This is the extraordinary story of an
aircraft which was in good flying order except for a blocked pitot
tube which caused multiple problems.
Birgenair Flight 301 crashed shortly after take off from Puerto
Plata in the Dominican Republic for Germany on 6 February 1996,
killing 13 crew and 176 passengers. The aircraft was a fully
computerized Boeing 757-225.
As with the crash of AF 447 the black boxes were in deep water
and there was there was a race against time to locate them before
their signals faded and they were damaged by salt water.
They were found and brought to the surface by a US Navy CURV -
Cable Underwater Recovery Vehicle - a remote-controlled submersible
which could work at depths greater than a manned submarine.
From the flight data recorder and cockpit voice recorder,
investigators had learned that the air speed indicator failed to
work at first, then seemed to come alive as the plane climbed.
Unknown to the pilots, one of the aircraft's three pitot tubes
was blocked. Yet the air speed indicator appeared to work because,
as the plane climbed, thinning air trapped inside an one-ended
pitot tube expanded, causing a build up of pressure.
Inside the cockpit this caused the air speed indicator to
deflect. Even though it was altitude causing an increase in
pressure, the sensors mistakenly read this as an increase in air
speed.
On the 757 there were different sources of air speed to rely on
but investigators noticed that when the trouble started the captain
wasn't flying on auto-pilot.
Unless pilots reconfigured it, the auto-pilot got its air speed
information from only one main source: the blocked pitot tube.
Acting on faulty information the 757's auto-pilot system
calculated that the plane was travelling too fast and raised the
nose to slow it down.
"Both of them are wrong" says the Captain of the air speed
indicators. "What shall we do?" He had wrongly concluded both were
malfunctioning whereas the first officer's readings were always
correct. The aircraft was actually travelling much too slowly.
The pilots were overwhelmed by conflicting audible warnings and
caution lights. One of the warnings was "rudder ratio" - an alert
that if the rudder is fully deflected in high- speed flight the
plane could yaw sharply and violently, leading to loss of control,
damage to the aircraft or even a crash.
Believing he was going too fast, the captain made a grave error:
he pulled back on the throttle - which caused the controls to
shake, a warning that the aircraft was about to stall. He needed to
lower the nose to increase speed.
When the "stick shaker" began, the auto-pilot deactivated to
give the pilot full control to prevent a stall.
But the auto-pilot disconnected at a point of Captain's greatest
confusion.
The plane needed airflow over the wings from a lowered nose.
Instead the captain tried to get full power from the engines. At
the angle of the aircraft, the engines could not get enough air.
Applying full power was more than they could handle. The left
engine quit first.
With the right engine at full acceleration, the airliner swung
around as though a wing were caught on a branch. It went into a
full stall. Eight seconds after a ground proximity warning went off
in the cockpit, the plane entered the Caribbean Sea, klling all on
board.
The pilot's last recorded words: "Thrust, don't pull back, don't
pull back, don't pull back, don't pull back don't pull back, please
don't pull back what's happening?"
Robert Macintosh JR, an investigator of the crash of Flight 301
from the US National Transportation
Safety Board said: "That air speed warning horn combined with a
stick shaker was a tremendously mind-boggling experience to a
pilot."
National Geographic re-enactment part
one
The Federal Aviation Administration asked Boeing to change some
of the cockpit warnings which included the addition of a new one to
tell both pilots when their instruments disagreed and the ability
to more easily silence troublesome alarms.
The FAA also issued a directive that simulator training for all
airline pilots must include a blocked pitot tube scenario.
Boeing modified planes so that pilots could easily choose which
pitot tube the autopilot was choosing for air speed readings.
The Pitot tubes were never recovered so what blocked them is not
known for certain. But investigators believed that the
mud dauber
wasp, which is well known in the Dominican Republic, was the
cause.
Mud daubers looking for nests choose ones which are more or less
tubular; and when they make their nest the mud dries and
hardens.
A Pitot tube is perfect home for the wasp - especially as the
757 was lying idle at the airport for 25 days for its last flight -
which was more than enough time for the wasp to build its nest in
the uncovered pitot tubes.
National Geographic re-enactment part
two
Investigators concluded that mud dauber wasps blocked the
uncovered Pitot tubes which fed the captain's air speed indicator
which caused it to malfunction. [Ice can also block the tubes -
though they are supposed to have heaters which stop that
happening.]
The nesting of tiny insects led to a series of mistakes which
brought down a computerized passenger jet which was known for its
safety and reliability.
>> Go back to
Air
France Airbus: its last few minutes? or on to
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Top image fromWikimedia Commmons.
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AirDisaster.