Hangared versus outdoors, what changes and what does not

A hangar removes ultraviolet light and it removes rain. It does not remove humidity, it does not remove the salt already on the airframe, and it does not remove ozone. So a hangared aircraft needs its finish looked after less often and its structure looked after about the same.

This matters because hangar space is expensive and people reasonably expect it to buy them something. It does. Just not everything.

What a hangar genuinely removes

Ultraviolet. This is the big one for appearance. UV degrades the resin at the surface of a topcoat, and over years that shows as chalking, loss of gloss and color shift, worst on reds and dark blues. An aircraft that lives indoors will hold its finish visibly longer than an identical aircraft that lives on a ramp in Phoenix, and it is not close.

Rain and standing water. Water sitting in the places FAA AC 43-4B calls water entrapment areas is a corrosion mechanism, and the AC notes that low-point drains are frequently ineffective because they are plugged by sealants, fasteners, dirt, grease and debris. Less water arriving means less water to fail to drain.

Thermal cycling at the surface. Not the flight cycle, which happens regardless, but the daily ramp cycle of solar gain and overnight cooling.

Bird activity, tree sap, and most fallout. Worth more than people credit. Bird droppings are acidic and they etch, and they are the single most common cause of a localized correction on an otherwise healthy finish.

What a hangar does not remove

Humidity. A hangar in South Florida is a humid building. Corrosion needs an electrolyte, and humid air plus chloride already on the surface is an electrolyte. Enclosing it does not change the chemistry.

Chloride already on the airframe. This is the one people get wrong. Salt does not evaporate. An aircraft that flew a coastal leg and came home to a hangar has brought the salt inside with it, and now it is sitting in the seams in a warm, humid, still building. Arguably that is worse than a rain shower, which at least rinses.

Cessna’s own handbook language makes the same point from the other direction: operate in a seacoast or salt water environment and it must be washed and waxed more frequently. It says nothing about where it is parked.

Ozone. Relevant specifically to de-ice boots, which crack from ozone rather than from UV. A hangar does not protect a boot, and the manufacturer’s six month preservative interval does not get longer because the aircraft is inside. More on that here.

Dust, and the hangar’s own contribution. Hangars have their own dust, and if there is maintenance happening in the same building there is also grinding dust, metal filings and overspray. We have seen more finish damage from a neighboring aircraft’s paint work than from a season outdoors.

Condensation. An aircraft brought in cold-soaked from a flight will sweat in a warm hangar. That is liquid water on and inside a structure, indoors, with no airflow.

How this changes what we actually recommend

Two different clocks, and they do not tick at the same rate.

The appearance clock slows down a lot. Correction, protection and gloss work stretch out. This is where the money saved actually shows up. On a hangared aircraft in a mild environment we will happily go a long way between anything that touches the finish with a machine, and that is the right call because correction spends film thickness that does not come back.

The corrosion clock barely moves. Washing frequency in a coastal environment is driven by chloride exposure, and the hangar is not part of that calculation in the way people expect. We tighten intervals for coastal aircraft whether or not they are hangared, and we look harder at the seams, the gear wells and the belly.

The practical version: a hangared coastal aircraft on a long wash interval and a short correction interval is exactly backwards. It should be the other way around.

What to look at on a hangared aircraft

• Gear wells and the belly aft of the wing root. These do not get rinsed by anything, ever, on an aircraft that lives indoors.
• Lap joints and around fastener heads. The areas the Cessna language calls most susceptible.
• Low-point drains. AC 43-4B recommends daily inspection of low-point drains and notes that plugging a single drain hole can create a corrosion problem if water becomes entrapped.
• De-ice boots. On the manufacturer’s schedule regardless of storage.
• Anything unpainted or polished. Bare metal has no coating between the residue and the substrate.

The version worth saying to an owner

Hangar the aircraft. It is worth it, and the finish will last visibly longer for it. Then do not let the hangar become the reason nobody washes it, because the part of the aircraft that the hangar does not protect is the part that costs the most to fix.

If you want an interval that reflects both clocks rather than a number somebody guessed, that is what a scheduled program is for. At Miami and the other South Florida fields we set them differently from the way we set them in the desert, for exactly the reasons above.