Safety by Design

Aircraft designers have put new effort into accident avoidance and crashworthiness. If only the elements could be adapted to older airplanes


by Ken Ibold

With general aviation dominated as it is with decades-old designs, the generally declining rate of accidents and fatalities bears witness to the value of training and technology in keeping pilots and their charges safe.

However, the recent influx of clean-sheet aircraft designs helps illustrate how much designers have learned about safety since the birth of Cherokees, Skyhawks and Bonanzas. While performance continues to drive the sales of new airplanes, safety is a key component of a manufacturers long-term viability, thanks in part to product liability concerns. That reality has forced designers to pay close attention to both active and passive safety features in new models.

Industrial designers have long recognized that training is less effective in general at preventing accidents than is removing the threats through equipment design and technology. And once an accident happens, its up to the machinerys configuration to limit the extent of injury to the operator.

Applied to airplanes, that means an airplane designed with active safety in mind should have forgiving handling, good performance, good visibility and an effective man/machine interface.

Good handling includes positive stability, high maneuverability, controllability in a stall, stall resistance and a high crosswind component. Performance means short takeoff and landing rolls, low approach and stall speeds, good glide range, and enough speed to minimize a flights exposure to changing weather. Good visibility means limiting blind spots, to be sure, but also means the airplane should be equipped with recognition lighting and traffic avoidance equipment.

The pilot/machine interface is the most challenging. It requires reducing the pilots workload through thoughtful cockpit ergonomics and advanced powerplant design. However, it also requires powerful avionics that have a simple enough operating logic that training requirements are not excessive.

Despite the best efforts of designers, accidents will happen. And when they do the airplane design can help minimize injuries.

Passive safety elements include such things as protected fuel tanks that help limit the threat of post-crash fire. The airplane should have effective seatbelts as well as seat construction that absorbs downward impact yet restrains occupants in a frontal impact. Perhaps most importantly, it means the fuselage should have a design that absorbs energy up to the point where the passenger compartment is, and then it protects occupants vigorously.

Active Safety
In a nutshell, active safety means the pilot is able to avoid accidents in the first place. You see this in automobile commercials routinely, as the sports sedan swerves to avoid the debris that has fallen off of the flatbed truck. Psychologically this plays into the drivers (or pilots) sense of being in control of his own destiny.

Cirrus Design markets two main design aspects of its airplanes as active safety enhancements. The outboard leading edge of the wing is distinctly different from the inboard leading edge, a design that aims to provide better roll control authority and pre-stall warning at low airspeeds and high angles of attack. One result is that the airplane tends to resist inadvertent spins better than conventional wings.

The outboard leading edge extends farther forward than the inboard edge, creating a stall fence that blocks the detached airflow from progressing outboard. In practice, you can fly a Cirrus well into the pre-stall buffet and retain good aileron control.

Lancair has followed a similar strategy in its certified Columbia models.

The second active safety feature Cirrus has pursued is weeping-wing ice protection on its SR22. An antifreeze solution weeps from panels mounted on the propeller and the leading edges of the wings and horizontal stabilizer. The TKS system works as anti-ice and de-ice, although the airplane is not approved for flight into known icing conditions.

Diamond has designed what company president Peter Maurer calls the BMW feel into its DA40 and DA20 airframes, using responsive handling characteristics and excellent visibility to form the cornerstone of its active safety design.

The DA40, for example, features a stall speed of 49 knots – 10 percent slower than the Cirrus SR20 and more than 15 percent slower than the Lancair Columbia 350. The Liberty XL2 takes the notion further, boasting a stall speed of only 43 knots.

The slow stall speed does more than reduce the chance of landing accidents, however. Dialing in full nose up trim after an engine failure in a DA40 leads to a descent rate of about 700 fpm – less than a typical elevator and substantially less than the descent of a Cirrus under its parachute.

The notion of performance as an aid to safety is something of a two-edged sword. Like the motorcycle rider who understands that a crack of throttle can put the car alongside a block behind in an instant, pilots can use responsive handling and good climb performance to avoid trouble.

But high performance – especially cruise speed – does create a few problems as it solves others. First, the good news.

Speed sells airplanes because thats what makes them useful. If your mission is to fly 400 miles, your exposure to changing weather, mechanical trouble, fatigue and other hazards is only half as great if you fly at 200 knots as opposed to 100 knots.

However, that advantage turns into a disadvantage when you consider a few factors. Added speed makes the airplane more useful for longer trips, increasing your exposure again – especially as you cross multiple weather fronts or into geographic areas with which you are unfamiliar. In addition, the higher speeds usually translate into higher approach speeds as well. Its not coincidence that both stall speed and cruise speed are lowest in the Liberty XL2 and Diamond DA40 and greatest in the Cirrus SR22 and Lancair Columbia 300.

Man and Machine
The biggest challenge designers face is to make the airplane ergonomically friendly and still make full use of advances in avionics and instrumentation. It is here that the balance gets tough indeed. Advanced panels such as the Garmin G1000 and Avidyne Integra primary flight displays have steep training curves for users to take full advantage of their capabilities.

On simpler levels, other technology makes life easier for pilots through reduced workload, and that can only help.

For example, the Cirrus uses a mechanical linkage that ties the prop control to the throttle to remove a layer of complexity from setting the power. The Liberty uses the PowerLink digital engine control to adjust mixture and timing and automatically get the engine running at its peak.

And all of the new designs achieve good cruise speeds without the need for retractable landing gear, which reduces the complexity of the airplane as well as the pilots workload.

Despite the best intentions of pilots and designers, however, airplanes do sometimes crash. When that happens, the pilot is at the mercy of the decisions made by engineers during the design process. Its here that the unexpected can occur.

The canopy of the Diamond DA20 is designed for maximum visibility. The airplanes roots as a glider (which must maneuver abruptly to stay in thermals) and its mission as a trainer led designers to opt for maximum outside visibility. However, on three occasions the airplane has crashed because the canopy was not properly latched and came open in flight, adding drag it could not overcome.

Other design aspects to consider include the fuel system, seats and restraints, and the ability of the structure to protect the occupants.

The DA20 carries its fuel tank in the fuselage behind and below the cabin. Its a welded aluminum affair that is well protected by structure. All of the fuel lines are flexible and covered with braided steel. No fuel lines pass through the passenger compartment. Liberty has adopted a similar strategy on the XL2.

The design is similar to what race car designers have used for years. Perhaps the theory is that if the crash is bad enough to cause a fire, the occupants are moot anyway.

Diamond airplanes have never suffered a post-crash fire in 22 U.S. accidents. In only one accident did investigators notice a fuel tank breach of any kind.

Stretching the two-seat DA20 into the four-seat DA40 required designers to relocate the fuel tanks to the wings. So far no DA40 has crashed, so the results of that move are as yet uncertain. But in an effort to protect the fuel the DA40 wing has dual spars, one on each side of the tank.

The Cirrus carries its fuel in the wings, and in three of 17 U.S. accidents the fuselage was destroyed by fire. Two of those accidents involved cruise flight into terrain and the other was a spin. None of the three would likely have been survivable even without the fire.

Liberty also looked at the problem of engine compartment fire with a composite airframe and developed a proprietary ceramic firewall to help keep the fire from deforming the structure. As an added benefit, the improved firewall helps keep smoke and fire away from the cabin. Certification rules state the firewalls should be metal, but the FAA relented when it saw Libertys data and design, although the firewall is subject to a continuing airworthiness program and additional testing.

The Cabin Environment
Buckle up in a modern airplane design and youll likely find a four-point harness holding you into seats that are designed to withstand 26-g impacts. But youll find more than that.

The bottom edges of the instrument panel are not blunt pieces of sheet metal. There is no yoke to cause blunt trauma to the chest. The so-called headstrike zone – the area into which a properly restrained persons head would extend in an impact – is as free from obstructions as possible.

The seats represent the biggest change. Older aircraft designs call for seats to withstand a 9-g crash. That is, at 9 gs the seat should remain attached to the fuselage and the restraints should hold the occupant in place. Newer designs, however, recognize the different between a downward, spine-compressing deceleration and a forward deceleration that spreads the forces across the persons torso and hips.

In the case of Diamond airplanes, the seats are reclined and fixed; it is the rudder pedals that are adjustable. The reclined position prevents an occupant from submarining, or sliding under the seat belt, in the case of a frontal impact.

The rest of the modern cockpits are similarly designed with safety in mind. The side sticks favored by Lancair and Cirrus serve two purposes. First, they open up panel real estate, allowing the glareshield to be lower and still include a full battery of instrumentation. Second, they eliminate the chest-crushing yoke.

The side stick, however creates another potential hazard. (See how everything is a compromise?) The left seat pilot must fly with the left hand and the right seat pilot must fly with the right unless the autopilot is on. However, a left-handed pilot flying in the left seat or a right-handed pilot flying right seat is unable to switch hands while writing down amended clearances, for example. There is also something of a fatigue factor in using one hand for a prolonged flight, cementing these airplanes as ones in which a functional autopilot becomes a necessity.

The Diamond and Liberty use center sticks. These provide the advantage of allowing the pilot to switch hands when necessary at the cost of having an obstruction in front of the occupants. The stick provides less of an obstruction than does a yoke, however, so the compromise still works. One airplane, the short-lived Eagle 150B, uses a stick between the two front seats and moves the power quadrants to the sidewalls. This carries the same kinds of disadvantages as the Lancair and Cirrus.

Its important to note that the cabin environment is subjected only to static testing, rather than actual crashes. The exception to that comes from Diamond, which actually crashes fuselages occupied by crash-test dummies in automotive test facilities. While not a perfect simulation of an airplane crash, the tests do provide additional insight that allows designers to have more confidence in the airplanes performance.

Airframe Protection
Modern airframes are designed something like automobiles in that the structure is intended to absorb energy up to the cabin area, and then jealously guard the occupants. For composite fuselages, that means carbon-fiber roll cages – an advancement on Mooneys steel cage.

The Liberty structure is unique in that it uses a welded steel chassis and surrounds that with a carbon-fiber fuselage. Liberty augments that with a wide landing gear stance to help prevent rollovers.

The problem with designing crumple zones into the fuselage is the result of two factors. First, the airplane structure must be as light as possible, limiting the ability of engineers to work their magic. Second, the wings and their attachment to the fuselage demands great strength and rigidity – directly at odds with the notion of absorbing impact.

Liberty addresses that by mounting each wing directly to the steel chassis rather than to a spar carrythrough. This allows one wing to shear in a crash without destroying the integrity of the fuselage, while still keeping the wing attach point strong.

Its interesting to note that the new designs add a tool to the cockpit – a crash hammer – intended to break out the windows should the doors or canopy jam. The hammers are relatively small and the ability of an injured occupant to break out is still, at this point, theoretical, but we think its certainly better than being trapped in the wreckage with no recourse.

The Ace in the Hole
No discussion of airplane safety would be complete without addressing what Cirrus calls its ace in the hole: the Cirrus Airframe Parachute System.

When all is lost, firing the chute should bring you back to earth in a manner at least more sedate than crashing. The impact will be downward, which is where those 26-g seats come in handy, and not necessarily under control, but in the only operational deployment to date the pilot walked away without a scratch.

The parachute is somewhat controversial and had one early failure in which it failed to fire. It remains to be seen if it provides the kind of last-ditch out the company intends it to be. Military research on ejection seats has found that pilots are much more likely to use ejection seats in the face of an equipment problem than in the case of pilot error. Should that dynamic apply to civilian pilots, it should be noted that about 85 percent of airplane accidents are due to pilot error. That means some Cirrus pilots will not pull the chute despite every indication that they should.

Cirrus puts the impact under the chute as the equivalent of falling 10 feet. The landing gear, roll cage and seats then absorb most of the impact. Further experience will show if the parachute is as much a safety advance in the real world as well as it appears to be on paper, and we expect other manufacturers are looking at the results of the Cirrus experience before incorporating the design into their airplanes.

Modern airplane designs have as much in common with older designs as modern Volvos do with vintage Ramblers. While many pilots complain that advances in airplane performance comes at too slow a pace, we consider advances in safety to be significant, tempered only by how slowly they will be assimilated into the fleet as a whole.

Some of the advances can be retrofitted, however. Seat belts can be improved. Vortex generators can reduce stall speeds on some models. Seat rails should be inspected and maintained regularly.

You can also do your part by limiting cockpit clutter and items that would cause injury in a crash. Tie down luggage. Restrain pets. In short, stack the odds in your favor at the beginning of the flight, because at the end you wont have the time to spare.

Also With This Article
“Stop Right There”
“Lessons Learned”


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