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 Crash Two Aeroperú Flight 603.

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Top image from Wikimedia Commmons.

Bottom image from AirDisaster.