you to let the airplane and the environment absorb the energy of impact. The fact is that the human body is pretty resilient. Yet it makes no sense to subject yourself to trauma if you can avoid it. Therefore, our objective is to minimize the amount of G force we're exposed to during an emergency crash landing. We can do this by understanding how impact speed and G force are related.
A pilot traveling at 50 mph who stops in two feet experiences approximately 43G's. The human body can't withstand this amount of force without experiencing severe or fatal injuries. The same pilot traveling at 50 mph who stops in five feet pulls only 17G's. This is easily more survivable than a 43G deceleration. Three feet means the difference between a survivable and nonsurvivable accident.
The secret to surviving a crash is to let the environment slow you down in such a way that breakable parts of your airplane (horizontal stabilizer, gear, propeller, wings, etc.) absorb impact energy. Now you know why returning to the airport after an engine failure from too low an altitude is very risky. If you stalled and spun into the ground at 50 mph, you'd probably stop in less than two feet. In other words, there's probably less than two feet worth of crushable structure ahead of you. You'll pull over 43G's and get a chest implant to boot (the control column).
Personally, if I had a choice to stall into the ground or make a controlled crash into the side of an aluminum warehouse, I'd choose the building. No question about it (assuming, of course, those were the only two choices). Your job is to make sure those aren't your only two choices!
Most general aviation airplane cockpits are designed to give each occupant every reasonable chance of escaping serious injury in a minor crash landing under the following assumed conditions. First, seat belts and shoulder harnesses are properly used (more on this in Part 2). Second, the airplane experiences no more than 9G's worth of linear deceleration in the forward direction.
The secret to handling the emergency crash landing is deciding how to keep the deceleration down to 9G's or less. You'll be amazed at how little distance it takes to decelerate without pulling more than 9G's.
Figure 5 shows the 9G uniform deceleration graph. It provides you with the minimum stopping distance (left vertical axis) versus groundspeed (bottom horizontal axis) necessary to experience no more than 9G's during deceleration. At 50 knots groundspeed, in order to pull no more than 9G's, I need to stop in no less than 12.3 feet. At 60 knots, I need to stop in no less than 17.8 feet. Of course, this assumes uniform deceleration throughout the entire distance, not a sudden stop during the last foot of travel.
Consider the implications of this graph. At 60 knots (a typical groundspeed for most of the airplanes we fly), you only need to stop in less than 17.8 feet to keep the cockpit and its occupants relatively intact. That's less than the wing span on most of the airplanes found in the general aviation fleet. Isn't that amazing?
Sure, you may have bruises, blemishes and maybe even broken bones, but, relatively speaking, who cares? Your objective is to avoid serious injury. This is all that matters.
As you can see, there is a science to crashing an airplane. But there's more where this came from. We're not done yet. Next week, we'll talk more about the art of crashing. We'll talk about how to pick and choose those environments that give you the best chance of absorbing the energy of impact. Of course I'm not teaching you to do this for fun, even if you are flying a rental! My objective is to provide you with tools you can use in the unlikely event of such an emergency. Tools are important. As Abraham Maslow once said, "If the only tool you have is a hammer, then everything looks like a nail."
Stay tuned.
For more information on this subject, see "Learning Experiences: Emergency Landings" and "The Long Wait: What To Do Until Help Arrives."
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