COASTAL PATROL IS increasingly vital to countries with extensive shorelines, whether they be interested in smuggling, illegal immigration, unapproved fishing or sea pollution. Well over a dozen aircraft types of various configurations (mostly propeller-driven) are now offered in this role, and there is a wide selection of sensor equipment available.
Flight International met the Bombardier de Havilland Dash-8-202 MPA (maritime-patrol aircraft) in Oslo, during its delivery to Australia, where it will be one of three operated by National Jet Systems under contract to the Australian Customs Service. The Dash 8s, modified by Field Aviation in Canada, will be used on patrols covering some 270 million km2 (80 million nm2) a year.
With maritime economic offshore zones often extending to 370km (200nm), the tasks facing this type of aircraft can be complex: operations can be from basic airfields near to the patrol area or involve long cruises to station; hours can be spent patrolling at low altitude, and inspections can be at very low altitude. How does the Dash 8 MPA match up?
Its basic airframe and structure are little altered from the basic regional-airliner configuration, and easily reconfigured (say on resale) back to airliner status. Standard maximum weights are used and the trim is moved only slightly forward.
The standard modifications incorporated are: 2,540 litres extra fuel; Sundstrand Titan auxiliary power unit (APU) in the tail cone; heavy-duty batteries; and a rough-field kit. One non-standard modification - tested, cleared and ready for a customer - is a rear freight-hold door conversion for rescue air-drops of anything up to a ten-man life-raft.
The APU is not operable in the air. Some operators of big-engined turboprops use the core of one of the engines in the role of APU, but the added weight and complexity of the necessary propeller brake argued against this. Communications and direction-finding (DF) antennas are added, but the main changes are the fitting of an under-belly steerable forward-looking infra-red/television camera and 360¡-scan radar. Their drives are within the pressure hull, so reduction of ground clearance is minimised.
The half-teardrop-shaped radome, 2.5m long and 0.5m deep, encloses a 1.15m wide by 0.35m deep antenna. Its maximum depth is just ahead of the main wheels, giving ground clearance even with oleos fully compressed and a flat tyre on one side. Windtunnel trials confirmed that this radome gives modest drag, and minimal effect on airflow over the high tailplane.
Power and performance
The 1,600kW (2,145hp) Pratt & Whitney Canada PW123D engines give good hot-field performance: at a maximum take-off weight of 16,465kg in ISA + 30°C conditions, the field length is 1,100m (3,600ft). Cruising speed at 25,000ft is 255kt (470km/h) - a loss of just 20-25kt because of radome drag; long-range cruise is at 210kt. A patrol area 400km from base can be reached in 1h: endurance there, assuming a 30min hold and a 180km diversion, is over 8h - or some 2,000km distance covered - consuming 4,000kg fuel.
With the Dash 8 carrying extra tour equipment as well as 3,600kg of fuel, the take-off weight for our evaluation was near maximum. Take-off reference speeds, with 15¡ flap, were: decision and rotate speed V1/Vr, 93kt; safety speed V2, 96kt. With power levers set to give 95% torque and 1,200RPM propeller speed, the ground run took 15s. Flap-retraction speed was 101kt and initial climb-out speed 120kt; a power control unit automatically sets the engine regimes for route climb and cruise.
At 3,000ft, weather conditions down the Oslo fjord were realistic for maritime patrol. The sea was rarely visible and the radar was relied on down to 1,000ft. Even on inspection runs at 300ft, there was loose scud just above the aircraft. Roll rate is lively below 140kt, when the second pair of roll spoilers comes into operation; 30¡ bank reversals can be made in 3s. When flying even slower (below 120kt), control in pitch was noticeably looser. Engine slam accelerations cause little pitch change.
Cockpit noise levels are very low when patrolling at 135kt indicated airspeed for maximum endurance. The windscreen view is good for close manoeuvring: down the nose at over 15°, and backward to 125° outboard from each pilot. A few removable switch-panels are added in the cockpit for the mission systems. Electrical-load shedding (automatic on engine failure) and separable intercom/radio systems enable the crew to put a "wall" between the cockpit and the mission systems in the cabin in an emergency.
A dedicated bearing indicator on the captain's side works from the cabin DF equipment. Radio DF is available on distress frequencies and in the VHF/UHF bands. Most important operationally, the ground-proximity-warning system is inhibited, but automatically comes into full operation if a trigger height on the radio-altimeter is breached.
The search radar has weather-detection capability, but the standard aircraft weather-radar serves for tactical avoidance of stormy weather. The radar still has enough range at low speeds and heights, even though it is always tilted several degrees up to avoid returns from the surface.
There are only minor changes to the cabin furnishings, and the equipment consoles, well behind the propeller plane, are quiet enough for free conversation. Space in the radio suite on the left, housing a second independent Rockwell-Collins communications package, allows for options: VHF, satellite or HF datalinks; encryption and identification friend-or-foe. The black boxes and many switches have a military "add-on" look; the radar screen and controls could have a better multi-function display format, but the de Havilland operator used all controls with ease in choppy turbulence close to the water.
The under-wing Type III emergency exit doors are replaced by full-length observation windows which have electrically demisted glass of similar thickness to the cockpit windscreens, to withstand the pressurisation.
Forward and aft view from these windows is clear at quite shallow angles, and the downward view is deep; as they are shaped conformally to the fuselage, there are no distortions to the view. The high wing allows all-around view in level flight, but the massive engine cowlings begin to intrude into the far view in turns. The Australians have chosen a layout in which the console operators sit by these large windows in large and manoeuvrable "executive" seats.
Mission plans and Jeppesen navigation data are uploaded by disk into the flight-management-type functions of the Universal UNS-1C; this incorporates a 12-channel global-positioning system (GPS) for primary navigation. A further stand-alone GPS and an inertial-reference input to the radar are optional. A short demonstration was given of the use of the radar (see box). The search radar tracks returns from patrol heights of about 2,500ft, then inspections are made at 200ft. Interesting target areas - 22° wide by 4km deep - can be "windowed" in the screen's top right corner. Homing tracks to targets are relayed to the cockpit's relative-bearing indicator.
The UNS "multi-mission management system" inputs air data and navigation information to the radar display. It can provide the autopilot with a suite of search patterns: race-track; expanding square; rising ladder; sector search; orbit and border patrol. The software allows pattern interrupts, with same-point rejoins, so that the aircraft can leave for refuelling, and pick up a given patrol pattern where it left off.
In misty conditions, small fishing boats not obvious from the cockpit were picked out by the infra-red (IR) sensor with over a minute to run. Once a target had been spotted with the IR, the display could be switched to TV and zoomed in for better identification. At this stage, the intercom between cabin and console became busy.
The TV zoom was rapid and steady, and the picture stabilised within 1s after the camera was slewed. An IR picture at maximum range is, of course, indistinct, but definition usually improves rapidly over the last kilometre or two - enough to identify a vessel at night.
We made an instrument-landing-system approach to Torp, south of Oslo's Fornebu Airport, for a visual break-off just below the 1,000ft cloud-base, and continued downwind at 120kt. The good cockpit visibility and agile manoeuvring showed to advantage in close circuits. The large propellers are supremely effective as air-brakes at the 1,050RPM approach setting; prompt reductions in airspeed and descent angles of around 6¡ were easily achieved.
On early Dash 8s, the inner spoilers went immediately to full-up deflection on mainwheel touchdown. It proved an unnecessary complication; lift is destroyed quickly as the propeller blades reach a fine angle. The brakes can be applied rapidly and aggressively for full stopping.
Capt Bob Gumbinger demonstrated a "performance stop", with a ground roll of under 200m. I repeated this, but went easier on the brakes; flare, when the nose is pulled firmly up, was remarkably easy from Vref -5kt (86kt) , to a precise touchdown.
Searching for searchers
A maritime patroller is more than just an aircraft: it is an integrated system of vehicle, search equipment and communications. Potential customers can choose from well over a dozen converted civil types - as well as from more dedicated designs, such as the Dassault Atlantique or the four-engined Lockheed Martin P-3 Orion and C-130MP. Steady reductions in equipment size and weight have now ensured that mission capability can be accommodated in much smaller aircraft than was possible before.
The Dash 8 200 series is claimed to be an optimum size for present-day equipment in a wide MPA role, with space for up to 16 airline seats at the back of the cabin, and other equipment for search and rescue operations. Further sensors can be fitted, such as side-looking radar. The company argues that even though the necessary equipment will fit in smaller turboprops, their confined cabin spaces means they can be trying on long missions.
Area coverage must be matched to price. De Havilland claims that the Dash 8 MPA can sweep 315,000km2 per mission with full fuel. This implies an operating cost of less than $7 million a year, for a baseline of 150 7h missions. Maximum fuel allows a payload reserve of 440kg; loading to maximum zero fuel weight gives a payload of 3,050kg, with 2,040kg fuel. The -102 and -103 models, which have lower basic empty weights, may also be used as MPAs.
The limited changes to airframe and avionics from a standard Dash 8-202 mean economies of design and resale costs. It is capable of Category II all-weather operations, and of Cat IIIa, with a head-up display.
The Dash 8 is far from alone in the MPA market, but it does offer a flexible combination of features which are attractive for the patrol of long, sparsely populated coasts.
Source: Flight International