On Monday, June 14, 1993, at 1702 eastern daylight time, a Short Brothers SD3-60-200, N711HJ, operating as Allegheny Commuter Flight 3575, made a precautionary landing at Atlantic City, New Jersey, due to a fire warning in the aft baggage compartment. There was no damage to the airplane or injuries to the crew and 18 passengers. Visual meteorological conditions prevailed and flight 3575 was operating on an instrument flight plan under 14 CFR 121.

Flight 3575 was a scheduled flight operating between Philadelphia, Pennsylvania, and New Haven, Connecticut. While cruising at 6000 feet, the aft baggage compartment fire light illuminated in the cockpit. Fire extinguishing agent is directed to the aft baggage compartment by placing the nozzle of a hand held fire extinguisher through a receptacle with a sealing "O" ring on the aft cabin bulkhead. The fire extinguisher is then actuated and the extinguishing agent travels through a line to the baggage compartment.

In a written report, the captain stated:

...the smoke aft light illuminated...Jim Town, my first officer, pulled out the Emergency checklist. He notified Donna Smith, the fight attendant, to empty two fire extinguisher bottles into the back bay port...A few minutes later Jim informed me that Donna had trouble with the extinguishers blowing back when used. I gave her the flight compartment extinguisher to use. She came back later, she said she used about half a bottle and had the same problem. She informed us that no smoke was coming from the back and no heat was detected at the aft wall....

The flight attendant stated:

...I did as I was told, the fire ex.[extinguishing agent]

blew back into my face. I reread the instructions to make sure I was operating it correctly and tried again. Went to the second one with the same result. Called the Captain & asked if my actions had any effect. Told the FO[First Officer] I'd need his fire extinguisher & told Captain that there was no smoke or heat...I forced the third extinguisher mostly in & wrapped the fire extinguisher handle with paper towels to try to force more in. It was blowing back so much that it was frostbiting my hand through the paper towel. I stowed extinguishers....

After landing, the pilot stopped the airplane on the runway and deplaned the passengers through the passenger entry door. No evidence of a fire was found. The source of the false fire warning was found to be a failure of the Aft Baggage Bin Smoke Detector.

In a telephone interview, Mr. Bob Walling, of Short Brothers (USA), said other operators had reported difficulty in getting a good seal with the extinguisher nozzle in the "O" ring. The fix was to lubricate the "O" ring with Dow Corning compound # 33, (DC-33). In addition, he said the fire extinguisher manufacturer had issued a service bulletin to modify the nozzles for easier entry. This was Service Bulletin 26-107, Revision 1, dated November 2, 1992, and issued by Fire Fighting Enterprises (U.K.) Ltd. A check with the operator revealed that neither of these had been accomplished.

On June 25, 1993, the FAA conducted testing at the operator's facility, using a fire extinguisher with the nozzle modified in accordance with the service bulletin. In addition, the "O" ring the nozzle was placed through, had been lubricated with Dow Corning compound # 33. According to their report:

...Testing using a nozzle modified in accordance [with] the FFE Service Bulletin did not require excessive force and fully engaged the fire extinguisher with minimal effort, with no damage to the "O" Ring Seal.

The use of lubricant with the modified nozzle did not appreciably change the force to engage the nozzle in the fire extinguisher system port....

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