Power - Would you like one or two engines Sir?

I think most people assume that, in the case of helicopters, more engines are better as it will give the pilot more power to play with and, of course, a second engine to use in case one fails.? The problem with this perception is that whilst the second point is definitively true, under the first point there is way more to think about and sometimes it is necessary to have experienced both engine solutions, in the same basic airframe, to realise why.? It helps to explain why some manufacturers turn single engine into twin engine helicopters and why, in some rare cases, the exact opposite is true.

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I first started to fly the AS350 back in 1997 when the UK MOD transitioned from using the Gazelle for basic helicopter flying training to a contractor provided leased platform in the shape of the AS350 B2 or Squirrel HT 1 in MOD terms.? The AS350 was a larger helicopter than the Gazelle where you could easily carry 4 passengers (5 if someone was in the left-hand pilot seat), full fuel and still be within the Max Take-Off weight.? It also had plenty of luggage space spread around the helicopter with plenty of newer technology components like hingeless main and tail rotor systems. ??I was an instructor for a while and became very comfortable with the aircraft’s capability, manoeuvrability and performance.

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After that I went to test pilot school and was exposed to a wide range of different aircraft types, including the AS355N, which we used to evaluate what was then quite a new autopilot system alongside a full single pilot IFR kit.? The two aircraft then evaporated from my flying career for nearly 10 years, first when I completed a short helicopter refresher course on the Squirrel in 2009 and then when I did my CPL Instrument Rating on the AS355F1 in 2014.

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During that latter course I did some limited single engine failure activity as part of the training, but since it was a rather cool April, I really didn’t pick up some of the aircrafts real limitations over what was a rather pressurised 3-day type rating course. ?We were then into the rather more intense Single Pilot Instrument Rating course where the previous autopilot didn’t quite seem so good (probably because I had lots of time on the Merlin).? It was then over to the EC135, for which there never was a single engine comparison to deal with.

My next encounter came when I was lucky enough to be offered a Type Rating renewal on the AS355F1 as part of a business activity we were involved with at the time.?

It was early June and, whilst it wasn’t hot, temperatures had begun to climb into the low 20s centigrade.? Because we were going to be operating in a commercial activity the big focus was how I was going to be deal with the Performance Class 1 regulations and therefore how to best use the aircraft’s Category A performance capabilities.? ?Although I initially used the Flight Manual charts to try and work this out, the company helpfully provided us with an iPad app which certainly took a lot of the pain of pre-flight planning away.? What it didn’t do was make the aircraft perform any better and it became rapidly clear that I would have to trade away fuel and/or payload in order to deal with the aircrafts (lack of) performance in the weather conditions.

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With planning complete it was time to put both the aircraft’s performance and my ability to manually fly the one engine inoperative techniques to the test.? We taxied out to the southern, helicopter side of Thruxton aerodrome, and began to run through each of the Category A flight profiles for the aircraft.? It rapidly became clear why we had done all the number crunching on the ground as we failed to leap effortlessly into the air each time the instructor cut back the power on one of the engines.? It all became a game of balancing where the collective lever was positioned against the pitch attitude as we forced the underpowered helicopter skyward at vertical speeds barely above 100-200 feet per minute trying to accelerate to the best climb speed whilst avoiding houses and other obstructions in our way. ??All the while I was thinking why is this so different to my experiences in the single engine aircraft and why was this twin engine development of the original AS350 so popular up until the early 2000s.? Having successfully gained my type rating I started to dig into the history surrounding the AS355, where it had come from and why it seemed really not worth having the second engine.

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Originally developed by Aérospatiale, now part of Airbus Helicopters, the AS355 series, including the AS355F1, was introduced in the early 1980s as the twin engine version of ?the AS350 écureuil which was certified in the late1970se . The primary reason for the introduction of the second engine was a regulatory one since at that time regulations concerning commercial helicopter flights in Europe, and other parts of the world, had started to include the need for twin engine helicopters when operating over cities or other similarly challenging areas for helicopters.? Aérospatiale’s answer was to change out the powerful Arriel engine with 681 horsepower in the single and put in its place two two Allison 250-C20F turboshaft engines, each producing 419 horsepower. ??

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Now one might think that this extra 157 horsepower would make all the difference but the problem in the AS350/AS355F1 aircraft is that the difference in mass by adding the extra engine is approximately 130 kilograms.? As a result most of the benefit of the second engine becomes lost since the rest of the helicopters’ aerodynamic components remain unchanged and from a pure payload perspective there isn’t really much to gain.? In addition that second engine is also eating more fuel (and maintenance money)than just one so there are economic factors at play here too.

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Therefore the AS355F1 is useful if you need to fly to commercial standards (nominally with passengers) into heliports or perhaps as a parapublic aircraft but not much if you are flying cargo around the wilderness or conducting survey operations that really do not need two engines.? This is why the AS350 has sold 7000 up to 2022 and the AS355 around 4000 albeit that other models have come along which have taken the latter’s place in the twin engine light helicopter space.? In fact, the AS350 has been further developed into the H125 by Airbus Helicopters and remains the world altitude record holder from a flight in 2002 by the famous film pilot Fred North who reached 42,500 ft.

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Of course it hasn’t been just Airbus who developed twin helicopters from their original single design.? Bell helicopters did it with the Bell 212 coming from the Bell 205, and the Twin Ranger from the JetRanger.? Perhaps the most interesting though is the reverse change that Leonardo Helicopters have completed with the AW109, where they developed the aircraft into the single engine AW119K and then proceeded to make this aircraft a major success by selling it to the US Navy.? Perhaps with this explanation it is easier to understand why.

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