Twenty years after aviation authorities bumped up the requirements for aircraft seat certification, nearly doubling the force of gravity that seats must be able to withstand and remain protective, questions are being raised about just how well those strengthened seats perform. The average weight of air travelers in several western countries has surpassed the weight of the dummies used to test aircraft seats. The 77.1kg weight of the dummies was selected by the FAA in 1946, which means there hasn’t been a reassessment of passenger weight and seat certification since practically the beginning of commercial aviation. Despite the lack of tests to demonstrate how increased weight will affect seats and seatbelts, some experts say the safety of overweight travelers – and those seated around them – is at risk.
“We are beginning to move in the direction of acknowledging our inherent obesity but it is happening faster than the industry can keep up,” says Dr Robert Salzar, chief scientist at the University of Virginia’s Center for Applied Biomechanics.
Yoshihiro Ozawa, an engineer whose company, Jasti Ltd in Japan, has been manufacturing crash dummies for transportation safety certification tests for 20 years, says he is concerned that there is no data proving that seating is safe for larger passengers.
“If we don’t test with heavier dummies we won’t know if seats and seatbelts are safe enough,” says Ozawa. “The regulation says the manufacturer is responsible to test for the 50th percentile male and they all do that. There is no regulation that says they have to test for heavier.”
The 50th percentile male weighs 77.1kg and seat manufacturers test their products using dummies of that weight, which regulators have said represents the median weight for men. But the latest numbers from the US National Center for Health Statistics shows the average man in the USA weighs 88.5kg – some 11kg more than the regulatory median. It is widely reported that in North America 33% of people over 20 years of age are obese, but larger girth isn’t confined to one region of the world. Back in 2000, the World Health Organization called obesity a ‘global epidemic’. Seat certification, however, has failed to keep up with the size of the person occupying that seat, says Dr Salzar, and as a result the airline industry is facing a ‘huge’ problem.
“If a heavier person completely fills an aircraft seat, the seat is not likely to behave as intended during a crash,” explains Dr Salzar. “The energy absorption that is built into the seat is likely to be overwhelmed and the occupants will not be protected optimally.”
There is most potential for incident in economy seats, Ozawa says, as the close proximity to other passengers increases the likelihood that the heavier passenger will become a hazard by colliding with those sitting nearby.
Airline seats have been under increased scrutiny since 1988, when regulators determined that safety could be enhanced if obstacles to escape were eliminated. Accidents involving aircraft take longer to play out than with other modes of transport and, even after the aircraft comes to a stop, surviving passengers need to be able to evacuate quickly. It is for this reason that the 1988 regulations gave the prevention of passenger incapacitation and seat collapse greater emphasis.
Seat design takes into consideration how best to protect heads, spines and limbs while the seat remains connected to the aircraft floor. The 1988 rules dictated that new seats should be made even more resilient by requiring them to withstand 16-times the force of gravity applied with energy; previously, seats needed to hold up under 9g of static force. So contemporary seat certification is intended to improve survivability on two fronts: protecting the occupants during the crash and giving them more time to get out afterwards.
“From a survivability point of view, the first thing you want to do is protect the occupants during the crash by ensuring that people and seats stay connected to the aero,” states Nora Marshall, a senior advisor for human performance and survival factors at the US National Transportation Safety Board (NTSB). “The new 16g seats are tested dynamically to improve everyone’s chances for crash protection.”
In her job, Marshall takes passenger lists from aircraft crashes and studies medical reports and injury patterns with the aim of determining which factors turn an accident lethal. In 20 years she says she has never encountered a crash in which the weight of the passenger appeared to be a factor in survival.
Still, Dr Salzar says regulators need to catch up with changing physiques of air travellers and that present testing does not provide information about a growing class of passenger. “They know they’ve got a problem,” Dr Salzar says of the Federal Aviation Administration certification criteria, “and like all large government entities, they don’t know what to do about it any more.”
Although passenger size and how it affects seat restraints is being discussed regarding aircraft for the first time, it is an acknowledged safety issue in cars, according to Andrew Walton, senior project engineer at Cranfield University’s Impact Centre in the UK. “We’ve actually tried to raise this in the automotive sector. We thought we’d get a better response. People are getting heavier but all the certification requirements are based around a standard 50th percentile male.”
Dr Salzar is also concerned that without testing on larger dummies, there is no way of knowing if seat restraints will adequately confine heavier passengers. “You’d be amazed at how a large person can blast through that restraint,” he says.
Technology is being developed for automobiles that can sense the weight of the seat occupant and calculate the correct seatbelt feed rate and airbag inflation for that person’s size and weight. This technology is not presently in use in aviation.
But seat strength and restraint effectiveness aren’t the only factors affecting safety when the passenger is overweight; many overweight air travellers do not wear their seatbelts except when required at take-off and landing, and when specifically asked to do so during turbulence.
Deitrich Jehle, professor of emergency medicine at University at Buffalo in New York, conducted a national study of more than 300,000 serious automobile accidents to see how passenger size affected crash mortality. He found that obese people were 21% more likely than people of average weight to die in automobile accidents. It was a consequence of many factors, Dr Jehle said, but he believed his study was the first to look at the effect of body mass on crash outcome.
Dr Jehle said that in the accidents he studied, obese people were 67% less likely to be wearing seatbelts, a situation that he said was probably applicable to air travel. There appear to be no studies of how common it is for people to keep seatbelts buckled when usage is optional. Dr Jehle suspects that obese air travellers are probably taking their belts off, taking on greater risk to themselves and becoming a hazard to others.
“Force = mass x acceleration, and when someone is heavier and unbelted, much more force is applied,”
Based on tests that Ozawa has seen, he said he believed heavier passengers could injure other travellers, especially in areas of the aircraft where seats are smaller and more closely spaced. “In economy, the backs of the seats may not be strong enough and the spaces between seats not sufficient to protect against the impact of heavier passengers behind or beside,” he says.
Marshall at the NTSB said she had not yet investigated any accidents in which seatbelt failure was a factor in survivability. “I think theoretically it could have an impact but we haven’t seen that in our accident investigations,” she says, adding that if such evidence showed up in an investigation, the NTSB would take a closer look.
The sensitive subject of passenger size is something that airlines avoid discussing, although a few have policies about when large travellers are required to purchase two seats. In a newly published booklet, the USA’s National Association to Advance Fat Acceptance provides an airline-by-airline listing of these policies. But Peggy Howell, the group’s public relations director, says she is unaware of safety issues regarding heavy passengers.
In a paper published in 2006, Christian Olschinka and Axel Schumacher, scientists at the Hamburg University of Applied Sciences, Germany, wrote, “Seats are the most influential mechanical components on the safety of flight passengers.”
A study of airline accidents by the NTSB published in 2001 showed that in 568 accidents over a 17-year period, 95% of passengers lived. “Most accidents we investigate are survivable,” Marshall says. “There is the misconception among the public that the things you do to protect yourself are meaningless because there’s nothing you can do if an aircraft crashes. That’s not true.” The evolution of safety equipment including seats and restraints has had a role to play in this success, asserts Marshall. With further developments in safety tests and equipment in line with growing passenger sizes, perhaps these survival statistics can get even better.
Pain in the neck?
An idea for aircraft passenger safety could come from the automotive world, with a seat that designers at Chard Safety claim eradicates whiplash as well as generally enhancing occupant safety in motor vehicles.
“Integrated seats and seatbelts have been investigated in the past,” says Chard Safety’s Michel Coenen, the creator of the DRS-Albatross seat concept. “But for one reason or another they have never really taken off. Crash forces – particularly at the back of the seat – are extremely high so such designs had to be structurally strong, and consequently were heavy and more expensive to manufacture. With OEMs looking at reducing vehicle weight, particularly for reasons of fuel economy, their take-up has been virtually non-existent.
“But what we have with our Deceleration Responsive Seat (DRS) is an entirely new seat frame concept. It is assembled to the vehicle floor, and has three legs instead of four – two at the front and one at the rear. Essentially we’re integrating the seat within a seatbelt, rather than the other way around. We have relocated the seatbelt anchor points from the sides of the occupant to points in the floor, in front of and behind the seat. The seat itself is made up of three sections, kept rigid and in place partly by the seatbelt webbing, which is threaded through the seatback and seat section, across the occupant’s shoulder and around the lap area.”
As Coenen reveals in further detail, the webbing is an integral part of the design. “In the event of a crash, an inertia release function transforms the seat from a solid structure into fully supporting sections, with the exerted crash forces being transferred directly to the belt. All of these design changes mean that the seat itself doesn’t need to be that strong, hence why we can offer reduced weight and lower production costs for Tier 1 suppliers and OEMs.”
The upshot of this unique construction is that the vehicle occupant is maintained in the optimal position in the event of a crash – the normal sitting posture – and decelerates with the vehicle, rather than at a different velocity away from the seated position.