Are Singles Safer than Twins?

Data

The statistics for the past five years (1986-90) for charter operations (including aerial ambulance) in Australia were dissected and tabulated as follows: (Helicopters have been included for purposes of comparison) Interpretation of data

In Charter Operations (including Aerial Ambulance) over the five year period examined:

1. Per accident, Single engined aircraft flew 18% more hours than did helicopters and twin engined aircraft 15% more than singles, or 37% more than helicopters. This is the only statistic which would, before further analysis, favour twins as “safer” than singles.

2. Per fatal accident, helicopters flew 123% more hours than twins and single engined aircraft 31% more than helicopters, or 293% more than twins.

3. Per accident involving serious or fatal injury, twin engined aircraftfly 87% more hours than helicopters and single engined aircraft of 134% more than twins, or 338% more than helicopters.

4. Per fatality, single engined aircraft fly 134% more hours than twins and helicopters 71% more than singles, or 301% more than twins. However, this apparent non fatal record of helicopters has been shattered in the first six months of 1991.

5. Perrerious or fatal injury, twin engined aircraft fly 55% more hours than helicopters and single engined aircraft 131% more than twins, or 259% more than helicopters.

6. Per aircraft destroyed, twin engined aircraft fly 34% more hours than helicopters and single engined aircraft 30% more than twins, or 75% more than helicopters.

It is interesting to note that no serious or fatal accident involving single engined aircraft was attributed to engine failure. This was not the case for twin engined aircraft, where at least two and perhaps three fatal accidents were attributed to failure of an engine. This would tend to dispel the myth that “two engines are safer than one”. Two serious helicopter accidents were also attributed to engine failure.

Claims some might make that twins carry more passengers than either singles or helicopters, hence the greater number of fatalities become baseless when one realises that singles attracted in this period 4. 5 fatalities per fatal accident while twins attracted only 3. 6! (and helicopters 2. 0)

It is a fact that, in Charter Operations, single engined aircraft (including helicopters) are not permitted by Regulation to carry passengers at night or under InstrumentFlight Rules. The claim, as a result of this, that twins therefore fly in more hazardous conditions does little more than reinforce passenger safety in singles and support pilots’ command decisions to fly under Visual Flight Rules. In truth, however, the degree that Night VMC or IFR (Instrument Flight Rules) may be more hazardous than VFR is offset by the fact that pilots operating under these rules are usually more highly trained in such operations and are operating aircraft with more sophisticated navigational equipment.

To the converse, single engined aircraft can and do operate in and out of marginal “bush strips” which would totally exclude twins and this, combined with the fact that singles are generally lighter and less powerful than most twins (ie more easily affected by wind gusts and shear), would incur at least as much potential “hazard” as IFR in twins. In spite of this, however, the overall accident rate of single engined aircraft is statistically very similar to that of twins. However, the “survivability” of accidents in single engined aircraft makes them very substantially safer than twins. Asymmetric flight – flight in a twin engined aircraft with one engine inoperable – substantially increases both the cockpit workload and the minimum controllable airspeed while significantly decreasing the controllability of the aircraft. Any one of these factors would detract from safety.

For the cynic who would reject the inclusion of aerial ambulance operations on the spurious grounds that such operations “necessarily” occur in sub-minimal flying conditions “in the interests of saving lives”, the data are reproduced below for Charter Operations only (ie without aerial ambulance figures): * The writer has taken reciprocal liberty in separating the (Soulia, Qld) accident of 7 April 1989 where five persons lost their livesincluding the pilot who was allegedly conducting prohibited night passenger charter operations in a four seat single engined aircraft. The pilot was also found to have had a “high blood alcohol reading” although consumption of any alcohol in the eight hours prior to flying is prohibited as is flying while in any way affected by alcohol.

** The first six months of 1991 have seen a spate of helicopter accidents, serious and fatal, which have put the lie to these figures. As can be seen above, notwithstanding the Soulia tragedy, there is no significant change to the data with the deletion of the aerial ambulance category. Apparently single engined aircraft remain much safer than twin engined aircraft and, where the unfortunate occurs, very much more survivable.