 
|
|
|
Hi all.
A colleague has drawn my attention to this discussion, and I would like to contribute constructively if I can to the fixed wing part of the question.
Firstly, let me declare up front that I have a commercial interest in single engined aeroplanes: I run Pilatus Australia, and we have 16 Pilatus PC-12 air ambulances currently in service with three sections of the Royal Flying Doctor Service of Australia. The first two were introduced in 1995 and the fleet has continually grown since then. The oldest aircraft now has 9000 hours total time.
As an aircraft marketeer, I researched this subject extensively during the period 1992 to 1997 while I was working with our CASA (sort of equivalent to the FAA) to change the rules to allow commercial passenger operations under the IFR and at night in single turbine powered aeroplanes, and I have retained an interest in the subject.
During that period neither we nor CASA could find any statistical proof that twins are safer than singles (nor the other way round). What we did find was that when we tried to compare apples with apples, it was certainly clear that two engines did not necessarily add safety.
Consider charter operations. This is the most representative example, because there is a common regulated standard of equipment, maintenance, training and operation. Australian statistics were provided by ATSB for the period 1986 to 1996.
Fatal accident rate single engined aeroplanes: 0.31 per 100,000 hours.
Fatal accident rate multi-engined aeroplanes: 0.98 per 100,000 hours.
(Note that a fatal accident for the purpose of the rate above is an accident in which there was at least one fatality. It does not reflect total people killed)
Total accident rates were similar at 9.54 per 100,000 hours for singles and 8.39 per 100,000 hours for multis. The difference is that accidents in multi-engined aeroplanes were around 3 times more likely to result in a fatality.
Total reported hours flown were 1,299,900 for singles, and 2,647,300 for multis.
The statistics above have one variable: the singles in those days were VFR (except freight) and the twins were generally IFR, so we are not strictly comparing apples with apples. However, I have yet to find any professional who will argue that IFR is not as safe as VFR: quite the contrary. The singles also were almost all piston powered, whereas the multis included a mix of piston, turboprop and pure jet aircraft. So, as I said before, the statistics do not constitute proof, but it makes the argument that "twins are safer" a bit suspect when the actual result has been the reverse over this 11 year sample. The history of the Cessna Caravan, a single turbine aircraft, also supports this view. FEDEX has a particularly large fleet which operates on freight duties around the clock in virtually all weathers. If there is a Cessna rep on the forum, he might have the latest accurate data, but I understand that the engine failure related fatal accident rate for the Caravan is one in more than 5 million hours.
I think the truth is that in some circumstances twins may be safer than singles of similar airworthiness standard, particularly in cruise over water or extremely rough terrain. However there are not a lot of twin turboprops of similar airworthiness standard to the modern single turbines out there, and unfortunately engine failures on take-off or go-around in twins all too often result in loss of control with tragic consequences.
A modern single like the PC-12 which incorporates FAR23 Amendments 1-42 is arguably safer than most GA twins because the design standard against which it is certificated addresses the major causes of accidents and injuries to a much higher standard. For all of you who have the back of an aeroplane as a workplace, FAR23 Amendment 36 is very important, because it requires a very significant increase in crash worthiness over previous designs. This is critically important when you consider that the major risk in aviation today is generally agreed to be CFIT (Controlled Flight Into Terrain).
As for the motivation for aeromedical agencies buying PC-12s, I can not speak for them, although as has been pointed out previously on this thread, the PC-12 is substantially cheaper to buy and operate than the competitor. With ever increasing pressure on health budgets, that is surely a good reason to at least consider the aircraft. Its cabin size, layout, noise and vibration levels, access and turn-around time etc also make it substantially better as an ambulance than its twin engined competitors, and this is surely a major consideration.
In summary, if comparing two identical aircraft, one with one engine and one with two engines, and the engines in each have equal reliability, you can make a case for the twin being safer particularly in the cruise. Sadly you don't get that choice at the moment.
I am sorry if this has been a bit long winded, but it is a very interesting subject and I thought that maybe my two cents worth might be of value.
Safe flying.
Terry Wesley-Smith