It is important to note that statements relating to safety, when made in comparison terms, must be considered statements of opinion only, because there exists today no clear cut, objectively based and quantifiable criteria by which the relative safety of two or more helicopter operations can be measured. The determination of the relative safety between single and twin engine helicopter operations includes considerations of many factors, such as the number of incidents and accidents; the injuries and loss of life resulting from those accidents; the time period under comparison; manufacturer quality, pilot training and supervision, and locale or regional considerations.
Indeed, even the federal government in the U.S. have been unable to reach an understanding of the term “safety” that is objective or quantifiable. For example, a report prepared for the Federal Aviation Administration, Office of System Safety, stated:
"In the broadest sense, safety data include reports of events, such as accidents and incidents, inspection results, reports of enforcement actions or other sanctions, and other data which characterize the activities of the air transportation system. It must be noted that only accidents (and some incidents) involve measurable harm to persons or property, and that many types of incidents are reported to FAA by the carriers themselves.
It should be noted that there is no consensus among researchers and participants in the aviation industry about what exactly constitutes "safety data." This lack of consensus was strongly expressed in the comments to the initial draft of this report. Although accidents are universally regarded as events that should be avoided and eliminated if possible, there is little or no statistical evidence for U.S. domestic commercial aviation that other forms of "safety data"-incidents, surveillance results, or enforcement actions-serve as predictors of future accidents or are correlated with accident rates for individual carriers….the exact nature of the relationships between these data and the safety of airline operations remains the subject of research in the aviation community. (GRA Inc (under contract with the FAA), “Safety Reports-Aviation Safety Data Accessibility Study Index”, Washington D.C. (January 20, 1997))
In the U.S., the FAA has not decided that two engines are safer than one engine. Indeed, a legitimate on-going debate exists within the helicopter industry and between the aviation safety and regulatory agencies of different countries as to the relative safety of Single-Engine and Twin Engine helicopters.
As noted above, helicopter experts have yet to define the most basic terms of the safety equation. Reasonable people in the industry, and international helicopter experts and regulators often draw valid, but completely different conclusions regarding the relative safety of Single-Engine and Twin-Engine helicopters.
FAA Administrator Jane Garvey, in a published response to a question by ROTOR in 1997 in opposition to European conclusions that single-engine turbine helicopters pose an unacceptably higher risk than twin-engine helicopters, stated: “Only two states provided all of the data necessary, but it was enough to show that the accident rate for single-engine turbine helicopters was, in most cases, better than twin-engine helicopters”. She also stated that “[a]n analysis of the accident data revealed that single-engine helicopters were either as safe as or safer than twin-engine turbine helicopters”.
However, the NTSB reported for the years 1991-1998 that the single-engine turbine helicopter fatal accident rate was 0.1.41 while the twin-engine turbine helicopter fatal accident rate was 0.88 per 100,000 flight hours. Clearly, the same safety data is being interpreted differently between the U.S. and Europe, as well as within governmental agencies of the U.S. (Harris, Joel S. “Data Show Downward Trends in U.S.-registered Helicopter Accident in 1991-98”, Helicopter Safety, Volume 27 (January-February 2001).)
Roy G. Fox, the Chief Safety Engineer for Bell Helicopter Textron, authored a document in August 1991 entitled “Measuring Safety in Single- and Twin-engine Helicopters”. In that article he presents data that the fatal accident rate from all causes for single-engine helicopters was 5.49 and twin-engine helicopter was 4.37 accidents per 100,000 flight hours between 1984 and 1988. From that data a reasonable person might conclude that it is safer (less chance of dying) in a twin-engine helicopter rather than a single-engine helicopter. Yet in the same article he concludes with the same data that the relative risk of serious injury to the individual from all causes was the same in generic single-engine turbine and twin-engine turbine helicopters. It is possible that a reasonable person might conclude that it is safer (less chance of injury) in a single-engine turbine helicopter rather than a twin-engine helicopter (less chance of death).(Fox, Roy G. “Measuring Safety in Single- and Twin-engine Helicopters”, Flight Safety Digest, Volume 10 (August 1991))
Joel S. Harris reported in an article for the Flight Safety Foundation in 1999 that “Data show same U.S. Fatal-accident Rate for Single-turbine and Twin-turbine helicopters”. The statistics for 1993, through 1997 showed a fatal accident rate of 1.4 per 100,000 flight hours for both single and twin-engine helicopters. (Harris, Joel S. “Data Show Same U.S. Fatal-accident Rate for Single-turbine and Twin-turbine Helicopters”, Helicopter Safety, Volume 27 (January-February 1999))
Clearly, there are legitimate differences of opinion regarding the relative safety between single-engine and twin-engine helicopters. However, it remains true that the FAA has not stated that twin-engine helicopters are safer.
There are a number for reasons why twin-engine helicopters may not be safer. First, the existence of two engines may be irrelevant given that the greatest cause of helicopters accidents is pilot error. In the NTSB study cited above, 66.5 percent of all accidents were caused by pilot error. Engine failure was a cause in 20.8 percent of the accidents in the study. When fatal accidents only are considered, engine failure or malfunction as a contributory cause totaled only 9.9 percent of the total while airframe failure and prop or rotor failure were a contributory cause in 96.2 percent of the accidents. (National Transportation Safety Board, “REVIEW OF ROTORCRAFT ACCIDENTS 1977-1979”, NTSB Special Study NTSB-AAS-81-1 (August 11, 1981))
Furthermore, the addition of a second engine does not in, and of itself result in a ‘safer’ helicopter. A second engine increases the overall system complexity. Carrying an extra engine means carrying the extra weight of the engine, heavier system and airframe, and additional fuel. This requires larger and more complex drive systems. Rotor and anti-torque systems must be larger and often their size requires the use of hydraulic systems. As the systems become more complex and non-linear they are more difficult to operate and pilots require more training. The possibility of pilot error, already the greatest cause of helicopter accidents, increases.
A twin engine helicopter does not always cost more to purchase than a single engine helicopter, but it will always cost more to operate. This additional operational cost can effectively reduce safety if maintenance or servicing intervals is deferred because funds are not available. (Perrow, Charles. “Normal Accidents; Living With High-Risk Technologies”. New York: Basic Books, (1984))
The subject of engines when considered alone creates confusion and uncertainty when safety is considered. There are significant differences between helicopters in the broad categories of single-engine turbine and twin-engine turbine. As an example, a single make of helicopter can be equipped with two or more engine types. These engine types can be from the same or different manufacturer. Each engine will have different operational limits, performance and failure rates. Another problem results from the misconception that all twin-engine helicopters are capable of flying on one engine. This is not true. Many twin-engine helicopters are incapable of flight on one engine in many phases of flight, such as hover or take-off. Many twin-engine helicopters are incapable of single-engine flight at high weights regardless of the mode of flight. The addition of a second engine may seem safer at first glance, but this is not always true. (Harris, Joel S. “Implications of the Power Curve on Single-engine Flight in a Twin-engine Helicopter”, Helicopter Safety, Volume 18 Number 2 (March-April 1992)