What happens to an aircraft that sits

An aircraft that is not flying is not an aircraft at rest. The documented hazards of low utilisation start within days rather than months, and the two regulators who have written about it most recently both found problems on aircraft that had been parked for less than a week.

This matters in January and February, when a lot of aircraft fly four hours a month instead of forty, and it matters any time an aircraft sits between trips for longer than usual.

Days, not months

The UK Civil Aviation Authority issued Safety Notice SN-2021/014 on pitot blockage events in June 2021. The aircraft involved were of different types, had been in what the notice calls “low utilisation” operation, and “had been subject to parking for periods of 3-7 days at various locations within the airport.” A form of insect infestation may have contributed.

EASA went further in Safety Information Bulletin 2020-14R1. Most reported occurrences concerned foreign objects such as insect nests accumulating in the pitot static system, obstructing probes and ports, “in some cases on multiple systems, even when the covers were installed.” The bulletin notes events after the aircraft was parked for less than 48 hours, and in aircraft in transit on the ground for less than 12 hours.

The UK Air Accidents Investigation Branch investigated a cluster at Heathrow between June and July 2021 in which several aircraft suffered abnormal pitot and static events, two of which resulted in rejected takeoffs. The cause was identified as nesting by wasps and bees. The Natural History Museum identified the species. None of the aircraft had pitot and static covers fitted while parked.

That is three authorities on the same point, and none of them is talking about long term storage.

What gets in, and what it does once it is in

FAA Advisory Circular 43-206 is about avionics rather than appearance, but paragraph 309 is the clearest official statement on the subject we found. Insects and animals “may enter through vent holes, open cabinets, or torn packaging material. Once inside the equipment, they can build nests which will hold moisture. This moisture plus excretions can cause corrosion.” It goes on to describe insects and animals eating electrical insulation, varnishes and conformal coatings, removing the protection and allowing direct corrosive attack. The same AC includes a figure captioned rodent droppings in bilge area, which tells you this is not a theoretical concern.

The drains nobody is checking

AC 43-4B paragraph 4.8 covers water entrapment, and it is worth reading in full because of what it assumes. Design specifications usually require low-point drains wherever fluids can collect. In many cases those drains are ineffective, either from location or because they are plugged by sealants, fasteners, dirt, grease and debris. Then: “Plugging a single drain hole or altering the level of the aircraft can result in a corrosion problem if water becomes entrapped in one of these ‘bathtub’ areas, and we recommend a daily inspection of low-point drains.”

Daily. The paragraph is written for an aircraft in service with someone walking it every day. An aircraft that sits for six weeks is not getting that, and nothing about the drains got easier in the meantime.

What grows, and the advice the FAA rebuts

AC 43-4B paragraph 2.8.3 gives the conditions microorganisms want: temperatures between 68 and 104 degrees Fahrenheit, and relative humidity between 85 and 100 percent. A Florida hangar in August sits inside both.

Paragraph 2.8.5 gives the menu, and it is the sentence that should interest anyone who owns a cabin: “wool, cotton, rope, feathers, and leather provide sustenance for microbes, while metals and minerals do not.”

Now put that next to what the manufacturer says is in the aircraft. The Gulfstream Completion Center Maintenance Handbook states that carpet fibre content is 100 percent wool, on the same page as the warning that wet carpet may cause severe corrosion to aircraft flooring and structure. The carpet is food. The seats are food. Neither of those is a criticism of the materials, which are chosen for good reasons. It is a statement about what happens when they are damp and nobody opens the door for a month.

Paragraph 2.8.4 adds the part that catches people out: spores stay dormant while dry and become active when moisture arrives, and when desiccants saturate and can no longer absorb, microorganisms begin to grow.

And then the FAA says something genuinely counterintuitive in 2.8.3, which is that this is “counter to the former belief that operators could prevent fungal attack by either applying moisture-proofing coatings to nutrient materials, or by drying compartment interiors with desiccants.” If your winter plan is a bag of desiccant and a closed door, the FAA has already told you that is not the answer.

Leather, and a claim we are not going to make

You will read that leather dries out and cracks because an aircraft is not being flown. We went looking for a source for that and there is not one, in aviation or in the leather trade. We are not going to assert it.

What is documented is humidity, not disuse. The Canadian Conservation Institute, which is a government conservation body rather than an aviation one, states that relative humidity above 65 percent favours mould growth on leather, that below 30 percent moisture loss leads to loss of flexibility and risk of physical damage, and that leathers exposed to a change in humidity lose or gain internal moisture and distort. That is museum guidance about tanned hide, and we would not present its numbers as a cabin specification. The mechanism is sound and the mechanism is about the air, not about the aircraft sitting still.

The practical version is the one Gulfstream gives in its own handbook, which is to use leather restorer periodically to keep leather soft and supple. Periodically does not mean when you next fly it.

The tyres

Goodyear’s aircraft tyre care guidance is specific where most things here are not. Loaded tyres left stationary for any length of time can develop temporary flat spots, and an aircraft parked for long periods, which it defines as 30 days or more, should be jacked to take the weight off the tyres where possible. It also notes that losing up to five percent of inflation pressure in 24 hours is within normal, which is worth knowing before you conclude you have a leak.

What we would actually do

Cover the probes and ports, and treat that as a days decision rather than a months one. Get the drains checked rather than assumed. Get air moving through the cabin rather than sealing it and hoping. Deal with anything damp before it sits, because a damp wool carpet in a closed aircraft in a warm hangar is the exact condition the FAA describes.

None of this is exotic. It is just the maintenance an aircraft stops receiving the moment it stops being somewhere on a schedule.