Michael Gubisch, Aircraft Technology Engineering & Maintenance, Farnborough 2010 Spec. Ed.

More than four decades in service, and still going strong - the Boeing 737 family is the benchmark for short to medium range narrowbody aircraft.  What does it take for MRO companies to keep the twinjet flying?

 The 737 generally is a robust, reliable and easy-to-maintain aircraft. After 42 years of airline service and more than 6,400 units delivered around the globe — the total orders currently stand at over 8,000 — the bestselling twinjet family has more than proven itself, not just from an airline operations perspective but also from the point-of-view of the hangar floor. The good maintainability is best reflected in the easy access to the aircraft without specific ground support equipment thanks to its comparatively short landing gear and compact dimensions.

 When Boeing launched the Next Generation programme (737-600, -700, -800 and later the -900) in 1993, one of the central objectives was to improve maintainability and lower MRO costs over the previous Classic generation (-300, -400, -500), which was produced between 1983 and 2000. The CFM56-3 powered Classic series was already the second generation after the original 737-100 and -200 models. Those had been manufactured from 1967 until 1988 and were equipped with Pratt & Whitney JT8D engines. However, the 1990s rejuvenation from the Classic (CL) to the Next Generation (NG) series — prompted by the arrival of the clean-sheet A320 in 1988 — was a much more profound change than the previous update. For instance, Boeing enlarged the NG’s wing area by approximately 25 per cent and significantly increased the use of composites to reduce the aircraft’s weight. The airframer employed the latest generation CFM56-7B engines and fundamentally modernized the flight deck and systems throughout the aircraft.

 Dr Ismail Demir, GM of Turkish Technic, reports that changing engines and accomplishing the related tests on the NGs have been considerably improved over the Classic series. The same can be said for replacing the landing gear and servicing the nose wheel steering mechanism. The actuation system for the flight control surfaces was also simplified, which resulted in easier adjustment and testing. Another maintenance advantage of the NG series is the vacuum-operated lavatory system that replaced the former, gravity-based tank principle, according to Demir.

 The ability to simply interchange components across different models is one of the NG-family’s major strengths. The digital cockpit, onboard maintenance computers and use of built-in testing equipment (BITE) throughout the aircraft’s systems architecture have made troubleshooting and post-maintenance functionality checks much more straightforward.

 Heavy maintenance NANCE

Moving on to heavy maintenance, Demir states: “Since THY [Turkish Airlines] is one of the first operators who integrated the 737NGs into their fleet, Turkish Technic had the chance to perform many of the very comprehensive C6 and C7 checks, including structural task cards. These ended up with less problems compared to the 10-year D checks for the 737 Classics.”

 One reason for the improved structural performance is that Boeing used thicker skin panels for NG airframes than on the CL series. This has resulted in less corrosion and fatigue cracking and, consequently, less need for repair. Another factor is that the fleet is still relatively young. Just one 737NG has thus far been scrapped, discounting aircraft damaged beyond repair in incidents and accidents. This passenger jet is a 1999-vintage 737-600 that belonged to the former UK based charter carrier FlyGlobespan, which entered administration in December 2009. Landon Nitschke, VP operations at Aviation Technical Services (ATS), a MRO provider right on Boeing’s doorstep at Paine Field in Everett/Seattle, says: “In ATS’ experience, even with the improved structural performance and reduced corrosion potential of the 737NG, as with any aircraft, I expect we will see some amount of corrosion occurring in areas of high susceptibility such as doorways, wet areas, cargo bays, etc.” The company has serviced approximately 2,600 aircraft since 1996, two thirds of which were CLs.

 While airlines will be pleased about any reduced need in heavy maintenance, the structural improvements and greater reliability have put under pressure those MRO companies that specialised in airframe work and built their business case upon it. “NGs are less labour-intensive when performing certain airframe-related tasks. Thus they decrease the natural cost advantage of airframe- focused MROs located in lower cost regions like Eastern Europe, Central America or Asia,” explains Jonas Butautis, CEO of FL Technics, a maintenance specialist for 737, 757 and Saab turboprop aircraft in Vilnius, Lithuania. “In addition, servicing NGs is a much more capital-intensive activity for any MRO, as compared to Classics. These two forces — less labour [and] more capital — bring back the competitive edge of major global MRO players, backed by capital rich shareholders.”

Butautis concludes that while the NG series delivers much higher reliability to the operators than the Classic models, the share of heavy airframe tasks in the overall maintenance work volume has shrunk. At the same time there has been a shift in the aftermarket potential from the airframe work to the maintenance of engines, components, avionics, etc., due to the  NGs are less labour-intensive when performing certain airframe-related tasks. Thus they decrease the natural cost advantage of airframe-focused MROs located in lower cost regions like Eastern Europe, Central America or Asia.  FL Technics gained EASA approval to provide MRO services for 737NGs in April 2010. The Baltic company has 20 years of experience in line, base and heavy maintenance of Boeing’s narrowbody family, including modifications, structural inspections, corrosion prevention & control programmes (CPCP), composite and structural repairs.  

Scribe marks

Scribe marks have been a structural issue of both Classic and Next Generation 737s. These are narrow, shallow distinct scratches in the fuselage skin or structure which, if not repaired can develop into premature fatigue cracks. They are typically created when the aircraft is stripped and painted, or during the application/removal of graphic decals. For example, the use of razor blades to remove sealant or trim decals on the fuselage skin can cause scribe marks. The same is true for using metal scrapers to remove sealant from the fillet seals at lap joints or gaps at butt joints. Aircraft that frequently change operators and receive new liveries, which is typical for the volatile 737 redelivery market, are therefore more likely to suffer from scribe marks than aircraft that stay within one fleet for many years.  According to Boeing, scribe marks are not exclusive to its aircraft or the 737-family in particular.

If the aircraft has been stripped, scribe marks can initially be detected by visual inspection. Otherwise it is possible to regularly use phased array equipment to conduct NDT for scribe lines. It performs an optical evaluation of the affected fuselage area by taking a digital image and mapping every blemish. During the following inspection, the tool will go back over the same area and remap the aircraft based on the last mapping. It will alert the user to any changes. If any signs of damage have been found, high-frequency eddy current (HFEC) or ultrasonic (UT) inspections become necessary to detect any cracks. Laser measurement or optical micrometer inspections are employed to determine the scribe line depth. There are manufacturer service bulletins (SB) and guidelines in the structural repair manual (SRM) to assess the damages and decide about further actions.

“The common repair is to cut-out the damage, repair it or perform a modification that requires removing the entire lap joint,” says Nitschke. “The scribe line issue is a recurring issue with all aircraft, but is more common to older aircraft.” Another deterioration typically found on Classic aircraft with high numbers of flight hours (FH) and cycles (FC) numbers is fatigue cracking in the radius of chemically milled pockets in the airframe structure. The ATS vice president expects that this will likely be an issue with ageing 737NGs too as they accumulate high FH/FC levels. He reports that corrosion within the flap tracks is a present issue with NG aircraft.

From letter checks to task-based maintenance

One of the most consequential changes between Classic and 737NG aircraft has been the switch from the traditional letter checks to task-based maintenance — in other words, the change from the MSG2 (maintenance steering group) logic to MSG3. While operators in the past had to adhere to rigid, uniform work programmes for comprehensive checks, they can now group the single tasks into different packages for each individual maintenance event in a manner that is most efficient to them. This freedom allows the operators to take factors such as aircraft downtime, accessibility for maintenance, particular operating conditions, manpower and availability of resources into account. Boeing only provides recommendations, and doesn’t impose a set maintenance schedule on the airlines.

“Maintenance planning capabilities vary greatly from one owner/operator to the next,” states Danny Martinez, VP and GM of AAR Aircraft Services in Indianapolis. “While most large fleet operators have very robust maintenance planning infrastructures and well established reliability programmes in place allowing them to package maintenance tasks efficiently, the small fleet and single aircraft owner/operator typically relies heavily on the OEM’s intervals and recommendations.”

For instance, Turkish Airlines (THY) has currently scheduled A events (comprising certain task packages) to take place after 150FH. Base maintenance, the traditional C check, becomes necessary after 7,500FH or 730 days, whichever occurs first. The Istanbul-based carrier has a fleet of approximately 70 737NGs, which it operates 12FH during five FC on an average day. However, Demir stresses that these arrangements are subject to possible adjustment in the future if operating conditions change.

On 737 Classic aircraft, A checks are typically done after 250FH, C checks after 4,000FH, and a D check (overhaul) including all structural inspection (SI) tasks after 24,000FH. On Next Generation aircraft, all maintenance tasks are arranged as multiples of a basic 500FH phase interval. In other words these tasks have to be accomplished after 500 FH, 1,000FH, 1,500FH depending on their urgency.

Base checks are formed by grouping tasks with intervals of 8, 10, 12 times the basic phase interval, i.e. 4,000FH, 5,000FH and 6,000FH. It is also possible to accomplish tasks, which were traditionally part of a base maintenance event (C check), into an A event. FL Technic’s Butautis reports that the MSG3 philosophy of the 737NG resulted in a reduction of required manpower and aircraft downtime by up 25-30 per cent. This allowed the Baltic MRO company to maintain more aircraft during the same period compared to the traditional A and C checks on Classics.

This is echoed by Leonard Kazmerski, VP marketing and business development at TIMCO, a US MRO provider in Greensboro, North Carolina: “It can certainly be argued that aircraft reliability has improved with the movement from the traditional letter checks to task-based maintenance. Since the aircraft is seen in scheduled maintenance events more frequently — albeit for a shorter duration — more items and nonrecurring findings are identified and can be corrected before they accumulate and combine into time- and cost-driving events at less frequent scheduled letter check visits.” Kazmerski states that the change in maintenance philosophy not only saved costs for the operators but also created a more regulated work flow and greater efficiency for full support MRO providers.

Future outlook

The sheer number of active 737s and continued, strong sales for the model have created a huge market potential for MRO companies. “There are more 737 aircraft delivered and in service today than any other aircraft in the world,” says Nitschke of ATS. “Combined with high reliability, there will continue to be a very large number of 737 aircraft needing to be maintained for many years to come.” The main change, from a technical point-of-view, will be the decreasing number of Classics in the established aviation markets. As the series matures, many of these aircraft will be retired and sold to operators in other regions, be converted to freighters, or used as spare parts sources and finally be scrapped. Many of these aircraft were already parked during the downturn. “The biggest challenge a North American company like ATS faces in light of the ever-growing population of international MRO providers in countries, such as Latin America and China, is maintaining competitive advantages. We must continually find ways to be more efficient while focusing more on customer needs and offering added value to our services.”

Catering for the increased maintenance requirements of ageing 737CLs still in service will be one of the main technical challenges, according to TIMCO’s Kazmerski. “This will likely include more findings related to heavy corrosion, for example,” he says. “It is even possible that, at some point, the Classic fleets could become candidates for OEM-recommended life-limiting thresholds, particularly as more of the NG models become available as economic used and new replacements.” On the other hand, he expects that working together to plan the best possible schedule for maintenance flow will be one of the biggest opportunities for 737NG operators and MRO companies. “Moving to a more harmonious partnership in approaching optimal visit times can ensure the actualisation of efficiencies that promise to reduce cost for operators and improve profit for [MRO] providers.”

Turkish Technic’s Demir anticipates that the falling number of Classics will be balanced out by the proliferation of 737NG aircraft. However, as the latter require less maintenance, the position of the MRO companies will remain unbalanced; the main beneficiaries of the fleet change will be the airlines and not the MRO companies. He expects that, while the Classics disappear from the company’s main market in Europe, a number of them will fly in Africa and the CIS countries. Parts of these regions would still be within reach for the Istanbul-based MRO provider. “[However], the steep increase of MROs — either airline or independent — will obviously lead to a more competitive environment to the benefit of the airlines,” says Demir. “This will eventually lead MROs to revisit their pricing structure. Obviously, the MROs with back shop capability, who can still provide competitive prices, will be the winner in this competitive environment.” He expects a growing trend in airlines asking for full support services. One indicator for this development is the greater number of NG versus Classic operators who buy into component pooling programmes.

With the overall aviation market picking up momentum again in 2010, Butautis believes the worst of the downturn for the MRO companies is over. “FL Technics is positioned on the edge of one of the world’s fastest growing aviation markets — the CIS region. Thus we are optimistic about the future. In a way, we are also helped by one of the deepest economic recessions in the Baltic countries, which will be helping us to sustain labour costs for the next few years while catching the opportunities that global growth delivers. We also believe that further consolidation in the industry is imminent as the MRO business is becoming more and more capital intensive.”