FIREFIGHTING IS A matter of timing - getting sufficient water to the right place (even remote places) early, and keeping it coming. The perfect delivery machine for that task is an aircraft, which can link water supply and fire directly and rapidly. Over the years, numerous aircraft have been adapted to a fire-fighting role, but Bombardier's twin-turboprop Canadair CL-415 is unique as a new-build amphibian primarily dedicated to the task.
Flight International, evaluated the CL-415 as a follow on from the test, made 30 months ago, of the CL-215T turboprop modification of the original CL-215 piston-engine water-bomber.
By no stretch of the imagination could the CL-215 be described as a thing of beauty, and its development into the CL-415 has done little to change that. The airframe is festooned with airflow fences and vortex generators, to optimise the low-speed aerodynamics of the wing and empennage, which live in an asymmetric prop-wash from the two large propellers which have the same direction of rotation.
A huge fin and rudder are supplemented by "finlets" to each side of the tailplane, which are angled 5¡ from the fore/aft line. Deep baffles below the wing protect the outer sides of the engine intakes.
The only clear external difference, between the CL-215T and the CL-415, is that there are four drop doors, instead of two. The release setting of their pre-loaded door actuators has been increased to resist a flight vertical load of 2.2G, to cure a slight weeping found during manoeuvring during flight tests.
Those doors serve a drop system, which has been increased in capacity and flexibility. The upper parts of the water tanks are reshaped to bulge forward, adding 15% volume for a total of 6,140litres. The overfill/air relief ducts are larger than before and float-type content sensors settle more quickly to a reading than potentially more accurate pressure sensing.
A flatter foam tank, holding 600litres, is now located under the starboard four-seat bench (up to 14 firefighters can be carried), and holds concentrate for 19 drops. The Quebec Government's aircraft have two foam tanks. Protective coatings and paints have been further improved.
The optional auxiliary power unit (APU) is housed in the cabin and exhausts to the left, under the wing. It supplies extra electrical power, but is not operable in flight.
Piping for washing-down the aircraft is fixed in the engine nacelles, removing the need for the crew to climb up through the cockpit roof hatch and manhandle panels and hoses - important in maintenance afloat. Engine oil can also be replenished remotely from a cabin unit, from where filter condition and chip detection are also monitored.
A new forward electrics-centre houses the batteries: numerous off-load devices protect them, especially when the generators are idle. An electric pump powers a separate hydraulic line, for emergency gear-lowering or door stowage, and maintenance.
A fuel drain pump, operable from the refueling panel aft of the right gear, eases the task of draining the eight-cell fuel tanks in each wing for maintenance.
Under the eye of Yves Mahaut, Canadair's senior pilot on amphibious aircraft, I started the first engine on external electrical power. The fuel lever is set to start/feather at 10% high-pressure rotor-speed (Nh), and the starter cuts out at 46%. In sub-zero temperatures a "rich" start is made by temporarily selecting flight idle for 10s.
Inter-turbine temperature (ITT) is displayed on the primary panel, Nh and oil pressure on the secondary and fuel flow on the third. With familiarity, a pilot could get used to this, but it might be better if there was at least one composite reversionary display for start-up, to save the eye waving.
Idling parameters were, Nh 78%, ITT 445°C and fuel flow 130kg/h (280lb/h) for each engine, with Np (propeller speed) at 830RPM. Should Np drop to the restricted range of 500-780RPM, a "high cam" switch biases the mechanical fuel-control unit to lift idle Nh to 84% and thus raise Np.
The parking and standby brake handles behind the control wheel are easy to reach, for the control columns are canted outboard, as on the Douglas DC-3 and de Havilland Comet, exposing the ledge space under the instrument panels.
After initial fierceness, from lack of recent use, the brakes settled down to progressive operation. The free pedal travel before brakes initially respond may be an advantage. Braking at high speed during landings, when there may be residual lift, is discouraged. Control of taxiing speed by propellers alone is excellent: the power levers are cranked back and at less of an arm-stretch than they were on the earlier aircraft.
The maximum take-off weight on land is 19,930kg. The payload can be 5,250kg over short distances, or the maximum fuel load of 4,650kg can give an empty ferry range of up to 1,300nm (2,400km).
Our take-off weight was 15,820kg, with 2,700kg fuel and a mid-centre-of-gravity of 30.17%. C-GKEQ (the eleventh CL-415) had flown 112h, and was due for delivery to the Securit, Civile in France.
For our first take-off, 100% torque (Tq) would be used. The torque setting for the actual atmospheric conditions is taken from reference cards. Vernier screws below each lever quadrant set a soft stop, using a counter built into each power lever. The stop can be over-ridden with a push to give full power.
There is no take-off configuration warning - it would certainly be difficult to design one for an amphibian. Neutral trim in fact, represents a 3° rudder offset to the right. Power, rather than speed, affects trim, and rudder trim is automatically balanced over a small range of authority to contain any small thrust imbalance - but not a swing on engine-failure.
Mahaut held the control wheel forward, with aileron to counter a slight crosswind, while I steered along runway 06 at Montreal's downtown Dorval Airport. At 50kt (92km/h) I took over, rudder control having been effective by 20kt.
After lift-off, I noted a slight pilot-induced oscillation in pitch and a high pitch-trim rate. Trim changes little with airspeed, but this rate was suited to the change of trim with flap retraction: I was to appreciate this during manoeuvring near terrain. Fuel flow was 500kg/h on each engine at take-off.
The flaps were retracted from their 10¡ take-off setting at 100kt. The best-gradient climb speed is slow for a big aircraft at 110kt, and the best-rate-of-climb speed is 135kt, but 140-150kt was better for fitting in with the surrounding traffic. Hydraulic assistance to the rudder is halved above 145kt IAS (indicated airspeed). Vmo (maximum IAS) is 190kt.
The precision of torque setting is outstanding. At 85% Tq, fuel flow decreased to 450kg/h in the climb at 2,500ft (760m).
During a brief hold, in the base of summer cumulus at 5,000ft, we checked the de-icing. The engine inlets and all empennage leading edges have inflatable boots, but the wing is not ice-protected - instead, there is a performance allowance of 180kg. The thick wing-section is claimed not to be sensitive to light icing.
The high-speed cruise at 8,500ft, with 82% Tq, was 175kt IAS - 200kt TAS (true airspeed) - with the fuel flows at 420kg/h. With Np reduced to 900RPM, and Tq reset up to its flight limit of 96%, fuel-flow was 385kg/h each for 165kt IAS (188 TAS). The long-range-cruise TAS is a low 145kt.
In a simulated go-around at 95kt, a touch of pitch-trim balanced out a rapid application of full power and raising of the flaps. In a simulated steep descent at 110kt with 10¡ flap, the aircraft reached 8¡ nose-down at flight-idle thrust, with a descent rate of 2,400ft/min (12.19m/s). It balloons up briefly if 10¡ flap is lowered at high speed, but at lower speeds there is just a slight airflow rumble.
With a rapid roll-rate when entering turns there is a slight adverse yaw, but a steady 30¡ banked turn can be flown hands-off. The ailerons always gave full control during stalls, made with landing flap at 25¡, and at 15¡/10¡/0¡ settings.
Without flaps, the ailerons had to be used vigorously to keep the right wing from dropping at the stall. As speed reduced with 25¡ flap, I was steadily applying a lot of rudder, because of the asymmetric geometry of the empennage, and there was much heavier warning buffet. All stalls ended in a high-drag flattish descent with the control column held fully aft; direct recovery could be made on thrust alone - with a loss of just 200ft after a full stall.
Pulling hard into rapid reversals of 45° bank, at approach speed, I saw the angle-of-attack indicator tremble at the edge of its amber warning sector. In a sideslip at 100kt, with 15¡ flap, the CL-415 was taken to 20° bank at full rudder. With the propeller of the critical port engine feathered, minimum control speed (Vmca) was confirmed exactly at 84kt, with full rudder and 5° right wing down.
TESTING THE WATER
At Lake Mont Tremblant, we met our photo-shoot helicopter. The excellent view in steep turns, through the side-screens and eyebrow windows, soon revealed this small grey target. The water, wind and obstacles were checked in a run at 500ft radio altitude, with a 10kt wind from the left.
Mahaut made the first landing, while I took note of the flare attitude and visual cues in preparation for my own efforts. My water take-off gave a feeling for the changes in pitch control - first pulling, to lift the hull onto the step, then easing forward to accelerate without lifting off prematurely.
The lake is 600ft above sea level and nearby wooded hilltops top 2,500ft. Judging the best working path and available escape routes (for the extremely unlikely event of engine failure), I flew various paths to a left-hand descent on to the lake, from around 1,500ft.
The presence of houses on the hillside above the lake meant that a shallow approach was out of the question, and that altitude had to be held late. Then, with 15° flap and idle power, we lost height dramatically in a 25° banked turn, at the target angle of attack for the approach, on to a reciprocal landing heading. Using sideslip sharply increased the descent rate. The flaps are left at 15° for a water landing, as the impact might be heavy and the flaps when extended to 10° are stressed to only 2G, rather than 3.25G.
Over the water, the pilot's eyes must turn from the baro- to the radar-altimeter, whose scale was running down like the reels of a "fruit-machine". A touch of power - up to 20% Tq at 200ft - in the 500ft/min final descent, and then airspeed steadily reduced for a gentle flare from 50ft.
Mahaut says that the secret is to contact the water at low speed, with enough pitch-up to protect the nose wheel doors from water damage and the hull's chines from digging in deeply, with the attendant risk of a "water-loop".
After a firm and steady pull on the control column, I was still feeling for the height of the keel above water when my first touchdown made itself. It was unbelievably smooth and I was destined not to repeat it. For this water-based touch-and-go, the flaps are raised to 10¡. Skimming "on the step" at 70kt, near to free-air stall speed, needs fine attitude judgement to keep the aircraft kissing the water. A pilot still on the learning curve might lift the aircraft up a metre or two in "ground effect", by inadvertent control back pressure or in a wind flurry. With height increasing and speed dropping after a bounce, I had to quickly lower 25° flap to preserve stall margins.
We tried several full stops: in each case the water run was kept to a couple of hundred metres. Reverse pitch can be used to help here, and propeller response is rapid. The landing distance from 50ft at maximum weight is quoted as 675m, and take-off to 50ft less than 850m.
The touchdown attitude is 5° and floating attitude 3° nose-up, but when the CL-415 sinks off the step at low speed the nose briefly pitches up to 7-8°, like a landing duck. In water taxiing, directional control depends on differential forward and reverse propeller pitch once the speed decays far enough to make the rudder inoperative.
The water became more choppy and started to slop over the windscreen at take-off, meaning that wipers were needed. Canadair has answered the prayers of every pilot here with an intermittent wiper switch mounted on the control wheel. Such switches are often unsuitably placed.
For a scooping run, I touched down on the stall warning, at 70kt. The two cast-metal scoops were already pivoted down and forward, ready to fill the two pairs of tanks. The extra capacity and slight increase in internal water drag in the new aircraft have meant an increased uplift-time for a full load from 10s to 12s - still an impressive 500kg/s, over just 350m. The total clear distance needed to fill the tanks with a scoop is 1,200m.
The scoops can be extended once on the water, but landing with them down is more efficient. Again, low speed is the key; a water speed of 90kt is allowed, but touching down above 80kt means a steep increase in drag, and marked changes to the control forces and engine torque needed.
Even at the maximum landing weight of 16,800kg, touching down at less than 70kt is feasible - as long as a close eye is kept on the yellow low-speed and warning sectors of the angle-of-attack gauge.
Warnings may start at only 5kt above stalling speed but, with the hull now close to the water, any adverse gust would merely drop it on to the surface.
A take-off weight of 20,900kg is allowed off water. An automatic scoop- control targets this figure, or limits uplift to a selected volume. There is an irritating delay between movement of the multi-function selector knob of the glare-shield controller and the reaction of its associated display. This control also allows the number of drop-salvos and the intervals between them to be preset; one press of the dump button on the pilot's control wheel is sufficient to set off the sequence of four closely spaced drops.
After a full load of water was dumped from 300ft, the doors re-closed in 5s. The sudden feeling of updraft gives a temptation to over-control in pitch. With the photographic helicopter off our port wing for close shots, we continued in low-speed runs, at 25° flap, 15ft above the water.
The CL-415 in its natural environment is the very model of precision in speed and height. Precise control in attitude comes from the powered controls. Gentle turns could easily be made, to slowly weave around tree-covered islands, with the helicopter holding station. With the aircraft afloat, the wing-tip floats clear the water at up to 2.5¡ of bank angle; on the step a bank-angle of 2-3°, leaves a 1m float clearance, the bank angle margin at 10ft is 15¡.
Our final departure encompassed a towering right turn at 30° bank, keeping well inside and climbing above the up-slope of the hillside. Leaving the flap at 25°, this turn could be made at 90kt with a radius of 200m.
POWER TO THE ELBOW
With the controls unpowered, the pilot has to apply much more effort, but manoeuvrability overall is little different. At the start of a rapid roll, the ailerons display the merest snatch of overbalance, but in wing-overs left and right to 90° of bank the feeling was of well-harmonised control.
The trim operates differently in powered and manual-control modes: with power, trim commands are executed through moving the main control surfaces, but in manual mode, trimming relies on trim tabs on one surface in each axis.
Two landings were made back at Montreal - the landing checks include a reminder to switch from "SEA" to "LAND" mode, on a panel near to the gear handle, to restore the gear-up warning. Flight at 150-160kt is comfortable for integrating with traffic, and slowing down for the landing configuration can be left until near the threshold. After extensive water work, height judgement over a wide concrete runway at low landing speeds needed some mental re-adjustment.
The large low-pressure tyres and the long vertical travel of the gear allow minimally prepared surfaces to be used, but they must be sufficiently compacted for the smaller nose gear not to dig in.
I applied the brakes at about 60kt after touchdown, but Mahaut made it very clear that this was unnecessary. I then taxied and parked without any use of brakes, save for final engagement of the parking brake. The maximum nose-gear steering angle is 65°.
On shutdown, I tried the emergency manual release of the water-drop doors, a large red handle each side of the centre pedestal. It moves easily halfway and then must be heaved fully back - a pull and a tug, then the doors bang open, driven by pre-loaded spring actuators. A stack of rubber discs acts as a damper to prevent door damage.
A GOOD TOOL FOR THE JOB
By mid 1994, over 1 million drops had been made by CL-215s, with 125,000 water landings; 125 CL-215s were built and 15 have been converted to turbine power. The first batch of 24 CL-415s are being made at a production rate of nine per annum.
The low-speed characteristics of the CL-415 are outstanding, and make it far more suitable for fire fighting than converted military types. It is truly an impressive specialist aircraft.
Source: Flight International