Criticism by a former Boeing engineer has brought renewed scrutiny to the crashworthiness of the 787's composite airframe. Are the concerns real or just out-of-date thinking
Everything is new and unknown as least once in its life. Aluminium airframes are well into their maturity, their strengths and weaknesses well understood. But, when Hugo Junkers built the duralumin J4 in 1917, the new wonder metal was regarded with suspicion by aeronautical engineers used to wood and fabric.
Ninety years later, composites are regarded with similar suspicion by some aerospace engineers. An ex-Boeing engineer has gone public with his concerns about the crashworthiness of composite airframes, prompting newspaper articles and television programmes questioning the safety of the "plastic" 787.
But composites are far from newborn and unknown. Fibre-reinforced plastic materials have been used in aircraft for decades. And over time composites have migrated from non-structural fairings to load-bearing assemblies as the performance of both the plastic matrix and reinforcing fibres has improved.
The first all-composite aircraft certificated by the US Federal Aviation Administration was the glassfibre Windecker Eagle in 1969. Boron-fibre composites were used in the tailplane of the Grumman F-14 in 1970. And the all-carbonfibre Beech Starship, with its filament-wound fuselage, was certificated in 1988.
Airbus introduced a carbonfibre fin on the A310 in 1985 and has used more of the material with each new airliner. The A380's empennage, wing centre box and outer panels are carbonfibre. The new A350 will have an all-composite wing and - in a new development - carbonfibre frames as well as skins in the fuselage.
So why all the fuss about the 787? The best explanation is that engineers are conservative creatures who want to base their decisions on a database of design allowables supported by exhaustive and documented experience ranging from coupon tests in the laboratory to aircraft crashes in the real world.
The crux of the ex-Boeing engineer's concerns over the 787 is whether Boeing has done sufficient testing to be absolutely sure it understands how a composite airframe - and particularly a carbonfibre fuselage - behaves in a crash and the fuel-fed fire that could ensue.
The consensus of composites and certification experts consulted by Flight International is that the ex-Boeing engineer is in a minority. Most believe there is a sufficient database of knowledge on composites crashworthiness within the industry for Boeing to understand how the 787's airframe will behave in a crash.
The criticism being levelled at Boeing is that the company is endeavouring to demonstrate the 787's survivability through modelling, analysis and testing of pieces of airframe, rather than conducting an expensive and time-consuming full-scale crash test.
It is easy to see an old-school engineer being uncomfortable with this approach. After all, the industry's understanding of how aluminium airframes deform and burn has been built up over decades and validated by many real-world events. A look through NASA's history of full-scale aircraft crash testing shows how that database of knowledge was built up over the years.
But while the industry was learning about how aluminum aircraft perform it was also gathering valuable data about the forces on airframes in crashes - information that can be used in modelling the behaviour of composite structures. And there have been full-scale crash tests of composite airframes - admittedly smaller aircraft and helicopters, but the results have confirmed that carbonfibre structures can be designed to match or exceed the impact survivability of metal aircraft.
To argue that it is not possible to take this data, perform additional tests on critical structures, and analyse and assemble the results into a rigorous demonstration of the 787's crashworthiness is to call into question Boeing's ability to exploit the advances in computing power that are the basis of modern aircraft design.
It also requires the FAA's competence and independence in overseeing certification of the 787 to be called into question. Boeing's response to the concerns is to emphasise that it is required to demonstrate the 787's crashworthiness to the FAA. To argue that the 787 will be less safe than the 777 (which has never crashed) is to suggest that the FAA will allow Boeing to cut corners in demonstrating its airworthiness.
As the 787's first flight slips, and its certification schedule compresses, that oversight becomes critical. But there is no reason to believe a process that has worked well for decades will fail us now.
Source: Flight International
