Blocked - the Boeing 757 disasters of 1996

December 19, 2018

The aviation industry had a bad year in 1996.  ValuJet 592 and Trans World Airlines 800 both came down due to onboard infernos, while major aircraft collisions occurred in Chakadari, India and Quincy, Illinois.  Then the infamous hijacking of Ethiopian Airlines 961 ended with a 767 cart wheeling into the Indian Ocean.  But two other notable accidents occurred at either end of the year - Boeing 757-200s were involved in both and each was caused by a similar problem.

 

Boeing 757s such as this, operated by United Airlines, have sold well and been popular with passengers.  The pilot-static ports can be seen on on the nose below the flightdeck windows.

 

The Boeing 757 was in use with many airlines in 1996.  It was an ideal replacement for aging Boeing 727 trijets, as well as four-engined Douglas DC-8s and Boeing 707s.  The 757 and its larger brethren the 767 were the first twin-engined types certified for long over-water operations under newly introduced ETOPS (Extended-range Twin Operations).  These rules that allowed twin-jets to be used on services over the Atlantic, opening up new routes that were uneconomic to operate with three- and four-engined airliners.  They were also the first medium- / long- haul aircraft not to need a Flight Engineer on the flightdeck, as tasks traditionally performed by a third crew member were automated.  The Boeing twins entered service in 1982 and proved popular, filling a niche between larger widebodies such as the DC-10 and 747, and short-haul aircraft including the Boeing 737 and Airbus A320.

 

Birgenair 301

Birgenair was a charter firm based in Turkey that flew on behalf of tour operators in its home country and in Germany.  The story of flight 301 began on the February 6, 1996.  Boeing 757-225 TC-GEN was allocated to the charter originating at Puerto Plata in the Dominican Republic and terminating at Frankfurt, with en route stopovers at Gander (Canada) and Berlin.  The aircraft and crew had been in the Caribbean for a while and there were 189 souls on board on the fateful night, so the flight was effectively full.

 

As TC-GEN roared down the runway at Puerto Plata the three pilots on the flight deck (a captain, a first officer and a another first officer who was riding back home) noticed the captain’s air speed indicator (ASI) was not providing normal readings.  Instead it was displaying an unusually high and increasing speed.  The first officer’s instruments were behaving more normally but - possibly influenced by the older and higher ranking captain who was in command and eager to return home to Turkey – he elected not to intervene.  The captain decided to press on and the 757 got airborne.  It was a fateful decision that would sign the death warrants of 189 people.

 

As the aircraft powered into the Caribbean skies things got worse with the left side air speed instruments continuing to provide erratic readings.  The captain’s ASI was showing speed increasing rapidly while one on the first officer’s panel was registering a gradual slowing.  The departure had been in the dead of the night so there were no ground references to help with situational awareness, and faced with two conflicting sources of information the crew became confused.

 

Captain Ahmet Erdem made ta decision to reduce thrust from the engines, mistakenly believing his ASI was correct. This proved to be the final and fatal blow to the doomed jet.  The aircraft’s automated warning systems soon started to emit out noisy alerts, while the stick shakers fitted to the control columns began to vibrate vigorously.  The plane's nose reared upward then the aircraft stalled and began to lose altitude.  Realising his error a little too late, Captain Erdem applied power to the 757s engines again in an attempt to recover from the stall.  Unfortunately the aircraft’s high angle of attack and the sudden change in throttle settings caused a flame out [uncommanded shutdown] of the left engine.  With a lack of power on one side the stalled aircraft entered a corkscrew dive with the left wing down.

 

The 757 and its hapless crew and passengers plummeted down from 5,000ft – by then there was little that could be done to save the aircraft.  Inverted, it plunged into the North Atlantic ocean, killing everyone on board during the impact.  The smashed airframe ended up resting on the sea bed, presenting the joint Dominican and US investigation team with a significant recovery challenge.  But the black box flight data recorders were located and taken to be downloaded.

 

The readings confirmed the erratic ASI readings.  Investigators explored many possible explanations, but eventually concluded that the only thing that could have caused such behaviour – and ultimately made the crash almost inevitable – was a blocked pitot-static port (a little blade / tube on the outside nose of the aircraft that is used to collect airspeed and altitude data).  Further research revealed that the aircraft had been parked in the Dominican Republic for some time before the fateful flight.  It was thought that the most likely cause of a blockage was a black and yellow mud dauber, a wasp that could might have settled on the airframe while it was on the ground.  The insect was known for using tubular shaped hollows to make its nest, and 757 pitot ports very much fitted the requirement.  Although the consensus was that a wasp plugged the port, investigators were unable to recover the vital components from the sea bed to draw a definitive conclusion.

 

The disaster tarnished Birgenair's reputation, especially in Germany & Turkey, and within months it had ceased operations due to a significant loss of business. 

 

Aeroperú 603

Birgenair 301 was not the only deadly accident to involve a Boeing 757 in 1996.  However, solving the riddle of flight 301 came too late to save the lives of 70 people travelling to Chile during October of the same year.

 

Aeroperú 603 was a regular service from Miami, Florida to Santiago in Chile with a stop at the airline's base in Lima, Peru en route.  The flight from the USA during the evening of October 1, 1996 was uneventful but an aircraft change took place in Lima and Boeing 757-23A N52AW was allocated to the second leg of the trip.  Shortly after midnight, on October 2, the aircraft took off for the trip to the Chilean capital carrying 61 passengers and nine crew.  Just like Birgenair 601, the climb away from the airport was in the dark and over an ocean denying the crew visual most references.  Just minutes after getting airborne all hell broke loose on the flight deck.  Alarms started blaring out contradictory warnings - rudder ratio, over speed, stall ,stick shaker, and mach trim.  The crew had no reliable altitude indications and the other instruments were behaving erratically.  A frantic radio call was made to the air traffic control tower at Lima asking for some assistance but, unbeknown to the controller at the time, the information passed to the pilots was incorrect.

 

Another attempt to assist was quickly arranged and a Boeing 707 was instructed to depart from Lima and to fly alongside the stricken Boeing 757.  However, this last-ditch action came too late.  Within half an hour it was all over.  Although the crew and the air traffic controller did not realise it immediately, the 757 had started to descend rather gracefully and eventually its left wing tip struck the sea.  The crew quickly applied power to the engines in an attempt to climb away from the water, but to no avail.  The Boeing 757 stalled and inverted before plunging into the cold waters of the South Pacific.  Autopsies revealed that all 70 on board died of drowning.

 

A team from the US National Transportation Safety Board (NTSB) joined Peruvian investigators in Lima with the aim of establishing why another Boeing 757 had gone down.  It soon became apparent that there were eerie similarities between Birgenair 301 and Aeroperú  603.  The problems faced by the crew departing from Puerto Plata were familiar to the investigators, and their suspicions were proven correct - blocked pitot ports downed the Peruvian 757 too.  The root cause though was somewhat different, and the tragic accident in South America was found to be the result of a small error which had very serious consequences.

 

Just before leaving Lima for Santiago, the Aeroperú Boeing 757 had undergone a maintenance check.  When investigators located the wreckage on the ocean floor, they found a six inch strip of silver tape blocking the all-important ports that allow air to reach the instrumentation on the flight deck.  An untrained and poorly educated maintenance worker had incorrectly used adhesive duct tape to cover the pitot static ports on the Boeing 757 while the aircraft was being washed and had neglected to remove it.  There are purpose-designed covers available for pitot tubes which usually have large red ‘Remove Before Flight’ tags attached to them, but they were not used.   It was thought that the crew’s pre-flight inspection of the airframe, a task performed before every flight, failed to detect the tape as the check had been completed at night with the aid of only a flashlight.  From eight feet below on the ground, the silver tape would have looked very much like the natural metal pitot port.

 

The victim’s families brought a huge lawsuit against Aeroperú and similarly to Birgenair, the Peruvian firm folded in 1999.

 

Ramifications

The Turkish and Peruvian Boeing 757s were both ultimately brought down by faulty instrument readings, caused by blocked pitot-static ports.  More than 270 people lost their lives in the two incidents.  Since then the 757 has not been involved in another disaster attributable a similar cause, despite 1,050 examples having seen service with the world’s airlines.  The type has now been retired by many front-line carriers, although it is still popular with major US airlines and freight operators worldwide. 

 

Delta Air Lines and several other major carriers still use Boeing 757s on passenger services.  The type has been a reliable and economical workhorse for airlines. 

 

Thirteen years after flights 301 and 603 were lost, in June 2009, an Airbus A330 operating Air France 447 came down in the South Atlantic ocean off Brazil.  Its pitots were blocked by ice, which caused erratic instrument readings and abnormal autopilot behaviour.  Two fairly inexperienced pilots then made some simple mistakes that claimed 228 lives.

 

Over the years aircraft manufacturers have striven to ensure pilot error is not a cause of accidents.  But could their efforts indirectly be encouraging another risk – over-dependency on  automation?   As aircraft systems become ever more sophisticated, perhaps these past  disasters should be a wake-up call?  Instrument flying and the use of electronic systems have made travelling by air safer and more dependable.  But will additional automation cause more accidents if pilots become too reliant on systems?  With the crash of the Indonesian 737-MAX airliner in November 2018 thought to be related to erratic instrument readings and the misunderstood behaviour of an automated system, could this be a close repeat of the previous losses?  More importantly, could the next take off become a repeat the 1996 Boeing 757?

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