Surviving Catastrophe

Air Safety Week, Oct 25, 2004

The following items were culled from the Oct. 18-21 course taught at the National Transportation Safety Board (NTSB) Academy, "Survival Factors In Aviation Accidents." As the class was under way, two passengers amazingly survived the fiery Oct. 19 crash of a Corporate Airlines Jetstream 32 twinturboprop with 13 passengers and two pilots aboard. The survival of two passengers was deemed a "miracle" in media accounts. However, very specific factors made the difference between one person's death and another person's survival in this accident. Below, some of the overall factors and considerations in crash survivability covered by the Academy's course:

Crashworthiness acronym. CREEP is the acronym used to delineate and describe the crashworthiness of an airplane. Crashworthiness is defined as how well it protects occupants in the event of a "survivable" accident. The elements of CREEP that sum to the airplane's general crashworthiness are:

Container: The fuselage's resistance to structural collapse, and the survivable volume in occupied spaces. According to Rick DeWeese, from the Federal Aviation Administration's (FAA) Civil Aerospace Medical Institute (CAMI), sufficient volume has to remain in the fuselage, and the impact must involve low G-forces. "Less than 100 Gs and down," he said. He pointed out that sufficient volume remaining and the G forces "are related."

Restraints: The ability of seat belts and seats to help passengers survive the sudden force of deceleration during impact. Inertial loads must be transferred to the skeleton rather than to soft tissue, and flailing must be controlled to prevent injury to limbs.

Energy management: The energy dissipated by the crushing, deforming, sliding and tearing of materials and structure (e.g. the "crush zones" in automobiles). The fuselage should provide energy absorption through controlled deformation, and it should be reinforced to resist penetration. Seat cushions, seat pans and seat legs are major components of energy management.

Environment: The cabin interior design must minimize the potential for injury during a crash (e.g., overhead bins do not collapse onto passengers).

Post-crash factors: Surviving occupants must be able to perform multiple functions, to include unfastening restraints, maneuvering through post-crash mayhem and opening emergency doors. Given the potential for pain and shock (reducing normal motor skills), obscured vision from smoke, breathing distress from toxic gases and panic, the designer must keep necessary functions for evacuation as simple as possible.

Truly crashworthy. Lisa Jones of the National Aeronautics and Space Administration's (NASA) Langley Research Center said, "The seat is the only crashworthy part of the aircraft." It is specifically designed to control the forces of impact.

Anatomy lesson. Many occupants do not survive the blunt force trauma associated with a crash. The term "thoracic transection" refers to cases where the victim's heart is separated from its mountings. Jones said the heart and its interior attachments to the body may be likened to a "lump mass on a string."

"The victims may look almost normal externally," she said, "but the internal organs are displaced." The heart, for instance, may be ripped from its attachments and found in the abdomen.

Seat protection. Presently 16G seats are being installed in newly built big jets. That's a substantial upgrade from the previous 9G standard. A 16G horizontal deceleration equates roughly to traveling at a speed of 30 mph and stopping in a distance of four feet, DeWeese explained. The 16G seat must also withstand a combined vertical (60[degrees]) and horizontal deceleration of 14 Gs. This standard, DeWeese quipped, "Can be met with a well-built lawn chair."

Add shoulder belts for more protection. Some advocates assert that lap and shoulder belts in airliners, akin to the three-point lap and shoulder restraints in automobiles, would provide more protection. Because the loading area is a factor, lap and shoulder straps (a 3-point restraint) have a larger peak allowable force than lap belts only (a 2-point restraint). However, lap and shoulder restraints are not found in airliners for a simple reason - the shoulder belt attachment point much higher on the seat increases the leverage (the moment arm) on the seat-to-floor attachment points. As DeWeese explained, shoulder straps would require "a drastic strengthening of floors."

Exemptions and exceptions. DeWeese asserted that the current 16G seat requirement contains "a bit of a loophole." It does not apply to FAR Part 23 commuter category aircraft. Nor does the 16G seat requirement apply to existing transport-category aircraft - only to new aircraft designs certified after 1988. The retrofit lies in limbo because of the cost. DeWeese said, "Since the floor strength of the current fleet is compatible with the loads produced by the improved seats, there is no technical reason why the existing aircraft could not be retrofitted. This rule is still under consideration."