AB139: first flight impressions

 - March 28, 2007, 6:39 AM

fter flying the Bell/Agusta AB139, it is easy to see why Amedeo Caporaletti, president of Agusta and CEO of AgustaWestland, believes that this helicopter sets new standards for medium twins. The 13,227-pound-mtow AB139 meets the stringent standards imposed by both the European JARs and FAR Part 29, including all amendments.

A collaborative program between Agusta and Bell Helicopter, the AB139 was also developed with risk-sharing partners Pratt & Whitney Canada, Honeywell, GKN Westland Aerospace, PZL Swidnik, Liebherr and Kawasaki. Built with state-of-the-art components based on proven technologies, the helicopter was designed, tested and certified in Italy in only 42 months.

Designed to adapt to a wide range of missions, passenger seating, cargo and fuel capacity, the AB139’s 282-cu-ft cabin has a flat, unobstructed floor. The 120-cu-ft, flat-floor baggage compartment is accessible from the cabin for maximum flexibility. To put the cabin size in perspective, with standard fuel the helicopter’s five rotor blades could easily be removed and stored inside the cabin/baggage area without modification.

The AB139’s primary structure is made of aluminum alloy, with sections such as the cockpit constructed of composite materials for weight savings. Its forward fuselage contains the instrument panels, pilot and copilot adjustable crashworthy seats– which meet the requirements of JAR/ FAR 29.562 and 29.785–flight controls, cockpit doors and steps. The central fuselage includes the cabin and rear fuselage.

The cabin section holds up to 15 passenger seats and/ or cargo, the fuel tank housing, the main gearbox and engine attachments and main landing gear attachments and housing. It includes two full-size (66 inches wide by 53 inches high) sliding doors, fixed steps for easy cabin entry/exit and maintenance steps for easy access to the systems on the upper deck.

The cabin has six pop-out windows that help ensure a quick evacuation of the helicopter in case of an emergency. The standard seating arrangement of the AB139 includes three rows of four seats, and a high-density 15-seat configuration is also available with three rows of five seats each. The cabin is contiguous with the large baggage compartment, which is accessible from a left outer door (a baggage door on the right side is optional). The baggage compartment is also accessible from inside the passenger cabin when the optional auxiliary fuel tank is not installed.

The tricycle energy-absorbing landing gear has two hydraulically actuated brakes on the main landing gear, and the nosewheel swivels 90 degrees and features a centering lock.

Drive System

The drive system, which consists of a main gearbox (MGB) and a tail-rotor drive system, has provisions for the optional installation of health-monitoring and diagnostic system sensors (HUMS). All of the drive-system gearboxes are able to operate for at least 30 minutes after complete loss of oil.

The main gearbox is mounted on the main cabin by means of four struts and an anti-torque device, and is driven by the two engines at 21,000 rpm. It has three stages of reduction: one spiral bevel gear as first stage; a spiral collector stage; and an epicyclical reduction system as last stage. The main gearbox includes its own autonomous oil lubrication system. It also provides the attachment points for a hydraulically actuated rotor brake, coaxial with the tail-rotor drive output.

Additionally, it drives three hydraulic pumps, has two spare pads for dual AC generators, a fan for main gearbox oil cooling and a spare for the twin compressor unit of the air-conditioning system.

The main gearbox is provided with three chip detectors/debris collectors with burning capability. The tail-rotor drive system consists of two driveshafts turned by the main gearbox, an intermediate gearbox, an interconnecting shaft and a tail gearbox. Both gearboxes have four isolated attachment points, are oil-splash lubricated and have a sight gauge and one magnetic drain plug/chip detector.

Rotor Systems

One of the major hazards associated with rotorcraft is the possibility of the blades contacting objects or people on the ground. The AB139’s main rotor tip path plane is almost 10 feet high at flat pitch, with the tail rotor sitting at a comfortable 7.5 feet off the ground. Its main rotor is a five-blade, completely articulated system consisting of a main rotor head, rotating controls and blades.

The main rotor head consists of hub, elastomeric bearings, tension links, dampers, droop stops, pitch change levers and blade connection bolts. Articulation hinges are provided by the elastomeric spherical bearings, which transmit the blade loads while allowing the flap, lead-lag and pitch motions. The rotating controls are obtained by means of a rotating and a stationary swash-plate separated by grease-lubricated ball bearings. The stationary swash-plate is connected to the main rotor servos and is able to slide along the upper gearbox, while the rotating swash-plate case is connected to each blade by means of an adjustable track rod, and to the rotor head by means of two scissor links.

Each blade has a completely composite structure, with a fiberglass epoxy spar and a parabolic tip. The leading edges are protected by a stainless-steel erosion shield, and the trailing edges are of continuous carbon-fiber/Nomex construction. The rotor blades are protected against lightning damage by a grounding system connected from the tip to the root of each blade.

The tail rotor has a four-blade articulated rotor, and the leading edges of the composite blades are protected by a metallic strip. The hub is metallic with elastomeric bearings carrying the centrifugal loads, while providing for pitch, flap and drag blade motions with a damper for each blade.

Rotating controls are internal to the mast and consist of a control rod connected on one side to the yaw control lever and on the other to a four-arm pitch change lever, which in turn is connected to the blades by four track rods. Interestingly, the tail rotor sits atop the vertical fin, with the upper driveshaft inside the vertical fin instead of attached on the outside of the structure. This design, in addition to saving weight, greatly reduces the stress concentrations in the fin root section.

The tail rotor is also canted, allowing a shorter tail rotor mast. Because there is a vertical component to the tail rotor thrust, it makes the helicopter more efficient at low speeds; and as speed increases and the fin becomes more effective, the vertical component is phased out.

Flight Controls and Hydraulic Systems

The control system is operated by the pilot, with dual copilot controls in a side-by-side cockpit. The hydraulic system is divided into four systems and subsystems: two flight-control systems, a wheel brake and a rotor brake. The flight-control hydraulic system has two separated and redundant circuits, supplied by independent hydraulic pumps. Each of the two circuits is capable of providing the power to the helicopter’s flight control and auto stabilization functions.

On the second hydraulic circuit, the pressure line going to the tail-rotor actuator has an isolation valve, the T/RSOV (tail-rotor shutoff valve). This valve closes automatically when the fluid in the circuit reservoir reaches the minimum level.

The wheel brakes are an independent hydraulic system, operated and controlled by the pilot or copilot. The system uses a toe-operated master cylinder for each mainwheel, providing differential braking. The rotor-brake hydraulic actuation system is separated and self-contained, and it is operated and controlled by the pilot via a control lever installed in the cockpit upper panel. The brake works only when the engines are off and weight is on the wheels. This safety device prevents unintentional activation of the rotor brake in flight.

Fuel System

Instead of being positioned on the bottom of the aircraft underneath the passengers, the two main fuel cells are wrapped around the baggage compartment behind the main cabin and are crash resistant as per JAR/FAR 29.952. Each tank contains booster pumps, engine feeder and a fuel/water drain valve. Total usable fuel capacity is 2,756 pounds.

Two boost pumps guarantee operation of both engines supplied by one tank in cross-feed mode, and the cross-feed valve opens automatically if one pump fails. The fuel lines are fitted with frangible self-sealing couplings. An internal 882-pound-capacity auxiliary fuel tank can be placed in the baggage compartment.

Electrical System

Two 30-volt, 300-amp generators provide power and are controlled by independent generator control units. Each generator is connected to its main distribution bus and feeds the non-essential busses. The connections between these busses are achieved through relays that open during engine start to reduce the load on the generators.

In case of a generator failure, the non-essential busses are automatically disconnected, but all the primary and emergency loads are still powered by the remaining generator. In case of dual generator failure, all the emergency loads are still powered by the main and auxiliary batteries. During ground operations with the rotor not turning, electrical power can be provided by the ground generator through the external power receptacle. A DC ground connector near the fueling receptacle is also provided for an external electric fuel pump.

Powerplant and Avionics

An aircraft’s success is sometimes determined by its powerplants. In the case of the AB139, one of the most reliable and powerful engines was chosen–the Pratt & Whitney Canada PT6C-67C. The AB139’s two P&WC engines are capable of churning out a combined 3,358 horsepower on takeoff, or 1,872 horsepower for 30 seconds OEI. Considering that the transmission is rated for much less horsepower, 2,200 and 1,600 respectively, high-altitude performance and safety are all but guaranteed. The engines are FADEC-controlled during normal, emergency and training operations.

The electronic display system (EDS) carries out the dual function of presenting flight and navigation data, engine and systems parameters and the related caution, warning and advisory annunciations.

The EDS collects, integrates and presents data and information from systems housed in modular avionics units and from the FADEC. It can also present the output of optional mission equipment packages.

The EDS includes the two primary flight displays, multifunction display and an optional fourth display for the copilot.

Display units contain autonomous processing capabilities and are connected directly to the ASCB-D and LAN networks. An electronic standby instrument system (ESIS) provides separate and autonomous information on flight parameters for IFR applications, presenting attitude, airspeed, altitude, vertical speed, compass indication from a remote AHRS and ILS data.

It is a stand-alone system to ensure redundancy in case of failures in EDS functions provided by the Primus Epic avionics system. The Goodrich GH-3100 ESIS is a 3-ATI self-contained solid-state standby instrument featuring a color active-matrix LCD. It contains built-in air data and attitude reference sensors and provides attitude; side slip; airspeed with Vne and autorotation limits; altitude; barometric correction in HPA or inches of mercury; vertical speed; heading; ILS needles from remote VOR/ILS receiver; and failure flags.

An aural warning generator (AWG) integrated into the Primus Epic system provides a combination of both tones and synthesized voice warnings via the intercom. AWG logic is performed by a dual monitor warning system (MWS), which collects data and provides its output via ASCB-D to drive the alarms in AWG and annunciations on EDS.

A three-axis dual channel automated flight-control system is provided based on the Honeywell SPZ-7600 architecture. Both channels can perform the same functions, providing redundancy. The package comes standard with several useful features such as altitude pre-select, deceleration and integration with the FMS to fly Vnav approaches. An optional four-axis autopilot is available with search-and-rescue capabilities, such as two-phase approaches, mark on target and depart from a hover.

The radio system provides communication and navigation functions to the flight deck and is based on the modular radio cabinet (MRC) concept. Two MRCs are connected to the Primus Epic system via the ASCB-D and LAN.

Two multifunction control display units (MCDU) provide the central control for the modular radio system. They have color flat-panel displays with multiple line select buttons and dedicated function buttons. Each MCDU is capable of tuning pilot and copilot radios by either keyboard or concentric knobs. The bus structure allows safe operation of the radio system in case of multiple failures.

Reliability and Maintainability

The Primus Epic provides an integrated maintenance system, including a central maintenance computer (CMC) installed in one MAU, which can be connected to a ground remote terminal via the LAN. The CMC hosts the central maintenance function (CMF) and the airplane condition monitoring function (ACMF), and can also be used for general-purpose functions such as data loading.

The CMF provides aircraft fault recording and maintenance-access functions, while the ACMF monitors and records aircraft parameters for post-flight analysis.

The helicopter’s CMC gathers and stores reported faults in flight, and the fault history can be displayed either on the remote terminal or on the cockpit displays while on the ground.

Specifically, it monitors and records aircraft system failures (engines, transmission, hydraulic, electrics, airframe and so on); engine exceedances; time; engine low-cycle fatigue (LCF); and avionics failures.

To prevent loss of stored data, the CMC is connected to the auxiliary battery to allow proper shutdown in the case of power interruption. The system can be integrated and completed with the optional installation of HUMS, further enhancing the system’s capability. Access to the main data useful for maintenance (hours, status of equipment, anomalies encountered, diagnostic information and so on) is possible both by displaying them on the cockpit screens and by downloading the needed information on a laptop.

First Flight Impressions

My flight in the AB139 began at Agusta’s plant at Malpensa Airport at the base of the Italian Alps with test pilot Guiseppe “Pino” Lo Coco in the left seat and flight engineer Bernardino “Dino” Paggi riding shotgun. Lo Coco allowed me to fly from the pilot seat and go through the pre-start, start and runup checks.

Upon climbing into the cockpit, I felt at home. Because the Agusta engineers used existing and proven technology, the engine controls, fuel panel cyclic and collective are remarkably similar to those of the 109E Power, which I fly regularly. In fact, any pilot with previous A109E experience would feel comfortable in the larger AB139 cockpit, and any operator flying a mixed fleet would find it easy to cross train pilots between the AB139 and the A109E.

The main difference between the two models is the instrument panel. The AB139 that I flew had two PFDs, an MFD in the middle and two ESIS displays flanking the PFDs.

Starting and runup are simple and straightforward due to the FADEC–basically battery, fuel and turn the rotary switches to idle, then flight. Systems checks also require minimal ground time.

Pick-up to a hover was extremely stable. One benefit of the high, canted tail rotor is the lack of a pendulum effect, with the aircraft lifting off in a level attitude, although slightly nose high. According to Paggi, we were about 500 pounds under mtow because we had a lot of test equipment on board.

One nice feature while hovering is the collective force trim, interrupted with a trigger on the collective. With the force trim turned on there is no collective droop, a trait common to some helicopters with dual controls.

On departure and climbout from Malpensa, Lo Coco had me hold 80 knots and 100-percent torque (top of the green), with a resulting steady-state climb of 3,000 fpm. Upon level off, I discovered that the AB139 is quite fast.

Lo Coco demonstrated one interesting feature of the digital airspeed indicator. As he brought the aircraft to Vne the entire airspeed display turned red, providing an excellent visual cue to the pilot. Vibration levels were so low that it would be easy to exceed Vne in level flight in the absence of some of the other cues that would normally alert the pilot.

During a simulated engine failure in cruise, the helicopter required very little collective adjustment to maintain OEI limits. With an OAT of 16 degrees C at 5,000 feet, Lo Coco was able to maintain a 1,500-fpm climb at the top of the green.

After landing at a small airfield for some hover work, I attempted to reach the limits of sideward flight. Going to the left, I felt pretty good about attaining 40 knots steady state (off the GPS). Lo Coco and Paggi felt that I was being too timid with the aircraft, so going to the right (the more demanding side for torque) I was able to achieve 50 knots sideward while maintaining full control of the nose.

I was also interested in seeing some Cat A engine failure demonstrations. Lo Coco explained that they had not yet worked out the exact profiles. However, he demonstrated a likely profile from an airport departure. Using hover torque plus 15 percent (60/75), he began a vertical climb, rotating at 30 feet. Upon rotation he pulled one engine to idle. The aircraft needed absolutely no collective or attitude adjustment, continuing the climbout as if both engines were turning.

Considering that there is no HV curve below 7,000 feet at ISA+20, it’s no wonder that Cat A operations may be relatively unnecessary. Lo Coco said the goal will be to make Cat A engine failure recoveries as easy as possible, requiring minimal pilot action.

Twin-engine helicopter pilots know how difficult some aircraft can be to maintain OEI limits on the remaining engine after a failure. Some aircraft tend to over-temp, while others will over-speed or over-torque. Often this limiting factor changes with the season or density altitude. Agusta believes that in an emergency the pilot should concentrate on flying the aircraft instead of trying to interpret a number of different gauges.

To make this possible, there is a rotor tach on the lower right side of the PFD and a power index on the lower left. The FADEC will save the remaining engine by limiting the engine to maximum power for that day, bleeding the rotor speed down if any engine parameter is about to be exceeded. Consequently, the Agusta theory is that the rotor tach is the primary gauge when dealing with an engine failure. Since the pilot is looking out the right side of the aircraft during a single or dual engine failure, the rotor tach is always in view.

During a single engine failure, the pilot pulls as much collective as needed to deal with the emergency. If the rotor RPM remains at 100 percent, the limits will not be exceeded. If the rotor begins to bleed off, the pilot is pulling too much collective. In this situation, the pilot can accept a lower rotor speed or reduce the collective to regain the rotor. If the pilot wants to know how much available power is remaining, a quick glance at the power index will show how much is left.

One other invaluable feature to the pilots is the MFD. The MFD is the equivalent of having a ground instructor in the aircraft, complete with visual aids. While on the ground or in flight, either pilot can access the systems pages of the MFD, providing an instant graphic representation of how the systems are functioning, whether normally or in a reduced capacity. If there is a problem with a system, the graphics make it easy to diagnose the cause. Additionally, the MFD displays the engine and transmission instruments, navigation data or weather.

It is hard to believe that such a well thought-out and engineered aircraft was designed and built in only 42 months. After experiencing the aircraft in flight for only a short time, I would have to agree with Caporaletti that the AB139 will indeed be the new standard for medium-twin helicopters.