WASHINGTON - A day before the door plug blew out of an Alaska Airlines flight on Jan 5, engineers and technicians for the airline were so concerned about the mounting evidence of a problem that they wanted the plane to come out of service the next evening and undergo maintenance, interviews and documents show.

But the airline chose to keep the plane, a Boeing 737 Max 9, in service on Jan 5 with some restrictions, carrying passengers until it completed three flights that were scheduled to end that night in Portland, Oregon, the site of one of the airline’s maintenance facilities.

Before the plane could complete that scheduled sequence of flights and go in for the maintenance check, the door plug blew out at around 4,900m, minutes after embarking on the second flight of the day, from Portland to Ontario International Airport in California.

The plane landed safely, and no one was seriously injured, but the incident focused new attention on Boeing’s manufacturing processes and the safety procedures followed by airlines.

The scheduling of the maintenance check on the plane has not previously been reported. It demonstrates that the airline chose to keep the plane in service while it made its way towards the maintenance facility rather than flying it to Portland without passengers.

Alaska Airlines confirmed the sequence of events. But the airline said the warnings it had on the plane did not meet its standards for immediately taking it out of service.

Mr Donald Wright, vice-president for maintenance and engineering for Alaska Airlines, said the warning signals – a light indicating problems with the plane’s pressurisation system – had come on twice in the previous 10 days instead of the three times the airline considers the trigger to take more aggressive action.

Alaska Airlines has repeatedly asserted that there is no evidence that the warning lights, which could also be caused by electronic or other problems, were related to the impending plug blowout.

“From my perspective as the safety guy, looking at all the data, all the leading indicators, there was nothing that would drive me to make a different decision,” Mr Max Tidwell, vice-president for safety and security for Alaska Airlines, said in an interview.

The airline’s engineers had called for the plane to undergo a rigorous maintenance check on Jan 5 to determine why the warning lights were triggering based on their use of “a predictive tool” rather than on the number of times the warning lights had gone off, the airline said.

While it kept the plane in service, the airline did put restrictions on it following the recommendation of the engineers. It restricted the plane from flying long-haul routes over water, like to Hawaii, or remote continental areas in case of the need for an emergency landing.

Extensive evidence of a potential problem with the plane had been accumulating for days and possibly weeks, according to interviews with the airline and records of the investigation into the blowout.

In addition to the flashing lights, investigators say the door plug had been gradually sliding upward, a potentially crucial link in the accumulating string of evidence. The airline said its visual inspection in the days leading up to the blowout did not reveal any movement of the door plug.

A door plug is a panel that goes where an emergency exit would be located on a plane with the option of expanding the number of passenger seats.

A preliminary report released by the National Transportation Safety Board in February said that four bolts meant to secure the door plug in place were missing before the panel came off the plane.

It outlined a series of events that occurred at Boeing’s factory in Renton, Washington, that may have led to the plane being delivered without those bolts being in place.

Mr Mark Lindquist, a lawyer representing passengers on the Jan 5 flight, said the series of mishaps involving the Alaska Airlines jet were alarming, adding that both the carrier and Boeing, the 737 Max 9’s manufacturer, would struggle to explain the events in court.

“When jurors find out they had actually been cautioned by engineers to ground the plane, and they put it into commercial rotation instead, jurors will be more than mystified – they will be angry,” Mr Lindquist said.

In his court filing, Mr Lindquist said that passengers on a previous flight heard a “whistling sound” coming from the area of the door plug. The documents say passengers brought the noise to the attention of the flight attendant, who then reported it to the pilots.

When asked about the report, Alaska Airlines said it could not find any record of a report of whistling coming from the plane.

Almost a week before the blowout, the 737 had been taken out of service on Dec 31 because of an issue with the front passenger entry and exit door. Records show the plane resumed service on Jan 2.

However, on Jan 3, a pressurisation warning light was triggered during at least one of the plane’s flights. Alaska Airlines officials said the plane was inspected by engineers, and the carrier determined it was safe enough for it to continue flying.

The next day, the same light was again triggered.

Alaska Airlines said it was then that engineers and technicians scheduled the deeper inspection for the night of Jan 5 in Portland. But the airline chose to keep the plane flying with passengers as it made its way across the country that day. NYTIMES