FOR most of us, the idea of being trapped, helpless, in a stricken airliner
is the ultimate nightmare. The only crumb of comfort is that when crashes do
occur, at least everything possible is done to find out what went wrong and make
sure it never happens again. Or is it?
The key to discovering the cause of a crash is usually the flight recorders
that record flight data and the crew鈥檚 cockpit conversations. Surprisingly,
crucial as they are to crash investigators, these instruments do not have a
built-in battery backup in case power from the aircraft is cut off.
This issue was thrown into sharp relief last month when Swissair Flight 111
plunged into the ocean off Halifax, Nova Scotia, killing all 229 passengers and
crew. The Transportation Safety Board of Canada found that both flight recorders
had 鈥渟topped at about the same time鈥攕ome six minutes before the aircraft
struck the water鈥�
(This Week, 3 October, p 4).
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The investigators don鈥檛 yet know why power to the aircraft鈥檚 recorders was
lost. But the mere fact that the electricity supply can be severed without a
battery cutting in worries the global airline safety regulator, the
International Civil Aviation Organization in Montreal. 鈥淭hose recorders need to
work until impact,鈥� says ICAO spokesman Denis Chagnon. He expects the Flight 111
accident report to recommend 鈥渟ome sort of mechanism鈥� to ensure that recorders
always capture the vital information about a plane鈥檚 final moments.
Regulators such as the US Federal Aviation Administration (FAA) and Britain鈥檚
Civil Aviation Authority base their rules on the ICAO鈥檚 Convention on
International Civil Aviation. This demands only that the power lines to the
flight recorders provide 鈥渕aximum reliability for operation鈥� without
jeopardising any other flight control systems. There is no requirement for a
built-in back-up battery.
This might seem an obvious omission, but the reason the FAA and aircraft
makers haven鈥檛 wanted a battery backup is that both the flight recorders have a
continuous loop of recording tape. If they continued recording after a crash,
all the data would be overwritten.
Albert Reitan, a voice recorder analysis expert with the National
Transportation Safety Board in Washington DC with experience of 150 crash
investigations, says that while the idea of a battery backup has been 鈥渂atted
about for years鈥�, it has always been stymied by concerns about overwriting. But
now, he says, some modern light aircraft use a gravity switch that senses the
force of a crash and cuts the power to the recorders a few seconds later.
But overwriting is not the only problem with battery-backed recorders, says
Peter Waller, technical director of the Flight Data Company at Heathrow. There鈥檚
also the sheer size of the battery that would be required, and the problem of
maintaining it in service. Nor is there any point in having the recorders
running if all the other instruments have gone dead. Other systems, such as the
cockpit audio and radio system, would also need a battery backup to ensure there
is something to record, he says.
When it comes to power, the priority must be keeping the aircraft safe, says
ARINC of Anapolis, Maryland, a company that specialises in turning the ICAO鈥檚
rules into electronic specifications. 鈥淭here are plenty of trade-offs, but it鈥檚
critical to keep an airplane flying,鈥� says Dan Martinec, the company鈥檚 director
of avionics engineering, and chairman of the Airlines Electronic Engineering
Committee. 鈥淚f an aircraft is on its last source of power, what鈥檚 more
important? Keeping the flight critical avionics working or the recorders?鈥�
However, despite the technological challenge, he thinks it is possible to design
just such a system.
And this, surely, is a challenge that the aviation industry must take up. To
build flight recorders that can withstand a crash impact of 3400 times the force
of gravity, but fail to provide back-up power, seems somewhat bizarre. The air
travelling public deserves better.