Noise and vibration have always been a problem with turboprops, but the Dash 8 Q400 from Bombardier is a breakthrough designed to challenge jet popularity
Marketing a turboprop 70-seat commuter aircraft when there is a surge in the popularity of regional jets, Bombardier acknowledges it has a battle on its hands. The economics still make sense, however: turboprops win hands-down on flights of up to 460-555km (250-300nm), but their reputation for noise and vibration works against them.
Turbojets are achieving such levels of popularity that airlines have to accept higher operating costs on short flights because of passenger demand. The Embraer RJ-135 and RJ-145 and Bombardier's own Canadair Regional Jet (CRJ) series are examples of successful regional jets that are often used on uneconomically short sectors.
Bombardier believes that, if the turboprop can be revolutionised by reducing noise and vibration, its economics once again become irresistible for short-haul work. The company's market research has identified the need for a 70-seat regional airliner on high-density routes. The latest technology, its argument goes, can give a turboprop low levels of cabin noise and vibration comparable with that of a turbojet. A truly new image for turboprops could re-establish them on short-haul routes as part of a logical family mix of turboprops and jets to cover various sector lengths - Bombardier hopes.
This stretched Dash 8 has been referred to as the -400, but is now being marketed as the Q400 - Q, of course, being for "quiet". Quiet versions of all the Dash 8 series are being offered: Q100 and 200 (37-39 seats), Q300 (50-56 seats) and the Q400 (68-78 seats). The Ultra Electronics noise and vibration suppression (NVS) system is at the nub of the "quiet" revolution.
This technology moves from the traditional approach of masking noise, with copious layers of insulation material, to countering it. Microphones concealed about the cabin send noise signals to a computer which also receives propeller speed information. The computer then signals tuned vibration absorbers mounted on fuselage frames. These absorbers generate vibrations to diminish the original resonance. Bombardier claims that NVS in the Q400 reduces the cabin noise levels to 75dB- the same as in the CRJ cabin.
Flight International tested a Q400 (registration C-GIHK) used for development flying and crew training. After undertaking conversion of SAS Commuter aircrew in Scandinavia, the aircraft was back at its Wichita base, still wearing the Scandinavian livery. In addition to the NVS (which is largely transparent to the pilot), the Q400 has a glass cockpit, full-authority digital engine control (FADEC) and digital electronic propeller control. The reversible pitch propellers are six-bladed Dowty R408s, designed to reduce blade loading. Advanced synchrophasers minimise simultaneous shock loadings from the propeller tips against the fuselage sides. The Q400 is indeed a revolutionary turboprop.
An initial disappointment was that the pilots' seats are not easy to get in and out of. A large centre console fills the space between the seats and, although the overall width of the cockpit would allow the seat rails to swing outwards at the rearward extreme of their travel, an avionics housing is in the way. Consequently, the centre console and seat must be scrambled over as in a much smaller business jet cockpit.
Once in the seat, however, the cockpit's appeal becomes instantly clear: a clean, crisp, uncluttered appearance is dominated by the five 150 x 200mm (6 x 8in) vertical-format liquid crystal display (LCD) screens of the Sextant electronic flight instrumentation system (EFIS). The seat is comfortable, adjustable fore and aft, for height and recline, and has a five-point harness and adjustable arm rests. The rudder pedals are adjustable for reach via a small crank handle between the pilot's knees. The field of view is excellent through the large curved non-opening windows; each pilot can see the wingtip on his side of the aircraft. There is ample room, outboard of each seat, to stow a flight bag; document stowage is in a bin under the cockpit floor.
The emergency oxygen masks are above and behind each pilot's outboard shoulder and there is an escape hatch and associated rope in the cockpit roof. A large control quadrant stretching the width of the centre console neatly houses the handbrake, power, propeller and engine condition levers.
I asked Wally Warner, chief engineering test pilot, who commanded the flight from the right-hand seat, about the need for propeller levers in view of the electronic propeller control, which could allow single-lever control for each engine/propeller unit. He said a great deal of thought and discussion had preceded the decision to retain the levers. A strong influence was maintaining commonality with earlier Dash 8 models, but it was difficult to think of a better or easier way to control the propellers.
The Q400's engines are Pratt & Whitney Canada PW150As, the latest in the well-established PW100 series, rated at 3,420kW (4,580shp) each for normal take-off conditions, with an emergency rating of 3,780kW if an engine failed during take-off.
As well as FADEC (which does not include auto-throttle), the PW150A has an engine monitoring system (for maintenance purposes) and electric starter motors. The engines were started using a ground power unit. The start control is in the cockpit roof - one cockpit panel that is not at the leading edge of technology - retaining switch panels from earlier Dash 8 variants. One important reason for this is, again, fleet commonality, to allow the Q400 to be flown on the same type rating as other Dash 8s.
After the start button had been pressed and the engine condition lever set to introduce fuel, the start cycle was automatic, including automatic start-abort, had there been a malfunction. When both engines were stable, the noise and vibration levels in the cockpit were remarkably low and similar to a turbojet.
The Hamilton Sundstrand T-62-T-46C12 auxiliary power unit in the fuselage tail-cone supplies bleed air for the cabin conditioning and 28V DC to the electrical system, but cannot be used in flight and was not started.
Despite the fuselage stretch of 6.8m (22.4ft) over the Q300, giving an overall length of 32.6m, the Q400 was agile to taxi. The changing of propeller pitch with power lever angle could be heard, but did not seem any more intrusive in the cockpit than similar power changes in a jet. The carbon wheel-brakes were smooth, but not as obviously powerful as I had expected. There was a nose-wheel steering tiller on the captain's side only; it was precise and smooth in operation with strong self-centring.
While taxiing, an area was found to run up the engines and check the propellers, including the overspeed governors, and arm the auto-feather system. The aircraft must face within 30° of any significant surface wind and a clear area is needed behind in the region of the propeller slipstream.
Once the first-flight-of-the-day run-up has been done, subsequent take-offs demand a shorter check and experienced crews could accomplish it quickly and smoothly. This chore is inescapable with a turboprop and puts it at a disadvantage compared to turbojets, which generally do not require a run-up.
Mid-Continent's runway 19 right was designated for take-off. The surface wind was 090°/10kt and the temperature +2°C. The aircraft weight was 21,300kg (47,000lb) compared with a maximum permitted 28,920kg. With the flaps at 15, the relevant speeds were V1/Vr 100kt (185km/h) and V2 104kt. Before take-off, the spoilers and elevators were checked for full and free movement by looking at the pilot's multifunction display (MFD).
All Q400 take-offs have to be brakes-off, rolling start. The rudder-pedal nose-wheel steering keeps the aircraft straight as the power levers are moved to full power. Because of the Q400's length, care must be exercised not to rotate to more than 8° nose-up until the aircraft has unstuck, to avoid scraping the rear fuselage on the runway.
Once airborne, the pitch attitude can be increased to 10° for the climb. After take-off, the undercarriage and flaps were retracted and the propeller rpm reduced from 1,020 (take-off) to 900 (climb) by moving the propeller levers to the climb detent.
The engine bleeds were selected "on", which caused no discernible pressurisation surge. Because of weather, it was not possible to climb above 3,000ft, so the aircraft was hand-flown straight and level at 220kt. The remarkable quietness in the cockpit was almost indistinguishable from what could be expected in a turbojet under the same conditions.
Eventually, it was possible to climb to 9,000ft and manoeuvre the Q400. The primary flying controls consist of mechanically operated ailerons and hydraulically powered spoilers, elevators and rudder. The four spoilers (two per side) assist the ailerons in providing roll control and kill lift after touchdown on landing. The break-out force in roll was higher than would probably have been the case with powered ailerons, but the force became lower as the spoilers exerted their authority. The control force was a little higher in pitch than roll, but the artificial feel in both axes was unusually nice.
The aircraft was easy to trim, particularly longitudinally, using the control wheel two-pole electric trim switches. At 1,000ft and 210kt, a full control deflection roll from 45° of left bank to 45° right bank produced roll acceleration and rate of roll to be expected from this class and size of aircraft. Also at 210kt, at 14,000ft, a rudder doublet to excite dutch roll was met with good damping - both natural and with the yaw damper operating.
At 14,000ft (not a representative altitude, but the weather precluded going higher), a brief cruise at 850 propeller rpm (the cruise setting) was established to check noise levels. The indicated airspeed settled at 270kt, where the quietness and smoothness were again impressive.
Next, two stalls were tried. The Q400 has a stall protection system which includes a stick shaker and a stick pusher. The stick pusher is required only to meet the aft centre of gravity, power-on stall condition. To experience the full stall characteristics, the stick pusher was switched off.
For both stalls, the same technique was used: power "off", wings level and reducing the airspeed at 1kt/s from the trim point. For the clean stall the trim speed was 135kt. Natural buffet occurred at 112kt, very closely followed by the stick shaker.
The recommended technique as the aircraft approached the stall was to hold the wings level with roll control and maintain heading with rudder. This worked well but practice would doubtless make it work better. Roll control was effective at and near the stall, but the Q400 displayed some propeller effect and agile use of roll control and rudder was called for.
The stall was defined by the left wing dropping at a nose-up pitch angle of 12°. Roll control and rudder successfully opposed any further tendency to roll. The aircraft characteristics remained benign in the stall and releasing back pressure on the control column caused it to fly itself out of the stall.
A little unpredictable
In the second stall, with the undercarriage down and flaps at 35, entry was better managed. The trim speed was 104kt, the stick shaker operated at 88kt, there was obvious natural buffet and a g break defined the stall; the minimum speed was 80kt. The Q400 is obviously well endowed with natural and artificial stall warning.
At the stall the aircraft could be a little unpredictable, but showed no vicious tendencies. With the stick pusher in operation, the angle of attack would have been reduced before the stall occurred.
Because of the extensive cloud cover between 3,000ft and 10,000ft, an engine shutdown and auto-feather was flown at the unrealistically high altitude of 14,500ft. The auto-feather system provides automatic propeller feathering and engine shutdown if an engine fails on take-off. Simultaneously, the live engine power is increased by about 10% through an uptrim signal sent to the FADEC.
The auto-feather system has to be switched on for take-off and, if the ARM light does not appear as the power levers are advanced through 60° of power lever angle, the take-off would have to be rejected. The system was now switched on and the aircraft flown at 130kt. with full power selected and the undercarriage and flaps up. With the flap up, the rudder deflection is restricted to 12°, but with flap selected to 5° or greater it increases to 18°.
Warner triggered the auto-feather by moving the number one engine condition lever to cut off the fuel. The propeller feathered and the engine stopped; the uptrim raised power on number two engine to maximum.
There was a momentary swing towards the dead engine after which the aircraft was kept straight with full available right rudder. The hydraulic rudder power meant that the foot forces at full deflection were reasonable and easily sustained. During the Q400's development testing, the six-bladed propeller slipstream was found, during side-slips at V2, to wrap round the fuselage and impinge on the opposite side of the fin and rudder. This tendency was cured by fitting the two long ventral strakes under the fuselage's rear half.
A return to Mid-Continent was now started using the autopilot, eventually to make a coupled instrument landing system approach. The Sextant avionics are well laid out and easy to use. Each pilot's EFIS consists of a primary flight display, with the attitude director indicator displayed above the horizontal situation indicator, and an MFD. The presentations in this Q400 were the optional vertical tape displays for airspeed, altitude and vertical speed (analogue displays are available).
The MFD normally shows the powered flying control synoptic across the bottom of the display on the captain's side while the first officer's shows flap position and hydraulic pressures in a similar display. The main portion of the MFD can be used in MAP mode (when weather, traffic alert and collision avoidance system and enhanced ground proximity warning system displays can be superimposed) or for aircraft systems synoptic pages (doors, electrical system etc). Displays can be moved from one LCD to another if a display fails.
The central LCD panel clearly displayed the engine indications, with the left engine parameters on the left half of the screen and the right engine on the right. The indicators were sized in order of importance. The major indicators had analogue arcs and digital readouts. This central display does not include a full engine indication and crew alerting system (EICAS). In the interests of economy and commonality with earlier models, there is a separate central warning panel.
All the LCDs were clear and sharp, adapted well to changes in ambient light levels and could be clearly read across the cockpit from an acute angle. The flight guidance control panel is at the centre of the glare shield, logically laid out and easily reached by both pilots.
The autopilot flew the aircraft smoothly and the mode switches were easy to use. The autopilot did not compensate for changes of directional trim with power and speed so the pilot had to retrim the rudder as required.
The lack of an auto-throttle meant the pilot had to regulate speed manually by resetting the power levers.
A learning curve
When the Q400 entered cloud, a slight whistle became audible. Warner said that the precise cause of the noise has not yet been established, although it appears to be associated with the presence of water vapour.
The Q400 was not easy to land. Presumably because of the high wing, ground effect was not strong and the long slender main undercarriage legs extending from the engine nacelles were stiff and relatively unyielding. As when taking off, care had to be exercised over the aircraft's attitude; it was important not to exceed 6° nose up in the flare to avoid fuselage contact with the runway. There was a 10kt crosswind and the technique recommended was into wind wing low, rather than a crab, approach.
In all, I attempted four landings and none was smooth or precise. A learning curve was clearly at play and honing the technique required practice. Probably keeping a little power on during the flare and touchdown would help when the landing run was not critical. At 20kt for wet or dry runway conditions, the Q400 has comparatively low crosswind limits for a modern aircraft. After touchdown, drag from the propellers at ground-range blade angle and the spoiler deployment to dump lift and increase wheel brake effectiveness noticeably aided slowing the aircraft. Reverse pitch is available and can be used to reverse taxi. When taxiing with the tail towards the 10kt surface wind, slight exhaust fume smells reached the cockpit.
After only a couple of hours flying the Q400, it was clear that the aircraft sets new standards for turboprop smoothness and quietness. The latest technology has succeeded in combating a turboprop's two worst vices: noise and vibration. The Q400 has good handling qualities and its cockpit ranks among the most modern in any class of airliner in terms of instrument displays and flight management.
In a brand-new design it would be fair to criticise pilot seat access, the use of traditional style levers for electronic propeller control, a dated roof panel for systems control, and a crew alerting system that is not a full EICAS. The Q400 is not, however, a new design. It is a sensible development of a well-established model and fleet commonality has had to be considered.
The Q400 is young - certification from Canada, Europe and the USA was gained in the last nine months and it is too early to judge how well it will succeed in selling against turbojets for short-haul sectors. As of March 2000, Bombardier claims it has 125 orders and options against a break-even target of 400.
Source: Flight International