Birds' nests in engine cowlings, insect larvae, flat tyres – the challenges of getting aircraft out of mothballs and into the air again are daunting. Right now, for the engineers, mechanics, cleaners, pilots and aircraft certifiers who make that happen it's all hands on deck.
The sheer number of aircraft stood down as the pandemic sent the world's aviation industry into a tailspin is enormous. According to aviation analytics company Cirium, by April 2020 more than 16,000 aircraft had been taken out of service as a result of the pandemic. That's more than 60 per cent of the world's passenger aircraft fleet. There were fewer passenger jets in the air than at any time in the previous quarter century, including during the complete aviation shutdown experienced in the US in the immediate aftermath of the September 11, 2001 terrorist attacks. At the end of 2020, Cirium estimates over 30 per cent of the global passenger jet fleet was still in storage.
But now, airlines are recovering fast, especially in the USA where the number of domestic air travellers in July 2021 was more than three times the number in the previous July. For international air travellers, the increase is close to five times greater than the number in July 2020.
That has meant a lot of aircraft that were laying idle are returning to service, and that's a big job. Essentially it requires reversing the processes when the aircraft were decommissioned, including measures that were done to preserve the engines and auxiliary power unit, draining any moisture and remaining fluids, lubricating bearings and reactivating the avionics. All the aviation regulatory authorities – the Federal Aviation Administration in the US, the Civil Aviation Safety Authority in Australia, the European Union Aviation Safety Authority (EASA) have stringent regulations governing the checks that need to happen before an aircraft can be returned to service, right down to lavatory fire extinguishing equipment on aircraft stored in high-temperature environments. According to one engineer at American Airlines' largest maintenance base in Tulsa, Oklahoma, it takes anything from 300 to 1000 hours of reactivation work to return an aircraft to commercial service. At the end of that process the aircraft must pass a maintenance check flight to confirm its airworthiness.
When the time came to return aircraft to service some were found to be suffering from corrosion during storage. This wasn't such a problem for aircraft stored in the warm, dry, low-humidity environments such as the Asia Pacific Aircraft Storage facility at Alice Springs, which hosted more than 100 jet aircraft at the height of the pandemic, including Singapore Airlines Airbus A380s.
Among the problems, aircraft tyres develop flat spots if they're left in one place too long. That means a shortened lifespan for the tyre and possibly the landing gear, and a bumpy ride on the runway. Some airlines routinely tow their mothballed aircraft to prevent that from happening. Batteries don't love being left idle and nor do APU units, which provide auxiliary power when the aircraft is on the ground with its engines shut down. The level of detail extends to life jackets that need to be checked for their expiry date. Even opening the door of a cabin that has been sealed for several months can be a nasal knockout, especially in a warm environment.
One particularly vulnerable component is the pitot tubes, which provide data for measuring airspeed and altitude. These tubes have a small diameter inlet which makes them attractive to insects, and the EASA reported a surge in the number of unreliable airspeed and altitude readings during the first flight after an aircraft leaves storage. Desert environments have their own problems, including aircraft landing gear proving a popular hideaway for rattlesnakes and scorpions.
The importance of the aircraft rebooting process was underlined in dramatic fashion by the crash of Indonesia's Sriwijaya Air Flight 182 in January 2021. Just four minutes after takeoff from Jakarta the aircraft plunged more than 10,000 feet into the sea, with the loss of 62 lives. The 26-year-old Boeing 737 had only returned to operations at the end of 2020 after spending the previous nine months in a hangar. Between December 17, 2020, when it was given a new airworthiness certificate, and its crash on January 9 the aircraft completed 132 flights. A final report has yet to be issued but preliminary findings suggested a malfunctioning automatic throttle might have caused the aircraft to roll and dive, with air-valve corrosion resulting from the long layoff as one possible culprit. In October 2019, 18 of Sriwijaya's fleet of 30 aircraft had been grounded by Indonesia's Transportation Ministry over airworthiness concerns.