What do you get if you fill a helicopter with dummies and drop it from a great height?
No, it's not the opening line of a joke. On the contrary, it's a serious question – the answer is data, potentially vast amounts of it, on how to make accidents more survivable. Which is why, at the end of last month, a team from Nasa and the US Army, Navy and Federal Aviation Administration slowly hoisted the fuselage of a CH-46 tandem rotor transport helicopter containing 15 crash test dummies nine metres (30ft) off the ground. And then let it go.
Thanks to design changes driven in part by rigorously testing new cars with crash test dummies, drivers and passengers are far more likely to survive road accidents than they were 30 years ago. But because of the increased cost and complexity, such testing has hardly ever been carried out on aircraft.
There are occasional "barrel drops" in which sections of fuselage are dropped from the height of a second-storey window – which are designed to simulate the effects of rough landings, rather than crashes. The most notable recent passenger plane crash test was carried out last year when several television production companies teamed up to film a Boeing 727 loaded with dummies and sensors hit the ground in the Sonoran Desert, Baja California, Mexico, at 140mph (225 kph).
This latest test, the first of two planned by Nasa, is designed to simulate a severe, but survivable crash. Having been stripped of its rotors and stub wings, the sacrificial CH-46 was effectively just a stubby tubular fuselage, with a few notable extras. "We had over 350 channels of data in terms of instrumentation and sensors on board,” says Martin Annett, Nasa’s lead test engineer. “The majority of that data came off crash test dummies or the airframe itself.”
The test was carried out at the Landing and Impact Research (LandIR) facility at Nasa's Langley Research Centre in Hampton, Virginia, which has a rich and distinguished history going back 50 years. It was first used to train astronauts how to land on the moon for the Apollo missions, before being converted into a crash-test facility. It still has space links. The water-landing capabilities of Orion, Nasa's new spacecraft designed to be used in missions to the Moon and beyond, are being tested there.
The 14m- (45ft-) long CH-46 fuselage was suspended on cables before being swung down towards a bed of hard soil. Just before impact a series of pyrotechnic devices fired to release the cables, allowing it to hit the ground at about 30mph (48 km/h). It was kitted out with a vast array of sensors such as accelerometers, and the dummies were monitored by almost 40 cameras both inside and outside. One side of the fuselage was painted with black polka dots on a white background to aid high-speed photography. The dots were used as data points and with 500 images captured per second, the researchers are able to observe precisely how different it bent, buckled, cracked or collapsed at the moment of impact. (The video above shows some of those pictures for the first time.)
The dummies were arranged in a number of positions to simulate different scenarios. Some were seated, others were standing, and one was even arranged on a stretcher to see what would happen to a patient being transported in that position. Analysing the data will take months, however initial results reinforce the warning usually given on planes to stay seated with your seat belt fastened.
“The test went really well in terms of the data we were able to collect,” says Annet. “The dummies that were seated in what are considered energy-absorbing seats fared pretty well, but “a lot of bad things happened to those in the standing position.”
Annet hopes the lessons learnt here will be relevant to other aircraft, including small passenger planes. "I wanted to do something larger that could be applied to business jet and commuter jet size, and even bigger transport size,” he says. Having a cabin with a lot of sections to mount seats or experiments allowed the team to get a more detailed picture of what happened to the dummy passengers within.
For their next test, the Nasa-led team will replace some of the metal components in their test aircraft with composite materials, such as plastics reinforced with carbon fibre, which are increasingly being used in aircraft construction. Carbon-fibre-reinforced plastic is used in the fuselage, wings, tail and doors of the Boeing's new 787 Dreamliner, for instance.
Composites are strong and lightweight, but some are also potentially more brittle and less shock- or energy-absorbent than traditionally used metals. Manufacturers can carry small-scale impact testing of these materials, however, not on the scale of the recent LandIR test. So the next test could answer some fundamental questions about composites, such as whether you lose the “crashworthiness” of a metallic airframe, and if so whether there is anything you can do inherently to the design of the airframe itself to maintain or improve it.
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