A recent technical bulletin issued by Boeing advising pilots to "closely monitor the airplane's state and flight path to prevent a loss of control in flight" has highlighted concerns about pilots returning to work after being grounded for months due to COVID-19.
The bulletin was issued after Indonesian authorities published a preliminary report into the Sriwijaya Air crash in January that killed all 62 people on board a 737-500.
"Continual crew awareness of airplane attitude, airspeed, flight control position and thrust settings is fundamental for airplane upset prevention and can reduce the effect of startle or surprise caused by rapid unexpected changes," the bulletin said.
Boeing has previously been criticised for blaming the pilots of a 737 MAX crash in 2018, also in Indonesia, which was later linked to faulty systems onboard.
The Sriwijaya preliminary report found the plane had an imbalance in engine thrust that eventually led it into a sharp roll and then a final dive into the sea.
An investigation by the Los Angeles Times also uncovered a series of non-fatal incidents as pilots get back to work amid the pandemic. They included a pilot forgetting to disengage the parking brake while attempting to pull back from the departure gate, resulting in the damage to a towing vehicle.
In another incident, a pilot had "so much trouble" landing in strong winds, that it took three attempts before a landing was made.
In a third and potentially more serious incident, a cockpit crew member forgot to turn on anti-icing systems on the exterior of the aircraft. These are designed to ensure the altitude and airspeed sensors don't get blocked by ice, something that has been labelled one of the main causes of several fatal plane crashes.
The LA Times says there were "at least a dozen" flying errors since May 2020 that have been attributed to pilots being out of practice.
"Because I had not flown in a few months, I was rusty," an unnamed pilot said in a report to the NASA-run Aviation Safety Reporting System, which allows people to report incidents with all identifying information removed.
"This was my first flight in nearly three months, I placed too much confidence in assuming that it would all come back to me as 'second nature'."
The Federal Aviation Administration (FAA) has a rule in place to avoid situations like these where a pilot cannot fly a commercial jet unless they have performed at least three takeoffs and landings in the previous 90 days, either in the air or in a simulator.
However, the FAA changed these rules last year due to COVID-19. Pilots were given an additional 60 days to achieve the requirement, then added a further 30 day grace period in December.
In the US, airlines are currently flying about 45 percent fewer flights than usual, according to the industry group Airlines For America.
A safety analysis report issued last year by Airbus says loss of control in-flight makes up 33 percent, the largest single cause of all accidents since the beginning of jet flight.