Preventive Maintenance for Thrust Reversers
Rugged and reliable, few mechanics consider thrust reversers as a threat to the aircraft's safety. For the airlines, however, they can be a threat to other valuable resources: time and money. Preventable reverser problems can delay flights and, when the problem is significant, force cancellations.
To avoid these problems, and save the carriers needless expense, OEMs stress the need for regular, preventative thrust reverser maintenance. Too many airlines fly reversers Ôon-condition' — meaning little or no attention is given to them until a serious problem occurs.
"It's really no different than your own car," says John Timko, a product support engineer for Middle River Aircraft Systems. "You can never eliminate the chance of getting a flat tire, but if you rotate your tires when you should, keep them properly inflated, and follow the manufacturer's guidelines, then you can greatly reduce the chances that they will leave you stranded."
Timko works directly with the mechanics that service his company's reversers. In addition to the largely composite reverser for the PW4168 engine, Middle River Aircraft Systems (MRAS) is the sole-source supplier of reversers on GE's entire line of CF6 engines. Additionally, the company runs a complete reverser overhaul and repair facility. Everyday, Timko takes calls from mechanics and engineers with questions about reverser's maintenance and performance.
Like most other product support engineers, Timko represents the most direct link to up-to-the-minute, first-hand information about servicing and repairing thrust reversers. Support engineers are most frequently asked about repairs, processes, and general information, or even if a repair or replacement is really necessary. Sometimes a call can save time and money.
Timko remembers a call from a mechanic who noticed cracks along the outside of the transcowl. "It looked like it could be serious, but when he described it, I noticed that it was just the paint cracking in a hidden seam where the transcowl is jointed to the outer surface. Fortunately, this was just a cosmetic problem and was no real threat to the unit."
Clearly the best tool in preventive maintenance is information. Most fleet libraries keep updated Aircraft Maintenance Manuals (AMM) and Component Maintenance Manuals. In addition mechanics should be on the distribution list for the manufacturer's service bulletins and Commercial Engine Service Memorandums (CESM). CESMs and service bulletins are distributed as needed and are the best vehicle for communicating directly with mechanics and service providers. These bulletins may contain information updates on anything from inspection changes to new spare parts.
Like many OEMs, MRAS is working to discourage fleets from flying reversers on-condition. "Planning is the key to saving," says P.K. Bhutiani, vice president of engineering at MRAS. Bhutiani says he understands the time and money constraints that airlines are under, but believes that an on-condition maintenance program is counterproductive.
"Maintenance doesn't cause delays and cancellations. Unplanned maintenance does," says Bhutiani. He believes that an on-condition maintenance program with no periodic shop visits produces some short-term benefits such as less work for shop crews and lower initial cost. But, these savings are eventually far-outweighed by the additional expense incurred in delays and cancellations, greater effort placed on line maintenance crews, and more expensive overhaul or repair once the unit comes off-wing. In addition, fleet operators often
must lease spare thrust reversers due to several units being unserviceable within a short period of time. While regular reverser inspections can help keep planes in the air, the inspections themselves use precious time — time out of service and time that a mechanic could doing something else. To avoid wasting time on unnecessary checks, ask your reverser OEM to provide a Work Scope Planning Guide (WSPG). These guides provide a comprehensive checklist for routine inspection and maintenance based on the needs of each fleet. "Every fleet flies different," says Timko. "Mechanics need to be aware of how reversers are being used if they want to inspect and maintain them efficiently."
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.
Translating Cowl (transcowl)
First, a general inspection of the transcowl should include checking for loose or missing or cracked rivets. Look for surface damage such as cracks, holes, dents, and delaminations. Also check for damage to the graphite C-Ring as well as the mateline of the inner and outer bondments.
Next, visually inspect the Main Air Seal for cuts, splits or punctures.
Cowl guides on the outer bondment of the transcowl have a Teflon® strip bonded at the contact area to the fixed structure. Check this strip for wear, and replace cowl guides that show worn or missing Teflon. This will prevent unnecessary wear on the guide track on the hinge/latch beam. This guide track is integral to the beam, and protecting it can avoid a costly and time-consuming beam replacement or specially manufactured detail to replace the cowl guide track.
Be sure to check the transcowl T-track liners. These are the plastic liners that are in the hinge and latch beams where the transcowl T-hinge slides. Loss or severely worn liners can cause wear on the hinge beam and/or T-hinge, both costly repairs or replacements.
Also, check the bearings of the blocker door hinge blade and replace worn hinges on the blocker door and/or transcowls. Worn hinges will allow blocker doors to vibrate and may cause damage to both the doors and the transcowl.
Check for cracks in the torque box actuator casting. Cracks not repaired according to OEM guidelines may cause gearboxes to become detached from the torque box.
Whenever possible, look for damage to the fire coat. Loss of firecoat can cause premature bondline failures on the sidewalls and core cowls, and should be scheduled for out of service maintenance as soon as possible.
But, as reliable and rugged as large engine reversers are, periodically they do suffer from serious failures. A significant portion of all unplanned maintenance comes from operational damage (activating wing flaps while reverser is open, striking the reverser; engine fires; runway debris, etc.). After that, the causes range from general hydraulic leaks and actuation system failure to handling damage (fork trucks, baggage carts, ladders, etc.)
The average service life of a large engine thrust reverser is in excess of 100,000 flight hours, so if properly maintained, most reversers will last through several overhaul intervals.