: This report presents the results of a study, funded by the Federal Aviation Administration (FAA), of helicopter Crash-Resistant Fuel Systems (CRFS). It covers the historical efforts that led to the current state of the art in military helicopter fuel systems and the more recent modifications to civil certification standards in Title 14 Code of Federal Regulations (CFR) Part 27 (Normal Category Rotorcraft) and Part 29 (Transport Category Rotorcraft). It describes the basic research, testing, field investigations, and production efforts that have led to the highly successful CRFS, that have saved many lives and have reduced the costs of accidents. While the hardware and fabrics are available today to create the CRFS, the adequacy of the integration of these items into existing and new civil fuel system designs cannot be assessed because of the lack of cuiTent field investigation data on civil helicopter crashes. This report reviews this problem, including the forms used for reporting and the current level of available data, which is essentially nonexistent. Training of field investigators in specific crashworthiness technology is of great importance, as well as the need for trained engineers in the design and certification process. A discussion of the civil and military crash environments is provided to give a background for the discussion of the need for reevaluation of the rationale used in establishing the current civil regulatory standards. The value of full-scale crash testing during the early development of the military CRFS is reviewed. The lack of any planned tests for the CRFS in current civil helicopters is an area of concern. A section of the study discusses the individual components of a CRFS, with guidance on the application of each item to the overall system design. This report provides guidance to designers looking for information about CRFS design problems and analytical tools for use in product improvements.

A Study of Helicopter Crash-Resistant Fuel Systems

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Incendiary Gunfire Evaluation of a Condensate-Formed Fuel Fog Inerting System

Hot-spot and puncture ignition of fuel vapors by simulated lightning discharges was studied experimentally. The influences of skin coating, skin structure, discharge polarity, skin thickness, discharge current level, and current duration were measured and interpreted. Ignition thresholds are reported for titanium alloy constructed as sheets, sheets coated with sealants, and sandwich skins. Results indicated that the ignition threshold charge transfer for coated sheets, honeycomb, and truss skins is respectively about 200%, 400%, 800% that of bare alloy sheet of .102 cm (.040 in.)-thickness. It was found that hot-spot ignition can occur well after termination of the arc, and that sandwich materials allow ignition only if punctured.

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Ignition of Fuel Vapors Beneath Titanium Aircraft Skins Exposed to Lightning

Aircraft Kerosine vs. Wide-Cut Fuel-Safety Considerations - (Report of the Aviation Fuel Safety Report Task Force of the Coordinating Research Council, Inc.)

: The objective of the study was to determine fuel tank vapor space characteristics for a simulated helicopter fuel tank and to evaluate the potential hazard which exists. Fuel/air ratios were measured as a function of time and position within the ullage of the fuel tank for specified flight profiles. These results were compared to published flammability limits as a basis for assessing flight hazard potential. The flight profiles were simulated by withdrawing fuel (at rated engine usage) from a vibrating tank held at constant pressure and temperature. Parametric variations were made in fuel temperature (40 to 100F), flight altitude (0 to 15,000 feet), vibration environment, and fuel properties (liquid JP-4 versus JP-4 emulsion EF4-104H). Another important variable not considered initially but which was uncovered during the course of this investigation was the effect that the rubberized tank liner could have on the measured fuel/air ratios. The extent of this effect was found to be related to fuel temperature and exposure time of the liner to the fuel. The experimental results showed those ranges of the test variables which had a significant effect on the measured fuel/air ratios. They also demonstrated that fuel/air mixture gradients do exist in fuel tanks under flight conditions. It was found that tanks which would be considered safe as determined by calculations for equilibrium conditions actually contain flammable regions, even for level flight. An analytical model for the ullage space was written which included transient fuel vapor diffusion and convection which was brought about by venting of the ullage.

N L Helgeson

Papers

Flight vibration and environmental effects on formation of combustible mixtures within aircraft fuel tanks