Final cockpit moments with blocked pitot sensors

News Analysis

Final cockpit moments with blocked pitot sensors

Tony Collins

Two fatal crashes of passenger aircraft show that it is dangerous for pilots to have conflicting information and warnings in the cockpit.

The recovered black boxes from two computerized commercial airliners that crashed in the sea in the 1990s show what it's like in the cockpit when Pitot tubes are blocked. Investigators of the crash of an Air France Airbus in the sea off Brazil say that blocked Pitot tubes could have been a factor in the accident.

The two earlier crashes were Birgenair Flight 301 and Aeroperú Flight 603. Both planes were Boeing 757s which crashed into the sea in 1996.

In each case the onboard systems told the pilots that they were are flying too fast and too slow. Visual warnings were backed up by audible warnings, and the shaking of their flying controls - the stick shaker.

If pilots fly too slowly the aircraft may fall out of the sky; if too fast the aircraft may break up.

The Pitot tubes are sensors that feed data on air speed to the onboard systems which display information to the pilot.

The auto-pilot also gets its information from the pitot tube.

The pilots in the aircraft did not know that the Pitot tubes were blocked. This was established only after the black boxes - the cockpit and flight data recorders were recovered from the sea.

Investigators of the loss of an Air France Airbus in the Atlantic Ocean on 1 June 2009 say that the pilots might have received conflicting information on their air speed from the aircraft's Pitot tubes.

There are similarities between the crash of Air France Airbus, Flight AF 447, and the losses of Flights 301 and 603, all of which claimed a total of 487 lives.

With the three crashed aircraft:

- There were concerns about the performance of the Pitot tubes

- There were inconsistencies in the cockpit air speed data

- The auto-pilot was off (in the two earlier crashes this because the auto-pilot switched itself off when reaching the limit of its authority and then passing control to the pilots)

That said, it is impossible to say what caused the loss of 288 lives on Flight AF 447.

Air crashes are usually the result of a chain of events. If the Pitot tubes were blocked and caused conflicting air speed information to the pilots of the Air France Airbus that might have had nothing to do with the accident, or might have been only one link in the chain.

In the losses of Flights 301 and 603, the blocked Pitot tubes were important factors, in part because of the confusion of the pilots.

This is what happened in the cockpit when the pilots of Flight 301 and 603 were faced, unknowingly, with blocked pitot tubes.

The information is taken mainly from the documentary series "Air Crash Investigation" on National Geographic Channel, and from the official accident reports, together with transcripts of the cockpit voice recordings.

Crash One:

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."

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.

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.

Crash Two

The crash Aeroperú Flight 603 is the story of another aircraft which was in good flying order except for a blocked Pitot tube which led to onboard systems giving the pilots unreliable and conflicting information about their air speed and altitude. Again, the pilots were overwhelmed with inconsistent information.

As in the crashes of the Airbus AF 447 and the Birgenair Flight 301, the pilots of Aeroperú were flying over water.

Flight 603 was a scheduled flight from Lima, Peru to Santiago, Chile. It originated at Miami International Airport. It crashed on 2 October 1996, killing nine crew and 61 passengers. The aircraft was a Boeing 757-23A, one of what was then a new a new generation of computer-controlled aircraft in which pilots were trained to rely on a flight data system which was designed to reduce errors, both mechanical and human.

Problems began within two minutes of take-off. The altimeter read zero but the plane was clearly airborne. The 757 had three altimeters, one for the pilot, one for the co-pilot and one for back-up. All three seemed to be dead.

"The altimeters are stuck," said the co-pilot. The landing gear had only just been raised.

The pilot replied: "This is really new."

As in the Birgenair Flight 301, the pilots faced a "Rudder ratio" alarm which warned them to avoid large or abrupt rudder movements. It's a warning that the plane is flying too fast.

Then the air speed indicators seemed faulty. Confused with bewildering number of warnings the Captain decided to land. He was flying at night, over water, with no visual reference points and not able to trust his instrument readings.

With various alarms in the cockpit the co-pilot told air traffic control: "We declare emergency. We have no basic instruments. No altimeter. No [air speed indicator]. We declare an emergency."

There was confusion between the pilot and co-pilot as to whether the auto-pilot was engaged or not. At one point the pilot notes with concern that the auto-pilot had switched off.

Having warned the pilots that the aircraft was going too fast, the systems gave a warning that it was in danger of hitting the ground. An electronically-activated voice sounded a "too low terrain" warning.

Within six minutes of take-off the pilot said: "We don't have control, not even the basics."

The co-pilot saw that the speed was shown as zero. But air traffic control told the pilot that the plane was going up. This was wrong. Onboard systems were relaying faulty height readings to air traffic control.

A ground proximity alarm warned the pilots they were dangerously low. But air traffic control told them they were at nearly 10,000 feet.

As on the Birgenair Flight, the pilot's controls were shaking to give the pilots an unmistakable warning of an imminent stall, though there was a high indicated air speed. Another over-speed warning sounded.

The pilot said: "We are not stalling. It's fictitious. It's fictitious."

The last recorded words of pilot: "I have it. I have it we are going to invert!"

The aircraft bounced on the water, fell back and sank.

Tape was later found to have covered the 757's Pitot tubes, causing the instruments to go haywire. Maintenance workers had cleaned the jet, covered the ports with tape and forgotten to remove it.

Unnoticed by the pilots or air traffic control, the plane had been slowly descending.

National Geographic's documentary on the Aeroperú crash concluded that it was a "deadly lesson in how reliant pilots have become on their automated flight systems and how helpless they can be when the systems are crippled".

Air France Airbus part recovered by merchant ship


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