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BOEING 737-900 IN FLIGHT TESTS

After 260 flying hours, Boeing's 737-900 programme manager, Jon Robinson, and his director of flight testing, John Corrigan, are extremely pleased. Flight trials of the 737-900, whose fuselage has been stretched to an impressive 42.1m, are proceeding as hoped, and despite the significantly longer fuselage as of half-way through the test programme there had been no unpleasant surprises, as the two Boeing experts explained in an exclusive interview with FLUG REVUE.

"The aircraft rotates a little more slowly than an -800, pretty much like a -400, but it flies completely smoothly and also achieves all the expected performance data," enthuses flight test director Corrigan. Fears of tailstrikes, i.e. where the tail comes into contact with the runway on take-off or landing due to the unusually long fuselage, have proved unfounded in practice. "As a precaution we have strengthened the tail skid somewhat, but up to now tailstrikes have not been a problem," explains Corrigan. "The changes in flying behaviour tend to be more subtle," he explains. "We did have a problem with vibration during trimming of the elevators, which meant we had to change the trim tab. But we have already successfully modified a component on the prototype aircraft, which will now need to be certificated for the series."

"As the fourth member of the family of Next Generation 737s, the 737-900 is already like an old friend to us," says programme manager Robinson. "The overwhelming majority of the certification data it has been possible to calculate, and during flight testing we are just finding those calculations confirmed." For this reason Boeing is using only a single prototype, with the second test aircraft already earmarked for delivery to a customer later on.

Following the maiden flight on 3 August 2000, the prototype and its production mate are required to spend a total of 438 hours in the air plus another 120 hours undergoing ground tests before joint FAA/JAA certification can be achieved as planned by the middle of March. Since January the second test aircraft, with full cabin furnishings, has therefore been supporting the flight programme for a month. In particular, it has been used for smoke alarm tests, cargo tests and to check other cabin systems.

The prototype on the other hand, crammed full of test equipment, spends most of its time attended by between 12 and 20 flight test engineers, often operating out of Edwards Air Force Base in the Mojave desert of California. "For flight trials with higher levels of risk, as a precaution we often take only a basic team of eight engineers on board," explains Corrigan, adding, "while on some other flights, for example to LAX [the international airport of Los Angeles - Ed.], even customer airline crew were allowed to control and land the -900."

The extra length on the 737-900 is not the only record likely to be achieved on this programme, but, according to Boeing, the expected savings should also be of record proportions. The manufacturer is hoping that its latest offering will not only undercut the seat mile costs of the other Next Generation 737s  the 737-600, -700 and -800  but also those of the A320 family of arch-rival Airbus. If it succeeds, then the -900 will be the most economical short- and medium-range jet around. Boeing's latest "baby jet" is almost 11m (35.7ft) longer than the 31.2m (102.5ft) "short" 737-600, and can accommodate 177 passengers in a typical two-class configuration, rising to a maximum of 189 seats. With a maximum cruising altitude of 41,000ft or 12,500m, the 737-900 outdoes all its older predecessors, the 737-300, -400 and -500, as in fact all the Next Generation 737s do, in the present case by 4,000ft. And even the sophisticated A320 family cannot climb higher than Flight Level 390.

With a maximum number of passengers of 189, Boeing has reached the final upper limit for the 737, whose basic design dates from the 1960s. In those days a 737-100 carried only 103 to 115 passengers.

The 189 passengers permitted today is the maximum number able to pass quickly enough through the doors and evacuate the relatively narrow emergency exits over the wings. All Next Generation 737s are already routinely fitted with improved, automatically outwardly opening escape hatches over the wings to enable today's larger passenger complements to exit the aircraft rapidly in the event of an emergency. From the certification viewpoint, 189 seats are actually already permitted in the 737-800, which is only moderately stretched However, only in the larger -900 can they be transported in real comfort. The new aircraft offers 9% more cabin floor space, 18% more cargo space and 15 seats more than a normally configured -800.

"Even if from the structural viewpoint it were feasible to consider yet another stretch, a 737-1000, the evacuation requirements rule this out," explains Jon Robinson, adding, "Instead, we are already thinking about a possible ERX version of the -900 with a particularly long range. The -900 can be supplied today with blended winglets, if a customer so wishes. And of course we are playing theoretically with all the possible variants, for example, variants with increased take-off weights or a -900 business jet, in fact with all the growth options that are possible." However, according to Robinson, there are currently no plans to use the alluringly large fuselage for a pure freighter, although it would appear naturally suited to that role.

The main decks of the 48 aircraft which have been ordered to date will thus be carrying passengers. And their customers are certainly intent on filling 177 to 189 seats right away with fare-paying passengers. The first airline customers, Alaska Airlines, Continental, Korean Airlines and KLM, are all looking primarily to increase their short- and medium-range fleet capacity economically with their "giant babies", for all members of the Next Generation 737 family have the same cockpit, the same engines, the same wings and the same systems. They can therefore be flown by pilots with a common type rating. This means savings in training and personnel and increases the flexibility of 737 operators in the event of fluctuations in demand. Maintenance is also simplified, and already in the Boeing factory all 737 family members, in their mixed colours, roll out off a single final assembly line in Seattle. Even the -900 prototype is a product of that line.

Apart from its length, the -900 does actually have a few other differences: the cockpit is the first 737NG flightdeck to have state-of-the-art flat panel displays rather than CRTs. The screens are programmable, so that they can be made to look exactly like previous 737NG screens, which is important as far as the pilots' common type rating is concerned, or, alternatively, as in the Boeing 777, they can be easily adjusted so as to display additional information and alter the presentation. The design of the passenger cabin has also undergone a major revamp and reminds one strongly of the design of Boeing's giant twin, the 777.

Meanwhile prototype "N-737X" is flying test runs between Washington State and California. This includes at present flying different speed ranges, calculating the minimum speed on take-off and landing and ground clearance, plus flight stability tests. The next item on the test programme agenda will then be fine tuning of the autopilot, especially for range optimisation, automatic thrust control and various cruise profiles.

If certification is plain sailing as expected and all the indications are that this will be the case Jon Robinson and John Corrigan plan to then deliver their giant baby to its launch customer, Alaska Airlines, in April.

From page 24 of FLUG REVUE 3/2001