Aviation Safety Issues Investigation Report SII A05-01
Safety issues investigation report SII A05-01
Post-impact fires resulting from small-aircraft accidents
For aircraft with a maximum certified take-off weight of 5700 kilograms (12 566 pounds) or less, post-impact fire (PIF) contributes significantly to injuries and fatalities in accidents that are otherwise potentially survivable. A potentially survivable accident is one in which the impact forces are within the limits of occupant tolerance, the aircraft structure preserves the required survival space, and the occupant restraint is adequate.
This investigation examined Transportation Safety Board of Canada (TSB) data and the history of PIF safety action to become more informed and to provide discussion material with the intent of mitigating risks surrounding PIF in small aircraft, specifically regarding design certification. The historical incidence of PIF occurrences in aircraft weighing less that 5700 kilograms demonstrates a high probability of future similar occurrences, resulting in adverse human consequences, if current design standards are not addressed.
Statement of the Problem
PIF continues to contribute to injuries and fatalities in otherwise survivable accidents involving small aircraft. Both the National Transportation Safety Board (NTSB) and the Federal Aviation Administration (FAA) of the United States have attempted to address this issue through special studies and Notices of Proposed Rule Making.
In 1994, amendments to the United States Federal Aviation Regulations (FARs) introduced comprehensive fuel system crash resistance certification standards for normal and transport category helicopters to minimize the hazard of fuel fires to occupants following otherwise survivable impacts. Technology and design concepts intended to reduce the incidence of PIF have been demonstrated to be effective in helicopter, race car, and automotive applications. However, there is no requirement to incorporate these countermeasures into new or existing small aeroplane types or into small helicopters certified before November 1994.
Scope of the Project
The selection criteria for this investigation were accidents that occurred in Canada between 1976 and 2002 inclusive, that involved powered, small aircraft, and that resulted in PIF. The investigation reviewed TSB PIF statistics, the history of PIF, existing certification requirements, currently available PIF prevention technology, survivability thresholds, cost-benefit analysis barriers, retrofitting of existing types, and potential future design requirements.
The investigation identified 521 PIF accidents listed in the TSB Aviation Safety Information System (ASIS) database. Cause of death statements were reviewed on available autopsy reports, coroners' reports, and registrations of death to determine if fire had contributed to fatalities. Additional data sources were reviewed to determine if fire had contributed to serious injuries. Of the 521 accidents, 128 were accidents in which fire or smoke inhalation was identified as either partly or solely the cause of death or serious injury. An extensive research of past actions by the FAA, the NTSB, and the TSB regarding PIF safety issues was conducted. As well, a detailed examination of a cost-benefit analysis conducted by the FAA in response to previous NTSB recommendations was reviewed.
In all 128 accidents in which PIF contributed to serious injuries or fatalities, the aircraft occupants were in close proximity to fire or smoke for some time following the impact. The investigation identified four conditions that were essential for this to occur.
- There was an ignition source in proximity to a combustible material, such as fuel.
- There was combustible material in close proximity to the occupants.
- Occupant egress was compromised.
- The fire was not suppressed in time to prevent fire-related injuries or fatalities.
The data collected and analysed indicate that there is a significant risk for PIF with fire-related injuries and fatalities in small-aircraft accidents. Furthermore, information examined shows that past attempts to change certification requirements have been unsuccessful, largely because of insufficient data, which resulted in cost-benefit analysis conclusions that negated the proposed safety action.
The defences to prevent PIF and to reduce fire-related injuries should fire occur in otherwise survivable accidents involving aircraft weighing less than 5700 kilograms can and should be improved. PIF presents a great risk to the occupants of small aircraft because of
- the high volatility of aviation fuel;
- the close proximity of fuel to occupants;
- the limited escape time;
- the limited energy-absorption characteristics of small-aircraft airframes in crash conditions;
- the high propensity for immobilizing injuries; and
- the inability of airport firefighters and emergency response personnel to suppress PIFs in sufficient time to prevent fire-related injuries and fatalities.
Volatile liquid fuel is the combustible material of greatest significance in PIF accidents. Considering the propensity for rapid propagation and the catastrophic consequences of fuel-fed PIF, the most effective defence against PIF is to prevent the fire from occurring at impact, either by containing fuel or preventing ignition, or both.
The benefits of PIF-resistant fuel system technology have been proven in land vehicle applications and, recently, in certified civilian helicopter applications. A requirement for similar engineering countermeasures in existing, newly manufactured and newly certified FAR 23 and equivalent small aeroplanes, existing small helicopters, amateur-built aircraft, and basic and advanced ultralights would reduce the incidence of fire-related serious injuries and fatalities in otherwise survivable accidents, and could significantly increase the rate of occupant survival.
The implication of design improvements on new aircraft will be significant, and even more significant on existing designs. Enhancing current design standards will require considerable effort by Transport Canada and the FAA, and cooperation by their international counterparts to ensure harmonization of any new standards and guidelines.
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© Minister of Public Works and Government Services 2006
Cat. No. TU4-18/1-2006E
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