Irrespective of the pace of new aircraft deliveries, older aircraft remain a key part of aviation’s growth. Rick Adams looks at some of the considerations in maintaining a viable ageing fleet

Traffic growth and record orders creating production backlogs has made extending fleet life important.

There are challenges involved in the endeavor. Following the recent crash of a 34-year-old passenger transport, for example, Indonesia grounded aircraft more than 30 years old and banned importation of any commercial aircraft more than 10 years old.

But an analysis of aircraft accidents for the 53-year span between 1959 and 2012 “does not support simple age-based restrictions as the most effective means to maintain aviation safety,” according to Professor R. John Hansman, Jr., Director of MIT’s International Center for Air Transportation.

“Many states have rigorous processes to assure the continued airworthiness of older aircraft,” he says.


Major MRO programs

The rigorous assessment of older aircraft stems in part from the 1988 Aloha Airlines Flight 243 incident in which part of the Boeing 737’s fuselage peeled off in flight, as though opened by a can opener. The accident triggered regulatory and industry reviews that led to several major maintenance programs. Corrosion, fatigue cracking, repair assessment, and widespread fatigue damage (WFD) evaluation are all addressed under new initiatives.

Now, instead of applying the aircraft model’s Design Service Goal (DSG)—which typically correlates to 20-25 years of operation—the WFD rule establishes a Limit of Validity (LoV). The LoV is a value that can be demonstrated by test and analysis to ensure that catastrophic WFD will not occur. In essence, therefore, the LoV represents the end of an airframe’s life. For example, the US Federal Aviation Administration’s (FAA) default LoV for an Airbus A300 B4-2C model is 40,000 flight cycles. For a Boeing 737 Classic, it is 75,000 flight cycles.


Extending life

Whether an aircraft is worth repairing becomes a significant issue close to its LoV. A cracked part, most often due to its geometry, may already be damaged beyond repair and require replacement if the aircraft is to extend its life. This is especially true for large integrally machined parts.

It may be worthwhile for operators to inspect more regularly or use more elaborate inspection techniques, so a crack can be found when it is still repairable.

There are also options for improving the environmental performance of an older aircraft. Such emissions-reducing upgrades as winglets are a common feature of many older aircraft and airlines continue to invest in these and other appropriate developments.

“On a complex system like the engine, it is not simple to install major changes, but there are a few options available,” says Zoran Muratovic, Senior Aircraft System Engineer Powerplant & APU for Lufthansa Technik AG. “A good example is the installation of new acoustic liners on Lufthansa’s Boeing 737s to reduce noise.”

All 737 aircraft stationed at Frankfurt, Germany have been fitted with new sound-absorbing materials, or hush kits, reducing noise emissions from the engines. Muratovic also notes that software modifications can help to reduce thrust on the ground and save up to 10% fuel on the A321 fleet.


Rules of ageing

With Airbus and Boeing together having a backlog of more than 12,000 aircraft, the market for used aircraft remains strong, according to Airbus Group Head of Environmental Affairs, Jean-Luc Taupiac. He notes that fewer than 100 of the aircraft parked at the Tarmac Aerosave storage facilities in southwest France and northern Spain have been dismantled in the previous six years.

There is an abundance of spare parts available for older model aircraft, especially the common narrowbodies. Engines and engine components, landing gear, and avionics are the most popular commodities. However, Taupiac cautions: “Managing second-hand parts is really important. It’s a matter of safety. When you have second-hand equipment to replace a part on an aircraft, this has to be traced and you have to be sure this equipment is in good condition.”

The European Aviation Safety Agency (EASA) is considering a new rule on ageing aircraft structures. This was published for comment in 2013, though a final version is not expected to formally take effect before 2017. Its content has been harmonized, as much as possible, with FAA recommendations.

In any case, OEMs have already updated their instructions for continued airworthiness. As such, compliancy with EASA’s new requirements is already largely in place.


Maintenance status

Finally, one of “the fundamental factors to be considered when valuing an older aircraft is the condition of its maintenance status,” according to Shannon Ackert, Senior Vice President, Commercial Operations, at Jackson Square Aviation. “The sometimes wide disparity between appraisals for similarly aged aircraft can often be explained by differences in their maintenance condition,” Ackert explains.

The vast majority of aircraft appraisers and traders quantify the value of an aircraft’s maintenance status through analysis of certain, high-cost major maintenance events. These events generally consist of:

1. Airframe heavy check (heavy structural inspection)

2. Engine performance restoration and life limited parts replacement

3. Landing gear overhaul

4. Auxiliary Power Unit performance restoration.

Keeping an ageing aircraft going isn’t easy but a properly maintained older aircraft could offer a few more years of safe operation and a higher resale value.