Andrew Doyle/MUNICH Max Kingsley-Jones/STOCKHOLM Paul Lewis/WASHINGTON DC Guy Norris/LOS ANGELES

When Boeing began to study a replacement for its successful 737-300/400/500 "Classic" family of narrowbody twinjets in the early 1990s, it faced difficult decisions.

Airbus Industrie was on the scene with its technologically advanced fly-by-wire A320 family, but many of Boeing's biggest 737 customers said they wanted a high degree of commonality with their existing fleets - in addition to performance enhancements and reduced operating costs. "They wanted simplicity, reliability and low cost, not technology for technology's sake," Boeing says.

The Seattle-based manufacturer settled on a derivative approach for its Next Generation programme (737NG), launching the 126/149-seat 737-700 in November 1993 with an order for 63 aircraft from Southwest Airlines.

The aircraft introduced a larger and more efficient wing, giving a range of around 1,670km (900nm). Cruising speed also increased by around Mach 0.045 to M0.79, making the 737NG a suitable replacement for 727s in some markets. The new wing also allowed maximum cruising altitude to be raised by 4,000ft (1,220m), to 41,000ft. The CFM International CFM56-7 engine, an improved version of the 737 Classic powerplant, was selected as the sole option. A full technical description of the aircraft was in Flight International 28 August-3 September, 1996.

The stretched 737-800, with room for up to 189 passengers in a high-density configuration, was launched at the Farnborough air show in September 1994. The smallest member of the family, the 110/132-seat -600, was launched with an order from Scandinavian carrier SAS in March 1995. The final and largest member to be launched, the -900, was first ordered by Alaska Airlines in November 1997. Like the -800, this can carry up to 189 passengers, but in a less dense, two-class, configuration.

According to the airlines surveyed by Flight International, Boeing has achieved its aim of delivering a better-performing 737, while maintaining a high level of systems and operational commonality with the Classic variants.

The service introduction, which for early customers came months later than planned, because of Boeing's well-publicised production delays, was not trouble-free, however.

Few operators are as uniquely qualified as Southwest Airlines when its comes to an initial service assessment of the 737NG. Versions of the twinjet have formed the backbone of the Dallas-based carrier's operations for all its 28-year history. Among its almost 300 aircraft are examples of all three basic iterations, including -200s, -300s, -500s, and the first -700s off Boeing's Renton production line.

Southwest order

Southwest was one of 30 airlines approached by Boeing in 1991 to help define the then-designated 737X successor to the 737-300/400/ 500 series. The airline rewarded the Seattle manufacturer in November 1993 with a commitment for 126 aircraft (including 63 firm orders), all for the baseline 737-700 variant, the first example of which entered service in January last year.

About 60 of the -700s will be on strength by the end of this year and more will be added. The airline plans to introduce 25 aircraft a year through to 2006. "The pilots love it so much they want to see a lot more of them," says Greg Crumb, Southwest's director of flight operations.

As one of Boeing's "working together" carriers and, latterly, as a launch customer, Southwest has been heavily involved in the design of the 737NG from the outset. Its influence was most keenly felt in the cockpit's configuration. Boeing's dilemma was either to maintain some commonality with the second-generation 737 Classic series or to adopt an all-new digital cockpit and fly-by-wire (FBW) controls.

The compromise was a conventional non-FBW aircraft, but equipped with six liquid-crystal displays identical to those found on the 777. The common display system's (CDS) software dexterity allows primary flight and navigation data to be formatted in an electronic flight instrumentation system (EFIS) style, mimicking that of the 737-300/400/500, or in a configuration more akin to that of a 777/747-400.

Crumb explains: "What I wanted for myself and other Southwest pilots is to be able to sit down in a -700 and really not have to think about the differences. We've replicated round dials on a flat screen, so that when I'm in a -700, I've the same display in front of me as the -300. This allows us to step from one aircraft to another on any given day."

Southwest maintains a common pilot rating, not just between the 737-700 and -300/500 types, but across all three generations of 737s. The carrier plans to run on 33 hushkitted 737-200s until about 2005. The carrier has tailored its new aircraft with buyer-furnished equipment that provides standard operating procedures across all four types.

Every new 737-700 delivered to Dallas Love Field is fitted with an additional laptop computer and battery to calculate take-off criteria. The computer factors in the aircraft's actual loaded weight, prevailing environmental conditions and individual airport runway lengths.

"It's not only a safety benefit, but standardises the way we compute our performance data for all four aircraft. So, performance-wise, there is very little difference in our computation. The only thing that differs is that, in our -200s, we're looking at an EPR [engine pressure ratio] reduction," explains Crumb.

The -700s from this year are also being equipped with Flight Dynamics head-up displays (HUDs) similar to those being installed on all -300/500s, allowing manual Category IIIa minima approaches. The HUD software for the -700 has had to be modified to ensure that the capture and display of glidescope and localiser information is identical to that now shown on analogue-equipped 737s.

Southwest's determination to ensure cockpit procedural commonality with the 737 fleet has gone as far as a retrograde of some of the -700's new automated functions and systems. This has included de-activating the aircraft's vertical navigation mode and the auto braking and auto throttle systems. "We don't have the time or inclination to want to have odd aircraft. Every aircraft has to be standardised, even if we have to dumb down the aircraft, we'll do it in order to maintain standardisation," says Crumb.

While Southwest has striven to ensure maximum cockpit procedural commonality with older versions of the 737, the evolution in the -700's design has entailed some changes on the ground - requiring an extensive retraining of maintenance and ground handling personnel.

The biggest improvement as far the airline's engineering staff is concerned is the 737-700's built-in-test (BIT) capability, which has contributed towards lower maintenance costs. Southwest will not disclose the amount of money saved, although Boeing set itself a 15% reduction target, compared with that for the 737 Classic. It has also helped to ensure a healthy 98.6% maintenance dispatch reliability for the -700 by the first months of this year, compared with 99.37% for the more established 737-300/500.

"To have a quick turnaround on some aircraft in the past, our mechanics would literally go out and shotgun the aircraft with parts. Now, with only a few keystrokes on the same system, he can pull up the BIT and identify the box and replace the one component," says Bill Stolzer, Southwest's director of maintenance.

The airline's switch from an analogue to a digital aircraft raised early concerns that there would be major maintenance hurdles to overcome. Co-operation with Boeing starting as far back as 1995 on a service-ready programme has resulted in a relatively smooth introduction. The first three aircraft have passed through scheduled 400-day C checks.

"I was really quite surprised at how easily the aircraft went into service and how well our mechanics were able to handle it. We had them trained well in advance of the first aircraft's arrival and Boeing's production slippage doubly ensured that," says Stolzer. That effort has extended down to as far as comparatively simple cabin systems. "There is a different way to service the -700's vacuum lavatory to the standard recirculating toilet. Our ground operations people have had to go on a massive retraining campaign before putting the first aircraft in service," he says.

The introduction of the 737-700 into service has not been without its problems, albeit comparatively small ones. The "biggest headache" for Southwest and other 737NG operators has been the CFM56-7B engine. Signs of early wear had restricted the hydromechanical unit's (HMU) use to 300h before replacement.

Stolzer recalls : "As the fleet was ramping up in size, there were barely enough spares available in the world to keep all aircraft flying. There were some nights that AlliedSignal paid for jet charters to get the HMUs to our main base so that we could do the change at the schedule we had set." HMU restrictions have since been lifted and are now on condition with no prescribed time.

There is also some fine-tuning of cockpit instrumentation, notably fault indication lights on the CDS relating to the engine's electronic control. The power interruption warning is a malfunction that is common to an electronic type of aircraft, says the airline, but the fault signals are not being visibly detected soon enough.

These issues have been characterised as "minor problems" that might normally be associated with "having bought a new car", suggests Crumb, who adds that Boeing has been "very pro-active in seeking a resolution".

From an operational perspective, the 737-700's biggest improvement over the second-generation -300/500 is the aircraft's ability to fly higher, further and faster at less cost. It has permitted Southwest to open new 4-5h-endurance routes including, from January, non-stop services from Las Vegas to Baltimore and Orlando and from Nashville to Los Angeles.

Continental experience

Since receiving its first 737-700 on 30 March last year, Continental Airlines has begun amassing one of the largest 737NG fleets. At the current delivery rate of more than one a week it expects to have 78 in service by year-end. The airline plans eventually to stabilise its 737 fleet at around 311 by 2003, of which 180 will be the new models, if all options are exercised.

By late April, the airline had accepted 22 737-700s and 17 -800s. The airline will also be one of the first to take the stretched -900s, with the first of 15 due for delivery in 2001. Including the -900s, it plans to receive an additional 64 737NGs still on firm order. It also holds options on another 77, but these are interchangeable between all versions.

The Next Generation aircraft are replacing the airline's elderly Stage 2 fleet. They will also be flown by Pacific-based affiliate Continental Micronesia. "Air Mike", as the Guam-based operation is better known, will begin operating its first 737-800s in October. The Air Mike -800s will be cleared for 120min extended-range twinjet operations, allowing them to operate the many long over-water sectors that make up the airline's Micronesian network.

"We are very pleased with the aircraft," says Dave Lynn, Continental's 737 fleet manager. Dispatch reliability is up to 99%, he says. The airline has experienced software-related aircraft systems "nuisance warnings" that have afflicted other operators, but Boeing says it "is adjusting software to filter them out". In general, the faults are flagged because of transitory changes in system status. Boeing is reducing the warning software's sensitivity so that only events which last for more than around 0.75s are registered. "There is nothing wrong with the aircraft, just some of the system software," Lynn adds.

The only other significant operational headaches have been related to the CFM56-7B engine and its accessories. "We haven't had as much of an issue as other carriers with the HMU, but, yes, it has been a problem", it admits. The Continental fleet is expected to be fully retrofitted with the revised HMU by the end of the year. The airline has also experienced a bearing failure, but this appears to have been an isolated event, Lynn adds.

The 737NG has had a dramatic effect on the airline's overall operational flexibility and its ability to mix and match capacity to meet changing market requirements. "It adds another dimension to our 737 fleet in terms of performance," says Lynn. "For example, we take the aircraft out of Orange County's [California] 5,800ft [1,770m] runway, and fly fully loaded, non-stop to [New York] Newark and still meet the tight noise rules. By giving us transcontinental capability at off-peak times it adds a lot of strength to our capabilities." The extra range of the aircraft has also been put to good use on non-stop routes from Houston, Texas, to Caracas, Venezuela, and the "hot and high" (9,300ft elevation) airport at Quito, Peru.

Continental selected optional maximum take-off weight (MTOW) options for its aircraft. The -700s are operated with a 69,460kg (153,000lb) MTOW and engines rated at 24,000lb thrust (107kN), while the -800s are flown to a MTOW of 78,315kg with engines rated at 26,400lb thrust. According to Lynn, a big plus for the 737NG is the ease with which the airline has been able to transfer new crew on to the aircraft from its large 737 Classic fleet. The advanced display system, which allows the primary flight display to be tailored to replicate either conventional "round-dial" (electro-mechanical instruments), or later generation EFIS, playing a useful role. Continental has 58 "round dial" 737-300s, seven EFIS-equipped -300s and 66 EFIS-equipped -500s. "So we have three distinct types," adds Lynn.

Crew convert to the 737NG after a short computer-based training session and one-line check and after watching a training video. Continental was one of the major airlines involved in proposing this training method to theUS Federal Aviation Administration as an alternative means of compliance for conversion.

SAS' view from Europe


The securing of a major contract from SAS in March 1995 for the Next Generation 737 was an important coup for Boeing. Not only did it provide the US manufacturer with its first order for the new family from a major European flag carrier, but it also stopped its then rival McDonnell Douglas (MDC) MD-95 (now the Boeing 717-200) in its tracks.

MDC had been counting on SAS, a major DC-9/MD-80 operator, to launch its 100-seat advanced DC-9 derivative. An order was seen by those close to the deal as a foregone conclusion. "Every one was surprised when the 737-600 was ordered," says an SAS source, "Boeing had not even been included in the original evaluation." It is understood that Boeing's determination to beat MDC to the deal was reflected in a price which SAS found irresistible.

SAS placed orders and options for 70 737-600s, of which 35 were firm orders and, significantly, the airline had the right to convert to larger versions of the family later. The deal made SAS the launch customer for the smallest member of the NG family, and added to the MD-95 project's woes, helping to establish the 110-seat 737 derivative in the market. Four years on, both aircraft are being promoted alongside each other by Boeing as complementary.

As well as being a launch airline for the 737NG, SAS is one of the few major customers that was not a current 737 customer when it introduced the aircraft (others include American Airlines and Korean Air). The airline had a brief, but successful experience of the type in 1993-5 when it inherited ten 737-500s through the 1993 take-over of Linjeflyg.

"We were very pleased with the Linjeflyg -500s, but we disposed of the aircraft when we restructured the fleet as they were the easiest aircraft to remarket," says Jan Olson, SAS engineering manager for the SAS 737 and Fokker F28 fleets.

Olson is based at the SAS engineering base at Stockholm Arlanda, which has responsibility for maintenance of the airline's growing fleet of 737-600s, as well as that for the F28s, which are are about to be phased out. Olson says that some aspects of the new 737 introduction have been disappointing, but confesses that certain problems have been exacerbated by the airline's inexperience with the 737 since it is not a current operator of the type.

Since its order in 1995, SAS has added 20 orders, bringing its 737 firm purchases to 55.It has also increased its options to 40 aircraft.

SAS's first ten 737-600s were delivered as the "Euro" version - equipped with a large galley and toilets and B/E Aerospace variable-geometry seats throughout much of the cabin. The services have just begun with the SAS domestic version, which has a higher-density 116-seat cabin layout with fixed seats, and a smaller galley and toilets.

The airline has switched 11 of its 55 orders to the larger 737 variants, including six 131-seat -700s and five 179-seat -800s. Fifteen of the 44 737-600s on order had been delivered by early May. The first -700 is due in November and the first -800 in April 2000.

SAS' first 737-600 did not arrive in Stockholm until the end of September 1998, about six weeks later than planned, as a result of the Boeing production logjam and the need to install the new upward-opening overwing exits. According to Capt Göran Carlsson, SAS 737 assistant technical pilot, the delivery delay caused a few headaches. "We had to lease a 737-700 from Germania and a -800 from Sterling European Airlines, for crew training and route-proving, so that we would be ready to operate our own aircraft when they became available," he says.

The first 737-600 entered service at the end of October on the Stockholm-Paris route. SAS had been scheduled to take ten 737s by the end of last year, but received eight. Deliveries were back on schedule early this year.

"So far, our dispatch reliability performance has been among the worst [of the 737NG operators], but this is partly as a result of our relatively low experience with the 737 aircraft," says Olson. Dispatch reliability is steadily increasing from a low of 92.6% in January, achieving 97.2% by March this year.

"There have been a lot of nuisance warnings with various systems," says Olson. "Resetting the system will get rid of them, but this is not the official procedure, so we have to run through checks which has led to delays." The level of spurious warnings has diminished considerably, with "a positive trend" now showing, adds Olson.

Olson says that some reliability problems were exaggerated "through poor handling by SAS due to our lack of experience with the new systems on the aircraft". In many respects, the 737-600 is the most technologically advanced aircraft that SAS has operated. Some delay issues have been solved by service bulletins (SBs). Others are expected to be eradicated in the near future.

The relatively high flow of SBs from Boeing since the aircraft was introduced has been a concern, says Olson, "which makes us a little suspicious about the general quality of the aircraft". Olson also criticises the poor quality of Boeing's documentation, saying: "There has been some incorrect information in the illustrated parts catalogue and airplane maintenance manual."

Boeing's overall support of the aircraft has been good, says Olson, although spares delivery time in some areas has been slow.

Most of the systems, such as those of environmental control, hydraulic and fuel, have performed well, says Olson, as has the vacuum toilet system. Olson says the variable-geometry passenger seats are an element of the new aircraft for which SAS had pushed. While the design offers excellent cabin layout flexibility, Olson says that he has been disappointed at the high level of routine maintenance that these complex seats have required.

Performance of the CFM56-7B20s has beaten SAS's expectations, with fuel consumption "a little bit better" than the original brochure promises. Performance degradation has been minimal, says SAS 737/F28 powerplant engineer Ove Anderson, but "it is too early to establish the long term trend", he says. The -7B20 is nominally rated at 20,000lb thrust, but SAS says that it normally derates the engine to "18.5k" to reduce wear and fuel consumption.

SAS was the first airline to take delivery of an environmentally friendly double annular combustor-equipped CFM56-7, which produces lower nitrous oxides (Nox) emissions, "and we have had no problem with that", says Anderson. The engine's electronic chip detector on early aircraft proved to be too sensitive, resulting in a high number of false warnings. These are diminishing. A modification is being considered.

Being the last of the launch customers to take delivery of the 737NG, SAS was not directly affected by the major teething problems that afflicted the type during the early months. The most serious was a spate of engine-accessory gearbox starter-shaft failures, which resulted in an airworthiness directive. This was traced to a manufacturing fault, which was resolved before the SAS aircraft were delivered.

The CFM56-7's AlliedSignal-built HMU has had an unhappy history since its introduction, leading to uncommanded engine acceleration incidents. Anderson says interim modifications were installed on SAS HMUs before SAS' aircraft delivery, and it is awaiting the delivery of AlliedSignal's long term modification.

AlliedSignal recently proposed setting up a worldwide HMU maintenance pool to help speed up modification work, but this idea was met with a frosty reception from the operators, and the idea has been dropped. SAS was strongly against it as it would involve dispatching its relatively young HMUs for repair and replacement by other operators' older repaired units.

SAS's 737 flightcrews have come from the airline's fleets and have been impressed with the aircraft's handling and performance. Some pilots had flown the airline's 737-500s, and regard the new model, with its bigger wing and larger ailerons, as being a nicer handling aircraft.

"The -600 has better roll characteristics [than the -500], it feels more gentle and easier to fly,'' says Capt Hans Weckström, SAS chief pilot on the Stockholm-based 737 fleet. He adds that the take-off and landing speeds are lower on the new model, while crews like the higher cruise speeds and operating altitude that the new aircraft offers. "The -600 cruises at Mach 0.8-0.82, compared to M0.745 on the -500, and fuel consumption is better," says Weckström.

Pilots also like the -600's climb performance, says Weckström: "We can go straight to 41,000ft in 19min with the engine rated at 20,000lb thrust." Boeing's revised production technique for the fuselage rivets has also reduced cockpit noise, with levels lower than the -500 despite higher cruise speeds.

The SAS pilots praise the aircraft's Honeywell liquid crystal displays, but with no commonality issues to worry about they would have preferred Boeing to have gone the whole way and developed an all-new flightdeck. Although much of the flightdeck is new, the aircraft retains the overhead panel of the earlier models, which one pilot described as "jurasic". They also find the flightdeck cramped with insufficient stowage space, compared to other types in its category.

SAS has selected the Flight Dynamics HUD, with the system to become fully operational in October. Carlsson says the HUD will not raise the 737's minimum landing capability, which is cleared to Cat IIIa (75m runway visual range) with autopilot, it will enable departures with a faulty autopilot as the HUD permits manual Cat IIIa approaches to be flown. The system also permits manual Cat II level approaches to be flown at airports equipped only for Cat I automatic landings.

According to Weckström, most SAS cabin crew like the aircraft, particularly the large galley installed on the airline's "Euro" 737-600s. Cabin crew coming from the DC-9/MD-80 have been "shocked" at the weight of 737's passenger doors and the effort required when positioning them. Crews have also been uncomfortable when having to reach outside the aircraft to operate the doors, particularly when away from an airbridge, due to the height of the cabin above the ground. SAS crews are being instructed on a better door operating technique which should eradicate this problem.

Hapag-Lloyd charter debut


Though sales to major European scheduled carriers have been disappointing (only SAS and KLM so far) the 737NG, and the -800 version in particular, have proved popular amongst the region's charter carriers. Germany's Hapag-Lloyd was the worldwide launch customer for the -800.

Hapag-Lloyd managing director Wolfgang Kurth says the airline is generally pleased with the aircraft, though it has suffered many of the same minor problems as other carriers. "The aircraft itself, when it comes to economy, speed and altitude is well exceeding our expectations," he says. "Fuel burn is unbelievably low."

The first aircraft entered service on 23 April last year and another ten 737-800s have since joined the fleet. Eight more are on firm order. In the first year (to 22 April) Hapag-Lloyd's 737-800s performed 9,000 flights and accumulated 23,000h. During this time there were 81 aircraft-related dispatch delays of more than 15min, of which just under half were of less than an hour. Only eight were serious enough to delay departure by more than three hours. The 81 delays added together amounted to 115h, giving an average of just over 85min. The resulting dispatch reliability during the first 12 months was 98.9%, says Kurth, bettering the airline's target of 98.5%. It aims to hit 99% in the second year and 99.5% in the third. "The fleet is following Boeing's predictions when it comes to reliability," says Kurth.

Meetings recently took place in Europe between Boeing, engine manufacturer CFMI, the regulatory authorities and customer airlines to discuss the introduction of the 737NG and what could be learned from the experience. Hapag-Lloyd participated in such a meeting on 28 April, hosted by Germany's LBA and also attended by Air Berlin.

Boeing was keen to establish, he says, why certain European operators were achieving better dispatch reliability rates than their US counterparts early on. Kurth believes the disparity emerged because European carriers simply had to try harder to get their aircraft out on time because of the region's congested and unforgiving air traffic control (ATC) regime. "The European side had to care about the aircraft more, mainly because our flight schedules are so tight and if you are delayed by a few minutes you can easily run into a 2h ATC delay," he says.

Specific problems encountered by Hapag-Lloyd included the poor performance of the proximity sensor electronics unit (PSU), which provides the pilots with information on the position of everything from control surfaces to the passenger doors. The aircraft cannot be dispatched if the PSU is inoperative, and this device was responsible for most of the delays during the first year.

"During our early operations we had tremendous difficulties with that unit," says Kurth, who adds that the problem was traced to the "improper grounding" of the electronics.

A range of other minor issues are still outstanding. "Everyone expected some technical problems but what worries me is that it takes too long to fix those problems, some of which have been on the list since the first flight," says Kurth.

He adds that while Boeing and the airlines worked hard to ensure that the aircraft was "service ready" in time for the start of revenue operations, some key suppliers were not able to support this. The 737NG's CFM56-7 engines have also caused concern. Hapag-Lloyd operates some of the couple of hundred engines in service that are susceptible to high pressure turbine blade cracking, due to excessive vibration caused by the HMU.

"The manufacturer blames the HMU for a number of difficulties," says Kurth. "Everybody is waiting for the next [HMU] modification but that takes about a year to retrofit [to all in-service engines]," he adds. The revised HMU is being incorporated in new-build engines.

Some issues on the -700 have not manifested themselves on the -800, because of the latter's longer fuselage, says Kurth, including a nosegear "shimmying" problem.

He says pilots like the 737NG for its ability to fly at higher altitudes than older variants, thus avoiding much of the congestion in Europe's upper airspace. They welcome safety enhancements such as the traffic alert and collision avoidance and enhanced ground proximity warning systems, fitted as standard by Hapag-Lloyd.

Source: Flight International