limitations of black boxes

The mystery of flight MH370 is unlikely to be
solved until the flight recorder - known as a black
box - is found. It has two components - a cockpit
voice recorder and a data recorder. But these
devices have their limitations.
The voice recorder only captures the final two
hours
Listening to the last moments of Air France flight
447, which crashed into the Atlantic in 2009, gives
a chilling insight into the confusion that had
overcome the pilots. Such a record of what went
on in the cockpit would be a priceless tool for
investigators trying to work out whether the
Malaysia Airlines plane was the victim of foul play
or a mechanical fault. But it's not that simple even
if the black box is found. The cockpit voice
recorder continually records over itself as the flight
goes on. US firm Honeywell Aerospace says the
black box on the missing airliner - which it
provided - only retains two hours of recording.
That's the length of time that regulations demand.
The principle is in place because it is normally the
last section of a flight that determines the cause of
the crash. But in the case of the Malaysia Airlines
777 it might well be the case that the key events
happened long before the actual crash. On the
other hand, Steve Buzdygan, a former BA 777 pilot,
says the data recorder would provide a wealth of
useful information. "You can almost reconstruct
the flight path from it."
The battery life is short
The black box sends out a ping - after activation
by contact with water - that can be picked up by a
microphone and a "signal analyser". Both the voice
recorder and the data recorder each have their
own pinger. But there's a problem - the battery of
the pinger on MH370 will only last for 30 days,
says Steve Brecken, media director at Honeywell.
Some pingers last for 90 days. The variation
stems from the fact the rules changed after Air
France flight 447. It took nearly two years to find
its black box and new guidelines were issued that
the ping should last for 90 days to give search
teams longer to find it. Some planes have since
been updated, but apparently not the MH370. Even
after the batteries for the pinger run out, the
recorded data remains intact.
It is a small object to find
The black box is bolted into the tail of the aircraft
to avoid damage in a head-on crash. It is small -
about the size of a shoe box, says Dr Guy Gratton
of Brunel University's Flight Safety Lab. Contrary to
the name, it is bright orange. But it's not easy to
see it in the middle of the ocean. The search will
aim to try to locate the wreckage before moving in
to pinpoint the black box by picking up the ping. If
the pinger has expired then other techniques -
such as magnetic detection - are going to be
necessary.
It doesn't float
The box is made out of aluminium and designed to
withstand massive impact, fierce fire or high
pressure. That means it's heavy - about 10kg for
what is a small box - and will sink quickly. The
Indian Ocean has very deep sections. The search
area ranges between 1,150m (3,770ft) and 7,000m
(23,000ft), media reports suggest. So investigators
will be have to consider the prospect of it being
out of reach of many sonar devices. "You have to
ask if there's terrain in the way. The seabed could
be as mountainous as the Alps," says David Barry,
an expert on flight data monitoring at Cranfield
University.
The pinger's range is only a few miles
Honeywell, who made MH370's pinger, say the
signal can typically only be picked up a mile
away. But if it is deep on the ocean floor navies
have hydrophone technology that has a better
chance of locating it than conventional detectors.
The Air France black box was not found until after
its ping had expired. It was eventually located by
slow moving unmanned underwater vehicles. A
modern submarine - such as one of the Royal
Navy's hunter-killer models - could potentially at
least hear a ping from many miles away, Gratton
says. The US, China and Australia all have similar
submarines, he says. "By now there will be a
submarine down there. I'm certain the Chinese will
have put something out there." The US has
deployed a ship that will tow a special black box
locator through the water. According to the
Associated Press, "the Towed Pinger Locator,
which is pulled behind a vessel at slow speeds,
has highly sensitive listening capability so that if
the wreck site is located, it can hear the black box
pinger down to a depth of about 20,000ft
(6,100m)". However, there is a further
complication, says Barry. The black box may be
giving off pings from the ocean floor. But if those
pings hit a layer of warmer or colder water above,
the signal might be refracted or reflected.