Airlines with planes idled by the pandemic are cutting costs by delaying some maintenance tasks like changing life vests, testing oxygen bottles and checking emergency exits under Covid-19 waivers from airplane manufacturers and regulators.
The move allows airlines to stop the clock on a category of parts that would otherwise need checks or pass their "use-by" date without leaving the ground because of the huge number of planes parked during the pandemic.
In special guidance to airlines, Airbus SE, Boeing Co, Brazil's Embraer SA and turboprop manufacturer ATR say there is no risk to safety because the work will be done before the planes return to the skies.
But there are concerns that delays could trigger commercial disputes between airlines and their lessors and financiers if they breach contractual maintenance agreements.
"I think there is going to be some argument," said Phil Seymour, president of aviation intelligence and advisory firm IBA.
Delayed checks also mean it will take longer to reactivate planes.
In the highly regulated world of airplane maintenance, the frequency of many tasks is determined by the number of take-offs and landings or flight hours. But others, such as life vests and portable oxygen bottles, have a fixed schedule for replacement.
The world's two major aviation regulators in the United States and Europe have both provided Covid-19 related guidance on extending some time-related tasks while a plane is stored.
"Prior to an aircraft return to service from storage, all scheduled maintenance tasks that were scheduled during the non-operational status must be performed," a US Federal Aviation Administration spokeswoman said.
In a sign of the scale of the issue, Boeing put an engineering team to work analysing all of the roughly 1,400 tasks per model to help airlines keep costs down and delay work that was not deemed urgent or safety-critical.
That allowed the maximum extension of 10% beyond the scheduled time to be increased to as much as 12 months for grounded planes not subject to passenger foot traffic, cargo carriage, and other wear and tear.
"The unique, individual assessments for each Boeing model accounted for all climates around the world and can support all operators," a Boeing spokesman said.
Airbus also analysed calendar-driven tasks to give more flexibility to customers, said Airbus head of scheduled maintenance services Claire Kauffmann.
"This ensures that the task is performed in a safe way and that they have proper time and staff to do that," she said.
An ATR spokesman said airlines had used a three-month "clock stoppage" on parked planes to ensure time-driven tasks were not scattered over a longer schedule.
Consulting group Oliver Wyman said in a recent report that the mass return to service of grounded aircraft expected over the next two years could create a maintenance-demand bubble.
Although the airline industry does not expect passenger traffic to rebound to 2019 levels until 2024, Oliver Wyman forecasts spending on airframe maintenance will recover to 2019 levels by 2022.
The decision by many airlines to spread their limited flying among more planes than usual to be ready for a sharp increase in demand could also drive more maintenance spending as manufacturers consider requiring some work to be done regardless of flight hours.
For example, most of the tasks related to the air conditioning system need to be done every 2,000 flight hours, normally about six months. But if the plane only flies 400 hours in six months, the check might be moved up to 1,600 hours to account for the longer passage of time, Kauffmann said.
"Here we would actually recommend increasing the number of tasks to be performed if you have a low utilisation," she said. "This is always an engineering driven and safety driven analysis."