With first deliveries of HAL's Advanced Light Helicopter imminent, Flight International flew one of the prototypes



Hindustan Aeronautics (HAL) plans to certificate and deliver its first Advanced Light Helicopter (ALH) this year - although it still needs further development and more equipment, such as a health and usage monitoring system, flight data and cockpit voice recorders. The manufacturer aims for initial Indian military and civil approval of the medium twin machine, followed by US, European and other certification.

HAL will say only that its first deliveries will be to "a domestic customer". After the first 12 aircraft have been delivered, the manufacturer expects to build 24 a year, increasing to 36. The company has a letter of intent for 300 from the Indian Ministry of Defence, with the Indian army an anticipated customer. The first civilian version (priced at about $5 million) will go to India's Pawan Hans Helicopters.

HAL collaborated with MBB, now Eurocopter Deutschland, on design and development of the ALH, but the aircraft is essentially indigenous in conception and execution. The prototype flight-test programme is the largest undertaken by the company.

Two main pilots involved in the programme are HAL chief test pilot B S Choker, a graduate of the French test pilots' school, and deputy chief test pilot C D Upadhyay, a graduate of the UK's Empire Test Pilots School. Upadhyay was the pilot for my evaluation flight.

The initial ALH prototype first flew in August 1992 and three more have been manufactured. Three versions are represented: basic, air force/army and navy. Three prototypes have skid undercarriage while the fourth, destined for the navy, has retractable tricycle gear. Although the last bit of downwards collective-lever movement produces negative pitch on the main rotor, the naval version will also be fitted with a harpoon deck lock.

More than 800h of flight testing have been carried out, plus over 250h on the ground test vehicle (GTV), which is identical to the aircraft but firmly attached to the ground. The test programme calls for a total of 1,420 flight hours. The GTV was used to verify the design and testing of the components and systems.

Tests were also carried out in HAL's windtunnel and in a water channel which simulated sea-state conditions to determine flotation and ditching characteristics. A whirl tower was used for main and tail rotor testing and an environment chamber was used to test the tailboom and other parts of the fuselage. The landing gear was submitted to static and drop tests and the fuselage to breakaway tests.

The main gearbox was installed in a test rig for evaluation. Shake/vibration tests were performed on both the skid gear and wheeled undercarriage fuselages to help tune the anti-resonance isolation system (ARIS). Flight testing has covered expansion of the flight envelope, evaluation of systems, handling and control characteristics, and cold weather, high altitude, high temperature and shipboard trials.

The ALH was designed as a 4t, multi-role military helicopter, but has grown into a 5.5t aircraft suitable also for civil roles. Its standard engine is the Turboméca Arrius 2B1 (the 2B2 is on its way), but after 250h of flight trials it was decided to equip the naval version with the more powerful LHTEC CTS800-4P.

Major problems encountered during those early hours of testing halted further flying until they were solved. These problems included a leaking main gearbox, malfunctioning sliding door, high tailboom temperature, which caused a burning smell in the cabin, high vibration levels, pitch and vertical oscillations, and malfunctioning of the full authority digital engine control (FADEC). None of these problems were evident when I flew the army prototype.

Supercontingency rating

Upadhyay, who has been with the ALH programme since its beginning, showed me round the aircraft we were to fly. Main gearbox restrictions limit each engine to 620kW (830hp) for take-off and 535kW maximum continuous, whereas the Turboméca engine can deliver 740kW and 665kW, respectively, and the LHTEC engine even more - 1,000kW and 790kW, respectively.

The Arrius has a one-engine-inoperative "supercontingency" rating of 835kW, which can be used twice for 30s or once for 1min to get the pilot out of trouble. The gearbox restriction limits supercontingency power to 800kW. Afterwards, the engine has to be returned to the factory for reconditioning. Plans are afoot to increase the gearbox limits, but the present limitations mean the engines will continue to deliver full power in hot and high conditions.

We experienced a low cloud base on every day of my three-day sojourn in Bangalore, so we were unable to take the aircraft to altitude or even do an autorotation. For other technical reasons, HAL was not able to ballast the aircraft its maximum 5,500kg (12,100lb) gross weight.

The company says that only 73% of available tail rotor power is needed to hover at 17,300ft (5,280m) and 4,000kg. The ALH has been cleared to take off and land at up to 19,700ft at ISA+20ºC. At 5,500kg gross weight, the service ceiling is 17,820ft, HAL says.

The empty weight of the civil version will be about 3,100kg, a full fuel load of 1,140kg leaving 90kg each for 14 occupants. HAL hopes that the initial army version will weigh in at about 2,960kg, leaving 87.5kg each for the 16 occupants, still a satisfactory payload.

At 5,500kg and sea level ISA conditions, the aircraft reaches critical decision point during take-off at 61m and full take-off to a height of 50ft within 260m. With the LHTEC engine, full public transport category A performance is available at maximum weight up to an ambient temperature of 30ºC at sea level and up to 5,500ft at 0ºC.

According to the flight manual, an out-of-ground-effect hover is possible at maximum weight up to 7,500ft on an ISA standard day. The ALH will have an underslung load hook rated at 1,500kg, and will be able to lift this easily with a single pilot and fuel for 2.7h.

The manual gives one of the smallest height-velocity avoid curves I have seen. This is the low and slow area where a safe single-engined landing cannot be guaranteed. HAL, meanwhile, is advertising a range of 800km (430nm) or an endurance of 4h with a 20min fuel reserve.

At the time of my visit, one of the aircraft was undergoing maintenance and had all the covers off. Upadhyay showed me a typical pilot's pre-flight inspection, which was concise and covered all the basics. We then examined the aircraft in more detail.

The main gearbox contains the rotor head pitch change controls . This protects them from ballistic damage and eliminates maintenance. The gearbox has two lubrication pumps and is designed to run dry for at least 30min, although this has yet to be demonstrated.

Upadhyay pointed out the two DC starter generators and two AC alternators. When a generator fails, the other is capable of supplying all the emergency and essential loads; non-essential loads are switched off. In the event of a double generator failure, non-essential loads are shed automatically.

Similarly, the hydraulic systems are dual redundant, either one capable of giving the pilot full control power, including for the tail rotor. Loss of one system does not change the handling characteristics; the only restriction is a reduction in maximum g from +3.5 to 2.2.

HAL hopes that the redundancy in critical systems will provide high reliability, but this will be proven only when the aircraft goes into service, as will its calculations of direct operating costs and maintenance hours versus flying hours, both of which are unproven.

There are no bearings to lubricate in the main rotor head; blade movement is accommodated by elasticity of the composite material. The virtual flapping hinge position is offset by a hefty 12% from the centre of the hub. I anticipated lively handling as a result. The rigid rotor system allows a wide g range, from -1 to +3.5 at 4,000kg, reducing to -0.5 to +2.5 at 5,500kg.

The four main rotor blades, with their distinctive tip shape, are all-composite and are designed for high speed, low noise and efficiency. Most of the ALH's structure is also made of composites, which give strength with low weight, longer life and lower cost.


The four-bladed tail rotor is also of all-composite construction, a flexible beam connecting opposing blades. Again, blade control is achieved by elasticity. The tail rotor and vertical fin are slightly offset to provide a small amount of lift to help offload the tail rotor. In the event of a tail rotor pitch-change failure, HAL says it will still deliver 50% of the thrust required to hover, allowing the pilot to do a running landing at about 25kt (45km/h).

The fuel tanks, fuselage belly and undercarriage are crashworthy, as are the two front seats. The passenger seats have shoulder harnesses. The large (2.2m3/700kg) baggage compartment is at the rear, behind two clamshell doors. Four stretchers can be loaded through this opening. There is plently of clearance - 2.6m - beneath the tail rotor.

Despite the low cloud and showers, we decided to go flying anyway. HAL's operating base is at 2,900ft and an ambient temperature of 21ºC gave us a density altitude at ground level of 4,500ft, a good testing altitude from which to begin our flight. The wind was light, 10kt, and not so good for evaluation flying; I prefer stronger winds, which demand more control responses, both from aircraft and pilot.

The aircraft is cleared to hover in winds of 45kt from any direction and operations in headwinds up to 53kt have been demonstrated.

Our aircraft was the army prototype, and had all the telemetry equipment on board plus a flight engineer, but no room for ballast, so even with full fuel our weight was only 4,520kg, nearly 1,000kg short of maximum.

The pilot's seat was comfortable, with backwards and forwards adjustments. The pedals were adjustable fore and aft, too. Even so, I was unable to find a position where I could rest my right forearm on my right thigh, which can help when dealing with lively main rotors.

I considered the cyclic pitch stick a bit too long, but I was assured by HAL's chief test pilot that the pilot positions have been designed to fit 95% of human body shapes and sizes. Once I got flying, I did not have any difficulty in controlling the frisky rotor system.

The cyclic stick has built-in friction with no adjustment; the collective pitch lever also has built-in friction, but has a release button conveniently available just under its head.

The prototype has conventional analogue gauges - the usual blind flying instruments with temperature and pressure gauges for the engines and all the systems. The instruments are metrically scaled. The prototype also comes with a prodigiously long and complex malfunction/emergency procedures checklist.

All this will change, with production models having flat-panel displays presenting aircraft attitude, navigation and systems information and no temperature and pressure gauges as standard equipment. These functions will be monitored and the pilot advised only when something goes awry. The computerised system will also incorporate malfunction and emergency procedures, doing away with the hefty checklist.

There are the usual advisory, caution and warning lights on a centralised warning panel with colour changing from amber to red as the parameters get more critical. There are audio gongs and warnings for some of the more serious conditions, but HAL is going to revert to a descriptive voice warning.

I liked the ergonomically pleasing layout of the rest of the cockpit. All-round visibility is good and there is plenty of room for documents and other equipment. The ALH has a full set of radio, navigation and other essentials for instrument flight rules (IFR) operations.

The Arrius turboshaft is managed by a full-authority digital engine control (FADEC). In the event of a malfunction, the FADEC freezes the fuel flow at the point of failure, so there should be no engine run-ups or run-downs. There are two manual throttles overhead to deal with such a situation, the throttle taking over at the point of failure.

After the FADECs had self-tested, Upadhyay took me through a logical route system to set up for the start, dispensing with the cumbersome checklist. The first engine was started, with one hand on the switch ready to select stop in the event of a malfunction. The engines can be started to ground idle or flight idle.

The start was cool and slow. Starting with the rotor brake on is not approved, but the number one engine can be started with no rotor rotation and all the hydraulics and electrics brought on line. The rotor, because of its rigid characteristics, can be started in strong winds.

After the second engine start and with the rotors up to flying speed, we were ready to go. Reminding myself of the clockwise rotation of the main rotor blades, I fed in a little right pedal as I cautiously raised the lever, tripping the friction release as I did so.

My first hover with the rigid rotor was not so skittish as I had expected, and I was able to relax and check how much power we were using and how much we had available. The engine Ng limiter gauge (indicating the percentage of compressor rpm remaining for maximum speed) is to the left of the blind flying panel and above the dual torquemeter and triple tachometer. It told me both easily, although you have to know the readings for the various power levels - for example, that 1.5 is maximum continuous. The glass cockpit will incorporate Ng torque and temperature on one presentation, making it easier for the pilot to see which limit he reaches first.


Cyclic stick in neutral

As planned, I then landed while Upadhyay brought one engine back to ground idle on the FADEC control panel. Care has to be taken with a rigid rotor that the cyclic stick is placed in a neutral position as the lever is lowered to avoid excessive main rotor mast stress, so there is a mast moment gauge showing the level of load, clearly red-lined at the maximum.

I came to the hover again using just short of maximum continuous power on the remaining engine. Visibility all round was good and I was able to achieve an accurate hover with no difficulty. After restoration of the second engine, we had a vehicle on the runway go to 27kt, the provisional maximum sideways speed, and we kept level with it in sideways flight. There was still plenty of pedal and cyclic pitch control remaining at this sideways speed. The provisional maximum rearward speed is 16kt, which we achieved with no nasty nose down pitching and plenty of rearwards cyclic remaining. There should be no problem increasing all these limits. This is important for aircraft operating offshore, which can expect to experience strong cross and tail winds.

The aircraft is cleared for rapid, 80º/s, spot turns, so we went whizzing round - still with plenty of pedal remaining.

We took off and did a circuit, during which Upadhyay selected the training mode on the centre console engine/fuel controls panel. The selected engine immediately went to ground idle and the Ng limiter gauge went to the power setting you would experience on the remaining engine. Although the limiter was showing high power, in reality the engine had gone down one power level so that, for example, take-off power reverts to maximum continuous, but the limiter shows take-off. A thumb button on the collective lever lets the pilot select maximum power in the event of a real engine failure.

Although we had enough power to come to a single engine hover, Upadhyay asked me to do a slow running landing on to the grass, which we did using a lot less than maximum continuous power on our good engine.

Next, we left the airfield and climbed using 5min take-off power at the best rate of climb speed of 75kt. The climb rate was impressive, between 2,500 and 3,000ft/min (12.7-15.2m/s). I then reduced to maximum continuous power on both engines during straight and level flight. There was a 4kt difference between the pilot and co-pilot airspeed indicators, so we asked the telemetry section for our actual speed, which it reported as 143kt true.

This compares well with the other modern twins that I have flown, which nearly all settle between 140kt and 150kt. At ISA+20ºC, HAL has recorded 155kt at 4,000kg and 4,800ft. The sea level never-exceed speed (Vne) is 181kt, HALL test pilots have been to 202kt. At our density altitude and weight, Vne was down to 156kt, but due to the low cloud we had to make a couple of tries diving the aircraft before I saw 156kt briefly on Upadhyay's ASI. There was no significant increase in the level of vibration and the aircraft still handled well.

Upadhyay took over control and ran through various stability demonstrations, mostly without the automatic flight control system. The aircraft is adequately stable about all axes.

The automatic flight control system (AFCS) is still under development, but we tried it anyway. In cruise flight, it certainly helped smooth out the handling and attitude holding was good. No pedal was required when banking and it gave us an almost automatic hover capability. When fully developed, it will be managed by two digital computers and will provide stability and control augmentation and some autopilot facilities. Most of our flight was conducted with the AFCS off, with no handling problems.

We did sustained steep turns in both directions, pulling +2.5g at 80kt and 70º of bank, then 2.2g at 45kt and 80º. This is the performance one expects from a rigid rotor and gives pilots confidence. Visibility in the turn through the overhead panels was good.

At 80kt in the cruise, I asked Upadhyay to quickly pull back one engine, warning him that I intended to do nothing, unless the rotor rpm (NR) came close to the lower limit. He did so, and nothing happened. There was no rotor droop or loss of airspeed and the remaining engine stayed well below any power limits. We repeated the exercise at the maximum continuous speed of 142kt. Rotor rpm drooped to 96%, still above minimum, and stayed there. Again, this gives pilots confidence that an engine failure is not going to have drastic effects.

To check the ability of the FADECs to control power turbine, and therefore rotor, rpm, I asked Upadhyay to raise and lower the lever as quickly as he dare to allow me to check for any rotor droop. He went through the range for powered flight from minimum to maximum continuous. We got momentary droops of 1% before NR recovered - a satisfactory result.

A generator failure was simulated by switching one off. The warnings were adequate and there was no loss of electrical power. One hydraulic system was switched off and Upadhyay demonstrated that we could still pull 2.2g. We went through an engine fire situation. My attention was attracted to the flashing master caution and the fire light on the caution panel. When HAL installs the voice warning, it is to be hoped that it will announce that there is an engine fire and identify the engine. I liked the simplicity of the procedure, just three pilot actions, with no chance of shutting down the wrong engine in the process.


We returned to the airfield for some circuit work. I found that the right hand edge of the instrument panel protrudes slightly into the field of vision when doing an approach to the hover over a specific target, but not excessively. For a really steep approach, I kicked the nose slightly left to keep my aim point in view all the way down. I climbed vertically to 100ft to an OGE hover and, because of the good visibility, was able easily to stay over my target - likewise on the way down.

HAL is aiming to have the aircraft certified for single pilot IFR operations. With this in mind, I examined carefully the manual throttle operation, particularly since they are overhead. Upadhyay reached up and selected 25% torque on the "failed FADEC" engine. I flew the rest of the circuit. The power level of the good engine stayed ahead of the manual throttle engine, holding NR constant and in limits.

Similarly, during the approach to the hover, I lowered the lever cautiously, monitoring NR and power turbine speeds, because this is when you can get an overspeed. All went well, however, with no further movement of the manual throttle until we were on the ground. A single pilot, even in IFR conditions, will have no difficulty with this.

As Upadhyay is HAL's demonstration pilot, I asked him to show me some manoeuvres. In the cruise, he raised the nose hard up to 40º (I could feel a lot of g), then pushed hard over to 40º nose down (negative g), followed by a 180º turn. He then did some truly vertical wingovers. Not bad for an initial prototype just over half way through its testing programme.

Finally we went to find some sloping ground. The aircraft is cleared to land on slopes of up to 10º in all directions. The technique with a rigid rotor is different from an articulated or teetering rotor system because of the mast bending stresses. As the aircraft settles with the slope, the pilot needs to keep the rotor at right angles to the drive shaft. So as the aircraft topples, you move the cyclic to go with it, keeping the rotor disc level with the slope. The mast moment gauge showed that we stayed well within limits.

Low cloud meant we were unable to explore the autorotation characteristics or vortex ring/settling with power. Telemetry results show that, however, in autorotation, the rate of descent varies between 2,000ft/min and 2,300ft/min (10-11.7m/s).

Rotor brakes can be potentially hazardous. After shut down, Upadhyay asked me apply the brake as NR reduced to 40%. Although the brake pad has its own hydraulic cylinder, the pilot cannot apply pressure at his own discretion, but simply takes the brake lever out of the "flight" position and pops it into the "brake" position. This applies the correct pressure.

The flight/operations manual is extremely detailed and comprehensive, but I questioned whether it is was really necessary to include full-page diagrams of a fire extinguisher and axe.

My evaluations cover four points of view, that of the pilot/s, the passengers, the operator and the neighbours over or near which the helicopter will fly. The pilot requires sufficient engine power with some reserve to operate the aircraft safely and efficiently throughout its envelope. Similarly, he expects sufficient power from both the main and tail rotors. He needs simplicity of operation, particularly in single pilot IFR operations. Finally, he needs a safe aircraft, one that can suffer malfunctions and failures and without overwhelming him. I can state that the ALH fulfils most of these criteria - I was unable to evaluate the glass cockpit, however.

Passengers require comfort, acceptable levels of vibration, enough space and low noise. With development of the ARIS still under way, and in the absence of a passenger interior, I was unable to evaluate any of this but the large volume of the cabin - 7.33m3, bodes well for comfort.

The operator, like the pilot, requires above all a safe helicopter. He is also looking for efficiency, reliability and cost effectiveness. I can vouch for the safety aspects, but the rest will not be positively established until the aircraft has been in service for some time.

Modern helicopters need to be quiet. The modern design of the main and tail rotors looks encouraging from that point of view.

Source: Flight International