Cleaning frequency and corrosion control, according to the people who wrote it down

FAA Advisory Circular 43-4B names washing as an element of a corrosion prevention and control program and then declines to say how often. That gap is not an oversight. It is the document being honest, because the interval depends on the environment, and the environment is something you have to establish rather than look up.

Here is what the primary documents actually say, in the order they say it.

What AC 43-4B contains

Corrosion Control for Aircraft, dated 11 September 2018, is the current version. Paragraph 1.8.2 lists thirteen elements of a corrosion prevention and control program. Four of them are cleaning:

• Item 5: aircraft washing at regularly scheduled intervals
• Item 6: routine cleaning or wipe down of all exposed unpainted surfaces
• Item 7: keeping drain holes and passages open and functional
• Item 10: periodic thorough cleaning, lubrication and preservation

Paragraph 1.8.1 sets the variable: aircraft exposed to such environments as salt air, heavy atmospheric industrial pollution, or over-water operations, will need a more stringent program than an aircraft operating in a dry environment.

Nowhere does the document state a frequency. If you have seen an interval attributed to the FAA, it is not from this AC.

The part of the AC most people have not read

Paragraph 4.8, on water entrapment areas, is the practically useful section and it is not about washing at all.

Design specifications usually require low-point drains in all areas where moisture and other fluids can collect. But the AC then says those drains are often ineffective either because of location or because they are plugged by sealants, extraneous fasteners, dirt, grease and debris. It adds that plugging a single drain hole, or altering the level of the aircraft, can result in a corrosion problem if water becomes entrapped, and it recommends daily inspection of low-point drains.

Paragraph 4.5, on bilge areas, is the same idea: keeping bilge areas free of extraneous material, including water and oil, will ensure the best protection against corrosion.

Read together, the AC’s position is that the mechanism to worry about is water sitting somewhere it cannot leave. Which reframes what a wash is for. A wash is not primarily about the surface you can see. It is about not adding to the problem, and about the fact that a wash is the one routine event where somebody physically visits every part of the exterior.

The FAA has stepped back from the chemistry question entirely

Worth knowing alongside this. The FAA once published AC 43-205, Guidance for Selecting Chemical Agents and Processes for Depainting and General Cleaning of Aircraft and Aviation Products. It was cancelled in February 2017, with the cancellation memo citing outdated information and directing users to documents produced by the manufacturers of the aircraft and the products used.

So there is currently no active FAA advisory circular on selecting aircraft cleaning agents. The chain of responsibility runs through the airframe manual instead, and 14 CFR 43.13(a) is what gives that manual its force. The whole chain is here.

Who does publish intervals

Organizations operating fleets in known environments, because they have to write it down.

The Navy. NAVAIR 01-1A-509-2, paragraph 2-3, states that Navy aircraft shall be cleaned at least every 7 days when aboard ship and at least every 14 days when ashore. Paragraph 2-3.1 adds that when deployed within three miles of salt water, or when flown below 3,000 feet over salt water, daily cleaning or wipe down is required on all exposed unpainted surfaces.

The Air Force. A published corrosion research paper by Abbott, Jett, Statham and Sawinski describes Air Force wash intervals as determined by base corrosion severity, ranging from 30 to 180 days depending on whether the base environment is rated mild, moderate or severe.

Neither fleet is a business jet fleet, and Navy aircraft live in a far harsher environment than almost any corporate aircraft. The transferable part is not the number. It is the ratio: a six-fold spread driven purely by environment.

How to establish which environment you are in

ISO 9223 classifies atmospheric corrosivity from C1 to CX, and its named environments map usefully onto where aircraft actually live:

• C2, low: rural areas with low pollution
• C3, medium: urban areas, and coastal areas with low chloride deposition
• C4, high: coastal areas without salt water spray, and areas with de-icing salt exposure
• C5, very high: coastal areas, sheltered positions on a coastline
• CX, extreme: subtropical and tropical zones with strong chloride effect, coastal and offshore areas

Between C2 and C5 the published first-year corrosion rate for carbon steel differs by roughly an order of magnitude. Two honest caveats. Those published rates are for carbon steel and zinc, not for clad aluminum airframe alloys, so do not read a C5 steel rate as an airframe rate. And the classification depends on measured chloride deposition, not on distance to water alone.

The same research paper rated Daytona Beach as very severe, central Ohio as moderate and Robins Air Force Base as mild, and recorded a real operational contrast: an aircraft at a moderate base on a 120 day wash interval against one in a severe coastal environment on 30 days.

What the airframe manufacturers say

Less than you would want. Business jet maintenance manuals are proprietary and we have not been able to verify any published exterior wash interval for Gulfstream, Bombardier or Embraer. Anyone quoting one should be able to tell you the chapter.

What is public is still useful. Cessna’s own handbook language says that if the airplane is operated in a seacoast or other salt water environment, it must be washed and waxed more frequently to assure adequate protection, and that special care should be taken to seal around rivet heads and skin laps, which are the areas most susceptible to corrosion.

Rivet heads and skin laps. Not the middle of a panel. That is where the interval is actually being set.

What a corrosion-aware wash looks like

Different in emphasis from an appearance wash, though the two are done together.

• Fasteners, lap joints and seams get deliberate attention rather than being passed over.
• Unpainted surfaces get wiped down, per AC 43-4B item 6.
• Drains and low points get checked and cleared, per paragraph 4.8.
• Gear wells, wheel wells and the belly aft of the wing root are treated as part of the job rather than as optional.
• Rinsing is thorough enough that no cleaner residue is left in a joint. This is the situation ASTM F1110 sandwich corrosion testing exists to model, and it is why product residue matters as much as soil.
• The aircraft is dried rather than left to.
• Condition is recorded, so the next crew starts from a known state.

That last one is the reason every service we do is logged against the tail in our own system. Corrosion is a slow story and it is only visible as a story if somebody kept the chapters.

The summary an operator can use

There is no published interval because there cannot be one. Establish which environment your aircraft actually lives in, set the interval from that rather than from a number somebody repeated, and make sure the wash includes the seams, the drains and the places water collects rather than only the places people look.

If you would rather not manage that yourself, it is exactly what a scheduled program is: an interval set from the environment, applied without anybody having to remember.