The British Air Accidents Investigation Branch (AAIB) is investigating what they are calling a "serious incident" involving a Tui Airways flight with 67 passengers onboard.
The incident took place on September 11 at Aberdeen Airport and involved a Boeing 737-800 with registration G-FDZF.
While undertaking a go-around, or aborted landing, the aircraft deviated significantly from the expected flight path. After initially climbing, just before it reached the cleared altitude it began to descend.
"The aircraft descended from close to 3000 ft for 57 seconds before a climb was re‑established, and this represented a significant deviation from the crew's expected flightpath. There was a high rate of descent, which was reducing the aircraft's separation from terrain, and an uncommanded and undesirable increase in airspeed that were not corrected in a timely manner," the report says.
The aircraft's Cockpit Voice Recorder had been overwritten because the aircraft remained in service before the AAIB was notified of the event. The recorder essentially records over itself repeatedly.
With the recorder likely to stop working in the event of a crash, then the final hour or so of audio would remain accessible.
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The pilots of G-FDZF, like many other pilots, had not flown for significant periods during the 18 months before this incident. Although the investigation has not established a link between this incident and a lack of recent flying, the report says it is "clearly a possibility".
The amount of time each pilot had recently spent flying differed, but both had experienced significant periods without flying in the preceding 18 months.
The commander had flown 10 flights during the previous month. For the co-pilot, this was only his fourth flight in nearly 11 months having completed two flights with a trainer seven days before the day of this incident.
A Special Bulletin was published by AAIB to raise awareness of the event in Aberdeen.
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Both pilots had completed numerous simulator sessions during the 18-month period to gain or retain the required level of flying hours.
The report said airlines have faced significant challenges in the last 18 months to keep crews current.
"Whilst there are legal requirements for crews to complete three takeoffs and landings within 90 days, there are no regulatory requirements laid out for crews to have actually operated the aircraft, especially on commercial flights. Operators have had to adapt and develop their own programmes to ensure that crews are prepared and competent to fly, often after significant periods away from the aircraft," it said.
It goes on to say that simulators are great for practicing things such as take-offs, landings and even emergencies, but there is a challenge to try and replicate the "real world of flying" in a simulated environment.
"It can be difficult in the simulated environment to replicate moments of high crew workload caused by the effects of ATC instructions and background communications, the presence of other aircraft in the area, poor weather and other operational pressures," the report said.
"Regulators have been concerned that pilots returning to the flight deck following extended periods without flying could be at risk of performing below their normal standard during their first few flights."
As the aircraft involved is made in the United States, the US National Transportation Safety Board as well as the plane manufacturer, Boeing are also involved in the investigation.
