Several recent calls from Distributors have prompted this Tech Tip. All concerned the advisability of placing ABC type hand portable or wheeled extinguishers either on tarmac or in aircraft hangers. The best way to answer is to cite from NFPA Standards and other published documents.
NFPA 410 Aircraft Maintenance (Addendum) A-7-3.1
All-purpose (ABC) dry chemical-type extinguishers should not be used in situations where aluminum corrosion is a problem.
NFPA Fire Protection Handbook Chapter – Basics of Fire and Science
Extinguishment with Dry Chemical Agents
One reason that dry chemical agents other than monoammonium phosphate are popular has to do with corrosion. Any chemical powder can produce some degree of corrosion or other damage, but monoammonium phosphate is acidic and corrodes more readily than other dry chemicals, which are neutral or mildly alkaline. Furthermore, corrosion by other dry chemicals is stopped by moderately dry atmosphere, while phosphoric acid has such a strong affinity for water that an exceedingly dry atmosphere would be needed to stop corrosion.
Although a bit dated, we have extracted parts of an article published in the Nov. / Dec. 1983 issue of “Air Transport Newsletter” written by Ronald Horn, then Manager/Ground Safety for Eastern Airlines and titled “Class A-B-C Extinguishers Damage Aircraft.”
“The A-B-C extinguishers have excellent fire-fighting capability, but the mono-ammonium-phosphate chemical agent melts and flows when it comes into contact with heat. This is how it gets its Class A rating. This chemical is highly corrosive to aluminum and once it contacts hot aluminum and flows down into the structural cracks and crevices it cannot be washed out as the B-C dry chemical agents can.
Once an A-B-C extinguisher is used on an airplane, it is necessary to disassemble the aircraft piece by piece and rivet by rivet to accomplish cleanup. Failure to do so will result in destruction of the aircraft by corrsion.”
TO SUM UP: ABC EXTINGUISHERS (HAND PORTABLE AND WHEELED) ARE NOT PROPER AIRCRAFT (FIXED WING OR ROTARY) FIRE PROTECTION, ONBOARD, ON RAMPS OR IN HANGERS.