The art of crashing: it's all about dissipating energy over as long a distance as possible. Slow down, avoid the hard stuff and maintain control throughout.
If you are less fortunate, you will have to pick a field or road that may damage the aircraft a bit but mostly will end with a very awkward phone call. At general aviation speeds, making contact with unobstructed terrain at a shallow angle of impact should be eminently survivable. Then there is the outcome none of us wants to experience, where there is no clear landing site--no matter what you do, you are going to hit something. It just isn't your day.
This is where a pilot has a few seconds to demonstrate his or her mastery of the art of crashing. Understanding the physics of deceleration and the physiology of g-loading will help you make better choices in the moments that count. It also may result in different priorities when it comes to safety upgrades for your aircraft.
KINETIC ENERGY 101
To understand the art of crashing, we must get reacquainted with some basic physics. The first equation to know is the formula for calculating kinetic energy:
[E.sub.k] = 1/2m[v.sup.2]
Kinetic energy equals one-half the mass times the velocity squared, or in which m stands for mass and v is velocity.
What is key to understand is that the energy goes up as a square of the velocity. That means that when the velocity doubles, the kinetic energy quadruples, so even a small velocity increase results in a disproportionate increase in kinetic energy. In a crash situation, this is obviously very bad. To minimize the energy of the crash, controlling your velocity is important.
For example, a 10-knot groundspeed increase from 60 knots to 70 knots increases the kinetic energy by one-third. That means a 10-knot headwind will reduce the energy of the crash by one-third.
To ensure an artful crash, you should reduce your groundspeed before impact. Things to do: Point into the wind, have all your flaps down before you make contact, maintain the lowest controllable speed you can manage without increasing your sink rate. This will ensure first contact is as gentle as it can be. And whatever you do, do not stall your airplane.
DECELERATION AND ENERGY
High kinetic energy levels alone do not lead to disaster--it's how that energy is dissipated that determines the outcome. Having just explained the relationship of speed to kinetic energy, the speed of the crash isn't what kills you, it's the stopping part, or deceleration.
Deceleration is measured in multiples of the force of gravity, or G. During high-speed crashes, it is pretty easy to generate accelerations in the range of 25 to 50G. Unfortunately, that also is the range where severe injury or death is likely. The art of a good crash is to reduce G loads as much as possible. You do this by spreading the energy over as much distance as you can. Pardon the double entendre, but you want to leave a long skid mark.
In steep terrain, this means having the aircraft oriented to as low an impact angle as possible before making contact. Easier said than done, but a low contact angle will help you "skip" off the surface obstacles. Also, if you can aim for obstacles that have potential to give (tree tops, vegetation) vs. obstacles that don't give (rock faces, structures).
How much of a difference will skidding, bouncing or dissipating energy over a distance make? A lot. An aircraft traveling at 60 knots coming to a full stop in one foot (as when hitting a cliff face head-on), will generate 160G. It is not a survivable crash. In fact, at that G-loading, bones will break and the human body becomes a liquid--it is not pretty.
Spreading that same energy out over 10 feet drops the deceleration to 18G, which is bad, but survivable. Spreading the stopping distance to 30 feet takes you down to 5G, and if you can stretch the distance to 50 feet, where the deceleration is only 3G, you may have actually had worse turbulence than that.
Again, speed matters. If your initial speed was 80 knots instead of 60 knots, your 50-foot stop will be 5.5G and your 30-foot stop is 10G. Anything you do that will gain you a bit of stopping distance will reduce the crash loading. G-loading isn't strictly about initial speed, it also has a huge amount to do with how quickly you lose that speed.
Note the above deceleration calculations are not off-the-shelf numbers you can count on for your particular crash. They assume a uniform deceleration from first contact to complete stop.
Crashes are complex. When you hit something, you may come to a jolting stop until it gives and you move on, just much more slowly due to that initial impact. If you encounter a building, tree, large rock or some other immovable object earlier in the crash (when your speed is high), you will experience more Gs and decelerate more quickly than if skidding and bouncing and skidding again and then gently kissing the boulder just before coming to a stop.
Things like bounces, cartwheels, clipping a wing or hitting a movable object can radically change your particular crash's G experience. The point is to dissipate energy over as much distance as you possible can. That is much better than having a single object absorbing the full load all at once.
No matter what, you want to hit slowly and stretch out the crash's deceleration for as long as you can. But, the human body is not suited for uniform G loads in every direction. How many Gs can the human body take? The answer is not simple. It depends on where and how rapidly the Gs are applied on the body, in what direction they are applied (head-to-toe, front-to-back, side-to-side, etc.) and their duration.
We can deal best with smashing head on into walls or equally well spinning around and hitting the same wall backwards. Either way, we can tolerate as much as 45 G of deceleration before major injury. We are much less adapted to lateral, side-to-side motions and negative G. For side-to-side hits, we can go to 20 G before major injuries or 20-25 G for negative G load. But the human body simply can't take downward loads, as might be encountered when crashing with a high vertical velocity. We tend to break easily, suffering spinal compression or worse, under downward forces at 15 G.
So 45 G forward or 15 G down--either way, you will break, The takeaway is that I would rather hit something head on or even clip a wing and take my chances with a 20 G lateral acceleration from cartwheeling, over a stall and drop into a 15 G crush. The human body is the weakest in the vertical, or Z axis, and the resulting injuries carry the suffix--plegia.
Again, G-loads have a great deal to do with deceleration. Glancing an object that gives a little bit for 0.1 seconds and takes your speed from 60 knots to 30 knots is much better then having that same 0.1 seconds impact take you from 60 knots to zero. If you can, aim for softer things.
The single biggest tool you have to improve your survivability in a personal aircraft is its restraint system. A lap belt is better than being thrown out of the aircraft, but your face is still going to whiplash into the yoke, windshield or both.
That laptop and sledge hammer you have in the backseat are now projectiles. The laptop seemed innocuous, but at 20 G that five-pound laptop is now a 100-pound bag of cement heading for the back of your head. (The sledge hammer? What are you doing with a sledgehammer in your back seat?)
A single-belt shoulder harness is much better, and a five-point restraint system gives you the best odds of walking away, short of having airbags of some kind.
Are seat belts all the same? No. A rigid belt that does not stretch will let you experience full G. It may not seem like much, but material that is slightly elastic and stretches six inches during crash-loading versus three inches will give you twice the distance for deceleration. Not much, but what if I said I can drop a 100-pound bag of cement on your stomach or a 200-pound bag? Math never matters until it becomes your reality, then it makes a big difference. That extra three inches will save you a lot of pain and injury. It's physics.
Even better, airbags that spread the crash loading across a larger surface area and spread the load through time. Again folks, it is the physics. Firing off an inflatable milliseconds into the crash can make a huge difference. Airbags are an extravagance until you need them. The FAA/NTSB recently suggested seat restraint airbags would result in fewer deaths and injuries in general aviation. It should be obvious why.
If you don't see why it is obvious, consider this: As vehicle speed increases from 0 to 40 mph, the rate of injury in an accident increases by 50 percent and doubles again from 40 to 60 mph. Safety belts, when worn, reduce the number of deaths by 45 percent, and serious injury by 50 percent. The only difference between autos and aircraft is the Z-axis. Given the human propensity to break in downward forces, a better seat belt and minimizing downward G would definitely change the outcomes of most crashes.
RUN AWAY, RUN AWAY
The last thing we really haven't covered is the energy stored in your fuel tanks. Yup, even after your aircraft comes to a rest, there is still a great deal of energy that might be released. Post-impact fires are a huge killers. According to a NATO study, Injuries in Fatal Aircraft Accidents, "Fire occurs in 47 percent of commercial aircraft accidents, 32 percent of military accidents and 26 percent of general aviation crashes." The sooner you can get occupants away from all that fuel, the better off you and your passengers will be. That means opening the door just before crashing (so you are not pinned into the cockpit), turning off the master switch to eliminate those electrons and turning off the fuel.
Those seem like trivial details on the checklist, but the risk of a post-crash fire--at 26 percent--isn't trivial. If you can shut off a major source of spark and fuel flow, isn't it worth the two seconds of distraction to do so? There are reasons these items are on the emergency-procedures check list.
So, now you have arrived at the crash site. You managed to slow your airspeed to best glide (lowest sink rate), point the airplane into the wind (slowest groundspeed), get your flaps down before contact to drop speed but not increase sink (even less groundspeed), arrest your sink rate (no "--plegia" for me today, thank you), control your impact angle to the shallowest (maximize distance for crash zone) and you managed to aim for the softest objects in your trajectory--sure it's a billboard, but better than a brick wall. Before contact you switched the master off, killed the fuel flow and cracked the door.
On impact you were able to carom off the tree, hit your left wing against the boulder and gently came to a stop with the aid of your airbag-equipped five-point harness. "Only" 4.5 G; not bad. Nice job. You have demonstrated the art of crashing by walking away.
Now it's time for that awkward phone call after all. Wait, no cell reception. We'll cover that in the next article, the art of surviving the crash.
RELATED ARTICLE: MORE ABOUT THIS MONTH'S COVER IMAGE
On June 14, 1972, a tropical depression formed over the Yucatan Peninsula. By June 18, it had intensified into a hurricane, Agnes, which made landfall near Panama City, Fla. As it crossed Georgia, it weakened into a tropical storm and then marched up the Atlantic Coast, finally weakening over New York.
Along the way, however, Agnes dumped as much as 19 inches of rain on locations in Pennsylvania, causing the Susquehanna River to flood. Those flood waters had a major impact on people and property throughout the state, including an airplane factory in Lock Haven, located next to the river's west branch.
That factory belonged to Piper Aircraft Corp., and was the manufacturing site for the company's Aztec, Navajo and Comanche models. Flood waters destroyed some 100 airplanes and damaged or destroyed tooling for the three types, spelling the end of Comanche production, among other outcomes.
The flood was a major blow to Piper, but the company gave to NASA some 32 airframes for crashworthiness research, including the PA-23 Aztec pictured here and on this issue's cover.
The aircraft were transferred to NASA's Langley Research Center in Virginia, where a rig originally designed for testing lunar modules for landing on the moon was used to simulate various crash scenarios. That facility is known as the Impact Dynamics Research Facility (IDRF).
According to NASA, "Dynamic structural response data were obtained by conducting full-scale crash tests of GA aircraft under a variety of impact conditions. In all, 33 crash tests were performed during the 10-year period from 1974 through 1983." Subsequent testing at the IDRF has included Cessna, Cirrus, Lancair and LearFan aircraft, as well as a Beech Starship.
Mike Hart is an Idaho-based commercial/IFR pilot with 1000 hours, and proud owner of a 1946 Piper J3 Cub and a Cessna 180. He also is the Idaho liaison to the Recreational Aviation Foundation.
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|Date:||Sep 1, 2013|
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