Monday, September 3, 2012

Aged Aircraft Maintenance Program


                                   It’s the economy that is forcing owner to keep aging aircraft in service. With money in short supply, aircraft companies, owners, operators, and fleet managers are experiencing an increasing need for their fleets to maintain safe operations beyond their original design lives or service goals. This need results in a growing percentage of aging aircraft that must maintain their airworthiness and structural inte grity by using standard methods of inspection and repair.
                                    The fact that widely-used aircraft are getting older is not the problem. The issue is how much an airline/business owner has to pay to keep an older airplane airworthy – and at what point the hassle and cost exceed the benefits. Without a specified design life, or after the design life is reached, the cost of the required maintenance and inspections must be carefully weighed against the benefit of continued operation.
When this financial ‘point of no return’ is reached – as happened years ago with the iconic Boeing 707 – these aircraft begin to vanish from the skies. 
 Life Extension Strategies
                           Well-maintained aircraft can be kept in service almost indefinitely. Most aircraft are certified for a set lifespan known as the Design Service Goal (DSG). It is generally based on a predetermined number of flight hours and ‘flight cycles’ (pressurization/depressurization cycles) that the aircraft has been designed to endure.
                           The whole aircraft structure is designed to withstand every load which occurs during its operation at least for this time. “If the aircraft shall be operated beyond the above mentioned service goal, the whole aircraft needs to be certified for this life extension by the OEM.”
                       OEMs are amenable to providing such life extension programs. For instance, Airbus’s ESG1 (Extended Service Goal) program makes it possible for A320 operators to boost the aircraft’s life span from 48,000 flight cycles and 60,000 flight hours up to 60,000 flight cycles and 90,000 flight hours.
                                  Making this life extension requires “a huge effort. “(It) includes a new full scale fatigue test of the fuselage and the wing. The operator who wants to operate in ESG1 has to embody certain structural modification latest at the original DSG.”
                                    Engines do not face the same limits as airframes. “There is no restriction on how many times an engine can be overhauled.  There is no time restriction on when the engine should be removed from service.”
                                    To be specific, “since every part of the engine can be replaced by new or repaired ones during its overhaul, there is no point in time when you can’t ‘keep them running anymore.
Restrictions on Rejuvenation
                                   All of these positive points notwithstanding, aircraft are like cars: The older they get, the more problems they can present to their operators as parts wear out.
For operators and MROs, it is the number of flight cycles that can pose the most serious challenges. The constant pressurizing and depressurizing of the cabin puts the airframe under repeated stress. The more cycles, the more likely that metal fatigue will occur; just as a piece of strip of metal bent back and forth in a person’s hands will eventually crack and snap.
In new aircraft, this isn’t an issue. But as the flight cycles and flight hours accumulate, many aircraft will develop fatigue cracking in specific areas over time. 
                                 The design engineers, through analysis and testing, are able to predict the onset of such cracking.  Unanticipated cracking scenarios do arise, but these are often a product of change in the flight profile; namely different stresses than predicted, analyzed and tested for.”
                               The rule of thumb: “The older the aircraft, the higher the potential risk to encounter severe structural findings while performing fatigue test. The aircraft’s operating environment also has an impact: “Salty, coastal areas will create corrosion, as will flying through heavily polluted areas like densely populated areas in Asia. “Some corrosion issues arise with certain areas due to the design, where condensation water gathers in a low point in the airframe.”
                                  This brings us to a key reality of older aircraft. They tend to use a lot more sheet metal than modern aircraft fitted with composites. “For MROs, this means that they have to do a lot of work on these older aircraft looking for cracks, corrosion and fractures.”
That’s not all. “Since most older aircraft have surpassed their DSG maintenance inspection threshold, the aircraft have to be inspected more frequently. This adds to an aging aircraft’s operating costs. So does the need to replace older parts approaching end-of-life; again not an issue on a newer aircraft.
                                Put these factors together, and one can see why MROs cannot offer ‘fixed cost per flight’ maintenance on aging airframes. “They just cannot predict what kind of work an old airframe aircraft might require to keep it airwort. “As a result, they can’t cost out a safe price that makes them money while providing the customer with affordable maintenance over time.”
                              As for engines? Even though they can theoretically be kept running forever, in practice this isn’t the case. “Engines tend to boil down to the economics of operating them,” explains Menard. “Increased costs of fuel and costs of maintenance due to reduced competition in the marketplace and availability of parts usually make engines uneconomical — and the residual hull value of the older airframe rarely is enough to justify the costs of a re-engining upgrade for the fleet.”

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