One of the best ways to determine the requirements of your specific fleet is through an Overhaul Sampling Program. As the name implies, overhaul sampling looks at random reversers and determines the overhaul interval, which is the amount of service time between complete overhauls, barring any significant damage.
Different reverser models will yield different results, as well as similar models in service on long range versus short range aircraft. Generally, "long-range" aircraft are those that average more than three hours per cycle, with short-range aircraft averaging less than three hours.
Based upon CF6-50, CF6-80A3, and CF6-80C2 thrust reverser field experience, most long-range aircraft established an overhaul interval somewhere in the 18,000- to 25,000-hour range, while most short-range aircraft established an overhaul interval in the 6,000- to 13,000-cycle range.
For its reversers, MRAS recommends that the following sample schedule to help an airline establish its thrust reverser overhaul interval. If an airline's utilization is a mix of both long- and short-range, the airline will find it necessary to establish an Overhaul Sampling Program for both types of aircraft utilization.
To implement an overhaul sampling program, a fleet maintenance coordinator would remove the first reversers, say from an CF6-80C2 or CF6-80E1, which accumulates either 10,000 hours or 3,500 cycles. For an airline utilizing both long- and short-range aircraft, a sample from each should be taken. The first sample reverser would be forwarded to the airline's thrust reverser overhaul and repair shop for a thorough inspection to the CF6-80C2 or CF6-80E1 thrust reverser CMM, MM 92466, or MM 99440. Once the first sample reverser has been inspected, data gathered, and any damage repaired, the reverser can be returned to service.
If the first sample reverser shows no signs of unusual wear, then the next reverser that accumulates 15,000 hours or 7,000 cycles should be removed and the same inspection process repeated. If no unusual wear or damage is discovered, additional samples should be removed at the interval listed in the reverser overhaul sample schedule.
With the data gathered during the airline's sampling program, airlines can establish an overhaul interval appropriate for their fleet's thrust reversers. Results vary, but on average, transcowl removal is necessary every 15,000 hours or 7,000 cycles, whichever comes first, while fixed structure removal is necessary every 25,000 to 30,000 hours or 14,000 cycles, whichever comes first.
Since thrust reversers are used almost exclusively during landing, most parts, such as the actuation system, do not accumulate wear and tear during normal flight hours. Still, some parts, such as the wire mesh over the inner flow path, are affected by flight hours. Small holes in these mesh screens, that could be patched if caught in time, will grow simply from the natural force of air passing through them. FAA regulations allow just 18.4 sq.-ft of missing screen per reverser. Allowing these holes to go unchecked will not only lead to more costly and time consuming process of replacement, but loose or broken screens have been attributed to many cases of damaged or missing perforated face sheets. It is very expensive to replace the perforated face sheet on a transcowl.
Overhaul sample programs will help determine when is the best time to pull a unit out of service for a complete overhaul. However, fleet support coordinators should be keenly aware of what comprises a thrust reverser overhaul from their support service providers. A "thrust reverser overhaul" may vary among support service providers and mechanics should request a complete breakdown of exactly what functions are performed during an overhaul. This not only gives them a better reference for gauging future service, but also allows them to compare dollar for dollar and service for service the capabilities of each service provider.
To help maximize the time between overhauls, there are some basic things that mechanics can look for. Anomalies aside, the WSPG will provide a frame of reference for performing inspections on specific areas of the unit, and a very detailed list of what to look for. As an example, for the CF6-80C2, the most recommended inspections occur between 1,500 and 2,000 flight hours and should focus on two major areas of the reverser; the transcowl and the support assembly.
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