Cessna Pilots Association April 2010 : Page 34

Deterioration of Explosion Suppressant Fuel Tank Foam Revisited by Donald E. Nieser W e recently a c q u i r e d a n o t h e r Cessna O-2A Super Skymaster aircraft that needed some work. During the annual inspection it was determined that the aircraft was due for the re-occurring wing spar Airworthiness Directive (AD) inspection. At the same time, a few particles of deteriorated fuel tank foam were found while sumping the tank quick drains. These deteriorated foam particles aroused our suspicions, so it was decided to thoroughly investigate all fuel tanks while they were out during the wing spar AD inspection. Lo and behold, we found that the fuel tanks had not been opened up and the foam had not been removed in accordance with Cessna and U.S. Air Force instructions. In fact, when we cut open both the left hand and right hand inboard main fuel tanks we found that the tanks contained the original foam that Cessna has installed when new. When the Cessna directed cohesiveness test was performed on this foam, the foam crumbled and was completely deteriorated! An accident just waiting to happen! During the Vietnam War all military aircraft fuel tanks contained reticu- lated polyurethane foam for explosion suppression. In combat, the foam in the tanks prevents the tank from exploding if hit by gun fire. The foam reduces the likelihood of ignition in the tank and also slows 34 LH inboard main tank after cutting open finding full of deteriorated foam. flame propagation. Today all military aircraft, some military vehicles, boats, race cars and some spe- cialized vehicles have ex- plosion suppressant foam in their fuel tanks. Explo- sion suppressant foam is necessary for military aircraft in combat; how- ever, the foam does not have an infinite life span. Its useful life is finite and variable depending on temperature, humidity, type of fuel, etc. To ensure continued flight safety, a systematic scheduled replacement of the foam needs to be established. Back in the late 1970’s and early 1980’s as a civilian Aero- space Engineer working for the U.S. Air Force (USAF); I con- ducted accident investigation of USAF A-7D fighter aircraft that had a single TF-41 Rolls Royce engine. At least 7 dif- ferent A-7D aircraft crashes were finally attributed to dete- rioration of the explosion suppressant foam in the fuel tanks. The deteriorated foam clogged the fuel system with subse- quent failure of the TF-41 engine. A number of people were killed in these crashes. So here we are --- about 30 years later and deteriorated explosion suppressant fuel tank foam has raised its ugly head again! Around the early 1990’s, Close up of deteriorated foam in LH main tank. 26 Years of Support O-2A aircraft started to be released from the USAF into civilian hands. For- tunately, Cessna had produced a drawing and specifications (Dwgr. No. 10337-035) to convert O- 2A’s from military to FAA Standard Normal Category. One of the requirements was to “inspect the fuel Cessna Pilots Association -April 2010

Deterioration of Explosion Suppressant Fuel Tank Foam Revisited

Donald E. Nieser

We recently acquired another Cessna O-2A Super Skymaster aircraft that needed some work. During the annual inspection it was determined that the aircraft was due for the re-occurring wing spar Airworthiness Directive (AD) inspection. At the same time, a few particles of deteriorated fuel tank foam were found while sumping the tank quick drains. These deteriorated foam particles aroused our suspicions, so it was decided to thoroughly investigate all fuel tanks while they were out during the wing spar AD inspection. Lo and behold, we found that the fuel tanks had not been opened up and the foam had not been removed in accordance with Cessna and U.S. Air Force instructions. In fact, when we cut open both the left hand and right hand inboard main fuel tanks we found that the tanks contained the original foam that Cessna has installed when new. When the Cessna directed cohesiveness test was performed on this foam, the foam crumbled and was completely deteriorated! An accident just waiting to happen!<br /> <br /> During the Vietnam War all military aircraft fuel tanks contained reticulated polyurethane foam for explosion suppression. In combat, the foam in the tanks prevents the tank from exploding if hit by gun fire. The foam reduces the likelihood of ignition in the tank and also slows flame propagation. Today all military aircraft, some military vehicles, boats, race cars and some specialized vehicles have explosion suppressant foam in their fuel tanks. Explosion suppressant foam is necessary for military aircraft in combat; however, the foam does not have an infinite life span. Its useful life is finite and variable depending on temperature, humidity, type of fuel, etc. To ensure continued flight safety, a systematic scheduled replacement of the foam needs to be established. <br /> <br /> Back in the late 1970’s and early 1980’s as a civilian Aerospace Engineer working for the U.S. Air Force (USAF); I conducted accident investigation of USAF A-7D fighter aircraft that had a single TF-41 Rolls Royce engine. At least 7 different A-7D aircraft crashes were finally attributed to deterioration of the explosion suppressant foam in the fuel tanks. The deteriorated foam clogged the fuel system with subsequent failure of the TF-41 engine. A number of people were killed in these crashes. So here we are --- about 30 years later and deteriorated explosion suppressant fuel tank foam has raised its ugly head again!<br /> <br /> Around the early 1990’s, O-2A aircraft started to be released from the USAF into civilian hands. Fortunately, Cessna had produced a drawing and specifications (Dwgr. No. 10337-035) to convert O- 2A’s from military to FAA Standard Normal Category. One of the requirements was to “inspect the fuel tank assemblies for foam deterioration.” It further states that if the foam is defective or deteriorated either “remove all foam from the tank or replace fuel tanks and flush fuel system to remove contamination.” <br /> <br /> Then “inspect foam every 100 hours to prevent fuel system contamination.” Because of our first hand knowledge of the safety of flight risks of deteriorated fuel tank foam, during our restoration of a number of O-2A aircraft over the years, we took the conservative approach that all foam in O-2A fuel tanks was to be removed. The USAF provided instructions on how to remove the foam in USAF T.O. 1L-2A-3 ( O-2 Structural Repair Manual). It shows how to cut a big hole in the top of the tanks in order to completely remove the foam.<br /> <br /> There were and still are organizations like ours as well as individuals around the country restoring O-2A aircraft and registering them in FAA Standard Category. Some took the approach as we did and removed all the foam from all the fuel tanks. Obviously some did not take this approach and continued to operate the aircraft with foam still in the tanks. Some people thought they could wash the foam out of the tanks with chemicals without cutting a hole in the top of the tanks. Some, probably naively, left foam in tanks and did not make a log book entry to re-inspect the foam every 100 hours. So here we are full circle --- 10 years after an O-2A aircraft was registered in FAA Standard Category, containing deteriorated foam hidden in the fuel tanks---an accident just waiting to happen! Luckily we found the deteriorated foam and removed it before it caused an accident and possible loss of life.<br /> <br /> Our suspicions are that there may be other O-2A aircraft that did not have all the foam removed from the fuel tanks. We strongly recommend that all O-2A owners and operators inspect their fuel tanks for deteriorated foam as soon as possible. Using a strong light and a mirror or a borescope, inspect inside the tanks thru the filler cap hole and thru the fuel quantity sending unit holes. If any particles of deteriorated foam are found, we recommend that the tanks be removed, cut open and be cleaned out. It may save your life!<br /> <br /> We would like to know the results of your O-2A fuel tank inspections, please e-mail your results to: nieser.02.337parts@ juno.com. If you would like a sample of deteriorated foam please send a stamped self addressed envelope to Commodore Aerospace Corp, 6221 Commodore Lane Oklahoma City OK 73162.<br /> <br /> Donald Nieser spent 38 years as an Aerospace Engineer, spending the last 23 years at Tinker AFB managing the aging aircraft corrosion research program. Donald also spent time with Bell Aerospace, McDonnell Douglas, LTV and General Electric. He currently specializes in O-2 and 337 Skymaster parts, aircraft sales, and restorations through his company Commodore Aerospace, www.02337parts.com.

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