February 2012
Helicopters
  • Eurocopter AS355F1 twin Squirrel
  • 1 February 2012

  • By Helen Krasner

  • The AS355 has been around for a long time, and the type has been extremely popular ever since its first flight in 1979. A strong and reliable twin-engined helicopter, it has been in extensive use by organisations such as the police, emergency medical services, and charter operators; it has taken part in filming, photo sorties, and passenger carrying operations. Yet somehow I had never yet managed to fly one. So I was delighted when Michael Jupp, chief pilot of MW Helicopters, who are based at Stapleford Aerodrome in Essex, invited me to come along and see for myself what the ‘Twin Squirrel’ was like.

    MW Helicopters like the Twin Squirrel; they must do, for they have six of them, used for a variety of purposes. At the time of my visit, in April 2010, one was away load lifting somewhere in the Pennines, a second was in Holland doing some filming, and the others were in use locally. One was to visit a school in Surrey later that day for a company who fly helicopters into schools to tell the children about their use. Two others had ‘VIP fit’, that is, they were designed for comfort rather than utility, with leather seats and plenty of space for passengers and their luggage. It was clear that there wasn’t much which the Twin Squirrel couldn’t do; it is perhaps the ultimate utility helicopter, and an extremely versatile machine.

    G-XLLL, the AS355 we would be flying, was sitting on a helipad in front of the MW Helicopters’ main office. It was an ‘F1’ variant, built in 1981, and had originally been a police helicopter. My first thought was that, despite its age, it was an impressive looking and very attractive machine. It looked a little like a Gazelle, which is not that surprising, since both were Aerospatiale helicopters of around the same era. However, the Squirrel is rather more pointed in front than the elegantly curved Gazelle, and of course it has a tail rotor rather than the innovative – for that time - fenestron of its older brother. ‘LL’ also had a somewhat idiosyncratic paint scheme. At first glance it looked black, but as I got closer I could see that it was a very dark metallic purple. However, it changed colour as you walked around it, from purple to dark green and back again. I had first encountered this so-called ‘flip-flop paint’ when flying the Eurocopter EC135 some years ago; I had liked it then, and I still do.

    Of course, a colour scheme like this is not compulsory; and if buying a new Twin Squirrel you can have it any colour you like. You also have a huge amount of choice when it comes to other aspects of the helicopter. ‘Our’ Squirrel had high skids, but there is also a low skid version. There are a number of different door configurations, since people using a helicopter for photography find that sliding doors are useful, while others want more conventional doors; it all depends on the use to which you want to put the helicopter. As Michael put it: “VIPs want low skids, as it’s easier to climb into the helicopter. But some people prefer high skids, as you get more clearance from the rotor head. If you want to get a stretcher in it, sliding doors are useful, or at least one sliding door.” Similarly, you can decide how many seats you want to have. This particular AS355 was configured for six people, but a Twin Squirrel in use as an air ambulance would obviously get rid of some of those in order to have space for stretcher cases, and police helicopters would need room for all the equipment they have to carry. Walking around the helicopter, it was clear that everything would be easy to find and reach for the pre-flight check. There are large panels everywhere; so no need to shine a torch into odd corners as you have to do in some helicopters. Indeed the layout reminded me to some extent of the EC120 and EC135, and I suppose that this was to be expected, as both were to a large extent designed as more modern versions of the Squirrels. Clearly there had been something of an attitude of ‘if it ain’t broke, don’t fix it’ when it came to the new types, and in their modern aircraft Eurocopter had retained the things that worked well.

    One of the panels in the fuselage could be opened to reveal the two Allison 250 C20F turboshaft engines, placed side by side. There were also two fuel tanks, one for each engine. It is possible to swap the feed to the fuel tanks, if for instance you suspect fuel contamination, or for any other reason. The helicopter also two had fire extinguishers, and two hydraulic systems. Indeed, it appeared to have two of practically everything, so that in the event of a problem you always had a back-up.

    No ladders were needed, as a series of steps was provided so that the pilot could climb from the fuselage up to reach the three-bladed rotor head. This was of the elastomeric ‘Starflex’ type, which I had again become acquainted with when flying the more recently produced Eurocopter helicopters. However, it was something of a surprise to find a rotor head like this on a helicopter from the 1970s. In fact, the Twin Squirrel was the first aircraft ever to have a main rotor of this design, in which each rotor blade is squeezed between two flanges, enabling flexibility in all directions, but without the hinges necessary in conventional fully articulated rotor heads. The Twin Squirrel was definitely ahead of its time in this respect, although Aerospatiale used this design on the next variant of the AS350 ‘Single Squirrel’, then on their AS365 ‘Dauphin’, and later of course Eurocopter incorporated it in a number of their helicopters.

    Continuing my walk-around, I found that another couple of panels opened to reveal a certain amount of storage space for bags or suitcases, though less than in some more modern helicopters. Then came the tail rotor, which seemed extremely low, considering that this particular machine was on very high skids. From a safety point of view, I wondered why Aerospatiale had returned to a conventional tail rotor design for this helicopter, after using the more innovative fenestron for the Gazelle. Apparently it was because the early fenestrons had not really worked that well; they were very noisy, and there had been constant rumours of problems with tail rotor authority. When I flew the Gazelle I had noticed the noise, though there had not been any apparent problems with the way the fenestron worked. However, the company had decided that on balance it made more sense to return to the more conventional and long-proven design of the tail rotor. Of course, modern fenestrons have improved and do not have any of these problems.

    While Neil the photographer arranged his kit on the wide back seat, I climbed aboard and acquainted myself with the instrument panel and controls. The front cockpit was wide and comfortable, as would be expected in a helicopter of this type. Being shorter than average, I looked to see whether either the seat or pedals would move. In this case the seat moved forward, which enabled me to reach the pedals comfortably, but meant that the cyclic was right between my thighs, which is not ideal in terms of comfort. I prefer to have adjustable pedals, or – as in the Gazelle – to be able to move both seat and pedals to find the ideal position. However, it really wasn’t that big a deal...I’m being picky here! The four-point harness was easy to adjust, and all the other controls fell readily to hand.

    The instrument panel didn’t take a lot of studying, as it was extremely basic. The new Twin Squirrels of the latest version, the ‘NP’, have glass cockpits and instrumentation similar to the EC135, but this is not the case with the earlier models like our ‘F1’. Indeed, this looked very like a JetRanger instrument panel, but of course with dials for two engines instead of one. This is not that surprising, for the Allison engines are the same as those used in the B206. According to Michael, this is very useful for training, since for many pilots the Twin Squirrel is their first twin engined helicopter, the usual progression being R22, R44, B206 and then on to the AS355. With the same engine, all the newly converting pilot has to do is learn to start two of them rather than one.

    Michael now joined me in the cockpit. It should perhaps be mentioned that he had made it clear to me in advance that he was not rated as an instructor on the Twin Squirrel, and thus would be flying this sortie as Pilot in Command. Since each helicopter type requires a separate rating, I would be officially a passenger, although he was quite happy for me to handle the controls and see what the Twin Squirrel was like to fly.

    Start-up would be simple for any pilot used to a JetRanger, as it is almost the same. However, on this helicopter type the throttles were two large yellow handles on the overhead panel, rather than being the twist-grip type on the collective with which most pilots are familiar. The throttle was in a similar position on the Squirrel’s older brother, the Gazelle, so this again demonstrates the aircraft’s heritage. Once you have one engine up and running, you simply start the second one in the same way. Not a lot to it really.

    Taking off and moving into an open space well away from the crowded helipads, I started by investigating what ‘LL’ was like in the hover. I had heard that Twin Squirrels were difficult to hover until you became used to them, and I had asked Michael about this in advance. He had suggested that this might be because the type hovered markedly right skid low. And yes, I found that our helicopter definitely had one skid much lower than the other in the hover, almost to the point where I felt that we were going to over-balance! Of course, almost all helicopters hover with one skid lower than the other. This is caused by the position of the tail rotor with respect to the main rotor…the aerodynamics involved aren’t really relevant here. In most British and American helicopters the left skid is low, but in French helicopters like the Squirrels, the clockwise rotating rotors mean that they hover right skid low. Anyway, the pronounced sloping position of the machine meant that at first I found myself constantly moving the cyclic to the left to try to correct things; then I’d find we were moving sideways so I’d move the cyclic right, then I’d start the whole process all over again. Consequently it took me a few minutes to manage to hover the helicopter properly. But this is not that unusual when flying a new type of rotary aircraft, as all helicopter types do vary to quite a large extent. And once I was accustomed to it, spot turns, takeoffs and landings, and hovering sideways and backwards all presented no real problems, although when landing the right skid did touch the ground much earlier than I expected.

    I particularly wanted to find out about flying the Twin Squirrel in the way in which it is usually flown, and this meant doing a ‘Category A’ departure. This is the type of take-off used during public transport operations with twin engined helicopters in congested areas, the idea being that the helicopter can land safely if one engine fails during take-off. This type of departure is done by lifting into the hover, then climbing slowly backwards until the aircraft reaches 120 feet. Should an engine fail at any time during this manoeuvre, it would be possible to land back safely at your starting point rather than causing any damage to buildings or people on the ground.

    Category A departures are used by both the police and air ambulance for obvious reasons, and I had been shown this type of take-off when writing articles about both these types of operations. However, this was the first time I had tried one for myself. Basically you pick a point on the ground which you can see below you, then keep that in the same position on the windscreen as you climb and move slowly backwards. At 120 feet – the decision point - you move the cyclic forward and climb at 40 kts to 500 ft, then at 55 kts till you reach 1000 ft. The idea is that you get away from the ground as quickly and safely as possible, and while I didn’t find it that difficult, it was certainly a different departure from those I am used to. Once away from the airfield, we climbed to a safe height, which didn’t take long since the Twin Squirrel could climb at around 2,000 ft per minute. However, this was hardly surprising since we only had three people, in an aircraft designed for six or for carrying lots of gear. Leveling off, we soon reached the expected 115 kt cruise speed…but it wasn’t a comfortable ride. The helicopter really seemed to shake a lot, although Michael wasn’t sure whether it was indicative of the type, or simply a blade tracking problem with our particular machine. Either way, it wasn’t that big a deal, but it certainly made the aircraft feel more like the utility machine it often is, rather than one designed for long distance passenger carrying operations. Although with three hours endurance, it could certainly be used for that as well so long as you didn’t expect a smooth ride. However, this roughness was probably my only criticism. In terms of flying, the helicopter was comfortable and easy to handle – climbing, descending, and turns all required little effort, and you could easily take your hand off the collective, and even – if you trimmed it a little – off the cyclic for short periods. There was no autopilot, but the helicopter didn’t really need one. It was not particularly exciting to fly, but it was not designed for this purpose; it is supposed to reduce pilot workload and be useful, and in that respect I could soon see that it would work well.

    Still aiming to use the helicopter for the purposes for which it was intended, we flew towards Brentford, then simulated a photo sortie by coming to a high hover over the edge of the town, as photographers so often expect pilots to do. Bringing the airspeed back almost to zero, we had plenty of power and the helicopter was steady as a rock; in fact, far steadier than in the cruise. There would definitely be no problem using it for this sort of work.

    Next, I tried a Category A arrival. Picking a field, you make a normal approach down to 120 ft, then come slowly forward and down to a high hover. Again, the idea is that in the event of engine failure below 120 ft you could land on your designated area, while if above 120 ft you could climb away. But could you climb away easily on one engine? We simulated engine failure by rolling off one throttle, and at 80% torque we were still flying along happily, and were able to climb at 500 ft per minute. Admittedly we were very light, but after single engine failure you are permitted to use 100% torque, and 112% for five minutes. That ought to be enough to get even a heavily laden helicopter out of trouble and safely on the ground, but just to be sure, Category A regulations limit the take-off weight to 200 kg less than the normal permitted maximum weight.

    It was now time to go back to the airfield. We hadn’t tried autorotations or simulated hydraulics failure, but being a twin engined aircraft with two hydraulics systems, you really didn’t need to. I made a normal approach to the field, then hover-taxied to the helipad back at MW Helicopters, not for the first time marveling at the fact that new types of helicopters behave so much better after I’ve been flying them for half an hour! The trouble I had had with hovering had all but disappeared, and I felt quite at home in the Twin Squirrel. I can see why so many of them are still in use, especially as even the new models cost substantially less than the EC135, which is often seen as the AS355’s replacement.

    That being the case, why are so many police units and air ambulance operations flying the EC135, and replacing their Squirrels with them? It is partly because the EC135 has more room, and both these types of operations require space – for sophisticated kit for crime busting in the first instance, and for medical paraphernalia and patients in the second. Certainly the EC135 is a great machine and very necessary for certain types of flying. But so is the Twin Squirrel. Both have their place, and I can now see why so many old AS355s are still flying, and why Eurocopter is still producing new ones. The Twin Squirrel is a great aircraft, and I feel privileged to have flown it.

    HISTORY

    In the early 1970s, Aerospatiale in France began working on a new light utility helicopter which was intended to replace their popular Alouette series. The result was the AS350, with the first prototype flying on 27 June, 1974. Initial production of the AS350B ‘Ecureuil’ began in late 1977, with the first deliveries the following year. The new helicopter was powered by a Turbomeca Arriel 1B turboshaft engine, and was offered in a variety of different fits suitable for applications such as police work and emergency medical services. A number of variants followed, with different engines and improved rotor blade designs; these increased performance, particularly in ‘hot and high’ conditions. As popularity of the new helicopter grew, the British armed forces ordered a special modification of the AS350 B2 design for military training. They naturally translated the French name ‘Ecureuil’ into English, so that the helicopter became known in the UK as the ‘Squirrel’. As early as 1978, work began on a twin engine version, the AS355E. The idea was that two engines would provide greater lift capacity and increased safety. Initial flight was on 28 September 1979, and this early version was powered by two Allison 250-C20 turboshaft engines. In North America the AS355 became the ‘Twinstar’ and in the UK it was known as the ‘Twin Squirrel’. The first prototype was followed by the AS355F, which featured new rotor blades with a wider chord and an innovative elastomeric ‘Starflex’ head.

    The AS355F1 and F2 followed, each version with minor but significant improvements. In 1992 the AS355N was introduced. This had two Turbomeca Arrius 1A engines, and a Full Authority Digital Engine Control (FADEC) system; it also had better single engine performance. This was followed by the AS355NP, which was similar, but with an improved gearbox. This is the current version, and it is still in production.

    In 1992 Aerospatiale had merged with Messerschmitt-Boelkow-Blohm to form Eurocopter, which would still later become part of the European Aerospace and Defence Systems (EADS) group. As a result, the Squirrels are now part of the Eurocopter range of helicopters, despite their ‘AS’ designations.

    As an efficient utility helicopter, the AS355 Twin Squirrel continues to be used for a variety of purposes, including passenger carrying, aerial work, sling load operations, police work, news gathering, photo sorties, emergency medical work etc. It is well-known as a flexible, reliable, and reasonably economical helicopter, and it offers good one-engine performance. Despite more modern helicopters having been produced, the AS355 series have proved their worth and thus are still flying worldwide, many operators having seen no reason to replace something which works so well.