Low cost, low noise and low vibration led Eurocopter's design priorities for its AS350B3


Eurocopter's AS350B3 has a long pedigree. It is the latest version of a helicopter intended to succeed the popular Alouette and Lama, both of which have established a reputation for high reliability and good lifting capability, particularly at altitudes which few other helicopters could achieve.

Apart from retaining its predecessors' good performance, emphasis in the design of the AS350 was placed on low direct operating costs (DOC), low noise and low vibration levels. The latest B3 version competes with Agusta's new A119 and Bell's Model 407.

It was decided to develop the AS350 in 1973. The first prototype flew the following year with a Textron Lycoming turboshaft engine, followed in 1975 by a Turboméca Arriel-powered model. The Lycoming (now AlliedSignal) version was marketed only in North America, where it is known as the AStar. The Arriel-powered version is known elsewhere as the Ecureuil, or Squirrel.

French certification of the AS350 came in 1977 and deliveries began the following year. The aircraft has been developed steadily, with various upgrades. More than 2,000 have been delivered to over 800 customers, 90% of them civil, and 6.5 million flying hours have been accumulated.

Flight International flew the latest variant, the AS350B3, at Eurocopter's plant in Marignane, France. The main improvements, compared to its immediate predecessor, the B2, are a more powerful powerplant (+15%), full authority digital engine control (FADEC) and liquid crystal display (LCD) instrumentation.

The main gearbox is upgraded to take the additional engine power available and an improved tail rotor, the same type as that fitted to the twin-engined AS355, is installed. The gearboxes can run without oil for 45min.

While the maximum weight with an internal load remains the same as that of the B2 - 2,250kg (4,950lb) - weight with external load has increased from 2,500kg to 2,800kg, and the maximum load on the hook is up from 1,160kg to 1,400kg. In this area, the B3 outperforms its twin-engined counterpart.

The aircraft will lift 1,400kg and nearly 1h's fuel up to 1,000ft (300m) on a standard day - an impressive weight for such a small helicopter. The maximum weight with external load of 2,800kg is possible at up to 23íC at sea level. This lifting capacity is a result of the engine's high power and low weight, the main and tail rotor blade design and the main gearbox's ability to absorb the power available.

Hot, high and heavy

The design goal of the AS350B3 is to outshine other helicopters in its class in hot, high and heavy operations. The B3's main roles are aerial work and public transport. There are kits for emergency medical missions (there is no central upright pillar, so the whole cabin floor and left-hand cockpit floor is available for single or double stretchers with two medical attendants), and spraybars and tank for crop spraying, seeding, pest control and anti-pollution missions. This equipment can be fitted in less than 20min, says Eurocopter. With the high skid gear, there is a firefighting kit consisting of an external tank and pump which can pick up 830litre (220USgal) of water from almost any source in less than 30s, mix it with foam and drop it in 1s.

My pilot was Eurocopter test pilot Didier Delsalle. The weather was fine, with 10kt (18 km/h) of wind on an almost standard day, with the pressure and density altitudes about the same. The aircraft's centre of gravity was in the middle of its range and our weight was 6kg above the maximum take-off of 2,250kg. This extra 6kg would be burned off during start-up.

Because of additional equipment on board, the aircraft's empty weight was 62kg heavier than a normal visual-flight-rules-equipped machine. We had full fuel and 430kg of ballast to make up the rest of the weight. Many helicopters can carry only a partial payload with full fuel; the B3 can carry a full load with the highest density seating arrangement of pilot plus six passengers, provided that their average weight, with baggage, is no more than 81.8kg. The inside ground effect (IGE) hover ceiling at this weight on a standard day is 13,500ft.

Delsalle took me round the aircraft for a typical pre-flight inspection. All the relevant fluid levels are easily accessible and visible and access to the main rotor head is an easy climb up the side. I inspected the Starflex rotor head, which accommodates pitch changes through elastomeric twisting - no lubrication is required - and the three composite main rotor blades. The blade tips are high enough to be out of harm's way (2.6m above ground - and 200mm more with the optional high skid gear). The rotor can be started in up to 50kt of wind.

The Arriel 2B, also used in Eurocopter's AS365N3 and EC155 and the Sikorsky S-76C+, is installed in a fireproof bay. It delivers 630kW (850shp) take-off power for 5min and 545kW maximum continuous. The Arriel engine is modular, which means that individual sections can be removed and replaced.

The tail rotor is also composite and is all one spar. It turns fairly fast (2,086rpm), yet the aircraft is comfortably below maximum permissible noise levels. Eurocopter assures me that adequate tail rotor power is available throughout the flight envelope, including hot, high and heavy operations.

There are three separate baggage compartments, each with a tiedown net, one on each side of the fuselage and a rear hold. None holds very much - the largest is only 0.55m3 (20ft3) and the total is 0.97m3. The external power point is out of the pilot's sight and there is no cockpit warning light, so the pilot must be vigilant to ensure it has been removed before take-off. The aircraft can be started using the internal battery.

Seat layout

Inside, the standard layout is for a pilot and five passengers - or six in the high-density configuration. There is 1.65m of width for the rear seat to accommodate four passengers. Crashworthy crew seats are an option. All the seats have a shoulder harness.

With only the pilot's seat installed, there is 2.6m2 (30ft2) of floor area available for freight, with plenty of tiedowns embedded in the floor. If the high skid gear is used, there is the option of steps. Head and legroom in the cabin are adequate. Visibility all round, especially without a central pillar, is good, and an intercom system can be installed. A modified co-pilot's seat can be removed, folded and stowed in the baggage compartment, as can any stretchers.

I settled into the comfortable right-hand front seat and adjusted it fore and aft. There is no other adjustment and, unusually, the only adjustment available for the pedals is to turn them round to adjust their reach. The front doors have gas struts to hold them open.

The cyclic pitch stick has a comfortable grip. There is no trim, only built-in friction. The belly hook release button is guarded, to prevent inadvertent release of an underslung load. The collective pitch lever is also comfortable to manage and has adjustable friction.

I liked the simplicity of the instrument panel layout. Gone are the torquemeter, exhaust gas temperature gauge, gas generator tachometer, engine and transmission temperature and pressure instruments, fuel contents indicator, ammeter, voltmeter and outside air temperature gauge. These are all replaced by the dual-LCD vehicle and engine multifunction display (VEMD), supplied by Sextant Avionique and positioned in the middle of the panel so that both front-seat occupants have equal access.

A modification allows the aircraft commander to fly from the left-hand seat. This adapts the aircraft for vertical reference flying, when the pilot leans out to view the load on the end of a long line. AS350s are not suited for this role with the pilot in the right-hand seat, since the seat is too far from the edge of the floor. Eurocopter can install a floor window to facilitate right-seat vertical reference operations, but the best solution is to fly from the other seat.

The only instruments in front of the pilot are the usual airspeed indicator, altimeter, rate-of-climb, dual rotor and power turbine tachometers, compass, clock and warning panel. All red warning lights are accompanied by a "gong" alert through the headset, bringing the pilot's attention to this panel.

The VEMD shows only those parameters that the pilot needs in each phase of flight. After self-testing when the electrics are switched on to start the aircraft, it automatically goes to the start phase, then to flight and finally to shutdown at the end of the flight. The pilot can interrogate the VEMD, however, and bring up any information he requires.

The top screen is dominated by the "first limitation indicator" (FLI), a useful presentation with a large scale extending from 0 to 11, with 10 red-lined and yellow-arced. This senses the three basic power parameters of torque, compressor RPM and exhaust gas temperature, and shows that which is the highest at the time. A horn will sound if the pilot exceeds any of the take-off ratings and the exceedance will be displayed in a small window and recorded. This eliminates the need for instrument scanning or having to remember limitations; a glance will show how much power is being used and, most importantly, how much is in hand at the prevailing temperature and altitude.

There are digital readouts of these parameters on the right side of the display. They turn yellow when maximum continuous power and above is used. If one of the FLI inputs fails, the display reverts automatically to three analogue instrument presentations. The pilot can select a digital readout of the power turbine RPM. The VEMD shows remaining fuel level, fuel flow and remaining endurance. It will carry out an engine power assurance/trend analysis check and show whether the engine is above or below minimum specification.

Once the pilot has set the aircraft weight, a performance page will display maximum in-ground effect (IGE) and out of ground effect (OGE) weights for the prevailing altitude and temperature. This enables the pilot to overfly the intended landing area and see immediately from the VEMD whether he can carry out an IGE or OGE hover. This is a useful safety feature, as many pilots come to grief trying to land when, and where, they are too heavy, especially at high altitude.

The maintenance mode provides information on the various systems and any exceedances - a mini health and usage monitoring system. Plans are under way to incorporate more maintenance parameters.

A wide range of avionics is available for missions ranging from day-only visual flying to night and instrument flying (including a three-axis autopilot and global positioning system coupled to the horizontal situation indicator), with plenty of space to fit them and plenty of electrical power to run them.

With only the windows overhead in the cockpit, all-round visibility is excellent. The fuel cut-off and rotor brake can be repositioned overhead, however.

There is no need to touch the large throttle on the end of the collective lever - it remains fully open from start-up to shutdown, as the FADEC does all the engine management. The throttle is used only in the event of a FADEC malfunction, as we were to find out. I selected the fuel pump button (there are no switches) and let it run for 20s before selecting "Start". We watched the automatic start on the VEMD, which then switched to the flight display.

I came to the hover and experienced slight overcontrolling on the cyclic stick, probably caused by the rigid rotor system, but I settled down after a few seconds and the overcontrolling disappeared. I could rest my right forearm on my thigh, which helped. A glance at the VEMD showed we were using only 75% power, excellent at this maximum weight.

Crisp response

The usual hover manoeuvres of sideways and backwards flight, spot turns and out-of-wind landings showed crisp, precise rotor response, with no need for a trim facility. There are no limits laid down in the flight manual, so we went to about 45kt sideways in both directions before we approached pedal availability limits. Delsalle showed me extremely fast spot turns in both directions - I could feel the sideways g and the outside world started to blur - so there is no lack of tail rotor power. We went to about 50kt backwards - again without control problems or tucking under of the nose.

We left Marignane and, while still at nearly maximum weight, levelled out using maximum continuous power and achieved an indicated and true airspeed of 141/142kt - better than the brochure figure of 140kt. The VEMD showed a fuel flow of 186kg/h, also close to the flight manual figure of 180kg/h, and an endurance of 2h 16min. Flight was smooth with little vibration, thanks to the system of resonators (weights on arms under the cabin and an anti-vibration device on the main rotor head).

We zoomed up to 4,000ft, timing our rate of climb with the stopwatch. We got 2,100ft/min (10.6m/s), better than the flight manual figure of slightly less than 2,000ft/min. While still heavy, we went for the never-exceed speed (Vne), which, at this altitude, had reduced to 143kt from 155kt at sea level. A slight dive gave us this. There was no increase in vibration levels and 30í turns in both directions, rolling quickly from one side to the other, revealed no signs of stress.

At 60kt, I took my hands and feet off the controls. The aircraft continued with hardly any deviations from straight and level flight. A similar exercise at 80kt produced the same result. I did my usual steep turns, going to 60í of bank and rolling from one direction to the other. Delsalle took over and went to a sustained 90í bank at 45-60kt. There is no limit mentioned in the flight manual and this performance is better than that of some of the attack helicopters I have flown. Such benign and safe handling gives the pilot plenty of confidence should he inadvertently enter severe turbulence, for example.

We explored vortex ring/settling with power, that condition of helicopter flight at low speed (less than about 20kt), with power on and a descent rate in excess of 300ft/min, where the vortices around the main rotor blades destroy lift, causing blade stall, increased rate of descent and possible loss of control. The condition can often lead to an uncontrolled heavy landing.

Delsalle set up the conditions and we waited. The aircraft started to shake slightly, the heading started to oscillate and the rate of descent increased from 400ft/min to 1,100ft/min. A pilot would have to be particularly unobservant not to realise what was happening. Normal recovery is to push the stick forward to get some forward speed and leave the vortices behind. Pulling on the collective lever to reduce the rate of descent will almost always aggravate the situation and increase the vortices. The AS350 rotor disc is so forgiving, however, that Delsalle was able to recover just by raising the lever - a good safety feature.

I then asked Delsalle to raise and lower the lever as quickly as he dared to check engine governing and rotor droop. He did so brutally. I saw no more than 1% power turbine/rotor RPM deviation. There was no attitude change, which can sometimes occur.

I switched off the generator. The warning was obvious and the VEMD showed us what was happening. Delsalle switched off the hydraulic system, which powers the collective, cyclic and tail rotor pitch actuators and absorbs all the feedback forces. There is an accumulator which retains some pressure, but, as you move the controls, the pressure dissipates.

I tried not to move anything, but we lost all pressure about 60s later. I had to contain the feedback forces. Those from the tail rotor and collective pitch systems are comfortable. Those from the cyclic pitch system are not - the stick wants to go right and back quite hard. I had no trouble restraining the rearwards force since I could lock my forearm between my body and the grip. The sideways force was another matter, requiring a different set of muscles.

I slowed down to the recommended speed of 60-70kt and the forces reduced slightly. I set up a flat, decelerating approach to the runway, using minimum control inputs. We came to an acceptable hover and I landed safely. I would not want to fly the aircraft for long in this condition.

With the hydraulics restored, a steep approach to a target on the runway was straightforward, given the excellent visibility. A vertical climb to an OGE hover 100ft above the hover point and a descent back down again was similarly easy.

Manual throttle

Delsalle selected manual throttle and I used the thick twistgrip on the end of the lever. Subsequent handling took me back to my piston engine days: raise the lever, open the throttle; lower the lever, close the throttle. I made an approach to the hover and landed. I was going to do a go-around from the bottom of the approach, always a testing time when using a manual throttle, but the handling was so benign that I did not bother.

The usual criteria applied of being careful after touching down, to ensure that the dual tachometer (turbine and rotor RPM) is monitored to prevent an overspeed as the lever is bottomed. The facility is simple also from an instructor's point of view, since, if things are going wrong, he can restore the FADEC by pressing just one button.

I asked Delsalle to simulate a tail rotor emergency - a stuck, fixed-pitch situation. In the cruise, we took our feet off the pedals and accepted whatever pitch there was on the tail rotor. There is a test button which will reduce tail rotor pitch if it fails in a high pitch situation.

Cruise flight to a suitable area to carry out a running landing was not a problem. On final approach, we sideslipped down and, at the bottom, adjusted our speed so that the power required matched the torque reaction, keeping the aircraft heading straight. We touched down at about 20kt, still with our feet on the floor, then gritted our teeth as the lever was lowered and the aircraft started to swing to the right. We slithered across the grass and stopped 90° from our touchdown heading, a satisfactory result.

The flight manual covers all tail rotor failures - complicated and potentially hazardous malfunctions - in only five paragraphs, which I consider insufficient to give the pilot a good understanding of the many factors involved and how to deal with them.

Finally, we did some autorotations and engine-off landings. As an introduction, in the cruise configuration, the engine was selected to ground idle and I started the stopwatch. It was 4s before rotor RPM had drooped to minimum and the lever was lowered. This allows even the slowest pilot to analyse what has happened and enter autorotation.

Straight-in autorotations to the ground were entirely satisfactory and Delsalle let me try some. Control of the rotor speed within the generous range of 320-430RPM was easy, the rate of descent quite high at 1,800ft/min, but there was plenty of bite in the flare, giving me time to adjust to the desired touchdown speed, giving us some extra rotor RPM for the actual touchdown. In the case of an aborted touchdown, the FADEC will allow a rapid engine acceleration to restore the power, a comforting thought for student and instructor.

We positioned ourselves in the hover at the top of what is colloquially called the "dead man's curve". This is the region of altitude versus speed where there is no guarantee that an autorotation to the ground will be successful. Ours was. It involved a steep dive to pick up as much speed as possible before the flare. We touched down with full control, but with the rotor speed down to 250rpm, which showed the forgiving design of the main rotor. The dead man's curve is quite big in this aircraft, rising to 500ft with zero airspeed and extending out to 50kt at 100ft.

The aircraft has enough engine, main and tail rotor power to be flown quite safely at higher weights, but, to give the average pilot acceptable handling qualities in autorotation, the maximum weight is restricted to 2,250kg.

Finally, after all the items on my flight test list were accomplished, I handed over to Delsalle to show me the "extras" - manoeuvres not mentioned in any manuals. He pulled up the nose to about 45° and pushed it over quite hard. I could feel the low g. He then did several wingovers from a true vertical position.

On shutdown, the VEMD was interrogated to take note of the number of flights to date, the duration of our flight, the number of engine cycles and any maintenance messages such as exceedances or malfunctions. The system will retain this information for 32 flights and will record 256 failures and exceedances.


Pilots will be delighted with Eurocopter's AS350B3. It has enough engine, main and tail rotor power to operate safely, without concern, to the edges of the flight envelope. It is comfortable, with room for all the gear that pilots want to take with them, and simple to operate, especially with the VEMD. The pilot will appreciate the crisp handling, but not the feedback forces should he lose the hydraulics. Like me, he or she will want to land quickly.

Passengers will enjoy the comfort, and low noise and vibration levels. Furthermore, they will reach their destination quickly.

Operators will appreciate the safety features and track record. Their other major concern is cost. A standard aircraft costs $1.255 million, with direct operating costs of $184 per flying hour, excluding labour and fuel. With the fuel efficiency of the FADEC, they can expect to get nearly 2km/4.5litres, giving a range of 686km. There are only seven lifed components, maintenance of the rest being on-condition. The number of maintenance manhours required for every flight hour is between 0.5 and 0.7.

The AS350B3 has been sold to a variety of buyers for different uses, including operators in Canada and Scandinavia for low-altitude aerial work, companies in Japan and South America for high-altitude missions and police forces. I expect the AS350B3 to take Eurocopter well into the next century.

Source: Flight International