On the occasions when this question is asked (and it is asked often) I am often tempted to roll out again an article I wrote for the PPL/IR network as to why I took the decision. Apologies to those who have read it before.
Until about ten years ago I operated a Cessna 172 Skyhawk as my business aircraft. It was airways equipped, I was instrument rated (indeed had an ATPL/IR and had been employed as an executive jet captain in the recent past) and there should have been little except (relatively rare) 500m RVRs to prevent me from flying on business all over the country.
But I didn't. Almost every time a meeting or trip came up which obviously demanded aviation I found myself deciding that using the 172 was not appropriate. The reasons that the flight might not be possible multiplied in my mind...icing, low cloud, thunderstorms, long water crossings, night flights, electrical and avionics failures, strong headwinds en-route, strong crosswinds on arrival...the list seemed endless.
I was not cancelling flights on the day because of these reasons. Rather, I was making alternative plans because I feared that the trip would have to be cancelled at the last minute, thus jeopardising my business.
There were two seminal moments in my decision to dump the 172.
The first was when I was sitting on the threshold at Fairoaks and the thought occurred to me:
"Here I am with one engine, one alternator, one vacuum pump, one pitot/static source and one pilot. If I were in the jet and any one of these situations arose I would declare an emergency and land...yet here am I about to take-off."
The second was when I had to go to an important meeting with the Blackpool Pleasure Beach, which is in walking distance of Blackpool Airport, a highly equipped, multi-runway, IFR, GA friendly airfield. Yet I still decided to drive from home, past the hangar where the 172 sat and then a further four hours, "just in case" there was a problem.
As I drove up the motorway and thought of the 172 sitting in the hangar, costing me a fortune in insurance, hangarage and maintenance I took the decision...either I get an aeroplane that has a reasonable expectation of delivering me or I give up on business aviation.
So, why was the 172 not reliable enough to be trusted to deliver me? There were a number of reasons, some demonstrable, others based on preference and prejudice.
The demonstrable reasons were icing, crosswind capability and speed.
Icing
I have never been one to get over-concerned about icing, and have from time to time carried a fair amount of ice on a non de-iced aircraft. Indeed I might go flying on a day when light icing is present in a height band. But the problem is that if icing is going to be present from the MSA up to FL100 you really cannot even think of getting the SEP out of the hangar.
Crosswind
Fewer and fewer airfields are offering a cross runway. A crosswind capability of 15kts means that the number of days which are outside limits at one end of the flight or the other reach significant levels.
Speed
Although at first glance going fast seems to be a luxury rather than a necessity, it becomes very relevant indeed when faced with a strong headwind. 50kt winds are not unusual at airway levels. This is nearly half the TAS of the single, leaving me with 70kts made good, whereas in a twin the groundspeed would be 120kts or better...nearly double the speed.
The factors based on preference or prejudice were all to do with system failures.
Engine Failure
I have had nine engine failures in my flying career. I know that they happen. This is why I always want a plan B available to cover the eventuality. Plan B in a twin is to divert at leisure. Plan B in a single should be to make a forced landing in a field. But that's rather difficult in hill fog, or even a 200' cloudbase, or at night, or over water, or over mountains. So if you fly a single and want to ensure that there is a plan B you are limited to flying over farmland, during the day, when the weather is reasonably good. So why bother with an instrument qualification?
Electrical or other systems failure
Almost as serious as an engine failure is an alternator failure in or above IMC. With no way of navigating or communicating you are left with some pretty unpleasant choices...flying triangles in the hope that someone will notice, then unrehearsed formation flying in cloud, or maybe dead reckoning to where you hope the sea is and then hoping that your guess at QNH is reasonably accurate. Similarly, loss of your single vacuum pump can be pretty fatal, particularly if you identify the symptoms late.
I know that there are counter arguments to some of the above. There are singles with redundant ancillaries, particularly alternators and suction pumps, there are de-iced and fast singles, and all of these make them more suitable for business flying than their lesser cousins. Nonetheless, they remain dependent on one power unit, and therefore everything continuing according to Plan A. Also, these highly equipped singles are relatively rare and expensive.
There are also some arguments against twins on safety grounds. The commonest are:
Risks associated with single engine failures on a twin
Most GA twins are not certified to be able to continue flying in the event of an engine failure below 200'. Twin training concentrates on failures above 200', recovery from such a failure and continuation of the flight. Unfortunately not enough emphasis is placed on the fact that below 200' the most prudent action is normally to shut down both engines and make a forced landing. The argument continues that if either engine fails, the result is a forced landing, and therefore the risk is at least doubled (I say at least because engines are marginally more likely to fail on a twin because of greater vibration and longer control runs.)
This increased risk is real and must be taken into account by the twin owner, but, at least in my mind, is more than offset by having the second engine available in the cruise, especially when conditions dictate that a forced landing is unlikely to be successful (water, mountains, night, low cloud etc).
Furthermore the time for which the aircraft is exposed to risk is very small (less than 15 seconds per flight) compared to the time spent exposed to risk in a single.
Finally, the pilot can do a great deal to mitigate the risk (using the full runway, rotating at blue line, avoiding built up areas in the take-off path etc). I have written an article in Flyer magazine on the subject.
Less protection in a forced landing in a twin
People do argue that in the event of a forced landing, uninjured survival of the passengers is less likely because the aircraft is going faster and the momentum is greater, the occupants are not protected by the engine going ahead of them and that the gear may be up and therefore not in a position to absorb impact.
I do not know if this is true, but even if it is, I consider it mitigated by the fact that a forced landing is much less likely.
Greater risks taken by twin pilots
There is an argument, developed, I believe, by Prof.Adams of London University, that no matter how safe a piece of equipment or transport is made, overall safety is not affected because the user will take greater risks until the risk level reaches the same point as it had been on the less safe equipment. Prof. Adams believes that Volvos should be replaced with paper cars with spikes in the middle of their steering wheels.
Thus, a twin is not safer than a single, because the pilot will choose to fly over water, in icing, at night, to IFR minima, where the prudent single pilot would not.
My counter argument is simply "...sure, maybe I am taking the same risk as a single pilot, but I am able to operate in much more difficult environments at the same risk."
Which means I can get to my business meetings. QED