Flight International is first to flight test the Bell/Agusta Aerospace AB139, which offers impressive twin-turbine performance. Will it excite a market poised to re-equip?
A new medium twin-turbine helicopter does not come along every day and, judging by the healthy order backlog for the Bell/Agusta Aerospace AB139, the market is ready for a new entrant. The AB139 is coming to market just as its target customers - corporate, offshore and utility operators - are entering a long-overduere-equipment phase.
Starting life as the A139, a larger stablemate for Agusta's A109 light turbine twin using technology developed for the Italian manufacturer's A129 attack machine, the design was conceived as a quiet, fast and modern 12-passenger helicopter that would seat up to 17, including pilot(s), in a high-density layout. With the creation of the Bell/Agusta Aerospace (BAAC) joint venture in 1998, the helicopter became the AB139, stablemate to the BA609 civil tiltrotor.
Drawn up with input from potential customers, design goals for the AB139 included low cabin noise and vibration, high speed, good visibility and a high level of safety. Other demands were high weight and power reserves, adequate main and tail rotor power, good stability and manoeuvrability, and simplicity of operation, particularly for Category A (public transport) take-off and landings. Also desired were low direct operating costs with fewer scheduled checks and either on-condition maintenance or long overhaul intervals.
Agusta, which itself became part of the AgustaWestland joint venture in 2001, led development of the AB139. The helicopter first flew in February 2001 and Italian certification was achieved in June 2003. The first helicopter was delivered to Italian public-transport operator Elilario in December last year. US certification is expected by mid-year, and Bell Helicopter will deliver the first AB139 assembled at its Amarillo, Texas plant in the first quarter of 2006. The Italian and US assembly lines will have a combined production capacity of 50 aircraft a year, says BAAC.
Four flight-test aircraft have been produced, plus one for tie-down ground tests. Flight International was invited to evaluate the third flight-test AB139 with chief test pilot Bruno Bellucci. The helicopter was still equipped with test telemetry for the ongoing development programme, but I was able to examine a production model with a 12-passenger interior. Access to the passenger cabin is easy thanks to the low floor (there are no underfloor fuel tanks) and short undercarriage. This aircraft had an electrical retractable step. There is plenty of headroom, even for a six-footer, and the two banks of forward-facing seats have a pitch of 32in (81cm), which gives adequate leg room, while the rearward-facing bank has even more. Five of us tried the five-place seat bank: it was comfortable enough.
Payload plans
Empty weight for a typical fully equipped aircraft will be about 4,000kg (8,800lb), says Agusta. At the present maximum gross weight of 6,000kg, payload with a full 1,200kg of fuel and one pilot is 650kg. But plans are afoot to raise the maximum weight by 400kg, increasing the payload to 1,050kg. Fuel consumption is 350-400kg/h. A 400kg-capacity auxiliary fuel tank is available.
Bellucci demonstrated a typical pilot's pre-flight inspection, which proved to be precise, but easy and quick. All the relevant fluid levels are easily accessible for checking. For a more detailed inspection, access to the main gearbox, engines and main rotor hub is good, with built-in steps and retractable platforms. The voluminous baggage compartment - 2.2m3 (77.5ft3), which can be extended to 3.4m3 - has access doors on both sides of the fuselage, and there is no interference with the wide cabin doors. The panel between the baggage compartment and cabin can be removed.
The two Pratt & Whitney Canada PT6C-687C turboshafts produce 1,680shp (1,250kW) each at take-off (for 5min) power and 1,530shp maximum continuous. Single-engine power output is 1,870shp for 2.5min and 1,680shp maximum continuous. There are generous transient limits. Usually helicopter main gearboxes are unable to accept the maximum engine output, but on the AB139 the limits are identical, so the pilot does not have to guard against overtorquing the gearbox, although this might be difficult anyway with the helicopter's full-authority digital engine controls (FADEC).
All transients are recorded by the standard health and usage monitoring system, which records over 40 parameters. The gearbox is designed to run without oil for at least 30min, enough time to get to a proper landing site. This is one of many safety requirements manufacturers now have to comply with to achieve European and US certification. The high-mass components mounted overhead, such as the main gearbox and engines, are designed to stay in place in a crash or heavy landing and not penetrate the cabin structure. The initial impact is taken by the energy-absorbing undercarriage, then by the crashworthy subfloor. All seats and fuel tanks are crashworthy. The tanks are behind the cabin, hence the low floor.
The main rotor blades have swept tips, and all flapping, dragging and feathering movements are accommodated by one elastomeric bearing for each of the five blades so no lubrication is required at the hub. The main and tail rotors are mounted high enough to stay out of the way of passengers and others. A unique feature is the horizontal stabiliser with its upturned tips. These help to offload the tail rotor and assist with stabilisation at high speed, especially after an autopilot failure. Although Agusta still has to explore hot, high and heavy performance, the AB139 has been to 17,000ft (5,200m) density altitude and the company is confident that there will always be plenty of tail rotor power available.
Impressive hover
The rotors and engines give impressive performance. The preliminary flight manual which, at the moment goes only to 10,000ft, shows no weight restriction for an in-ground-effect twin-engine hover at maximum continuous power, nor for an out-of-ground-effect hover at standard-atmosphere outside air temperature (OAT). Single-engine hover performance is also impressive. The aircraft can hover on one engine at maximum weight at sea level using 2.5min power in ground effect up to an OAT of 40°C. This means that, if the aircraft is engaged in a rescue operation winching someone up, it can suffer an engine failure and remain in the hover, even at maximum weight. My own unofficial interpolation of the graph suggests that the AB139 will probably do likewise at the increased weight to 6,400kg to a maximum OAT of 25°C (77°F).
OAT was 25°C for our test flight, so we used the aircraft's air conditioning system. Windspeed was 10kt (18km/h), density altitude 1,500ft, and the centre-of-gravity position was mid-aft. At my request, the aircraft was at its 6,000kg maximum gross weight. As well as Bellucci in the left-hand seat, there was a photographer in the jump seat.
Initial impressions of the cockpit are of its user friendly and ergonomically pleasing design. The production aircraft will have plenty of space to install pockets and other receptacles for stowing the pilots' gear. This is important, as the AB139 will eventually be certificated for single-pilot instrument flight rules. Honeywell Primus Epic integrated avionics are standard. Our prototype had three 150 x 205mm (6 x 8in) multifunction displays (MFD), but production aircraft will be fitted as standard with the four displays required for US certification. There was bright sunshine throughout our flight, but there was no difficulty in reading the screens.
Extra console space
Because all essential information is condensed on to the MFDs there is plenty of instrument panel and centre console space for extras. Honeywell's Epic suite proved user friendly and multi-talented. Instead of emergency get-you-home instruments at the top centre of the instrument panel, such as conventional airspeed indicator, altimeter and attitude indicator, there is a small, all-encompassing attitude director indicator just outside the pilot's usual line of sight. The production aircraft I examined had one for each pilot. This instrument even has an instrument landing system presentation, and its information can be transferred to one of the MFDs.
The overhead console is kept intentionally simple - a single pilot in instrument conditions does not want to spend a lot of time looking up - just an electrics panel, rotor brake lever and engine speed selects.
The first engine start with the FADEC system is straightforward - switch fuel and pumps on, select ground or flight idle, then sit back and watch; from ground idle move to flight idle, watch the rotor spin up and then start the second engine. The flight manual has a quick start procedure. Our engine starts were slow and cool.
Normally, the test pilot would do the first take-off, or at least taxi from the parking spot outside the hangar and away from other expensive helicopters parked nearby, before handing over control. Bellucci had such confidence in his pilot-friendly aircraft that he invited me to take the helicopter up into the hover from where we were parked and taxi out to the clear area. His confidence was rewarded - I came to a neat hover, paused to check the power required against the power remaining, before turning and hover taxiing to a clear area.
The presentation of power required and power remaining parameters are unique to the AB139. The designers have integrated the three basic criteria of torque, compressor speed and hot-section temperature and provided the pilot with one single power index indicator (PI). He need not be concerned which parameter is uppermost, although a glance at the PI will show him. At 1,500ft density altitude and 6,000kg, we had plenty of power in hand.
While still at high weight, I asked Bellucci to put us on one engine. We stayed in the fairly high hover using only maximum continuous power. The MFD presentations reverted to single engine and we had the appropriate alerts from the sophisticated warning system, also developed by Honeywell. At least one flight-manual hover performance graph had been proven.
Sideways and backwards speed limits are listed in the flight manual as 45kt. In reality, says Bellucci, they are much higher. He allowed me to go swiftly in all directions. At no time did I reach any control limits. I handed over control to Bellucci to do likewise while I checked how much tail rotor pitch was being used, which was shown on test equipment in the cockpit, and how much engine power. We stayed well within all limits. This is encouraging for offshore pilots, who often have to land and take off in strong cross- or even tailwinds.
After my modest-rate 360° turns on the spot, Bellucci gave it some boot. Again, no limits were approached, and all this at just under 6,000kg. Landings and take-offs out of wind in all directions were benign even with our aft centre of gravity (CG), although there was only 10kt on wind blowing. A production aircraft should not have such an aft CG.
Maximum power
I transitioned into forward flight, levelled off and pulled maximum continuous power, straight and level. The airspeed indicator (ASI) crept up to 155kt. We were at 3,500ft density altitude, which gave us a true airspeed of 163kt. So the AB139 is quick. We achieved the never-exceed speed (VNE) of 167kt in level flight - it is unusual for a helicopter to be able to do this without diving. The ASI went to yellow and a computerised female voice called "Airspeed! Airspeed!" Vibration levels are benign.
The aircraft has two anti-vibration blocks under the floor, one under my seat position, which is the worst area for vibration, says Bellucci. Banks to 30° in both directions caused no increase. We were able to maintain 150kt during steep turns in both directions. Outside visibility around the thin window struts was adequate. Bellucci demonstrated sustained 90° turns, still with plenty of power in hand.
At 150kt straight and level, one engine was failed at my request with us doing nothing but watching. The rotor speed drooped 2% for a few seconds, then restored. Speed remained 150kt. We were at 3,000ft density altitude.
Bellucci says vortex ring state, or settling with power, has not been met on the AB139, and so has not been investigated. This is the condition that has caused many a crash when, at low airspeed but fairly high rate of descent - 400ft/min (2.03m/s) or above - with some collective pitch applied, the disturbed airflow over the main rotor causes it to partially or even fully stall. I would feel more comfortable as a purchaser of the AB139 if this condition was explored fully and the results published in the flight manual, since many pilots forget the entry conditions soon after leaving flight school.
I invited Bellucci to move the collective pitch lever as fast as he dare from fully down to maximum power in level flight. There was very little rotor RPM change thanks to the efficient FADECs. There was some fuselage roll, but nothing significant.
In the cruise, I switched off one of the two DC generators. Unusually, there are no AC generators. We got a good alert from the warning system, and the MFD showed us with a simple line diagram what we had lost (only some insignificant bus bars) and the revised electrical power route. There have been some accidents when an engine fire drill has been carried out improperly because of poor design, but a check of the AB139's fire warning system and procedure showed them to be virtually foolproof.
Our experimental test aircraft did not have the full four-axis digital autopilot installed. Under development by Honeywell, this will allow the pilot to program the autopilot and sit back and watch it fly the aircraft, and even perform an instrument approach or auto-hover. However, I was able to fly the AB139 "raw", disengaging all the stabilisation in straight and level flight at 150kt. Although I was relaxed on the controls, the aircraft waffled in roll slightly. This is not unusual with modern helicopters, and was evident in the AgustaWestland EH101, which Flight International has also flown. Manufacturers call it pilot induced oscillation (PIO). I went into gentle turns, changing direction frequently. As with the EH101, the PIO then disappeared. When I rolled out to straight and level, the PIO had gone. I do not know why. I would feel capable, after some practice, of flying the aircraft safely on instruments and returning to base or diverting safely in this condition. Later, on returning to Agusta's base, I did an approach, hover and spot landing with no stabilisation. With a little practice, I would be confident enough to land on an offshore platform with no stabilisation.
The aircraft controls have conventional mechanical runs, but do require hydraulic power to operate. There are two totally independent systems for all three controls (cyclic, collective and pedals). There are no restrictions to the flight envelope in the event of a single system failure and no difference of control feel. In the event of a system failure, the MFD will show the pilot a simple line diagram of the situation. He need take no action, apart from maybe modifying the flight plan.
We next explored a FADEC failure. Bellucci failed the control system on one engine. The warnings were explicit. The engine froze at that power level, while the other engine controlled rotor RPM. There is a conveniently located trim switch on the lever (good single-pilot IFR design) with which the pilot can trim the frozen engine to whatever power level is required. The pilot needs to remember that the engine is frozen to that power setting. Should he suddenly dump the collective pitch lever or forget during an approach, hover and landing, the frozen engine may overspeed the rotor. But this should be elementary stuff to the helicopter pilot. The important feature is that a single pilot in difficult instrument conditions will not be overloaded in the event of a FADEC failure.
Training mode
Bellucci demonstrated the FADEC training mode. By selecting either engine to "training", the pilot is faced with what appears to be a genuine engine failure. All the displays revert to single-engine configuration. The aircraft will perform as if on one engine. For example, if the pilot pulls up the collective pitch lever he will reach full single-engine power and may even droop the rotor RPM. In reality, he does droop the rotor RPM, but the good engine is only at maximum continuous power. This is so that single engine manoeuvres can be demonstrated and practised without entering contingency power levels and reducing the life of the engine.
We cruised at 150kt in straight and level flight, pulled maximum continuous power and got a rate of climb with no loss of airspeed of 600-700ft/min. This is exceptional performance for any civil helicopter. Rates of climb at VY (speed for best rate of climb) are in the order of 2,500-3,200ft/min.
The AB139 is stable in autorotation. It is almost impossible to exceed the maximum rotor RPM limit of 105%, with a transient limit of 110%. The test pilots have always to go to the limit +10% to ensure a buffer zone. They had to go to extreme positive g limits in the AB139 to exceed the published maximum RPM. Our rate of descent at the best rate of descent speed of 80kt was 2,500ft/min - an average performance. Modern helicopters with proven, reliable engines have little need to autorotate - only when there is a fire, in my experience. The FADECs gave a crisp, rapid recovery to powered flight.
Agusta has designed four Category A take-off and landing procedures:
From a helipad using a vertical climb to a take-off decision point from between 35ft and 70ft, where the pilot can decide either to land back or continue. The decision point is based only on the obstructions ahead, not the aircraft weight. This is unique in my experience. The technique can be performed at maximum gross weight. This also unique in my experience. The pilot can select the procedure and the MFD will show the relevant power indication and attitude.
A vertical take-off to between 70ft and 400ft with a reduced rejection distance. Again, the pilot selects the procedure and the MDF gives him all the cues he needs.
A rearwards take-off from a confined area. Again there are no weight limitations, only the obstacles in front. This is good only to about 75ft, where the helipad starts to disappear from view below, thus interfering with a land-back.
A normal forward speed and height gain from a runway or clear ahead area.
Follow the cues
Bellucci demonstrated the first three Cat A techniques. They were all simple, involving just following the cues. Apart from the simplicity, the other remarkable feature of the procedures is that in the event of an engine failure at any point, the technique remains the same. Just follow the cues and either land back or continue. In the event of an engine failure, the pilot uses only the one gauge, the rotor RPM indicator. Power required takes care of itself automatically - just a change of presentation on the MFD. There is no need to lower the lever as in many other helicopter types - on the contrary, to get maximum power and to get the benefit of maximum rotor inertia, he can droop the rotor RPM to 90% if needed.
Bellucci failed an engine at critical points on all three procedures and either landed back or continued. There was no change in aircraft handling from twin- engine performance - he just continued the take-off watching rotor RPM or landed back.
Back at base, I carried out a steep approach onto the 4.6 x 4.6m square on the runway. I had to cock the nose slightly to the left to keep my intended landing spot in view at all times, but the technique was simple and I was able to arrive at an accurate hover. I then climbed vertically to 100ft and back down again. Visibility was adequate.
The AB139 is impressive in all the areas I evaluated. Pilots will be delighted with its user friendliness, handling characteristics, single-pilot IFR qualities, and the performance margins on the engines and main and tail rotor. Passengers will appreciate the high speed, low vibration levels, good visibility and safety features. I was unable to evaluate cabin noise in the AB139. It is a little too early for BAAC to calculate an accurate direct operating cost, but its initial estimate is about $805 per flying hour.
BAAC says a lot of interest is being shown in the AB139 by corporate and offshore operators, as well as emergency medical service (EMS), law enforcement and search-and-rescue customers. Designed to carry 12 passengers in its baseline offshore configuration, which includes automatically deployable flotation gear, liferafts and emergency locator transmitter, the AB139 is also able to carry four litters plus attendants in an EMS layout.
The AB139 competes against the long-established Sikorksy S-76 and the latest member of Eurocopter's Dauphin family, the EC155. BAAC has about 100 orders including the potential sale of 30 helicopters to the US Coast Guard. Forecasts indicate the medium twin-turbine market is poised for growth. As the most modern aircraft on the market - until its stablemate the BA609 civil tiltrotor enters service in 2007 - the AB139 stands to do well.
PETER GRAY / CASCINA COSTA DI SAMARATE, ITALY
Source: Flight International
