Flight data recovered from a Boeing 767 that crashed near Houston in February indicates that the plane suffered a primary display failure that affected its attitude director indicator/horizontal situation indicator (ADI/HSI) display.

The crew apparently was able to restore the system, but moments later — at an altitude of around 6,000 ft — the air freighter stalled. It then began a rapid descent and crashed into Trinity Bay around 34 miles southeast of George Bush Intercontinental Airport. The two-person flight crew and one jump seat pilot were killed. The plane was destroyed and highly fragmented.

The accident aircraft. Sourcet: Nathan Coats from Seattle, WA, United States of America - Prime Air / CC BY-SA 2.0Facts surrounding the February 23 accident were made public by the National Transportation Safety Board (NTSB), which is investigating the incident. The investigation is ongoing and a report on the accident's probable cause will be issued at a later date.

The NTSB said the flight was operating as Atlas Air Flight 3591, a Boeing 767-375BCF with tail number N1217A. The airplane was operated as a domestic cargo flight, and originated from Miami International Airport. It carried cargo for Amazon and the U.S. Postal Service.

Accident timeline

According to documents released by the NTSB, the flight departed Miami at 1633 Zulu (Z) with the first officer as the Pilot Flying and the captain as the Pilot Monitoring. Zulu time is used in the military and in navigation as a term for Universal Coordinated Time, also known as Greenwich Mean Time (GMT).

The flight proceeded normally and at about 1809 GMT, a flight controller directed the flight to descend to 34,000 from 40,000 ft. Seven minutes later, according to the NTSB, the flight crew told the controller they were beginning their descent to the Houston airport.

At 1834:08 GMT, an air traffic controller told the crew to expect light to heavy precipitation about 35 miles ahead and moving eastbound. The controller told the crew that when they got in closer to the airport they likely would be detoured around the weather.

According to the NTSB, about 1836 GMT the first officer made a comment related to a possible primary display failure, and transferred control of the airplane to the captain. The captain became the Pilot Flying, and the first officer became the Pilot Monitoring, taking over radio communications.

At 1836:24 GMT, the flight crew told air traffic control that they would fly to the west side of the weather. The controller replied that the only problem was that there were a “bunch of departures,” so he needed the plane to descend and maintain 3,000 and to “expedite” the descent.

Recovery of the flight data recorder. Source: NTSBAt that point, the aircraft was about 48 south east of the airport at 10,000 ft. The flight crew did not initially respond, the NTSB said, so the controller repeated the descent clearance and told the crew to “hustle” down to 3,000 ft.

The flight crew confirmed the command, and about 10 seconds later the flight data recorder recorded the speed brake being extended.

According to the recorder, the first officer then commented that he experienced a potential failure of his attitude director indicator/horizontal situation indicator (ADI/HSI) display information. He made a comment about using the Electronic Flight Information (EFI) switch.

This was followed by the captain commenting on potentially getting the displays back.

At 1837:18 GMT, the air traffic controller instructed the flight crew to turn to a 270 heading. The captain confirmed the turn when the airplane was about 40 miles from the airport and descending through 8,500 ft. Shortly afterwards, according to recorded data, the captain transferred control back to the first officer, who became the Pilot Flying. The captain again became the Pilot Monitoring. The crew then began setting up the final approach to the airport into the airplane’s Flight Management Computer (FMC).

At 1838:37 GMT and while engine thrust was increasing, air traffic control told the flight crew to turn northbound in about 18 miles. The flight data recorder recorded the plane's speed brakes retracted to the near zero position, and the captain responded “sounds good.” Controllers then advised the crew that it was clear on the other side of the weather and that they should have no problem getting to the airport.

About a second later, the first officer made an "expression of surprise" in the cockpit, the NTSB said, followed by a comment related to airspeed. The captain then responded to the flight controller's previous transmission with “OK,” which was the last recorded communication between the plane and controllers. That exchange occurred as the airplane was about 35 miles from the airport and descending through 6,000 ft.

About 3 seconds later, the First Officer made a comment regarding the airplane stalling. At 1839:39 GMT, air traffic control lost radar contact with the airplane some 34 miles from the airport and at an altitude of 5,800 ft.

Roughly 16 seconds after the Captain replied “OK” to flight controllers, the flight data recorder stopped recording data with the airplane descending at an airspeed of about 433.5 knots and the autopilot engaged.