Airtest – Learjet 25 & 35

Learjet business aircraft, now split into the Century Ill and Longhorn series, have captured 26. 2 per cent of the world’s certificated civil bizjets as of January this year. The nearest rival is Cessna, with its Citation series cornering 17. 3 per cent, according to Aviation Data Service Inc.

The Bill Lear Sr story has been told many times – but it’s still hard to realise that the incredibly brilliant Learjet story only began 21 years ago this year. It was late 1959 when Bill Lear Sr, who had already dedicated 30 years to the development and manufacture of aircraft electronics, systems and airframes, inaugurated preliminary design studies on the Learjet 23 in St Gallen, Switzerland.

His right-hand man was Flug-und-Fahrzeugwerke’s chief designer Dr Hans Studer who had already masterminded the Sapphire-powered P. 16 ground-attack fighter. The final design was jelled in March 1961 and designated the SAAC-23, built around the primary objectives of optimum speed, comfort, utility, economy and integrity.

To add to design problems (which included the tip-tank idea forced on the team by an eight-spar wing giving a very stiff component), Bill Lear’s multinational manufacturing program hit turbulence. . . . He had production components in hand from Heinke! in Germany, the Swiss firm of Thommen making hydraulic system parts, Cleveland making oleos and Goodrich manufacturing brakes, engines and electronics from the US, and many other contractors in other parts of the world.

Getting it all together was a headache; the European sector wouldn’t work at the pace Lear was accustomed to, and there were customs delays in getting the US-made parts into Europe.

Lear did the obvious – and very expensive – thing; he moved the entire shooting match to North America following the completion of the first fuselage mockup at Altenrhein and the start of production tooling.

In 1962 the entire project was relocated to Wichita, Kansas. Concurrently the firm’s name was changed to Lear Jet Corporation, then to Lear Jet Industries Inc. , and in 1969 to Gates Learjet Corporation.

September 1963 saw the prototype Learjet 23 rolled out for final checkout, with the first flight accomplished on October 7 that year. Full FAA-type certification was won in July 1964, and the first Learjet to enter business service for a customer (Chemical & Industrial Corporation, Cincinnati) got going.

The rest, as the world knows, is stardust. . . .

Here are a few highlights of the Learjet story: May 21, 1965, three officials speed records for transcontinental hops; December 14, 1965, a standard production Learjet flies to 40,000ft in seven minutes, establishing a new bizjet time-to-climb record; March 1966, Learjet 24 gets FAA type certification in air transport category; May 23-26, 1966, a standard Learjet 24 with four pob circles the Earth in 50 hours 20 minutes elapsed flight time, setting 18 world records; August 12, 1966, a 10-plane Learjet, measuring 4ft 4in more than the earlier 24 Model, makes a perfect first flight.

This new variant, the 25, got FAA-type certification on October 10, 1967, and on February 20, 1968, a standard Learjet 25 bolted up to 40,000ft in six minutes 19 seconds, setting a new world mark.

Bill Lear Sr went out of production in April 1969, resigning as Board Chairman, but his brainchildren flew on. On August 30, 1970, a Gates Learjet 25C established a new round-trip speed record for ALL classes of commercial aircraft, setting business jet records en route. The 25C naturally enough won FAA certification as a transcontinental-range type in September of that same year . . . which, in a way, is where we come in.

Stillwell Aviation had positioned their golden oldie – Learjet 25C Tango November November – at Sydney airport in response to a client charter. What better way to sample this method of corporate travel. The AADR evaluation team joined Captain David Foster and First Officer Nina Plummer in an operational workout to see what this Learjet can really do for customers and/or owners.

This sortie was not an ordinary Point A to Point B flight; hence our notes are somewhat sketchy. But the overall evaluation proved beyond all doubt that the business world should be grateful to Bill Lear Sr for his convictions, and the courage he had to implement them.

The flight also made us more aware of the reason why Bib Stillwell lost his heart to the legendary Learjet.

Some of Australia’s leading companies have gone Learjet, among them being Katies (35A), Utah (35A), Mt Isa Mines (35A), Barclay Brothers and United Pacific Airlines also with a 35A apiece, Jet Charter operates two 35As and Arnold Glass with a 24D.

One of the classics in Australian utilisation is the current work done by Wards, with two 35As – each with seven hours’ daily use.

Stillwell’s 25C is also in regular demand for corporate charter, special missions and other work. Its operating figures give a broad hint why – 565 knots TAS top speed, 1765-mile range with four passengers and 45 minutes reserve, 45,000ft operating height and 15,000lb max take-off weight.

Powered by a brace of CJ610-8A turbojets, the 25 Series can tote up to eight passengers plus two crew (Learjets are certified only for two-pilot operations in Australia).

The T-tail 25C in which we made our evaluation made us aware of the direct, almost frighteningly simple manner in which Lear tackled the building of his concept:

• He produced a fuselage with a smaller, circular cross-section than competitive mini-jets, both light and super-strong. His reasoning? Simply passengers wouldn’t object to not being able to stand up in an aircraft any more than they would in a car.

• He evolved a Dutch-type door, the top half giving some protection from the elements. Both halves came together under the urge of a Lear-designed electric motor which pulled the pieces together tightly enough to align the locking pins with their respective holes.

• Under his sometimes fanatical leadership, parts were designed and redesigned to produce an item with maximum weight-saving built into it. One wondered, while eyeballing TNN, what would have happened if Lear ever had gone ahead with his Model 40 concept, the mockup of which showed it designed to carry 40 people. . .

Nina Plummer was pre-flighting by torchlight at Sydney airport when the team joined her, and waited for David Foster to come back from flight planning. The light of Nina’s torch highlighted the beauty of Lear’s dream – a dream which didn’t turn back into a· pumpkin at midnight but went on to capture an estimated 26. 2 per cent of the world’s jet-maker market.

Without giving vent to an excess of imagination, one could literally feel, by running a gentle hand over the 25C’s skin in that pre-dawn half-light, the strength and power of this subsonic creature.

Pre-flighting is necessarily lengthy when compared to a piston-powered twin-engine corporate aircraft.

We had five people on board for this sortie and take-off weight, according to David Foster, would be 15,000lb. Rotation speed was calculated at 138 knots, V2 at 147 knots.

Onboard the cockpit presented a surprisingly compact array considering the sophistication built into it. Another reminder of the Lear genius….

In essence, flight and nav dials were mounted in front of the left-hand seat, an annunciator spanning the coaming top, engine instruments centre, and nav and auto-pilot controls below the power console.

Among checks, with all on board, airstairs shut and locked, were pre-start routines, power off and power on drills. The start drill includes monitoring exhaust gas temperature (EGT) during the actual start, with the engine’s thrust lever at IDLE, tripping the generator switch at idle rpm, and checking fuel pressure light OUT (this means the jet pump is operative).

Anti-skid and yaw damper checks over, we taxied out to the holding point, holding down the wheel master switch. The ground ride was, as befitted an aircraft with such inbuilt strength and some rigidity, quite firm but infinitely within the Comfortable range.

Outside noise during taxi, climb, cruise and descent was never much above the average, while visibility through the knife-edge screen was reassuringly adequate. Through the passenger ports, it was pronounced excellent.

Twenty degrees of flap selected, trims checked and power levers forward, we rolled . . . disengaging the wheel master switch en route.

Acceleration was brisk indeed – a steady build-up of force expected with this type of aircraft. Vr and V2 came on cue very fast, and rotation produced an immediate reaction at about 3000ft of runway consumed.

David and Nina didn’t feel this takeoff was anything really spectacular but boring up through early morning layers of stable cumulus at 4500fpm, accompanied by a quiet electric-motor type hum proved to be one of the most exhilarating experiences we’ve had since flying the Macchi trainer and Navy TA-4G.

Pressure, pre-selected, came in smoothly, giving an average 8. 9psi differential on our way up. Controls, it was apparent, were positioned in this compact cockpit to maximum effect in reducing pilot workload, and passengers are more than adequately catered for in terms of comfort, noise levels, visibility, ability to work in a quiet atmosphere and also get the benefit of a mini-galley service.

Our first batch of readings included a climb rate of 4500fpm, 98 per cent rpm, EGT reading 640 degrees, and EPA (engine pressure ratio) 2. 35 – and all this added up to an indicated airspeed of 300 knots while pointing up towards the beyond.

Six minutes after lift-off we were at 20,500ft, in a shirtsleeve atmosphere belying the -10°C reading outside, ambling upwards at a comparatively sedate 2100fpm. EGT was then reading 650, still at 98 per cent revs, fuel flow notching 1400Ib/hr total.

And eight and a half minutes after the wheels left Sydney airport we sat at Flight Level 250, at 86 per cent revs, with fuel flow coming back to 750Ib/hr.

Our mission called for flight at varying heights, and it was not possible to make long-term readings. But a sample, at FL250, included 90 per cent revs, EGT 500C, EPA 2. 5 and gases going out the back end of the turbines after being used at 950Ib/hr. Speed was read out at Mach 0. 72.

A further reading came out at 300 knots IAS, outside air at -20°C, engine pressure ratio 2. 4, 91 per cent power and EGT at 500. Fuel flow was 900Ib/hr.

Manual study during this flight brought to light stress limits of +3G to -1G with flaps up, and +2 with flaps down; approaching the stall, the stickshaker operates with 80Ib forward pressure about seven per cent above the speed at which the stick-pusher comes into play.

Stick forces throughout this flight were light to moderate within the flight envelope, with stability good at all airspeeds flown. A yaw damper does a truly efficient job in ironing out lateral oscillations, but the aircraft will swing freely at the instant an engine goes.

In single-engine flight, with thrust close to the centreline, yaw can be trimmed out and the 25C put back on auto-pilot.

Figures to remember in the 25C include Vmo 306 kts IAS; Vlo 200; Vfe (full down) 153, and at take-off or approach settings 202; Vmca of 102 knots, and Vmcg (ground control, continuing take-off on flight controls only) 100 knots indicated. Demonstrated crosswind capability is 28. 5 knots.

The 25C is cleared in Australia for operating heights up to 45,000ft.

Clean stall is booked at 136 knots, with 111 listed in a dirty condition. And while on stalls, there’s a tail rumble apparent before the stick-shaker shakes and although this 25C is a T-tail there are conventional aerodynamic wing stall characteristics throughout.

Slotting into the Sydney approach pattern proved no sweat; pre-landing checks include gear down to 200kts IAS or less, spoilers check, and flaps down in stages. Trim changes were moderate to strong with gear extension or flap operation. Moderate trim changes are apparent with the spoiler pop-out.

Yaw damper is checked OFF at about 6ft above touchdown. There was a slight tendency to wander on finals; aileron control lessens at lower speeds, explained David.

* * *

Perhaps a better example of true corporate flying was shown by Stillwell Aviation during a later sortie, in Learjet 35A Uniform Papa Bravo, just back from a New Zealand charter and fleetingly positioned at Sydney airport.

The 35A, well into Learjet’s Century 111′ Series, is approved for a 45,000ft cruise, well into the stratosphere where really fast travel is a reality and the slice of sky occupied by more and more corporate characters who want to see Time compressed – and work. in near silent comfort while travelling from one place of business to another.

Boasting true trans-continental range – 2785 miles with four passengers on board – the 35A . is listed as an eight/10-place aircraft with a TAS up to 545mph, powered by bulbous (by comparison with the older CJ610-8As) Garrett AiResearch TFE731-2 turbofans.

The 35 was officially unveiled eight years ago in Wichita, along with the 36. FAA certification obtained in 1974, the 35 has gone from strength to strength and following an announcement in 1978 became available as a special mission type with underwing hardpoints and a highly sophisticated surveillance system.

One toured Australia earlier this year in the hope of interesting the Federal Government in buying a copy or two. Governmental reaction has been marked, as is so often the case when it comes to considering new aircraft, by silence.

The 35A represents probably the last of the conventional – as we know it – Learjets. The 36A followed the 35A, with a mind-stretching range of 3285 miles. After that came the Longhorns. . .

The Longhorn made its first flight late last year, powered by two Garrett AiResearch TFE731-3 turbofans and destined to operate at 51,000ft with exceptional performance. Figures include 534mph and max range of 3575 miles.

Dubbed the 54, 55 and 56 Series, the Longhorns will all have refined high aspect ratio wings plus winglets designed to harness complex wingtip vortices – result, the greatest fuel savings at high altitudes.

But the Longhorn story is yet to come. . . .

Our test 35A was typical of the current breed in Australia, designed for true transcontinental work. A ’79 model, it featured just about all the options available, including radar and long-range navigation systems. That the new Garretts are doing their job is proved by sideline noise tests – actual EPNdB of 83. 6 for take-off and 91. 3 for an approach make it ideal for curfew beating, among other things.

Bought about a year ago and used extensively on charter, UPS was flown on a route check for our evaluation with Captain Rob Scrivener in the left-hand seat and First Officer Stuart Stevenson with him in the front office.

Engine start in the 35A is slightly different from the 25C, because of the turbofans. Switches on – including inverter, battery and fuel computer switch – the starter-generator switch is engaged. The Thrust lever comes back to IDLE at 10 per cent turbine speed achievement. Oil pressure indication must register within 10 seconds. At about 50 per cent turbine speed the starter will automatically disengage.

The Garretts are quiet indeed. There wasn’t all that much difference in decibels within the aircraft from engine start, through taxiing out to take-off and climb/cruise.

We had four pob for this sortie, and take-off weight had been computed at 14,000lb about 4000lb under gross. Outside air temperature was a hot +29°C according to the gauge, wind light and variable.

Pre-take-off checks include stall warning switch ON, flaps selected (in this case 20 degrees), thrust levers to take-off fan speed before brake release, and once rolling, the nosewheel steering switch should be OFF by 45 knots.

We had earlier taxied out initially on the port turbofan these engines, as in all turbos, are thirsty beasts in a thick atmosphere.

Through V1 of 108 knots, V2 at 124 knots, the sleek 35A fairly leapt off the deck into the upper atmosphere. pegged by the bug to a healthy attitude of climb.

As in the earlier flight, acceleration was brisk without being “military” in any way a healthy sustained push back into the seat rather than a white. knuckle ramming effect.

The initial rate of climb was 3000fpm at 250 knots indicated, rising to 3600fpm at 260kts by the time we had bored up to 6000ft.

This rose fast to 4000fpm, with IAS going up to 260 at 98 per cent power. Three minutes after the beginning of the take-off roll we were climbing like the proverbial dingbat and going through t 8400ft.

Airspeed went up to 321 knots TAS (it’s so easy to calculate true airspeed when digital readouts do it for you!) and five short minutes from the start of the roll we 9 V were sitting at Flight Level 200 with -15°C outside, pressurised inside, and still going up at 349 knots TAS.

Ten minutes after the wheels started turning on the take-off run at Sydney airport, UPB had made it to FL 310 without panting and was still going up. passing through an outside air temperature of -43°C.

This was really Lear country – looking down on an RPT aircraft making for Sydney. . . .

Flight Level 340, OAT -49°C, turbine speed and fan speed at 96 per cent. EGT at 840C. . . .

Elapsed time 13 minutes; Flight Level 360, -55°C and TAS 404 knots and STILL going up. . . .

Fifteen minutes elapsed – FL 390, TAS back a little to 389 and climbing at 1500fpm. . . .

Sixteen minutes after roll start, we made a further reading while trying to tear Our gaze away from an utterly beautiful skyscape over Australia. FL 410, -63°C outside. TAS 410, 95 per cent turbine speed, 96 per cent fan speed, EGT 780, with fuel flow dropping back to 650b/engine.

Bringing turbine and fan percentages back to 92 and 94 respectively, we hopped the TAS up to 423 (Mach 0. 78) with a fuel flow 1200lb/hr total.

Thumbing the stick-mounted trim in small pecks, the 35A assumed a slight nose-down attitude. Looking far, far down we saw we’d made Canberra in 24 fleeting minutes. . . while bucking a wind of 45 knots almost right on the nose.

It seemed unfair to airlines.

We beaconed our way around the traps and all too soon began a long. raking descent under Sydney’s directions. While this began, Manual figures were extracted for your information:

Seventeen thousand pounds max take-off weight; balanced field length at gross, sea level, standard day, 4224ft: Vmo from s/t to FL 140 307kts indicated above FL 140 359 LAS: VIO 202kts, except for emergency descent; Vle 265kts; Vfe (full down) 153kts, eight degrees 198kts; Vmca 112kts, Vmcg 103kts.

Max overspeed for turbine or fan, 100-103 per cent for one minute; 103- 105 per cent for five seconds.

Aircraft operating limiting temperatures +50°C to -54°C which makes nonsense of any criticism about operations in outback Australia. Stress limitations are +3G to -1G with flaps up, and +2G with flaps extended.

The evaluation aircraft cranked up to Mach 0. 8 on initial descent, notching 421kts TAS in an OAT of -61°C, 87 per cent N1 (fan speed) and 80 per cent N2 (turbine speed).

With a decent rate pegged at 2500fpm, the 35A continued downstairs through FL 320 at Mach 0. 75 to FL 120, where Sydney advised getting down a bit quicker. No trouble -out came the spoilers with attendant airframe rumble. descent rate steepened to 4000fpm with power at flight idle and at 8000ft we were reducing speed from 246kts TAS with OAT going up to +6°C.

Not above 210kts, said Sydney, out came eight degrees of flap, with a moderate pitch change being trimmed out. On approach, speed was eroded to 160-170kts, with gear coming down plus an apparent pitch change at 1800ft.

Twenty degrees of flap, speed back to 145 indicated, 85 per cent N2, 66 per cent N1. . . .

Six hundred feet full flap. . . 130 knots, decaying to 118 at the touch.

The entire evaluation had taken 60 minutes, including going to Canberra via Wollongong and coming back by way of the Bindook beacon a significant dogleg flight from Sydney to the national capital and back to sunny Sydney.

As we have written before, corporate air travel in a company aircraft- be it leased or owned, Chartered or ad hoc hired is here to stay, albeit several decades behind most other sophisticated countries.

And proceeding about one’s business in a Gates Learjet is surely among the most supremely efficient methods known, seriously laying aside any shallow replies regarding snob value or keeping up with the Joneses.

This journal looks forward with a certain excitement to the coming Longhorn series arriving in this country. We know that Stillwell Aviation will be in the van when the first $3. 5 million Learjet 55s arrive from the US -In fact, we understand that some are already on order by prospective Australian corporate buyers.

Perhaps we aren’t as far behind the times as we think. . . .