A battery scare has gripped the aviation industry. Normally unfamiliar terms such as "over-discharge" and "thermal runaway" are on the lips of industry insiders and the public alike. When aviation executives meet with journalists, the first question is always devoted to which chemistry is inside their aircraft battery. US regulators are deeply alarmed about battery overheating and fires, but do not yet fully understand the problem or what to do about it.
This, by the way, describes the series of battery failures that rocked the aviation industry in the early 1970s, as mostly general and business aviation manufacturers transitioned from lead-acid batteries to more powerful and less mature nickel-cadmium (nicad) power sources.
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Boeing's 787 battery issues have gripped the aviation industry |
Forty-one years later, there are signs history is repeating itself in a tragically recognisable pattern of risk-management failure. This time the culprit is lithium ion, the powerful chemistry that eclipsed nicad batteries in consumer electronics at the beginning of the last decade.
The root cause for two incidents of lithium-ion battery failure on the Boeing 787 between 7 and 16 January has yet to be discovered. Manufacturing defects appear to have been ruled out and the focus of several, overlapping safety investigations in the USA and Japan are on the battery itself and how it interfaces with the overall electrical system.
It is a mystery that appears likely to keep the 787 grounded for several weeks, if not months, and jeopardises a wider industry transition from nicad to lithium-ion batteries. The US Federal Aviation Administration must now answer to Congress, Boeing customers and passengers about how it could have cleared the lithium-ion batteries as safe in the 787, which may not have been necessary in the first place.
In 2005, Boeing decided to make the 787 a trailblazer in the aviation industry for lithium-ion batteries. It was a curious choice for the normally risk-averse manufacturer.
The 787 was already making the leap to a bleedless engine architecture, replacing pneumatics and some hydraulics with a 1.5kVA-sized electric powerplant onboard the aircraft. It includes four engine-mounted generators, each with a 250kVA capacity, and two APU generators producing a further total of 450kVA. Compared with the Airbus A350 electric architecture unveiled four years later, the 787-8's power capacity still has twice the electrical capacity of its even larger rival.
By comparison, lithium-ion batteries on the 787 almost seem like an after-thought of technological innovation. Unlike the fuel-saving premise of the "more-electric" 787 architecture, the efficiency gains offered by the new battery technology appear marginal.
Lithium ion has become a preferred power supply in consumer electronics and electric automobiles, despite widely known risks, because it can produce almost twice the power of a nicad battery using the same comparably sized packages. Yet lithium ion was still an unnecessary risk for the 787.
"The 787's more-electric architecture has very little to do with batteries," Boeing vice-president of marketing Randy Tinseth wrote in a 19 January "Q&A" about the 787 battery failures on his corporate blog.
On 10 January, Boeing 787 chief project engineer Mike Sinnett said the company had other options besides lithium ion, as he spoke to reporters in the aftermath of the JAL battery fire three days earlier.
"The lithium-ion battery was the right choice given the design constraints that we had," Sinnett said. "That doesn't mean that it was the only choice. That means that it was the right choice."
Boeing's options still include switching to less powerful chemistries of lithium-ion batteries or to previous battery technologies based on chemistries such as nicad, says Cosmin Laslau, a Lux Research analyst for the electric car industry. As far as the 787's electric architecture is concerned, the only difference would be the size of the battery and the specific integration with the overall system. Although switching to less powerful batteries implies an increase in size, the difference is measured in the hundreds or even dozens of kilogrammes - barely noticeable on a 227,000kg aircraft.
The grounding has raised the stakes of Boeing's lithium-ion selection, but in other ways mimic the aviation industry's fitful transition to nickel cadmium in the 1970s. It was part of a new era in avionics, for small aircraft in particular. Aircraft electronic systems were becoming more sophisticated, overpowering the capacity of lead-acid batteries.
But the move to more powerful nicad batteries came with a heavy price. Eight incidents of battery failures were recorded in the first nine months of 1972, adding to a string of nicad battery failures on aircraft stretching back to the early 1960s. One operator attending the 1972 NBAA convention likened the nicad battery to sitting on a "time bomb".
Actually, nicad proved no less safe a chemistry than any other battery - once the industry learned how to design, install and maintain them properly. Any battery can fail if it is over-charged or over-discharges. The FAA was moved to first print an advisory circular for the industry in 1972, suggesting tips to airlines and aircraft owners on how to avoid a dangerous new phenomenon called "thermal runaway" on nicad batteries.
Some aircraft operators also identified a problem with the architecture of the early nicad batteries, which comprised 19 individual cells each rated to a charge of 1.5V. French company Saft introduced a nickel-cadmium battery composed of 20 cells each rated at 1.43V. By even slightly lowering the charge contained in each individual cell, the risk of thermal runaway was reduced significantly.
Nicad batteries quickly became the standard power source in the aviation industry, and the FAA finally adopted two rulemakings for installing them in aircraft by 1977.
It was these rulings the FAA cited when establishing the airworthiness certification standards for the 787's lithium-ion batteries 30 years later. As the previous standard specifically addressed the installation of nicad batteries, Boeing was required to meet new "special conditions" to demonstrate the safety of the 787. Lithium ion was one of 14 such special conditions imposed on the 787 by the FAA's airworthiness authorities.
Boeing was not the first to install lithium ion on a passenger aircraft - that distinction belonged to Airbus with the A380 - but Airbus had limited the application to a non-rechargeable battery for the superjumbo's back-up lighting system.
By contrast, the 787 batteries are more of a leap forward, as they are integral to the twinjet's super-charged electrical system, starting the auxiliary power unit, backing up critical systems in case of a generator failure and powering the flight-control electronics.
Whereas business jet makers were among the first to introduce nicad batteries in the early 1970s, most were content to follow Boeing's lead on the transition to lithium ion. Gulfstream, for example, selected a Meggitt Securaplane lithium-ion unit for the G650 as a main battery in 2011, but appears to have changed course. The G650 no longer includes a lithium-ion battery in the electrical system, a company official says, but did not elaborate on reasons for the change.
Cessna, meanwhile, remains committed to using lithium-ion batteries in most of its jet models, despite being forced to recall the battery in 2011 after it was introduced on the CJ4.
While a root cause remains a mystery, the 787's problems have raised awkward questions for other manufacturers, including Airbus and Cessna, which also wish to replace nicad batteries with a lithium-ion-based chemistry to start the auxiliary power units and serve as a back-up power supply on their new aircraft models.
However, the hardest questions are still reserved for Boeing and the FAA: both were persuaded by the airworthiness of the now-suspect batteries less than 18 months ago when the 787 achieved type certification. Suddenly, the 787's safety appears compromised not only by a faulty battery, but by the failure of the elaborate protections supposedly able to contain a fire even if the battery alone malfunctioned.
"The expectation in aviation is to never experience a fire onboard an aircraft," says Deborah Hersman, chairman of the US National Transportation Safety Board. "We have to understand why this battery resulted in a fire when there were so many protections that were to be designed into the system."
The ongoing NTSB investigation is already one of several. A similar inquiry is under way by the Japan Transport Safety Board on the 16 January main battery failure on the ANA 787. Meanwhile, the FAA is reviewing both the battery and the documentation submitted by Boeing to prove the battery complies with airworthiness standards.
Source: Flight International
