Advertising
  • News
  • FLIGHT TEST: Learjet 40 - Filling the gap

FLIGHT TEST: Learjet 40 - Filling the gap

Bombardier Aerospace has strengthened its light-jet challenge with the Learjet 40. But can it better its Cessna and Raytheon rivals?

Bombardier is unique in offering products covering the business-jet spectrum from light to ultra-long-range. But when the company stopped producing the Learjet 31A in 2003, it left a gap at the light-jet end of its product line - a crowded market segment with several strong competitors already in place, particularly the Cessna Citation Encore and Raytheon Beechjet 400A (now the Hawker 400XP).

Market research showed that any replacement for the Learjet 31A would need to offer a more-spacious cabin and modern avionics while providing lower direct operating costs. To meet this need, Bombardier decided to capitalise on its substantial level of investment in the development of the all-new, super-light Learjet 45, certificated in 1997 by both European and US authorities.

The decision to base the new Learjet 40 light jet on an existing aircraft was aided by several factors: the Learjet 45's speed and comfort; its modern systems, reducing pilot and maintenance workload; and its wide market acceptance - 204 were in service by the time of the Learjet 40's launch and a mature support network was already in place.

Pound for pound

The Learjet 40 is a straightforward shrink of the 45. To pare the super-light design down to fit the light-jet niche, 620mm (24.5in) of fuselage was removed forward of the wing, along with three of the original 16 cabin windows. The shortened fuselage seats up to seven passengers, two fewer than in Learjet 45. Fuselage fuel capacity is reduced by 285kg (630lb) for a total of 2,440kg. With its reduced fuel load, six passengers and two pilots, the Learjet 40 offers a range of 3,300km (1,780nm), 470km less than its larger stablemate. Maximum take-off weight is 9,230kg, only 520kg lighter than the Learjet 45XR. Perhaps the most remarkable difference between these two aircraft is their price. At $7.8 million, the Learjet 40 is nearly $3 million cheaper than the 45XR. If aircraft are, indeed, sold by their weight then the Learjet 40 is a bargain at $844/kg ($383/lb). The Citation Encore, perhaps its closest rival, is a touch more expensive at $1,045/kg. While the Encore and Learjet 40 offer comparable published levels of performance and direct operating costs, the heavier Learjet is faster and features more advanced and robust systems.

Robust light jet

Like that of the Learjet 45, the Learjet 40's wing has three main spars covered by machined skins, while the fuselage carry-through structure is bowed so that it does not impinge on the cabin floor. The 28.95m2 supercritical-section wing is swept 13.4° at the leading edge, with a straight trailing edge. The fixed leading edge has three segments with slightly varying degrees of droop, designed to improve slow-speed handling qualities. Supercritical 1.17m-high winglets reduce drag by around 20% at altitude and increase the range by 3%. Conventional ailerons and hydraulically actuated Fowler flaps make up the wing's trailing edge.

Two sizable ventral fins are located beneath the tail; these enhance lateral- directional stability, allowing the Learjet 40 to be dispatched with its yaw damper inoperative, with no restrictions on alternates. Additionally, they provide a nose-down pitching moment at high angles of attack (AoA). The ventral fins are not part of the aircraft's primary structure, and should they fall prey to hangar rash, they can be unbolted and replaced.

The oval-section fuselage is of conventional monocoque construction and is built for operation up to 51,000ft. Extensive use of single-part machined components has dramatically reduced the parts count. The passenger cabin directly benefits from the wing design, as it features a flat floor for its entire length. Maximum headroom, while it does not allow the passenger to stand up, is a reasonably generous 1.5m. The oval shape of the cabin and its slightly greater maximum width combine to give significantly more head and shoulder room than offered in the Encore.

The standard Learjet 40 interior has seating for six, four in a club arrangement and two in aft forward-facing seats. An externally serviced toilet is standard, and can accommodate a seventh passenger if the belted lavatory option is purchased. The aircraft seats two fewer people than the Learjet 45, yet has a cabin that is only 620mm shorter. Bombardier has put the extra cabin volume to use by installing a full-size galley and coat closet opposite the cabin entry door.

The Learjet 40's powerplants are the same as for the Learjet 45, two Honeywell TFE731-20AR turbofans. The engine is a two-spool design with geared fan and a thermodynamic rating of 4,460lb thrust (19.8kN). As installed on the Learjet40/45 they are flat-rated at 3,500lb thrust up to 31°C (88°F) at sea level. Each is controlled by a single-channel digital electronic engine control (DEEC). A conventional hydro-mechanical control module backs up the DEEC. Nordam-supplied hydraulically actuated thrust reversers are standard on the Learjet 40, quite a feature for a light jet. To increase directional stability on slippery runway surfaces, maximum reverse-thrust N1 is reduced as ground speed decreases.

Safety by design

Unlike some light jets, the Learjet 40, as a derivative of the recently certificated Learjet 45, meets stringent Part 25 requirements. One important area where this is apparent is in the design of the flight-control system. The primary flight controls are mechanically operated surfaces with integral balance tabs. In the event of a jammed elevator, the pilot and co-pilot control columns can be disconnected to allow each to operate one side (left/right) of the elevator. Redundant elevator and rudder control cables are routed on the top and bottom of the tailcone area, reducing the chances of a burst engine rotor severing all control runs to the tail surfaces.

Similar redundancy is provided in the roll axis. In addition to ailerons, the Learjet 40 has a single hydraulically actuated fly-by-wire wing spoiler on each side. A sensor in the captain's yoke controls the spoilers. In the event of jammed ailerons, the captain can disconnect his yoke from the ailerons with the flick of a column-mounted switch. The system operates like that in Bombardier's Challenger 300. Once disconnected the captain can maintain roll control solely with the spoilers.

The Learjet 40's heritage also pays dividends when the rubber hits the road. Its trailing-link main landing gear has dual tyres and brakes. The carbon brakes have brake-by-wire control with independent anti-skid systems for each wheel. As blown tyres are a common cause for rejecting a take-off run, dual tyres on each main gear assembly reduces the chances of a single blown tyre being misidentified as an engine failure. If a rejected take-off is initiated, dual tyres and brakes increase the odds of keeping the aircraft on the runway and slowing it to a stop without running off the end.

Overall cockpit design of the Learjet 40 employs the dark panel concept: lights are off for normal operations. System control panels are on the bottom of the forward instrument panel. System schematics are logically arranged, allowing intuitive operation of their respective controls. Notable for their operating simplicity are the electrical and ice protection systems. The 28V DC electrical system has two engine-mounted starter/generators, as well as two main and one emergency battery. Either engine generator alone can provide sufficient power for night instrument flight rules operations. Automatic load shedding and fault protection features make the electrical system's operation essentially automatic from the pilot's perspective. The anti-ice system uses engine bleed air to heat wing and horizontal-stabiliser leading edges as well as engine nacelles. A nose-mounted ice detect probe alerts the crew to icing conditions, and automatically turns on protective systems.

Enhanced avionics

The Learjet 40's avionics are Honeywell's Primus 1000, but not all such systems are created equal. The Primus 1000 in the Citation XLS has three 255 x 200mm liquid-crystals, while the Learjet 40 has four 200 x 180mm cathode ray tubes. Bombardier puts the extra screen to good use, displaying a full engine indicating and crew alert system (EICAS) as well as system synoptic pages. Two LCD radio management units (RMU) control dual navigation/communication radios, and act as back-up navigation and EICAS displays.

The aircraft comes standard with asingle pedestal mounted Universal UNS-1E flight management system (FMS) with embedded GPS receiver. A second FMS is optional, but was not installed in the demonstration aircraft. Rounding out the avionics package are two features to improve safety: a Honeywell enhanced ground proximity warning system (EGPWS) and a traffic collision avoidance system (TCAS).

US certification of the Learjet 40 was achieved in July 2003, with the first production aircraft entering into service with Bombardier's Flexjet fractional-ownership programme in January. Flight International was invited to fly Bombardier's smallest Learjet at its Wichita, Kansas production and flight-test facility. For the flight, I was accompanied by Learjet demonstration pilot Rodney Lundy. While I followed Lundy through the pre-flight inspection, Learjet demonstration pilot Owen Zahnle, observer for the flight, programmed the flight route into the FMS. All required inspection points are readily accessible from ramp level, and the door for the 1.42m3 (50ft3)-capacity heated baggage compartment in the tailcone is at chest level, allowing easy loading of heavier items.

View forward

Entry to the cabin is via a manual split clamshell door with integral boarding steps. The cockpit is separated from the cabin by a fabric curtain. There is no cockpit space for a traditional flight bag, but there is ample storage space for flight publications behind the captain's seat. I used the windscreen centre-pillar alignment balls to manually adjust the left seat to the correct position. Field of view through the four forward windows was good, my seated position allowing me to see the outboard half of the wing. The large winglets, in addition to reducing drag, provided a ready reference of wingtip clearance when taxiing in congested areas. While the engines can be started on battery power alone, an external power cart was hooked up to power the optional vapour-cycle air conditioner.

While the Learjet 45 is available with an optional auxiliary power unit, there are no provisions for one in the Learjet 40. Both engines were started using the external cart. The DEEC-controlled start brought each engine to idle in less than 40s. Peak inter-turbine temperature on both engines was 680°C, well below the limit of 916°C.

While Lundy used a paper checklist to complete the post-start checks, an electronic one is available as an option. A slight advance of the throttles started the aircraft rolling out of the chocks. Once moving, idle power was enough to taxi at a comfortable pace on level surfaces. The steer-by-wire electronic nosewheel steering is controlled by the rudder pedals and provides ±60° of deflection. The speed-sensitive system allowed me to negotiate slow, tight 90° turns precisely, while not being overly sensitive at higher speeds.

Lundy set the flaps to 8° in preparation for take-off. With a 20kt (37km/h) left quartering headwind and an ambient temperature of 28°C, the take-off speeds for the 8,727kg aircraft were: V1 115kt indicated, VR 117kt and V2 126kt. Computed balanced field length for these conditions was 1,481m. Once cleared for take-off, I advanced both throttles to the take-off detent. The DEECs stabilised N1 at the 90.1% take-off setting. As the aircraft accelerated down the runway, the nosewheel steering allowed me to track centreline accurately. At roughly 117kt, 15kg of aft-stick force was needed to establish the lift-off attitude, the aircraft breaking ground after a run of only 940m. Gear and flap retraction caused negligible changes in pitch forces as the aircraft accelerated to the initial climb speed of 250kt.

Passing 3,000ft above mean sea level the throttles were retarded to the maximum continuous detent for climb to our initial cruise altitude of FL450 (45,000ft). Even at maximum take-off weight the Learjet 40 can climb directly to FL450, above most turbulence and almost all other traffic. Passing FL325 the autopilot, engaged for the climb, tracked Mach 0.7 until we levelled off. Despite hotter than standard temperatures and ATC-directed turns, the climb took just 26min from brake release, burning 375kg of fuel. This compares quite favourably with the book values of 24min and 336kg. The Encore can also climb directly to FL450 at maximum weight, gets there 1min earlier and at an equal distance downrange has burned about 45kg less fuel.

Cruise altitude

Once level at cruise altitude I left the throttles in the maximum continuous detent and allowed the aircraft to accelerate to a high-speed cruise Mach of 0.8. A throttle setting just below maximum cruise held the Mach steady. At 214kt indicated total fuel flow was 500kg/h (1,100lb/h) and the aircraft maintained 452kt true airspeed. Slowing to a long-range cruise speed of 429kt true showed a Mach of 0.757. Total fuel flow was identical to the predicted book value of 445kg/h at 202kt indicated.

At altitude the relative merits of the Learjet versus the Encore shift to a question of time. According to Bombardier, with over 455kg of payload the Learjet offers superior payload-range capability for all missions up to 3,340km in length. While the two aircraft offer nearly identical direct operating costs (DOC) on a per-kilometre basis, the Learjet is 45kt faster at long-range cruise speeds.

While level at FL450, I unstrapped and went back into the passenger cabin. Ambient noise was fairly low and allowed for easy conversation at normal voice levels. Even near the cabin entry door, which employs a single passive seal, noise levels were remarkably low. Further contributing to the placid cabin environment were the 13 large windows. In addition to providing a light and airy feeling, they are mounted relatively high for easy viewing while seated. While the Learjet 40 is not an ultra-long-range aircraft, its pressurisation system maintains a sea-level cabin until passing 25,700ft. At the ceiling of 51,000ft, a pressure differential of 0.65 bar (9.41lb/in2) yields a cabin altitude of 8,000ft. The lower the cabin altitude, the less dehydrated and fatigued the passengers will be on arrival at their destination.

I returned to the cockpit and Lundy requested a descent to 15,000ft to investigate the aircraft's slow-speed handling qualities. Before initiating the descent, I slowed the aircraft in idle power until the red slow-speed marker came into view at M0.56 in the primary flight display's airspeed tape. The Learjet 40 has relatively light wing loading, 294kg/m2 for the current fuel load, and even at this high altitude the aircraft had a safe operation range of M0.25.

Next I advanced the throttles and lowered the nose to accelerate to the maximum operating Mach number of 0.81. Passing FL310 an aural "overspeed" warning sounded as the aircraft sped past M0.81. I stabilised the aircraft at M0.82, where a series of sharp control inputs in each axis elicited a well-damped response. To slow the aircraft, I retarded the throttles to idle and extended the spoilers. Spoiler extension caused the nose to pitch up slightly, while the ensuing airframe buffet provided a valuable tactile cue that lift was being dumped.

T-tail stalls

The first stall was in the clean configuration. An idle-power level-altitude deceleration of the 8,396kg aircraft approximated the certification-standard 1kt/s entry rate. In a 13° nose-high attitude at 121kt indicated, the stick-shaker activated. Control response in all three axes was good, as I jammed the throttles up to the take-off detent. The DEEC rapidly established take-off power and I relaxed aft-stick pressure to initiate a level flight recovery from the stall.

Pleased by the aircraft's ability to power out of the stall, I again retarded the throttles to idle and reapplied aft stick pressure to slow the aircraft below the shaker speed. Slowing through 110kt, moderate airframe buffet announced the approach of the full aft-stick stall at 105kt. At the aft limit of the stick, continual small deflections of the ailerons were needed to keep the wings level in the nose-high descent. Relaxing aft stick pressure let the nose drop and the aircraft to accelerate out of its stalled condition.

The final stall was in a landing configuration, flaps set to 40°. The power was set to 60% N1 to attain the desired deceleration rate. The shaker activated at 105kt. As with the clean stall, control effectiveness was good at shaker speed. Pulling the stick fully aft slowed the aircraft to 91kt and was again accompanied by moderate airframe buffet. While some T-tail aircraft have stick pushers to avoid a deep stall, the Learjet 40's ventral fins provide a nose-down pitching moment at high AoAs, alleviating the need for a pusher. Relaxing stick back pressure and advancing the throttles allowed the aircraft to fly out of the stall.

Before returning to Wichita, I had the opportunity to explore the Learjet 40's roll capabilities. At 15,000ft and airspeeds from 150kt to 250kt indicated, I did a series of half- and full-deflection aileron rolls. Assisted by the spoilers, roll rates exceeded 45°/s at half deflection. Full-deflection roll rates approached 80°/s. Control feel and aircraft response in all cases was exceptional. Even at the limits of their deflection there was no aerodynamic buzz or adverse feedback from the ailerons.

At 250kt Lundy held the yoke full right to demonstrate the aircraft's ability to recover from a jammed aileron condition. A single click of the captain's roll disconnect switch allowed me to use the spoilers to overpower the "jammed" ailerons. At this condition the spoilers had enough authority to reverse the roll and establish a left-hand turn.

Wichita touchdown

Recovery to Wichita was via radar vectors to an instrument landing system approach to runway 19R. With 1,501kg of fuel and flaps at 40°, approach speed was 116kt. Lundy computed a required field length of 820m.

I used the flight director (FD) to maintain assigned heading and altitude before localiser intercept. The FD's split-cue format was easy to follow, allowing me to precisely control the aircraft. Once established on the localiser, pitch changes caused by gear and flap extension were easily trimmed out with the yoke-mounted two-axis trim switch. Once established on the 3° glideslope, around 60% N1 on the engines held 116kt. The FD continued to provide excellent guidance during the final approach phase, significantly easing my task of staying on the localiser and glideslope.

Passing 30ft radar altitude (RA) I slowly started to retard the throttles to idle. At 15ft RA I initiated the flare manoeuvre. The trailing-link landing gear allowed the aircraft to touch down softly, despite my having rounded out a foot or two higher than desired. Once on the runway Lundy set the flaps to 20° for the touch and go.

The next approach was a visual one to a full stop. Again flaps were set to 40°. I found control harmony in the pattern and on final to be good, allowing me to easily fly a wing-low approach with the 18kt left quartering headwind. After touchdown the spoilers, armed on downwind, deployed. Moderate toe braking slowed the aircraft for runway turn-off and allowed us to taxi back for another take-off.

For the final take-off, Lundy again set the flaps to 8°. Take-off speeds remained unchanged from our initial departure. As the aircraft accelerated down the runway, Lundy rapidly pulled the right throttle to idle passing 115kt to simulate an engine failure at V1. The resultant yawing motion was easily detected, and about 35kg of force on the left rudder was needed to keep the aircraft tracking down runway centreline. Slightly after 120kt I initiated a slow rotation to establish the climb-out attitude. Once airborne the gear was retracted and an additional 5kg of pedal force needed to maintain wings-level co-ordinated flight.

Next Lundy turned off the Learjet 40's rudder boost system. Unlike some aircraft that use differential engine pressures to bias the rudder into the good engine, the Learjet uses a yaw damper. If more than a 13.6kg force differential is detected between the rudder pedals, the rudder boost system will use the damper to augment the pilot's pedal input. The boost system can apply a maximum of 40.8kg of force, but by itself is not sufficient to counteract the yaw caused by an engine failure. With the boost system off, an additional 20kg of force was required to maintain co-ordinated flight.

Keep on rolling

From my short experience with the Learjet 40, it seemed to have a fairly low ratio of roll angle to sideslide angle - the phi to beta ratio. The higher this ratio, the more a sideslip angle (beta) will generate a roll angle (phi). The conventional slip ball mounted above the primary flight display was useful for maintaining co-ordinated engine-out flight.

The flaps were set to the single-engine approach deflection of 20° on final. Once established on the glideslope, 64% N1 on the good engine maintained a final approach speed of 121kt. As was the case with the previous two landings, I found the aircraft very forgiving and easy to control during the roundout manoeuvre. Once on the runway I centred the rudder trim and advanced the power on both engines for a touch and go.

The fourth and final approach was a visual one, with flaps set to 40° for a short field landing. On this approach I established an aimpoint close to the approach end, ducking underneath the ILS glideslope. At 30ft RA I retarded the throttles to idle. Just before touchdown I brought the nose up slightly from the approach attitude to attenuate the sink rate. The aircraft touched down firmly and I applied maximum force on the toe brakes. The spoilers automatically deployed, and by the time I got around to pulling the engines out of reverse idle, the aircraft had slowed to below 60kt. The carbon brakes brought the aircraft to a stop after a very short ground roll of 400m. After taxiing back to Bombardier's ramp, I found the post flight and shut down checks were easy to accomplish.

Faster and cheaper

Bombardier's decision to base its newest light-jet offering on the successful Learjet 45 lets it bring to market a capable aircraft for a reasonable price. Like the Learjet 45, from which it is derived, the Learjet 40 has advanced avionics and robust fault-tolerant systems. Its ability to climb directly to FL450 gets it above most other traffic. Once there its long-range cruise speed of 421kt is 45kt faster than the Citation Encore's. And should time be even more critical, the aircraft can dash quite comfortably at M0.8. The six-seat standard cabin provides a comfortable environment that is larger than the Encore's. The Learjet 40's outstanding carbon brakes and wing spoilers give it remarkable landing performance, on a par with or better than the Encore's.

According to Bombardier the Learjet 40 is less expensive than the Encore, while direct operating costs for the Learjet are within 1% of those of the Citation on a per km basis. Not only does the Learjet 40 make sense on a rational basis, it isa delight to fly and, given the choice, operators may well want to accept marginally higher DOCs so they can enjoy the need for speed.

MICHAEL GERZANICS / WICHITA, KANSAS

Advertising
Advertising