The Pilot's Lounge #23:
Crashworthiness Improving Your Chances When You Have to Put It Down
Crashing. It's not something pilots like to think about, much less experience. But surviving an abrupt, unplanned end to a flight is something for which a well-prepared pilot should consider and equip. A lot of research has been done over the years and the fruits of that work have been incorporated into modern aircraft. Still, as AVweb's Rick Durden explains, there's much that a pilot and aircraft owner can do to help improve the chances that he and his passengers will emerge unscathed from an unplanned landing.
It was a grubby day outside the Pilot's Lounge here at the virtual airport. The VFR students had been scrubbed but a few of the IFR students were out flying with their instructors getting some actual experience in the clag. Most of the regulars were inside, sitting in the big easy chairs because someone had dug up a portable television and we wanted to watch the Indianapolis 500. It was delayed by rain, so we were doing what pilots do so very well — talking. During the delay we watched a short piece on crashes at Indy and the design of the cars to protect the drivers on impact. That triggered a very interesting discussion because one of the occasional visitors to the Lounge, Tom Tann, had stopped by to watch the race with us. He just happens to have an interesting background in crash research and high-speed operation in general, so, with the race coming on, we knew he'd have a unique perspective to add to our conversations.
Tom has a better understanding of speed than most of us, partly because he now teaches folks how to fly the Cessna Citation X — the only things in the sky faster than that airplane are built with government money. Tom also was in the pits at Indy with Team McLaren in 1973 and '74. The second year he was with McLaren, his driver, Johnny Rutherford won the 500. In '73, Tom saw Salt Walther hit the wall and spray burning fuel over many spectators due to an almost criminally poor design feature of the fuel system. That crash, and some other accidents he witnessed, must have influenced him, as he later went to what was then known as the Highway Safety Research Institute at the University of Michigan. There he worked with the very impressive John Melvin, the person who put accelerometers on Indy racers and obtained actual data on impacts when the instrumented cars hit the wall at 200 mph. Overall, Tom was around when some very sophisticated work was done on vehicle design to help the folks inside have a fighting chance of surviving a crash.
Naturally, with Tom here, we wound up talking about what a pilot can do to increase the chance of surviving when things get ugly and the arrival back on earth is probably not going to involve rolling serenely down a runway. As we talked I tried to make a list of the things a pilot and owner can do to increase the chances of surviving a crash.
The most important thing a pilot/owner of an airplane can do is to install shoulder harnesses in every seat in the airplane in which it's possible. Ideally, put in inertial-reel, double, over-the-shoulder harnesses. If that cannot be done, install adjustable, double, over-the-shoulder, harnesses on a fixed (not inertial-reel mount). Next in line is an inertial-reel, single-shoulder harness, followed by the adjustable, single-shoulder harness on a fixed mount. Anything that keeps your upper body from rotating forward and striking the aircraft structure helps tremendously in a crash.
There are some excellent double, over-the-shoulder, inertial-reel shoulder harnesses available for many general aviation aircraft. Raytheon sells one of the best for its airplanes and the Beechcraft it inherited (but it's hideously expensive and requires a field approval — Ed.). Piper and Cessna sell a single, over-the-shoulder, inertial-reel harness for most singles and twins; check the Parts Catalogue, it should be listed. You can buy an adjustable, single-shoulder harness for every seat, passenger or pilot, for virtually every Cessna single engine airplane built after World War II, as well as the Skymaster series. Cessna sells them at cost. There is no markup at all. The hard points are already in most all of those airplanes for every single seat. (They were put in at the factory because the shoulder harnesses themselves were always an option, but back in the bad old days, nobody ever bought them.) Installation is incredibly easy. When I owned a Cardinal I had the rear seat shoulder harnesses installed (it already had the front seat inertial-reels). Installation took 15 minutes. It's cheap insurance.
I'll emphasize this: If you own a single-engine Cessna aircraft made after World War II or a Skymaster, there is no excuse for you to not have shoulder harnesses in every seat in that airplane. Even the most tight-fisted aircraft owner can afford them. Buy the kits and put them in. It may save the lives of your family.
Wear The Restraint System
I am still stunned by the number of pilots who do not wear the shoulder harnesses that are already installed in their airplanes. One of the common excuses I hear during flight reviews is that the fixed ones prevent the pilot from reaching all of the controls and therefore the pilot doesn't have to wear his harness. While the FARs may say that, in a U.S. certificated aircraft the excuse is not true. For the manufacturer to certify the aircraft it had to show that the shoulder harness was designed so that pilots of short, medium and tall stature could still reach the controls while wearing the shoulder harness adjusted such that a fist could be inserted between the chest and the shoulder belt. If other pilots can reach the controls, you can, too. Having worked for a manufacturer and observed how the certification is done, I simply don't buy any excuse for not wearing available restraint equipment. Besides, most of the time when things go sour you don't have time to put on the shoulder harness before impact (plus, you probably won't even think of it.)
One of the fundamental rules of surviving a crash is to keep the occupants from hitting something in the aircraft itself during the impact sequence. Seat belts do the first part of that job by keeping the occupant's hips in or close to the seat. The problem is that the person then jackknifes over the seatbelt and the head smacks something more resistant to impact than flesh and bone. The shoulder harness is designed to keep the upper torso restrained so you don't flail forward so far. Sure, we can argue all day long about the quality of the systems in general aviation airplanes, but the bottom line is that they have saved one heck of a lot of lives, and they can't help you if you don't wear them.
There is continuing dispute as to what aircraft restraint systems will withstand. I'm not going to jump into the discussion, but I do recognize the certification requirements have changed. New aircraft seats will now absorb loads and more sophisticated upper body restraints are standard equipment. If someone is looking for a good reason to buy a new airplane over an old one, the crashworthiness improvements is one of the best I've heard. Older airplanes were pretty darn good, particularly if they have shoulder harnesses, but the new ones are better. Face it, we've learned a lot in the last 50 years. It's reflected in the new production airplanes.
Select The Surface
If you have any choice about where you are going to put the airplane, do a little thinking about the surface on which you are going to land. To the extent possible, pick the surface that will allow you to spread the process of coming to a stop over the longest distance. The difference between a landing and a crash is simply the distance over which the airplane can decelerate. Slowing from 60 knots to zero in 1,000 feet is no big deal; we do it every time we fly. Our bodies suffer no ill effects. Slowing from 60 knots to zero in one foot is a very big deal, indeed. I'm not convinced anyone, no matter how well restrained, could survive that stop. The longer the time and distance we have to spread out the stop, the better off we are. We'll talk about what the human body can withstand a little later.
There are some very interesting films made by NASA when it did full-scale crash tests of general aviation airplanes. The gantry arrangement that had been used for training Apollo astronauts for working under reduced G levels was modified to impact airplanes into the ground at specific angles and speeds. The initial tests crashed the aircraft on concrete. The data obtained showed that some impressively high-speed impacts were potentially survivable. At flatter angles the airplane would hit and then slide along the concrete. For a while though, everyone missed the obvious: The concrete simply redirected much of the energy of the crash. While it stopped movement downward, providing a significant deceleration in that direction, it did not absorb all of the energy of the moving airplane. The remaining energy was translated into a long slide. An engineer visiting from one of the manufacturers politely mentioned to the NASA scientists that, in his experience, not too many airplanes crashed on concrete. The light bulb lit and dirt was brought in and layered about three feet deep atop the concrete.
The same crash tests were rerun. The results were dramatically different. None of the impacts was survivable. The dirt compacted about six inches, and then stopped the airplanes cold. None of the impact energy was translated to the horizontal, as had happened with the crashes on concrete; it was all absorbed. Dramatically. The nose of the airplane crumpled as it was designed to do, absorbing a lot of energy, but the overall stopping distance for those inside the airplane was on the order of three feet. The resulting G loads were just too high for the body to tolerate.
The lesson for us as pilots is to choose a place that has the maximum amount of room to slide/decelerate after touchdown. Going with the rows of a plowed field is probably a good idea, going across them isn't. Flat ground is a lot better than hills. Concrete or asphalt is usually the best surface so long as it doesn't involve hitting a wall 50 feet after you touch the ground.
Fly The Airplane All The Way Into The Crash
Yes, you've heard that line before. It means you have more control over your destiny than you may imagine, so don't ever give up. I freely admit I stole the line from Randy Sohn, chief check pilot for the Confederate Air Force. He took it from Connie Edwards, who is one heck of a pilot. It means a few things:
Touch down as slowly as you can, but DO NOT STALL THE AIRPLANE BEFORE TOUCHDOWN. Far, far too many pilots, faced with what would be a routine forced landing, stall the airplane five to 10 feet in the air. That means you no longer have control of what has become a falling object. Even worse, it sets up a very high vertical component to the impact and can crush your vertebrae. It's why helicopter accidents are often so nasty; the large vertical G load means spinal column injuries.
Keep trying to control the airplane until it completely stops. If you are rolling and going to hit something, do everything you can to slow down as much as you can prior to impact. Impact energy is a square, not linear, function. You get a great deal of benefit for every knot you can slow the airplane down. That means when you blow a takeoff and run off the runway, stand on the brakes and pull the throttle to idle. I don't know how many pilots I've watched try to slow down an airplane while still carrying power. If you think of it, either pull the mixture control out or switch off the mags to get rid of any residual thrust.
Try to hit with minimal side loads. The most "crush" area for the airplane is directly in front of you and directly behind you. (It's a little tricky to hit going exactly backwards, although I did work on a case once where the airplane did just that after catching a wingtip on a tree.) The restraint system is designed to be most effective in a straight frontal impact, you have the most room to "flail" directly forward and there is a lot of airplane up there to crush and absorb energy. Take advantage of the airplane structure. Let it crush and deform to absorb the energy of impact so it is not transmitted to you. When TV reporters see the crushed remains of an airplane and make some sort of stupid comment that it's a miracle the occupant survived, it's no miracle; it's good design that allowed that structure to crumple and absorb energy, rather than transmit it to the people. If the airplane were designed to remain nice and pretty and intact after an accident it would kill the occupants in a very low-speed crash because none of the energy would be absorbed; all of it would be transmitted to the poor folks in the seats.
Wait until the airplane stops completely to undo the restraint system. Sure, that seems basic, but some folks think it's better to jump out when the airplane has "slowed down." While we used to hear of people being "thrown clear" and surviving, the reality is that you are a heck of a lot safer with an airplane around you than you are absorbing the impact with the ground all by yourself. I went off of the back of a pickup truck doing 20 mph once and the distance it took me to stop rolling and bouncing absolutely astonished me. I'd much rather have been inside the truck, firmly in a seatbelt and shoulder harness.
Gear Up or Down?
Over the years some general rules regarding whether to do an off-airport landing with the gear up or down have evolved, but it should be made clear that they are not hard and fast. If you are going to land on a surface where you reasonably believe the landing gear will catch on something while you are still going fast and either abruptly stop or flip the airplane, leave the gear up. In general, you want to take advantage of any part of the airplane that will absorb energy before it gets to you, the fragile human in the seat. If you are going to land on a hard surface or on a relatively smooth field, the gear will give you a lot of energy absorption for any vertical loads involved. It may also take a shock in a horizontal direction and snap off. That's good, because it just helped slow the airplane down without transmitting all of the energy to you. If you land gear up you hit on a fairly rigid structure that has only a few inches of crush distance before the load is directed to the seats themselves. The gear is designed to withstand and absorb a lot of impact load. If the surface allows it, having the wheels under you is usually a good idea.
Which Way To The Egress?
Open the doors of the airplane prior to landing if you can do so without seriously degrading the aerodynamics. It may mean a big difference in how rapidly you can get out after things come to a stop.
Assume The Position
Make sure everyone has tightened the seat belts and shoulder harnesses as tightly as possible and has "assumed the position" curled forward into the belts to take out the slack prior to touchdown. Slamming against the belts does reduce the impact forces a person can withstand.
If there is any sort of padding that you and your passengers can put in front of yourselves, be it a pillow, coat, soft-sided suitcase with no hard objects in it or a stuffed toy, use it. The idea is to spread the deceleration of as much distance and time as possible. When North American test pilot Al White ejected from the triple-sonic XB-70 and the bag that around the bottom of the ejection capsule designed to absorb loads didn't inflate, he actually disassembled parts of his seat so that he could put the padding where it would better support him when he hit the ground with a very high vertical G load. He was seriously injured but survived.
Turn off the master switch prior to touchdown. I've seen a lot of airplanes with the top half of the fuselage completely burned out after an accident in which the fuel system stayed intact and there was not an fuel-fed fire. The deformation of the electrical system started a fire that ignited the interior furnishings. (They are fire-retardant materials, not fireproof.) Shutting down the electrical system before impact can reduce the risk of an electrical fire substantially.
Turn off the fuel selector(s) prior to touchdown. A fuel-fed fire is a threat in any accident. Shutting off the fuel selector(s) and then touching down as slowly as possible (but not stalling in the process) after selecting the best site you can, goes a long way to reducing the risk of fire. The fuel lines in the fuselage of general aviation airplanes are routed to take advantage of the strongest portions of the structure, but as structure deforms to absorb energy, particularly forward of the firewall, there is the chance a fuel line will open up. Shutting off the supply of fuel to the engine area will minimize any fire that does start up front.
If you hit something with the wings, there is the chance you'll open a fuel tank. Once the fuel finds a source of ignition, the resulting fire can be impressive. Using the available restraint system may keep you conscious so you can get out before a fire can kill you.
Blue Side Up
Keep the airplane right side up. This may sound pretty basic for those who fly singles, but it is a killer in twins. Twins are much less likely to crash after an engine failure than singles, however, if they do, the accident is more likely to be fatal. The reason is that too many pilots let the airplane get below Vmc as they feverishly try to climb or just hold altitude. That means the airplane is going to roll upside down and crash. General aviation twins can take incredible loads when they crash right side up, but upside down the storybook usually closes immediately.
Years ago I worked on a case involving a freight pilot who lost an engine at about 200 feet up on takeoff. He hadn't yet raised the gear and had about 7,000 feet of runway in front of him. He retracted the gear, didn't feather the prop on the dead engine and made a turn while trying to hold altitude. Shortly after that the airplane rolled inverted and crashed. He died. Had he simply lowered the nose he could have landed. Even if he had rolled off the end of the runway, into the fence at 60 mph, he would have survived. A good friend of mine discovered he couldn't hold altitude following an engine shutdown shortly after takeoff. He kept up his speed, made a small turn and landed, gear up, in a plowed field. He slid through three fences, but once the airplane stopped, he unbuckled and got out, unhurt.
Don't wear nylon, polyester or other synthetic clothing when you fly. If a fire starts those clothes melt and stick to your skin, causing some hideous burns. Wear wool or cotton. Some pilots wear Nomex flight suits. That may buy you an extra couple of seconds in the event of a fire.
Fuel Tanks In The Fuselage
If you fly an airplane with fuel tanks in the fuselage, particularly if the tank is between the occupants and the engine, keep in mind that those airplanes have a horrible post-crash fire history. It does not take much of an impact to shove the hot engine back into the fuel tank and open it up. Yes, I fly those airplanes, but I generally do not fly them where I do not have a good spot for a forced landing and I am resolved never to stall one into the ground after an engine failure.
Every time I write about this subject I have Cub, Champ and Ercoupe pilots rush to defend their mounts as wonderful airplanes that have stood the test of time. I agree they are wonderful airplanes and I love to fly them. However, the test of time has shown they burn like blowtorches after even fairly minor crashes. They are the airplanes most likely to burn on impact. It's an unfortunate fact of life that pilots of these types must keep in mind.
The Gear Won't Extend, Now What?
During the course of the discussion we were having one of the regulars said that if he ever had to land gear up, he'd do it on a grass runway. He said he figured the grass was softer than concrete so it would be better for a gear up landing.
It sounded logical.
Tom Tann pointed out that one of the biggest challenges with crashworthiness research was that so many times the idea that sounded logical simply wasn't borne out in crash testing. The experience that has been gained with gear-up landings has also shown that what might seem logical is not how things turn out working in the real world. At some point during World War II the U.S. Navy's flight training operation at Pensacola started insisting that aircraft coming in with landing gear malfunctions land on hard-surface, rather than grass, runways. The results proved to be better in terms of damage to the aircraft and injuries to pilots.
A by-product of the NASA crash tests was to demonstrate that the risk of injury to aircraft occupants was less if a landing involving a landing gear malfunction were made on hard-surfaced runways. The reason is that if you mess up your landing on grass, and have a significant vertical descent factor in the touchdown, the turf may just stop you very rapidly. Grass may also ball up and cause the airplane to flip over. I was thinking about that recently when I looked at a film of a pilot landing his Cessna 210 after the gear could not be extended. He stalled the airplane about five feet in the air and really hit hard. In fact, he was high enough and deep enough into the stall that the nose started to pitch down before the airplane hit the runway. He was on a concrete runway. No one was hurt.
To put folks' minds at ease, there has been some research done on gear-up landings and injuries. Thus far a search of NTSB records has failed to turn up a single gear up landing in a civilian airplane (airlines and general aviation) since World War II where someone was hurt or killed. (I recognize that a gear-up landing does not ordinarily fit the definition of a reportable accident for the NTSB; however, many of them do get reported.) The only injuries located thus far involved passengers on airliners subsequently hurt going down the evacuation slides. It appears that while gear-up landings make quite the media event, the TV reporters road raging through traffic on their way out to the airport to get the "visual" are in much more danger than the people in the airplane involved. If you have hard data on a civilian gear-up accident in which someone was hurt or killed, please let me know. Part of the research was done to counter the FAA's claims of danger to passengers when they try to suspend the certificates of pilots who make an inadvertent gear-up landing.
A number of pilots I've spoken to express a concern for fire in a gear-up landing. After all, in the movies don't the airplanes always blow up when they make gear-up landings? Real life isn't that way. The fuel lines on airplanes are above the level of the belly skin of the airplane by some distance. The lines are also protected by some of the strongest portions of the fuselage structure. A gear-up landing may hurt the skin and lower fuselage structure a bit, but at least on concrete, the damage does not go up to where the fuel lines live. Interestingly enough, if you look at the belly of some jets you'll see some fairly robust strips that look a little like runners. Those are there just for gear-up landings, to help protect against damage in the area of fuel lines and tanks.
The fire concern is another reason one should not land gear up on grass. If the soil does cause the airplane to swerve abruptly in the slide out, there is the chance the machine will run off the runway and hit something that could penetrate a fuel tank or line.
Partial Gear Extension
Of course, the next question that came up among the pilots was what to do in the situation where only one or two of the landing gear legs will extend or the whole thing will only partially extend. Should the landing be made on those extended gear or should the gear be left up?
The answer is, by and large, when landing on a hard surfaced runway put anything you can under the airplane that will serve to absorb energy before the impact load gets to the occupants. Airplanes do slide on partially extended gear, sometimes on the sides or the tires and the gear doors. Airplanes do amazingly well on a nose gear, main gear and wing tip. Directional control usually isn't a serious problem. The operant rule is to do as much as possible to absorb loads. Centurions, Cardinals and other retractable single-engine Cessnas have made safe landings when the gear was stuck in the "trail" position, hanging down below the fuselage. It looks a little precarious, but things seem to turn out just fine.
If landing with partially disabled gear on grass or off-airport, the concern is with the gear catching on something, or hitting a hole or a ditch and flipping the airplane, something that could result in a very quick stop from about 50 knots, a most unpleasant experience. In those situations it is probably better to land with the gear retracted. Keep the descent rate low so as to have as much energy in a horizontal rather than a vertical direction. Again, that means a low speed, but not stalled, touchdown.
What Kind of "G" Loads Can Humans Withstand?
So, what is there to protect us if we do have to hit something that isn't soft and cheap? What can the human body withstand?
There has been research going on into the ability of the human body to withstand loads for well over 50 years. The first studies on human beings were apparently done by the Nazis as part of the "medical exam" atrocities perpetrated on the concentration camp inmates. Those "tests" were actually an evil combination of torture with a tiny bit of pseudo research tossed in to try to justify what was being done. The results were published but the data are considered unreliable and unusable from both ethical and scientific standards. I've known about these tests for about 20 years now, and I still shudder when I think of them.
Following World War II, various organizations in a number of countries began full-scale abrupt deceleration tests on human volunteers. The best known of these in the U.S. were simply referred to as the Stapp tests. In 1954, Lt. Col. John Paul Stapp and some other volunteers went through a series of tests riding an open rocket sled that shot down a rail before being stopped by a series of water brakes. Col. Stapp was the person who kept volunteering after others dropped out. He had some rides in which he was subjected to 20 Gs for a full second. On one ride the sled reached 632 mph and stopped in one and a half seconds. It was reported that he had experienced about 40 Gs for a brief period of time and, even though he was blind afterwards for a matter of hours, it was widely believed the human body could withstand 40 Gs.
Unfortunately, subsequent tests by other organizations were unable to reproduce the data. It was finally realized that after the rockets on the sled burned out, and before it entered the braking area, the air drag subjected the sled to significant deceleration, causing Col. Stapp to move forward into the restraining belts (they were very involved, far more so than anything found on any aircraft or automobile today) and "preload" the belts. Thus, when the serious deceleration occurred, he was not snapped forward any distance because the slack in the belts was already taken out. Accordingly, he could withstand, without serious injury, much more than someone who experiences rapid deceleration without warning and is flung into the belts. The rule of thumb now is that a healthy adult male can probably survive 20 Gs for a second, and maybe something more for a shorter time. A healthy woman can probably handle a little more as women tend to be able to withstand higher G loads than men.
A Little Planning Can Make a Big Difference
While crash survival can be a depressing subject, a little thought ahead of time can pay dividends should the real thing come to pass. The airplanes we fly do a pretty remarkable job in crashes when one considers they routinely see higher speeds at impact than do cars. What is a "low speed" crash in an airplane is going like crazy in a car. If you make use of the protection systems built into the airplane and fly the airplane all the way into the crash you have a pretty good chance of surviving the event.
By the way, one of the things I particularly enjoy about writing this column is that I learn a great deal from readers who have knowledge in these areas. I'm certain I haven't hit on everything that a person can do to protect the occupants (I didn't even mention helmets, which is a whole can of worms in itself) so I'm looking forward to comments and suggestions from readers on this subject.
See you next month.