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Most Recent Agent Guidance

This document was drafted by members of the task group to narrow the focus of work further. Their results were made public in September 2000. The group selected CF3I and HFC-125 for continued evaluation. Some remaining technologies do hold promise, however, economically, the gaseous systems still hold advantage.

View Options for Aircraft Engine Fire Protection, September, 2000.

"User Preferred Fire Extinguishing Agents for Engine and Auxiliary Power Compartments"

This document was written to direct the focus of this project regarding which agents ought be evaluated. The published work is a survey which was circulated amongst airline operators, powerplant, and air frame manufacturers to gather their feedback regarding the leading selections for halon replacement in this application. The conclusions resulted in (paraphrasing)...

  1. Work with compressible fluids, specifically, FIC-13I1 (CF3I), HFC-227ea (Great Lakes Chemical Corporation FM-200), and HFC-125 (DuPont FE-25), is of primary importance.
  2. Material compatibility with engine components must be better known.
  3. Gas generator technology is of interest; secondary to compressible fluids.
  4. Interest in water mist for this application received minor attention.
  5. Operation of these systems will not likely produce human exposure concerns regarding toxicological issues. However, concern during maintenance operations is voiced.

View User Preferred Fire Extinguishing Agents for Engine and Auxiliary Power Compartments, DOT/FAA/AR-96/80.

Minimum Performance Standard for Halon 1301 Replacement in Civil Aircraft Engine Nacelle and Auxiliary Power Unit Compartments

A task group within the International Aircraft Systems Fire Protection Working Group, formerly known as the International Halon Replacement Working Group for civil aviation (IHRWG), formulated a plan to address the issue of finding replacements for halon 1301 in civil engine and APU compartments. The approaches conceived, in various manners, were based on the principle of maintaining the current level of safety, as defined by the use of halon 1301 in these applications.

The group involved in this effort has been so since October 1993. The 1st effort coincided with activities conducted by the United States Air Force (USAF) at Wright-Patterson Air Force Base, Dayton, OH, which resulted in a design model for HFC-125, as used in these compartments. This effort was primarily funded by the 3 services of the United States Department of Defense (US DoD) with a smaller contribution from the FAA. Civil aviation interests were interested in further work with additional replacement candidates, thus additional work continued at the FAA W. J. Hughes Technical Center (FAA Tech Center) in Pomona, NJ. At various times in this history, a document has existed which describes the two facets needed to effect such an evaluation; a simulator and a procedure.

The 2nd revision (MPSe rev02) was accepted by the group during the July 1996 IHRWG meeting held at the FAA Tech Center. The series of tests run required delivering some amount of a replacement candidate which extinguished the majority, but not all of, a series of repeated fire tests for 4 major test conditions, where halon 1301 was used to define the fire threats. The primary step was the delivery of halon 1301 to the test section of the test article which met the intent of current FAA certification criteria. The next step required was to define the fire threats, based on the performance of halon 1301. The 2nd revision required finding a properly configured fire threat that was extinguished for 4 of 5 fire tests. Finding the proper configuration required that one manipulate fuel flow rate, fuel temperature, and hot surface temperature, while repeatedly delivering halon 1301. In April 2002, 5 initial tests were run using halon 1301 resulting with 5 fire extinguishments. Given the improbability of finding this "4-out-of-5" condition, a series of 15 tests were run to see if the hot surface alone could reignite a spray fire, while temporarily discontinuing the fuel flow so as to mimic the environmental exposure to a migrating agent pulse. Of the 15 tests, 1 reignited. Given 15 of 15 fires did not reignite under conditions thought to provide 100% reliable ignition, and in conjunction with US DoD experience and commentary regarding the fickleness of hot surface ignition, MPSe rev02 was terminated given the unreliability of hot surface reignition; i.e. the test process itself was unstable.

From 2002-2006, work continued under the guise of MPSe rev03. MPSe rev03 again described a test article and a process that could be used to demonstrate the equivalence of a candidate to the current level of safety, as defined by halon 1301. And again, the primary step was the delivery of halon 1301 to the test section of the test article which met the intent of current FAA certification criteria. The test environment was largely similar to that of MPSe rev02. However, the process changed from a "4-out-of-5" method to one which provided a measurable flame extinction duration created by delivering a pulse of fire extinguishing agent to the fire threats which possessed persistent fuel and ignition source presences. This flame extinction duration directly related to the quantity of agent delivered. MPSe rev03 was empirical and amended by those participating on an as-needed basis, and was not officially drafted until 26Sep2006, which incorporated all learned during the 2003-2006 test activities.

Equivalence was first achieved by establishing similar fire extinguishment behaviors between halon 1301 and the replacement candidate. Halon 1301 had a constant configuration which defined the threshold behavior. Equivalence was found by iterating the quantity of the candidate until an amount could repeat the fire-related performance of the halon 1301. Upon the replacement candidate achieving comparable performance, concentration measuring equipment was placed, where the distribution of the candidate was then quantified. The information from the fire extinguishment and concentration distribution behaviors were then used empirically to determine equivalent candidate concentrations. The largest of the 4 was moved forward as the recommendation for certification. Testing under the guise of MPSe rev03 was completed for HFC-125, CF3I, and FK-5-1-12. In 2007, testing began under the guise of MPSe rev03 with an aerosol, however, preliminary test results indicated the equivalent concentration would be less than something expected from a cup-burner assay.

The suggestion from the aerosol's MPSe rev03 preliminary outcome was paradoxical. The paradox lie with an outcome that was suggested less than an indication from a cup-burner assay. Two cases perhaps existed to explain this. First, MPSe rev03 was empirical in nature, built upon behaviors and observations related to gaseous fire extinguishing candidates. An aerosol is not gaseous. For these gaseous agents, many assumptions were built into MPSe rev03. Perhaps, due to the disconnect of the state between gas and aerosol, some assumptions were violated. Second, the empiricism may not have been the source of the problem, but some other unmeasured parameter was playing a role in fire extinguishment. If so, this poses a dilemma, as an FAA certification test only measures the concentration of the fire extinguishing agent over time in the compartment. The dilemma results because if some other parameter (variable) is playing a role in fire extinguishment, the FAA certification test would not track that variation, let alone quantify it. Regardless of these 2 cases, the paradox presented, and the FAA Technical Center acted to terminate future use of MPSe rev03 in 2008.

Work completed at the FAA Technical Center, with task group coordination, started in late 2008 and concluded in March 2010 with the posting of MPSe rev04. MPSe rev04 incorporated many aspects of MPSe rev03, including threshold criteria defined on historical halon 1301 behavior. However, some noteworthy changes resulted. The most important changes were :

  • discontinuing the use of halon 1301 in specified testing
  • the shift from an iterative search for successful candidate distribution criteria, as in MPSE rev03, to a pure proof-test, where some initially-identified, candidate distribution criteria are evaluated
  • the measuring of the distribution of future candidates, where 1 of the 12 sample points is in a wake region associated with flame-holding structure, where previously all concentration sampling was free-stream

In a broad view the MPSe requires :

  1. Fabricating a simulator which must be supplied with :
    • 2 airflow regimes
    • spray and residual fire scenarios
    • an ability to deliver a potential halon 1301 replacement candidate
    • an ability to capture the attendant behaviors with different data-capturing methods
  2. Following the procedure to demonstrate the acceptable performance of a candidate by :
    • delivering the candidate to the test environment by specified means
    • challenging its design criteria with the two fire scenarios
    • extinguishing the fire scenarios acceptably

The MPSe no longer reflects requirements to demonstrate equivalence for an alternate in the Auxiliary Power Unit (APU) compartment. The discontinuation of the APU effort reflects the fact that the nacelle criteria were found to far exceed what was actually needed to adequately protect APU compartments as they currently exist. For this reason, protection in this compartment is based on the criteria defined for the nacelle.

MPSe rev04 is an improvement on MPSe rev03, where it has been written broadly enough allow different technologies to be evaluated for this challenge. Such was the thought for MPSe rev03, however, given changes in the recent years, the realization that MPSe rev03 was inadequate fostered the development and creation of MPSe rev04, where rev04 is as simple in nature as can be for this environment, and is now a salient proof-test.

A working version of MPSe revision 04 can be accessed here.

Additional Information

For information contact:
Doug Ingerson