Features of SFAR 88
Fail-Safe Design Methods - The SFAR has the effect of mandating the use of "fail-safe" design methods, which require that the effect of failures and combinations of failures be considered in defining a safe design. Detailed methods of compliance with 25.1309(b), (c), and (d) are described in Advisory Circular (AC) 25.1309-1A, System Design Analysis, and are intended as a means to evaluate the overall risk, on average, of an event occurring within a fleet of aircraft. The following guidance involving failures is offered in that AC:
• In any system or subsystem, a single failure of any element or connection during any one flight must be assumed without consideration as to its probability of failing. This single failure must not prevent the continued safe flight and landing of the airplane.
• Additional failures during any one flight following the first single failure must also be considered when the probability of occurrence is not shown to be extremely improbable. The probability of these combined failures includes the probability of occurrence of the first failure.
Flammability Characteristics -The flammability characteristics of the various fuels approved for use in transport airplanes result in the presence of flammable vapors in the vapor space of fuel tanks at various times during the operation of the airplane. Vapors from Jet A fuel (the typical turbojet engine fuel), at temperatures below approximately 100 degrees F, are too lean to be flammable at sea level; at higher altitudes the fuel vapors become flammable at temperatures above approximately 45 degrees F (at 40,000 feet altitude).
However, the regulatory authorities and aviation industry have always presumed that a flammable fuel air mixture exists in the fuel tanks at all times and have adopted the philosophy that the best way to ensure airplane fuel tank safety is to preclude ignition sources within fuel tanks. This philosophy has been based on the application of fail-safe design requirements to the airplane fuel tank system to preclude ignition sources from being present in fuel tanks when component failures, malfunctions, or lightning encounters occur.
Ignition sources - Possible ignition sources that have been considered include:
• Electrical arcs
• Friction sparks
• Auto ignition. (The auto ignition temperature is the temperature at which the fuel/air mixture will spontaneously ignite due to heat in the absence of an ignition source).
Some events that could produce sufficient electrical energy to create an arc include:
• Electrostatic charging
• Electromagnetic interference (EMI)
• Failures in airplane systems or wiring that introduce high-power electrical energy into the fuel tank system.
Friction sparks may be caused by mechanical contact between certain rotating components in the fuel tank, such as a steel fuel pump impeller rubbing on the pump inlet check valve. Auto ignition of fuel vapors may be caused by failure of components within the fuel tank, or external components or systems that cause components or tank surfaces to reach a high enough temperature to ignite the fuel vapors in the fuel tank.
Fuel Tank Protection - Another source of information on fuel tank protection is the FAA – Fire Safety Section: Systems Fire Group web site. Here, you can find information on fuel flammability and inerting, either completed, or on-going, and planned research by visiting www.fire.tc.faa.gov/systems/tankprotection /intro.stm.
To learn more about this SFAR, you can view it on the FAAs website at www.faa.gov/avr/arm/nprm.htm.
New advisory circulars for fuel tanks
Two new Advisory Circulars (AC) entitled Fuel Tank Ignition Source Prevention Guidelines, AC 25.9811-1B, and Fuel Tank Flammability Minimization, AC 25.981-2, both dated 4/18/01, were released by the FAA and support the new SFAR. The first AC provides guidance for demonstrating compliance with the certification requirements for prevention of ignition sources within fuel tanks of transport airplanes. The second pertains to minimizing the formation or mitigation of hazards from flammable fuel air mixtures within fuel tanks. This AC has a section on fuel tank inerting using inert gas, like nitrogen, to keep the oxygen level in the tank ullage below a combustible level to prevent fuel tank flammability.
These efforts at minimizing the chances of fuel tank incidents will help Keep ’em Flying! AMT