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Flawed AoA sensor and maintenance led to Boeing 737 go-around

A dysfunctional Angle of attack (AoA) sensor and failure to properly follow maintenance procedures led to a Transavia Boeing 737-800 conducting a go-around in Paris, France, a newly published incident final report revealed.

The problems for the Boeing 737 started when a Transavia flight crew had to ferry another aircraft to maintenance in Norwich, United Kingdom from Paris, France. For the return leg to Paris, the flight crew hopped on the 737 in question, registered as F-GZHO, which left maintenance and was supposed to enter service the next day, according to a report released by the Bureau of Enquiry and Analysis for Civil Aviation Safety (BEA) on November 16, 2020.

The Boeing 737-800, which was delivered off the shelf to the Air France-KLM low-cost subsidiary Transavia in February 2015, concluded its C-Check on February 7, 2018, and was due to enter commercial service on the next day.

Ferry flight issues

As the aircraft departed from Norwich Airport (NWI) towards Paris Orly Airport (ORY), both pilots’ Primary Flight Displays (PFD) showcased the Indicated Airspeed (IAS) DISAGREE alert upon the rotation of the aircraft. Prior to it touching off the ground, neither of the two flight crew, in addition to a representative of the airline’s maintenance operations, noticed anything out of the ordinary.

When the captain pulled back the yoke of the 737 and began to lift the nose of the aircraft, the IAS DISAGREE alert appeared on both PFDs, with AoA DISAGREE AND ALT DISAGREE appearing a bit later during the initial climb. Both of the pilots acknowledged the appearance of the alerts and, after cross-checking their displays, noticed that the right-hand side PFD was showcasing erroneous speed indications while the left-hand one showcased data identical to the standby airspeed indicator.

Despite the erroneous data and the alert regarding the aircraft’s airspeed, the crew decided to continue the flight and to not open up the flight manual and carry out the indicated checklist in order to amend the faulty data and the alert. The captain argued that he took this decision due to the fact that his PFD was showcasing correct data and because the amount of air traffic within the London Terminal Control Area (TMA) was very high.

While the difference between the altitude and airspeed showcased on both PFDs continued to increase as the Boeing 737 continued to climb to FL200, which was its designated cruise altitude. At the cruise altitude, the flight crew carried out the AoA DISAGREE, ALT DISAGREE and IAS DISAGREE checklists, which led them to the Airspeed Unreliable procedure.

“Both pilots consulted this procedure but did not apply it as they had identified the erroneous indication and knew that the left PFD indication was correct,” concluded the French investigators.

The flight continued on to ORY without any issues or emergency calls to Air Traffic Control (ATC). During the descent, the captain contacted Transavia’s maintenance department in order to report the issue and requested the problem to be fixed. In addition, the problematic flight was logged in the Aircraft Flight Log (AFL).

Spotty maintenance and overweight go-around

Following the flight, a maintenance technician inspected the aircraft but found no issues. However, a go-around on the next day proved that the issue was yet-to-be solved.

The Transavia Boeing 737-800 was scheduled to depart Paris Orly Airport (ORY) towards Marrakesh, Morocco, in the early morning hours of February 8, 2018 ‒ a day following the ferry flight from NWI. During the flight to the North African country, the pilot flying (PF) was a trainee captain, while the pilot monitoring (PM) was a captain that assumed the position of a first officer in order to train the to-be captain. 

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