Science

What Can We Learn From Crashing A Plane On Purpose?

Why do some people survive plane crashes and others don’t? Having an entire aircraft at your disposal to deliberately crash under controlled circumstances, as they do in a new Discovery Channel documentary, would seem a great way to answer that question.

But is the crash of an old Boeing 727 a mere stunt, or a genuine attempt to tease some illuminating forensics from an event that just happens to make compelling television?

It’s a bit of both. Discovery’s forensic aim was to crash the plane by remote control to observe and measure how the aircraft breaks up. That would pinpoint the safest places for passengers to sit, find out if bracing before impact does any good, and what parts of the body are most at risk from injury. But for me, the choice of a crotchety old plane with a design that has little relevance to the planes we fly in today undermines the forensic value, for a couple of reasons.

First, the 727 was designed in the 1960s, is made from materials unrepresentative of the carbon-and-glass-fibre-rich designs of today, and has its engines bunched around its tail, rather than beneath the wings like modern airliners. History has shown that tails are a bad place to mount engines: if a jet turbine blade separates from the engine, it can sever the hydraulic control lines to two critical control surfaces, the elevator and rudder. This is exactly what happened in the 1989 Sioux City, Iowa, crash that killed 111 people. None of the major Airbus or Boeing planes of today has tail-mounted engines.

Secondly, the game plan for the show was not only to crash the plane, but to do so without frying it – as NASA tried to do when it purposely crashed an even older Boeing 707 in 1984. The aim there was to assess a new fuel that supposedly had low crash flammability. It did not, it turned out, and NASA’s plane went up in a dramatic fireball.

So, to get the forensic information they wanted, the Discovery Channel team of experienced military and Boeing pilots had to ensure the crash would break the plane, but not burn it to a crisp. But as a quiz on the show’s website even points out, post-crash fire is one of the greatest risks to passengers. Minimising this risk subtracts further from the flight’s forensic validity.

So why use the 727? Though cost isn’t given as a reason, $US400,000 of the show’s $US3 million budget was spent on the aircraft. The ex-Singapore and Alaska Airlines workhorse was probably available relatively cheaply because no western airline still flies the gas-guzzling, noisy beasts.

The main reason, though, we’re told, is that to crash the 727, three aircrew needed to set it on course to a pre-ordained spot in a Mexican desert and then bail out – a chase plane with a customised remote control unit flew the aircraft to its doom. The 727 is one of the few planes with air stairs that drop down dead-centre underneath the tail, allowing the pilots to parachute out safely behind, and immediately clear of, the plane. Try that from a side door and you’ll be sucked into an engine or smash into the fuselage.

(Strangely, the producers claim this is the first time an airliner has been flown via remote control. I happen to know first-hand that’s not the case.)

What do we learn as this 180-seat airliner bites the Mexican dust? As it hits, nose first with its wheels down, the cockpit fractures and separates from the plane. This is treated as a surprise, but it is precisely what happened in the aforementioned Sioux City crash. In that case the cockpit tumbled so far from the main wreckage field it wasn’t found for an hour or so. Astonishingly, all the crew in the Sioux City cockpit were alive, but the Discovery team reckon nobody in the cockpit, nor the people seated either side of the nose break-off point at about seat row 7, would have survived this crash.

They know this because the 727 was peppered with accelerometers measuring impact forces – and the producers conclude that in rows 6 to 8 the 12g forces would have killed any passengers. The impact forces declined the further back in the plane you went, reaching a survivable 8g just a few rows away and far less at the very back where the black box is kept – which is why it’s kept there. Bad news for first class.

How passengers seated in different parts of the plane, and different positions, would fare in the crash was analysed by positioning crash-test dummies in the cabin. One of the scientists contributing to the show was biomechanical trauma specialist Cindy Bir of Wayne State University in Detroit, Michigan. Bir and her team placed three $US150,000 automotive industry crash-test dummies about the plane, each instrumented with 32 sensors that give a pretty precise picture of the body’s reactions, from the top of the head to the ankle. They filled other seats with lower-tech dummy passengers.

The sensors revealed the brace position, in which you hunch over toward the seat in front of you, is worth adopting. A braced dummy suffered minor ankle damage, but an unbraced body swam in a debris-strewn cabin and suffered major damage to the lower back. Unbelted dummies sustained injuries “submarining” beneath the seat in front.

Still, the question of contemporary relevance remains. Latter-day planes crash differently to those heavy old beasts. When a bird strike forced an Airbus A320 to touch down on the Hudson River in 2009, nobody died. Brave flying and a lightweight modern design kept it afloat long enough for New York ferries to get everyone off. At Toronto’s Pearson airport in 2005, a four-engined Airbus A340 slewed off the end of the runway after a bodged landing, broke up and was entirely burned to cinders in the ensuing fuel fire. All 309 survived, with modern fire-retardant cabin structures, aeroplane seating and carpetting cited as a reason.

However, what Discovery has done, inadvertently, is highlight a need for crash-test data on modern aircraft — those with their engines slung beneath their wings, not clustered about the tail, and with ever more carbon-fibre structures, like the Boeing 787 and nascent Airbus A350. We simply do not know how many of the new carbon-fibre materials behave – either in service or in a crash. And more importantly, the industry doesn’t know. Explaining the source of cracks in Airbus A380 wing brackets, Tom Enders, then head of Airbus, told Bloomberg News in May:

We thought we understood the properties of the materials and the interface between carbon fibre and metal and found out the wrong way we didn’t know everything.

Those are the kinds of things we urgently need to know — even if they won’t initially make for such compelling television.

Images courtesy of Discovery Communications

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