Years ago a small airline started using a cleaner on their aircraft. It worked great and all were happy. Then the trouble began. An aluminum skin aircraft has thousands of rivet holes, laps, seams, and crevices that absorb and trap whatever liquids are running down their surfaces. The cleaner cleaned but it also caused airplane corrosion. Not outwardly on the sides of the fuselage, but deep in the rivet holes, and seams where it was hidden from view. By the time the airline noticed the corrosion their entire fleet of aircraft had severe corrosion - everywhere!
Since then I have always been very careful as to what chemicals are allowed on my aircraft. Is it safe? Will that chemical cause corrosion? Will it soften the wire insulation? Will it weaken the composite pulleys? What is the long-term effect of the chemical residue that I cannot remove? Remembering that although I apply the cleaner diluted, it becomes concentrated as it evaporates.
The same questions apply to composite built aircraft. Is it safe? What will long-term exposure to chemical residuals do to my airplane?
The loss of Army Chinook near Fort Richardson, Alaska.
Fast forward to just a few years ago. The Army lost a Chinook helicopter because they did not heed the above advise. They used a simple product that is found in every grocery store.
"It has been brought to the attention of the U.S. Army Aviation Missile Command (AMCOM) Depot Maintenance Engineering Team that numerous units are using the commercial product SIMPLE GREEN as an aircraft wash. STOP! This product has been through Department of Defense (DOD) testing and was determined to be highly corrosive on aircraft aluminum and also a catalyst for Hydrogen Embrittlement. Hydrogen Embrittlement brought down Chinook Aircraft on 10 October 1992, near Fort Richardson Alaska...
How to Protect Your Airplane
Guidence is provided in the FAA Advisory Circular AC43-205. The following acceptance tests methods are outlined:
- Sandwich Corrosion Test ASTM 1110
- Immersion Corrosion Test ASTM F 483
- Hydrogen Embrittlement ASTM F 519
- Effect on Painted Surfaces ASTM F 502
- Acrylic Crazing Test ASTM F 484
- Residue ASTM F 485
There are aircraft cleaning products that have passed all of these tests.
- Other tests that are sometimes required:
- Rubber Compatibility Test ASTM D2240
- Stress Corrosion of Titanium Alloys by Aircraft Engine Cleaning Materials ASTM F945
- Low-Embrittling Cadmium Plate ASTM F1111
Let a neutral party examine and test the product at their expense than then report back to you their findings. How do you do this? Most of the larger aircraft manufacturer's test and approve cleaners. For example, EXTREME Simple Green has a Boeing specification D6-17487P . Some products pass or meet a mil-spec. Your best protection is to only use products that have been "blessed" by a major aircraft manufacturer or the military.
As a general rule - Avoid Alkaline Cleaners on Aluminum
Alkalines dissolve aluminum. A strong alkaline, such as drain cleaner that contains sodium hydroxide will completely dissolve aluminum. There are alkaline cleaners that have been modified to be acceptable. Generally they use silicates to modify the corrosion film.
Avoid Fluoride based Cleaners on Aluminum
The edges of your aluminum propeller receive a lot of abrasive wear and it is difficult for a protective coating to remain intact. The rough abraded surface can allow chemicals to enter the grain structure of the propeller and cause stress corrosion cracks. For this reason you want to be especially careful that you follow the propeller manufacturer's recommendations and do not apply some chemical cleaning agents to the bare aluminum.
Examination of the wreckage of a light aircraft revealed that approximately 20 cm was missing from one tip of the aluminum alloy propeller. Fractographic and metallographic examination of the remaining portion of the propeller revealed extensive grain-boundary separation in the vicinity of the fracture, and grain edges and corners rounded by corrosion on the fracture surface. Energy dispersive X-ray analysis (EDXA) revealed fluorine on, and in the vicinity of, the fracture surface. In the ensuing litigation, it was asserted that the crash occurred because the propeller fractured in flight as the result of intergranular attack caused by the use of a fluorine-bearing cleaner.
Avoid Chlorine based Cleaners on Aluminum
As chemicals seep into laps and seems of your aircraft, the water evaporates and you get tiny small areas of very concentrated chemical. This is what caused the crash of the Army Chinook helicopter. It is also what caused the loss of Flight 529.
NTSB excerpts from crash of ATLANTIC SOUTHEAST AIRLINES, INC., FLIGHT 529
Laboratory examination of the failed blades indicated the presence of chlorine-based corrosion pits in both instances. The chlorine source was traced to a bleached cork installed in the taper bore to retain the lead balance wool. These findings were corroborated by Hamilton Standard engineers and the FAA
Results of investigations conducted in two previous propeller blade failures in 1994, one in Brazil with this model blade and the other in Canada with a similar model blade, indicated that corrosion was produced when entrapped moisture reacted with residual chlorine in a bleached cork used to retain the lead wool in the taper bore hole of the propeller. The accident blade exhibited a nearly continuous layer of oxide deposits on the initial 0.049 inch of the crack depth. These deposits contained a substantial amount of chlorine.
Stoddard Solvent - the traditional method
You have to wonder after all these years why there is so much confusion about cleaning aircraft and aircraft engine compartments? Before the rise of environmental concerns aircraft, engine compartments, engine parts, and almost everything else made of metal were washed in Stoddard Solvent. The mechanic walked out to the piston airplane with a 5 gallon bucket and a air siphon gun and would spray down the engine compartment and aircraft belly. Stoddard Solvent was a standard aircraft engine wash. Almost all aircraft facilities had "solvent booths" where they washed parts. You can still purchase Stoddard solvent, although it is hard to find.
Military Approvals for Aircraft Cleaners
MIL-C-22550 Cleaning Compound (Water-less) For Aircraft Surfaces
MIL-C-43616 Cleaning Compound For Aircraft Surfaces
MIL-C-87936 Cleaning Compound For Aircraft Exteriors
What Chemicals will your Repair Shop use on your Airplane?
Ask them beforehand rather than getting mad afterward.
Inspect the Airframe Before you Clean it
Unwashed airframes show problems better than washed airframes. Loose fasteners, fretting, fluid leaks appear as smudging and streaking ("smoking").