A Tui flight continued to descend for 57 seconds, following an aborted landing.
The incident occurred last year but the findings of an investigation were released on Thursday.
The investigation found crew likely allowed the plane to descend as they were overloaded with work.
A passenger plane descended for almost a minute without the crew realizing, despite its landing being aborted.
The findings were revealed in a government report published on Thursday.
The Air Accidents Investigation Branch (AAIB), which investigates aircraft incidents in the UK, said in its report that a Boeing 737-800, operated by German airline and travel agency Tui, significantly deviated from its scheduled flight path during a go-around at Aberdeen airport in Scotland. The incident occurred in September 2021.
The plane left Palma de Majorca airport at 10:47 a.m. with 67 passengers and six crew members onboard and was on its way to Aberdeen, according to the AAIB report.
Air traffic control at Aberdeen requested a delay to the landing because a search and rescue helicopter was about to take off at the airport and had priority over the passenger plane, the report said.
In response, the plane climbed to just under 3,000 feet above mean sea level (AMSL), but then descended to 1,780 feet AMSL for a total of 57 seconds before the crew took action, per the AAIB report.
"It is likely that the crew allowed the aircraft to descend unnoticed having become overloaded by the high workload during the go-around," the AAIB said in the report.
It was described as a serious incident in the report.
The AAIB also said the two pilots went through a period during the COVID-19 pandemic in which they didn't fly aircraft. The report added that this happened to the majority of pilots and air traffic controllers during the pandemic, presenting challenges for staff to maintain their skill levels.
In a statement to Insider, Tui said it worked closely with the AAIB throughout the investigation and has taken on board all recommendations and learnings from the report.
"We would like to provide reassurance that the safety of the aircraft was assured throughout this flight," Tui said, adding that it offers refresher training for pilots before they fly planes, post-pandemic.
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