During the latest economic crisis that developed in the beginning of this year a massive number of aircraft were parked long term as there was no use for them since traffic demand collapsed. Little by little aircraft are being taken out of storage and commissioned into service. There are different kinds of storage published in the Aircraft Maintenance Manuals; broadly described by short term and long term storage.
Every Aircraft Typer Certificate Holder (manufacturer) has different time limits for the categories of storage. Common issue is that short term storage requires more attention to the stored aircraft. On the flip side it takes less man power to re-commission the aircraft into service after short term storage.
Long term storage allows the aircraft to be unattended for longer period of time, but requires a substantial work pack to be conducted to re-commission the aircraft.
Long term storage for example, required all openings , orifices and slots to be sealed, this includes obviously all intakes and exhausts of engines, APU, Air conditioning packs, cooling inlets and exhausts, fuel venting slots, all pitot static sensors, door slots, space between landing gears and structure, blinding of windows, purging, the engine lubrication and fuel systems with conservation fluids and many more requirements.
There are infinitely more variants, like storage without installed engines or other components which introduces the requirements for approved deviations from published procedures.
And now the crunch of the story; now that aircraft are being re-commissioned after any kind of storage, it appears that there is an increased number of safety incidents connected to pitot static systems. Aborted take offs due to airspeed disagrees, air turn backs to to air data discrepancies. This can show up in various different ways, from autopilot disconnects to plain and simple altitude and speed indication problems. All of these incidents apparently occur despite adherence to the requirement in any kind of storage to cover the pitot static sensors.
From personal experience I have seen a couple of time that pitot probe covers were filled with insects that would have surely blocked the pitot system, see below picture. During previous employment, the operator advised to NOT install pitot covers in order to prevent certain insects to seek shelter in the covers. This was applicable for African regions where that particular insect was prevalent. Below picture was taken in an African country.
There is, in other words, a very high risk of pitot plumbing blockage, even when pitot covers are installed. Prudence would dictate a flush of the pitot system prior to re-commissioning an aircraft. On August 5, 2020, EASA has published a SIB (Safety Information Bulletin) addressing that very issue. A SIB is not mandatory but deserves to be reviewed and its recommendations implemented. Up to this time, FAA had not published a bulletin yet. If they have, please post a comment!
On 11-June-2021 UK CAA; posted a after notice telling interesting parties that between 9- and 11-June-2021 there have been three (3) separate reportable events of aircraft returning to flight after 3-7 days of parking, that had events requiring crew actions as a result of pitot blockage and are subject to investigation.
The EASA SIB and UK CAA Safety Notice are attached and downloadable below: