Now a Boeing product following the merger with McDonnell Douglas, the AH-64D Apache Longbow is a substantially improved version of the widely used AH-64A attack helicopter.
Changes include uprated engines, new digital avionics and the Longbow fire-control radar. This mast-mounted radar is associated withfire-and-forget Hellfire anti-armour missiles and gives the improved Apache its new name.
The US Army is upgrading its entire fleet of 758 Apaches to Longbow standard, and new AH-64Ds have been sold to the Netherlands (30) and the UK (64 - to be assembled under licence by GKN Westland as WAH-64Ds). The Apache Longbow is being offered in several attack helicopter competitions now under way from Australia to Turkey.
The opportunity to fly the AH-64D Apache Longbow at the Farnborough air show came after Flight International had evaluated several other leading Western attack helicopters: the Bell AH-1W , Denel Rooivalk and Eurocopter Tiger.
APPROACH WITH CARE
Approaching the aircraft with Peter Nicholson, Boeing production test pilot, I noted that the AH-64 is only slightly larger than a Bell 412 - with only another 0.6m more main rotor diameter and fuselage length. The Apache is a very different machine, however.
The basic design criteria for an attack helicopter were evident as Nicholson showed me around the Apache. These criteria are:not to be seen or heard; not to be hit; if hit, to stay airborne and get to somewhere safe; and, if this is not possible; allow the crew to survive a crash.
Minimising the chances of being seen, special paint on the narrow fuselage reduces radar signature, while infrared suppressors on the engine exhausts disperse the hot efflux. (Nicholson assured me that, when the engines are running, a person can stand at the exhaust outlets without getting burned). The double two-bladed tail rotors, set one atop the other, are offset by 55í to reduce noise. I listened as the Apache took off and flew away at low level - it was quiet and I can imagine that, if it were hiding behind a building or trees, the helicopter could not be heard even within about 2km.
To avoid being hit, an attack helicopter must be agile and the crew not concerned about taking the helicopter beyond its design and performance limits. The crew's job is to fly low and fast, hugging the ground while manoeuvring evasively, searching for and selecting targets, then firing and returning safely. The Apache has some operating restrictions which other attack helicopters I have flown do not, and which limit its agility, but these have been imposed by the USArmy, and not by the manufacturer.
The Army restrict the AH-64's maximum angle of bank and pull-up to 30°. It does allow rapid forwards and sideways acceleration and deceleration, however, as well as a sustained pushover at -0.5g and a +3.5g pull-up. We were to do all of these. Because of Army restrictions, we were not allowed to roll or loop the helicopter, manoeuvres which it is capable of performing. Test pilots happily exceed these limits during development flight testing.
Continuing our walk-round, Nicholson pointed out the 360í sensors which alert the crew both visually and audibly if the helicopter is illuminated by a laser- or radar-guided weapon system. The sensors will analyse the signal and, if necessary, prioritise it as an immediate threat. The crew can then select a suitable weapon, turn the aircraft towards the threat and either the pilot or weapons officer/co-pilot can pull the trigger. Self-protection systems include chaff and flare dispensers to divert any enemy radar- or infrared-guided missiles. There is also an active infrared jammer to help decoy incoming missiles away from the helicopter.
One of the aircraft's notable features is the Longbow fire control radar which sits atop the main rotor. This millimetre-wave sensor, produced by a Northrop Grumman/Lockheed Martin team, enables the crew to scan the terrain in all weathers, day or night, and in less than 30s detect up to 256 potential targets, stationary or moving, on the ground or in the air. These are then classified automatically as tracked or wheeled vehicles, air defence systems, helicopters or fixed-wing aircraft and prioritised in the order of the most dangerous. This information can be transmitted digitally to all interested parties such as a battlefield surveillance aircraft or command headquarters.
The aircraft can be positioned behind a building or row of trees at the limit of its weapons range (10km or more), exposing just the mast-mounted radar for a few seconds. Provided with a picture of the battlefield, the crew can then drop back behind cover to select the weapon or weapons matching the targets, then expose the top two-thirds of the helicopter above its hiding place, pull the trigger and depart. The Hellfire fire-and-forget missiles will home on to their individual targets.
Airborne platforms or command headquarters can also tell the crew which targets to go for, supplying coordinates via datalink. An accompanying Apache without the Longbow radar can be similarly directed. All signals are sent digitally (as burst transmissions using the improved data modem), which is both quick and intelligible only to designated receivers.
The Apache's ugly, but effective nose section houses the pilot's night vision sensor (PNVS) and the target acquisition and designation system (TADS), both supplied by Lockheed Martin. The PNVS is an infrared sensor in a rotating turret slaved to the pilot's helmet-mounted display. The TADS is another turret housing television and infrared sensors and a laser rangefinder/designator. Once locked on to a target, the system will track it automatically.
Design of Longbow systems has gone a long way to prevent fratricide by "boxing in" friendly troops and vehicles within the sensors' field of view and prohibiting them from being targeted by the aircraft. The same information can also be sent to other airborne platforms and command centres. Provided friendly forces stay together, this will protect them from Apache fire.
If the aircraft itself is hit, the chances of it being able to fly out of harm's way are good. Major systems have redundancy, shielding and duplication and positioned as far apart as possible. The two General Electric T700-701C turboshafts, which give 10% more power than the earlier -701s, provide a 2.5min single-engine contingency rating of 1,445kW (1,940shp) at sea level. UK Apaches will be powered by the Rolls-Royce Turboméca RTM322.
The engines are widely separated and will run for 30s without lubrication. Similarly the AC and DC electrical generators (the aircraft is fully operational with only one of each), the dual hydraulic pumps (again only one is essential), and dual-redundant avionics (including six computers) are physically separated. The two crewmembers are separated by a ballistic-proof transparent barrier and sit in armour-protected seats on an armour-plated floor.
Most of the critical components such as the undercarriage, rotors and drive train will withstand a certain amount of ballistic damage. If any of the flight control rods are taken out, the aircraft will revert automatically to fly-by-wire control. The main rotor driveshaft is contained within a sleeve which absorbs all the considerable feedback loads from the rotor system, instead of these being carried by the driveshaft and its associated bearings and gears as in most other helicopters. This also provides protection from enemy fire.
If the aircraft crashes, the crew is likely to survive, probably intact. The accident earlier this year in which a Eurocopter Tiger hit the ground at high speed and both crew walked away illustrates the crashworthiness of modern attack helicopters, and the Apache is no exception. In the Tiger accident the aircraft was destroyed, but the crew were protected by a rollover bar. The Apache Longbow's crew seats and undercarriage are designed to collapse slowly in a heavy landing, absorbing the impact energy. The gun under the belly will slide out of the way, the fuel system will contain itself and the main gearbox will not pierce the cockpit, which has a rollover bar. The whole fuselage is load absorbing and the cockpit windows are jettisonable.
The two tailrotor gearboxes, vulnerable parts of any helicopter, can withstand being shot at and use grease, instead of oil as is standard practice in other helicopters. The gearboxes have been tested to run without lubrication for at least 30min. The driveshaft is also designed to be ballistically tolerant. The tailrotor itself requires no lubrication.
Completing the survivability features are the wire cutters located above and below the Apache's fuselage.
The stub wings of our aircraft were bristling with a selection of weapons, none of them armed, in addition to the chain gun cannon mounted under the nose. The lateral centre of gravity limits are wide enough to not worry about exceeding them with asymmetric loads on the wings. For a ferry flight of, say, 800km, weapons can be replaced with external fuel tanks, leaving the cannon available for self-defence. The Apache carries 1,200 rounds of 30mm ammunition, enough for 2min firing, and the gun can be swung through +/-110° in azimuth, up 11° and down 60°.
As we continued our walk-round, I noted the many antennas which serve the Apache Longbow's communications, navigation and flight management systems. The suite is extensive, including UHF, VHF and HF radios, inertial navigation and global positioning systems, as well as ADF, Doppler sensor and radar altimeter. When we left Farnborough for RAF Odiham, however, we lost all contact with the control tower for several minutes while trying on both VHF and UHF channels and had to circle before getting landing clearance. This demonstrates the one weakness of most sophisticated systems - human failure.
Built-in maintenance platforms enable technicians to access the whole aircraft. A computerised maintenance programme has replaced 74 manuals weighing 30kg and containing 32,000 pages of technical information. The aircraft is designed to be deployed in the field with minimum technical support. Information on what the aircraft and its systems have been doing in flight can be downloaded from memory.
DOUBLY PLUGGED IN
I manoeuvred myself into the front cockpit, secured the five-point harness and put on the helmet, doubly plugged in (one lead for communications, the other for the integrated sighting system). Once settled, I looked around. For a such a sophisticated aircraft, the cockpit is simply provisioned and remarkably uncluttered. The HOCAS (for "hands on collective lever and cyclic stick") control philosophy means that aircraft systems can be operated without taking your hands off the two primary flight controls.
A unique control, not on other attack helicopters which Flight International has flown, is a CHOP button which brings both engines back to ground idle in the event of a tailrotor drive failure. This avoids the pilot removing his hands from the controls at a critical moment.
The front cockpit instrument panel is dominated by two multi-purpose displays (MPDs), separated by the gunner's sight and handgrips. When the sight is being used, the cyclic pitch stick is collapsed out of the way but is still usable to fly the aircraft. The rear cockpit is almost identical, less the handgrips, but with the addition of standby instruments (attitude indicator, indicated airspeed and vertical speed indicator) in case of a total failure of the MPDs.
The MPDs are colour liquid-crystal displays, instead of previous-generation monochrome cathode-ray tubes. LCDs give better resolution day and night, says Boeing, which helps the crew appreciate faster and easier what is happening - useful when flying low and fast. Flight, navigation, attack, weapons, fuel and communications information can be called up using the keys which surround the screens. There is a keyboard on the console alongside to enable communication with the flight management computers and other systems.
Nicholson overlaid on my screens a sensor image of the outside world, slaved to his head movement so that I could see what he was looking at, plus a moving map display to show the route to nearby RAF Odiham. There was also a mirror in which I could see Nicholson's head.
There are no temperature, pressure or other instruments. These are monitored automatically and the crew informed only if something goes wrong. The crew can interrogate the systems at will. Engine and main rotor parameters such as RPM, torque and temperature are shown as thin, green, vertical lines, some with a digital readout. Entry into the precautionary zone causes the line to change to magenta and thicken. Entry into a contingency or maximum permissible area causes the line to thicken more and turn red. If torque, engine gas temperature or rotor RPM approach within 2% of any transient limit, the crew is notified with a flashing then solid red warning in their helmet displays.
Use of the 2.5min engine rating starts an automatic countdown. Other critical conditions such as low or high rotor RPM, and engine failure or fire induce a computer-generated (female) voice warning. There is no need with this presentation scheme for pilots to remember power settings, temperatures and pressures when their minds are occupied with more important details.
The throttles were conveniently available on the left side console. Visibility from the front cockpit was generally excellent, though not so good forward over the sensing and sighting equipment in the nose. As the seat is raised, it tilts back. I would have liked a slightly higher position to be able to see over the nose.
The weather for the first of our flights was fine with a standard-day temperature of 15°C, a pressure and density altitude of 210ft (65m) and 18kt (35km/h) of wind. Our maximum permissible gross weight was 8,015kg, but our actual weight was 7,500kg.
Nicholson fired up the noisy built-in auxiliary power unit which gave us hydraulic and electrical power and provided air to start the engines. One of the MPDs showed the engine start page for monitoring its progress. The starts were semi-automatic, slow and cool. Everything switches itself on and self-tests automatically - even the controls go through a self test motion. The pilot will be informed if anything is wrong. This way, if the crew is in a hurry, they can get airborne quickly.
Seat and pedals adjusted to my liking, I was ready for my first hover. This was uneventful and, with little effort from me, we came to a neat hover. We were not able at this point to explore the single-engine hover capability, as we were still quite heavy and the aircraft did not belong to Boeing but to the USArmy, which restricts the use of single-engine contingency power.
The presentations of power being used (compressor speed, temperature and torque) were excellent and allowed me to see easily how much more power we had available: 1,410kW from each engine for 10min, 1,340kW for 30min and 1,240kW maximum continuous at sea level. As in all good helicopters transient overruns are allowed to get the pilot out of a difficult situation. These are recorded and downloaded later to ascertain if any special maintenance is required. These limits are generous, however. The readability of both MPD's was excellent in the bright light. They can be dimmed for night flying.
I selected attitude and altitude hold on the automatic flight-control system by using one of the HOCAS thumb switches on the cyclic and took my hands and feet off the controls. The aircraft swung slightly since I had not trimmed out the pedals beforehand. This I now did and the aircraft sat quite stable in a low hover. Taking advantage of the 18kt wind to explore out-of-wind characteristics, I deselected the auto-hover (again using HOCAS) and turned the aircraft 90° across the wind. I had no difficulty in holding a good hover. I explored the downwind and other crosswind position, again with good results. Some helicopters exhibit handling deficiencies when hovering with the wind in certain sectors. The Apache Longbow has none of these, Boeing says.
The US Army's operations manual clears the aircraft for 45kt sideways and backwards flight, so off we went. I still had plenty of pedal left in both directions in sideways flight and the aircraft stayed level and stable in fast backwards flight. The aircraft has been proven to go much faster by the test pilots, up to 100kt backwards.
To evaluate fast lateral accelerations, I had Nicholson throw the aircraft sideways from the hover up to 45kt. The result was impressive, with no control or aerodynamic difficulties encountered. The only restriction on the speed in turns on the spot is when recovering from a fast turn to the right, when a lot of left pedal may be required. Nicholson did his worst and the horizon became almost a blur as we sped round, I could feel my body being pressed sideways in the harness. This illustrates the large amount of tailrotor power available.
The aircraft will accelerate rapidly from the hover with the massive engine and rotor power available. We also tried some quick stops from fast forward flight - some straight ahead, which rather blocked my view, so we tried kicking the aircraft sideways, which improved my view considerably and also caused extra drag to slow us down more quickly. Again the large amounts of lateral main rotor and tailrotor power allowed this without running into any control limitations or restrictions.
While still in the hover I switched off all stabilisation and hovered the aircraft "raw", then did spot turns in both directions with the 18kt wind. Handling throughout these manoeuvres was benign, requiring no special effort from me.
Still reasonably heavy, we moved into forward flight and prepared for a single-engine landing back on to Odiham's runway. I went to the single-engine page on the MPD. After entering in the ambient air temperature and pressure altitude, this gave us a wealth of information including never-exceed speed, range and endurance speeds, hover capability in and out of ground effect and, most importantly, our single-engine flyaway speed - that minimum speed at which we could reject the landing and go round. This would also be the same as the critical speed during take-off at which you continue the take-off.
The computer gave us 31kt. I thought this to be a bit high and, sure enough, when we then did a single-engine approach to this speed, rejected the landing and overshot, we did not use anywhere near single-engine maximum contingency power. I felt we could have approached to a much lower speed and still have had enough power available to go around. A subsequent single-engine run-on landing was easily accomplished even in the 18kt crosswind.
I next pulled up into a vertical climb. Using 94% torque (6% less than maximum continuous), we achieved a rate of climb of 2,000ft/min. The aim of this test was to check the visible cues required to achieve a truly vertical climb, staying over our spot, to an out-of-ground effect hover at 200ft. The test results were adequate and a satisfactory performance achieved.
In the cruise at maximum continuous twin-engine power (100% torque), we obtained a healthy 145kt in straight and level flight. The other attack helicopters that Flight International has tested reached between 140kt and 150kt at maximum continuous power, so the Apache Longbow is comparable. Higher engine powers are available at the 30min and 10min ratings and there is even a 10s burst capability to 115%.
At speeds above about 120kt there was a vibration which made my teeth chatter slightly. Nicholson informed me that the main rotor had not been tracked or adjusted since the Atlantic crossing to Farnborough, and said that the Apache was usually much smoother. The never-exceed speed at low weight (6,630kg) and altitude is 197kt, achieved in a dive with or without weapons attached, but lack of altitude available prevented us exploring this. Nicholson showed me a quick descent from 2,000ft which consisted of a rapid deceleration to 60kt, lowering the nose to 30í and maximum bank. I saw a rate of descent of 4,500ft/min (23m/s) on the vertical speed indicator.
Steep turns were limited by the US Army operations manual to 60í, but these were benign, visibility remaining good throughout the turn with no increase of vibration levels.
Back in the cruise I explored a sudden engine failure, intending to do nothing to see what happened. Nicholson brought back one throttle to ground idle. My attention was directed first to rotor RPM - I saw a momentary droop of 1%. Our speed went from just over 100kt to 95kt, and the remaining engine went into the red momentarily before the collective lever was lowered slightly to reduce the power required. The engine presentation page showed what had happened. A box at the bottom analysed the problem and gave ways to deal with it.
A major part of an attack helicopter's role is being able to fly nap of the earth, hugging the ground and following the contours, so some agility is essential. Nicholson demonstrated this by pulling the nose up hard while at high forward speed, probably going to the aircraft's limit of 3.5g, followed by a hard, -0.5g push-over. There were no protests from the aircraft at this harsh treatment.
To check the effectiveness of the engine governors, rapid up and down collective lever movements were performed which caused only about 1% rotor RPM change and no change in attitude - a satisfactory result.
I switched off all the stabilisation at 120kt and flew the aircraft raw. There was hardly any difference of feel in the cruise at a variety of speeds, so an approach to the hover was made. Again, the handling was benign and I had no trouble hovering, doing spot turns and making a downwind landing.
It is impossible to switch off a hydraulic system in the aircraft, although this can be done in the simulator. Boeing assured me that the aircraft will handle identically, especially during the approach and landing, with no special recommendations to reduce speed in the cruise. Like most aircraft of its size, the Apache cannot be flown without hydraulic power, which is why special measures are taken to protect the critical components against ballistic damage. In addition to the two isolated hydraulic systems, there is an emergency tank which will deliver sufficient pressure for long enough to land the aircraft in the event of both main systems being taken out.
I did a really steep approach, probably about 60°, to a pre-selected point on the runway. By kicking the nose slightly to the left, I was able to see my intended landing spot all the way down to the ground.
The engines are normally digitally and electrically controlled to provide good engine matching, RPM control and speed and temperature limiting, as well as starting, but should this system malfunction, the pilot can select manual throttle, where all of the aforesaid facilities are managed by him. We went back into the circuit to try it. One throttle was taken out of the "flight" gate and moved quickly forward and back until that engine's torque was below that of the governed engine. Since the aircraft is designed to be flown by two crew, management was uncomplicated. I kept the manual engine's torque just below that of the good engine, while Nicholson flew the aircraft down the approach to the hover and landing. I invited him to give me a running commentary on his movements of the collective lever. Once on the ground, automatic throttle was selected by pulling the throttle lever all the way back to ground idle and then pushing it forward to "flight".
We then investigated a generator failure. The warnings were obvious and the recovery drill straightforward. An engine fire will produce an aural warning, illumination of the master caution light and of the appropriate discharge button on the warning panel. The subsequent procedure is straightforward with no chance of the wrong engine being shut down. After cancelling the master caution and adjusting the power on the good engine to avoid any subsequent excessive demand, the pilot only needs to press the illuminated discharge button. This will shut down the engine, arm the system, select the correct engine, fire the first of the two extinguisher bottles and shut down the air conditioning system.
Although autorotation is an academic exercise, since attack helicopters spend most of their working life at low level, Nicholson was happy to demonstrate one from 2,000ft with a powered recovery to the hover. He pulled one engine back, then the other. There was little rotor RPM change as he smoothly lowered the lever and adjusted our forward speed to 90kt. The presentation on the triple indicator (both power turbine speeds and rotor RPM) was excellent. The rotor RPM gradually increased to maximum and I felt Nicholson raise the lever slightly. We got a modest 2,200ft/min rate of descent. At the minimum rate of descent speed of 75kt, Nicholson assures me, it would be in the order of 1,600-1,800ft/min at our weight and height. This is remarkably low. I felt the flare bite at the bottom and we came to the hover, engines restored, with the minimum of fuss.
Fortuitously we found a slope on Odiham's airfield so I imposed on Nicholson's good nature to take the aircraft to its limits and land 10° left and right wheel on, 12° nose down and 7° nose up. If the slope is excessive, there will be no potentially damaging mast banging as the main rotor driveshaft contacts its surrounds; instead the pilot runs out of available cyclic stick.
Because the aircraft had not been refuelled prior to our flight, after 1.3h the master caution light illuminated and the warning panel showed "low fuel". As we returned to Farnborough, I brought up the fuel panel on one of my MPDs. It showed me everything I needed to know about the fuel system - the contents of each tank, total contents (all in amber), the remaining endurance to tanks dry, each engine's present fuel consumption, plus total fuel consumption and the state of the booster pumps, crossfeed and transfer operations, if any.
It is a pity that we were not allowed to take the aircraft to all of its aerodynamic limits, but those I did explore showed all aspects of aircraft handling to be benign - something less for the busy crew to worry about as they go about their most demanding of tasks.
After the initial flight of 1h 40min, followed by a shutdown, refuel, start, hover taxi and another flight of 30min, I felt confident enough to able to fly the aircraft to its limits and manage most of the operating systems, but not the weaponry, which we did not try. I also felt that, after some training and a little more practice, I could probably deal with malfunctions and emergencies. This speaks well of the Apache Longbow's design and state of development.
Source: Flight International