Guy Norris/LOS ANGELES
Between them, Boeing and CFM International faced the fastest ramp-up in production and delivery of any commercial airliner with the 737NG. Since delivery of the first aircraft to Southwest Airlines in December 1997, some 250 examples have been delivered to more than 32 operators. CFMI is to hand over 611 CFM56-7Bs this year alone.
The first example, the baseline 737-700 model, was flown in February 1997. US Federal Aviation Administration certification came in December 1997, about two months later than planned, mainly due to the need to incorporate last minute structural changes to the tailplane.
European Joint Aviation Authorities certification was delayed even further after lengthy discussions with Boeing regarding exit limit certification. JAA approval came in February 1998, after Boeing agreed to revise the 737NG's overwing exit design, adopting automatic upward-opening exits in place of the manual hatches of the earlier 737s. As a result, some aircraft completed, or being built to the old design, had to be modified.
The rapid production rate expansion during 1998 to 24 aircraft a month resulted in major delivery delays, and some of the in-service problems the aircraft experienced during its early months of operation. Despite this, scheduled mechanical reliability rates reached the 98.9-99% design target within the first six months, says John Hayhurst, Boeing 737 programmes' vice-president and general manager. "The aircraft is doing very well in service. We've had a few teething pains and we have had to address what issues have shaken out," he says.
Thanks largely to experience gained with the 737 Classic generations, Boeing avoided most pitfalls that accompanied the introduction of the -300, -400 and -500 throughout the 1980s. Airframe and aircraft system-related issues have emerged, however. One of the earliest being a problem of leaking helium gas, which led to the mandated inspection of fire-detection assemblies and parts of the auxiliary power unit (APU).
Some aircraft were also found to be leaking fuel, which led earlier this year to the mandated inspections of fuel-hose couplings. In March, the fleet was also inspected for improperly fitted insulation blankets around the aft strut area of the engine mounts. The US FAA says failure to correct this could have exposed the lower surface of the strut to extreme high temperatures and increased the chances of a fuel-tank explosion and fire. The following month, all power distribution panels were ordered to be inspected. This directive followed an incident in which all electrical power had been lost from an APU-driven generator, following the overheating, melting and subsequent failure of power feeder terminals.
Most service issues that could have become serious problems, were defused early, thanks to Boeing's newly developed aircraft reports process. "We have a process in place where the airlines report almost everything," says Hayhurst. "We measure all the of the issues that come up with each aircraft in the first 30 days of service. It helps us identify any kind of production issues that we need to address."
Causing the biggest headaches, however, were problems with the engines and their accessories. In some early incidents, number four bearing failures caused inflight shutdowns and diversions in Europe and Mexico. "It was an infant-mortality issue," says Wayne Adams, CFM56-7B programme manager. "There were hard particles creating a pit in the bearing, which flaked off. The residual compressive stresses in the bearing were not as high as we'd like and, if you introduce any sort of contaminant in the oil, it creates a defect in the bearing." The problem was traced to a particular supplier and affected 186 engines. Some inflight shutdowns were also caused by failure of the accessory gearbox starter gearshaft. This was traced to inadequate fatigue strength caused by high-tensile stresses introduced during manufacturing, coupled with the elimination of shot-peening in the gearshaft hub. In August last year, a month after the FAA issued an airworthiness directive (AD) calling for inspections of the shaft, the agency issued an AD instructing all 737NG operators to check the magnetic chip detector in the accessory gearbox transfer gearbox for "abnormal particles" that would give an early warning of an imminent shaft failure. The issue was solved with revised manufacturing practices.
More serious problems were encountered with the AlliedSignal-supplied hydro-mechanical unit (HMU) which led, in at least two cases, to uncommanded acceleration to full power in flight, and to flame-outs on the ground. The HMU problem is also blamed, at least indirectly, for the failure of a low pressure turbine shaft which caused substantial secondary damage to the engine of a Transavia 737 in December 1998. The failure of the HMU caused erratic fuel supply to the engine which resulted in excessive differences between shaft and fan rotation speeds, resulting in the failure.
The issues were related to "process problems and wear problems in various valves", says Adams. "The HMU has been a problem, but we have been introducing manufacturing process improvements and, in the long term, we have software and design fixes." A revised HMU design was certified by mid-April, and deliveries are about to start, says CFMI.
One of the uncommanded accelerations was caused by contamination in the HMU, while the second was caused by a problem with the fuel metring valve resolver. "We found a process issue which caused an intermittent failure, and we modified the software, so that, in the event of a failure, the dual track full-authority digital engine control would track the good channel. We also improved the manufacturing process so that the little wire [which caused the original failure] would not have the failure," says Adams.
With the bulk of the problems behind it, the overall performance of the CFM56-7B has reached exceptionally high reliability rates. CFMI says that, as of April, the inflight shut-down rate was 0.013 per 1,000 flight hours, with engine dispatch reliability running at 99.93% and shop-visit rate down to 0.016. Flight hours were rapidly approaching 650,000 and flight cycles were closing on 355,000.
Source: Flight International
