Self-Sealing Performance: A 45-Year Status Report

By: Marty Krammer

Self-sealing fuel bladders used on military aircraft (both rotorcraft and fixed-wing aircraft) serve as vulnerability reduction technologies to mitigate fuel leaks and fire caused by a ballistic hit to a fuel tank. Over the past decade, interest within the Department of Defense (DoD), the survivability communities, and the aircraft fuel containment communities has risen regarding deficient self-sealing fuel tank performance in fielded and soon-to-be-fielded U.S. aircraft. On several occasions, live fire test and evaluation (LFT&E) efforts performed on newly developed and older fielded U.S. aircraft fuel tanks (bladders), qualified to military specification (MIL-SPEC) MIL-T- 27422 (aka MIL-DTL-27422) and MIL-T-5578 (aka MIL-DTL-5578), have demonstrated self-sealing performance below their expected qualified and intended level.

The lack of self-sealing capability of a fuel tank can negatively impact military aircraft acquisition, affecting multiple factors, including aircraft vulnerability, key performance parameters (KPPs), cost, schedule, and weight. Accordingly, the causes as to why self-sealing performances witnessed in LFT&E are not in alignment with MIL-SPEC qualification continue to be questioned and studied within the DoD communities. In addition, the lack of understanding of the problem is causing a hesitation in making changes or improvements to the governing self- sealing MIL-SPEC standards.

Therefore, a Navy-led effort, sponsored by the Joint Aircraft Survivability Program Office (JASPO), was under- taken in 2014 by members of DoD’s Fuel Bladder Roundtable (FBR) working group to study and gain a greater understanding in the self-sealing performance of aircraft fuel tanks over the past 45 years. Goals were as follows:

Understand the history of U.S. aircraft fuel tank self-sealing performances.
Identify issues and potential causes for mismatches in performance between qualification, live fire test, and combat events.
Acquire information for mitigating future acquisition program KPP risks and impacts associated with self-sealing fuel bladders and associated vulnerability.
Provide recommendations for improving MIL-SPEC self-sealing standards to better align with the intended ballistic threat level capability of the aircraft.

As illustrated in Figure 1, the study brought together results from past component-level, MIL-SPEC qualification gunfire tests aircraft fuel tank LFT&E, and more recent combat incident damage reporting. The study evaluated performance of internal, external, and auxiliary self-sealing fuel tanks. These results were combined and analyzed to identify trends in self-sealing performances.

Figure 1. Comparative Understanding of Self-Sealing Performance.

Questions raised and addressed under the effort included the following:

What does “being qualified” and having a “self-sealing capability” actually mean? Are they the same?
Does the current level of self-sealing capability meet the performance expectations of today, and of the future?
Are the current military-standard gunfire qualification test methods adequate for addressing platform- level survivability self-sealing performance needs?

The study aimed to document recommended improvements to these military standard qualification tests to better address platform-level survivability requirements.

APPROACH

Navy and Army members of the DoD FBR working group performed literature searches that produced data dating back as far as 1971 and as recent as 2015. In total, 23 MIL-SPEC qualification gunfire test reports, 12 LFT&E reports, and 107 combat incident reports involving self-sealing fuel tank hits provided insight into the self-sealing performance of U.S. fielded aircraft systems over the past 45 years. The data obtained applied to self-sealing fuel tanks associated with U.S. military fixed-wing aircraft and small, medium, and large rotorcraft platforms. The literature searches produced data for 26 MIL-SPEC qualified fuel cell constructions on aircraft and their subsequent suppliers.

Navy members conducted a comprehensive side-by-side comparison of all data (qualification, LFT&E, combat incidents), reviewing in detail the following:

Aircraft fuel bladder specifications.
Aircraft vulnerability requirements.
Fuel bladder materials and constructions.
Aircraft fuel tank materials and constructions.
Data from various fuel bladder constructions and aircraft platforms:
- Qualification Phase I cube tank gunfire tests (50–100 °F and
  -40 °F temperatures).
- Qualification Phase II aircraft fuel tank gunfire tests.
- LFT&E (Title 10 U.S.C. § 2366).
- Combat: Afghanistan (Operation Enduring Freedom) and Iraq (Operation Iraqi Freedom).
MIL-SPEC cell qualification deviations.
MIL-SPEC cell qualification waiver by similarity.
Self-sealing performance against various projectiles (threats).
Self-sealing performance for types of tank penetrations (entrance vs. exit).
Self-sealing performance for various projectile (threat) orientations.
Fuel tank test conditions (warmer vs. cold temperatures, internal pressures).
Test conditions and execution (methods, equipment, setups, tank articles).
Fuel cell bladder damage (new and in-service fielded cells).
Other impacts on aircraft acquisition due to substandard self-sealing performance.

HIGHLIGHTS/FINDINGS

As shown in Figure 2, the study showed that, over the past 45 years, three fuel cell manufacturers have supplied self-sealing fuel cells to the U.S. military aviation community. Although names have changed through those years as a result of mergers and acquisitions, facilities, people, manufacturing methods, and capabilities have remained fairly constant. Furthermore, the majority of internal self-sealing fuel cells installed on or in U.S. aircraft during this time were supplied by two fuel cell suppliers or manufacturers: MEGGITT Rockmart (formerly Engineered Fabrics Corp.) and American Fuel Cell and Coatings Fabrics Company (AMFUEL).

Figure 2. U.S. Fuel Cell Manufacturers/Suppliers.

The study revealed that the technologies, materials, and methods used in manufacturing and ballistic testing have not significantly advanced or changed over the past 60 years. Fuel cell manufacturing remains a highly labor-intensive effort, requiring significant operator skill for producing consistent and ballistically tolerant fuel cells.

Per today’s standards, the study indicated that, on average, U.S. aircraft self-sealing fuel tanks, which had been considered fully MIL-SPEC-qualified, did not fully achieve the self-sealing performance specified within the MIL-SPEC prior to going into full-rate production. Additionally, findings indicate that changes are needed within the MIL-SPEC standards to meet the self-sealing fuel tank capability needs against threats of today and tomorrow.

The outcome of the study produced a comprehensive report titled “Understanding the Self-Sealing Performances of U.S. Aircraft Fuel Tanks.” The (limited-distribution) report can be obtained through the Defense Technical Information Center (DTIC) archives.

Conclusion and recommendation topics addressed in the report include the following:

U.S. aircraft fuel tank self-sealing capabilities, deficiencies, and gaps.
MIL-SPEC Phase I self-sealing performances for normal and low (-40 °F) temperatures.
MIL-SPEC Phase I/II test approach and execution issues.
MIL-SPEC Phase I/II projectile shot selections/distributions issues.
MIL-SPEC Phase I/II fuel leakage assessments and reporting issues.
MIL-SPEC Phase I qualification by similarity issues affecting multiple aircraft programs.
MIL-SPEC recommended changes and improvements.
On-Board Inert Gas Generator System (OBIGGS) effects on self-sealing performance.
Better and worse performing fuel tanks/cell constructions.
Vulnerability impact pertaining to fuel tank Pc/d methodology.
Age of fuel cell and self-sealing performance.

The report provides a single-source reference to the DoD and the Services for aircraft self-sealing fuel bladder performances and capabilities. In 2018, the DoD FBR working group plans to address the MIL-SPEC issues and recommendations identified in the study. The findings acquired will support future improvements and changes in the
MIL-DTL-27422 Phase I/II gunfire test and evaluation aspects of the standard.

ABOUT THE AUTHOR

Mr. Marty Krammer is an aircraft vulnerability engineer at the Naval Air Warfare Center Weapons Division in China Lake, CA, currently leading LFT&E activities on the CH-53K and CMV-22 programs. With more than 27 years of experience, he has supported numerous aircraft vulnerability reduction and live fire test programs, including AV-8B, F-15, F-14, F/A-18, JSF, AH-1, UH-1, H-60, V-22, and CH-53, and has provided subsequent recommendations to reduce the vulnerability of these aircraft. Specializing in aircraft fire, fuel tank self-sealing, and explosion protection, Mr. Krammer also serves as the Navy co-chairman of JASPO’s Vulnerability Reduction and Analysis Subgroup, as well as the Navy Deputy Test Director for the Joint Live Fire Aircraft program, investigating vulnerability issues associated with fielded Navy aircraft. He holds a bachelor’s and master’s degree in mechanical engineering from California State University, Chico and California State University, Northridge, respectively.