After the event pilots told investigators they were using personal laptops on the 21 October flight from San Diego to Minneapolis.
Missing were 17 of the 30min captured by Flight 188's cockpit voice recorder. According to the NTSB, the CVR was activated three additional times after the aircraft's engines were shut down in Minneapolis at 21:20 that evening.
Investigators say the aircraft was powered up at 21:54, 23:05 and again at 02:03 the next morning. When power is turned on, the CVRs begin recording at the start of the 30min window. The recorders will power down automatically 5min later if the engines are not started, which was the case that evening.
"It appears the airplane's electrical network was re-energised three separate times after the engine was shut down, before the CVR was removed or deactivated," said the NTSB's cockpit voice recorder specialist in a factual report issued on 16 December. "Each time, the CVR started and recorded 5min, causing more than half of the recording to be overwritten."
Airline officials will typically deactivate the CVR circuit breaker as soon as possible to preserve data. Northwest assistant chief pilot Todd Luebke, the first airline official to board the A320 when it landed in Minneapolis, recalled a complicated scenario in the cockpit as FBI, federal air marshals and Transportation Security Administration officials boarded the aircraft first to evaluate potential criminal or terrorist activity.
"While we never want to interfere with the duties of other agencies, this delay prevents the CVR circuit breakers from being pulled in a timely fashion and in this case, it was at least a causal factor in the flight documents being gathered," wrote Luebke in his statement.
The extra 17min of data would have included discussions held from the time the aircraft had descended below 11,000ft (3,350m) on approach to the airport. In the remaining 13min, which began when the aircraft was below 1,000ft for landing, investigators noted that "discernible" conversations were "related to the operation the airplane" and did not include a discussion of the communications blackout.
As to the missing flight documents, which include the flight plan, the pilots in their statements mentioned that they thought that Luebke had gathered the paperwork when he entered the cockpit or that it was discarded. However, Luebke reported that he did not "see any signs of the flight paperwork which would have been my reminder" to retain the documents.
"To my surprise [the pilots] had nearly completed their post-flight duties including flight bags packed," Luebke says of his entry to the cockpit. "I don't know if the crew retained [the paperwork] or due to their distraught state discarded them as we normally do on domestic, uneventful flights."
Luebke notes that, per regulation, the documents only need to be kept for international flights or for accident flights.
Earlier in the flight, the first officer had deleted the eight ACARS messages that dispatchers had sent to the crew in an attempt to get them to contact air traffic controllers.
"After they were back in radio contact, the [first officer] said he went to 'pull up' [automatic terminal information service] info and noticed there were several messages on the lower screen," the NTSB says in its documentation of the first officer interview.
"All he saw were the headings of the messages, which said 'contact ATC' or 'ATC is looking for you'. The [first officer] said he inadvertently pushed the 'delete all' button which erased all the messages." ACARS messages are retained by service provider Arinc, however.