A Behind-the-Scenes Look at Turbine Engine Overhaul

I am a helicopter — see me fly. I help people break their earthly bonds and soar like an eagle at performing tasks as varied as power line inspection, off-shore oil support, law enforcement, customs and border protection, executive transport and emergency medical transport, just to name a few of my capabilities. I can fly forward, backwards and sideways. I can even hover in place! Take that, you fixed wingers! I am truly a fantastic flying machine. Alas, I need power to fly and that comes from my engine. Yesterday, maintenance took my engine away and sent it in for an overhaul. Until it comes back and is reinstalled, I have to stay in my hangar and cannot fly. I know it’s only for a short period of time, but I was made to fly! I wonder what is happening to my engine while it is away.

Turbine engine overhaul is a series of steps that involves attention to many details. Removal of complete engines and engine modules for overhaul begins with a service call by a qualified mechanic. In many cases, an owner/operator has this capability and removes these items which are packaged and shipped to an overhaul facility for the required maintenance action. Upon arrival at the overhaul facility, a rather lengthy series of events is set in motion to return a unit back to airworthy status.


Individuals in the receiving area remove the freshly-arrived unit from the engine can or box and visually inspect it for shipping damage. Customer service is notified of its arrival, at which point a work order is opened in the computer system to track all aspects of the overhaul. Records are then sent to an analyst for a disposition of all life-limited components within the unit. This disposition is then sent to engineering for a work package to be generated.

An engineer reviews all life limits of the unit, then any instructions from the customer, customer service or regional engine manager are reviewed to determine the final work scope. All of this information is reviewed to help the engineer create a complete and accurate work package. The engineer also reviews the commercial engine bulletins (CEBs) to ensure the final product will be of the latest configuration required. Once these instructions are completed by engineering, the work package or book is sent to the receiving department to be placed with the unit for induction into the work cycle. 


Upon arrival in the disassembly queue, an inspector completes a “check in” sheet that inventories items on the unit. A complete set of photographs is taken of the unit for a historical record to reference a unit’s condition and configuration. These pictures are invaluable in answering future questions about possible missing parts or a unit’s condition upon arrival. The unit is then disassembled by a qualified mechanic. If the unit arrived as an engine, the modules requiring work are removed individually for disassembly. Non-disassembled portions of the engine are placed securely on a movable cart, along with the other parts. This allows all parts of the engine to travel together throughout the repair process. After a unit is disassembled, the inspector removes obvious discrepant material (such as time expired or damaged parts) and tags it as such. This prevents unnecessary cleaning and inspection of parts that clearly will be rejected.  


From the disassembly area, the parts filled cart is sent to the processing area for cleaning. This involves several processes including vapor degrease as well as mechanical cleaning methods. Modules or parts of the engine that have not been disassembled are cleaned as best as possible without risking contamination. Special attention is required on parts that will require nondestructive testing (NDT) to ensure defects are properly identified. Once cleaning is complete, the cart is sent to NDT inspection.

NDT Inspection

Upon arrival at NDT, the technician reviews the work package to determine which parts are to be inspected. Fluorescent penetrant inspection is the most common form of NDT on engines and modules. Other parts, especially gears from a gearbox, will have a magnetic particle inspection if required. Applicability of these processes is determined by the operational hours on the components as well as any customer requests. Some customers desire steps beyond actual criteria dictated by the OEM manuals. The NDT technician records the results of the testing on an inspection sign-off sheet or ISO, which is provided in the work package. This serves as a record for reference of each individual part. Parts are deemed “use as is” (no defects) or marked up to indicate any discrepancies noted. This information will be used by the dimensional inspector in the next step of the process to determine if a part can be repaired or requires replacement. All parts are grouped together on the cart and they are moved to the next inspection queue.


Dimensional Inspection

Dimensional inspection requires a considerable amount of time. At this step in the process, every part that is exposed is reviewed at some level. Parts not going through an overhaul inspection are visually examined for discrepancies. Items requiring overhaul inspection are carefully examined to the OEM manual requirements. Inspections range from diameter and concentricity limits to flatness on a wide variety of surfaces. Once a part is dimensionally inspected, NDT data on the part is cross-referenced to determine if a part is “use as is,” repairable or rejected. “Use as is” parts are returned to the cart for reinstallation during assembly. Their status is noted on the ISO to indicate they will not need further inspection or work for return to service. A part is deemed repairable if it can be fixed by a rework process that is considered approved. For an OEM authorized shop, the repair process must carry the approval of the OEM as well as the FAA. If a part is deemed repairable, the required rework processes are noted on the ISO. Then a router or work instruction is placed with the part to indicate which processes must be completed for the part to be considered acceptable for installation. Parts that cannot be repaired are rejected and a rejection tag is attached to the part. Again, the results are noted on the ISO. Once all the parts are inspected and the ISO forms are completed, they are forwarded to customer service for the creation of a cost estimate. 

Cost Estimate

Customer service representatives take all ISO information and create a detailed cost estimate. Discrepant parts are quoted individually as being replaced with new OEM parts, unless that particular item is deemed expensive as new, and is readily available for purchase in overhauled or serviceable condition. This helps keep costs down. In general, most rotating or life-limited parts are replaced with new parts to insure longevity of the component after installation. Repairable parts are listed in a separate section under rework. Parts are listed individually by part number and the required repair is explained in detail. Then the associated cost for that particular part and work scope is listed. The reason for the detail level is simple. Helicopter operators are much more mechanically involved with their aircraft than operators of many fixed-wing applications. Most have been or are currently involved in the maintenance of their machine. Many have worked in the repair and overhaul business and are familiar with their engines. Therefore, they dictate a detailed level of explanation to ensure the bottom line is not being compromised. Once the detailed cost estimate is complete, it’s sent to the customer for approval. 

Customer Approval  

Due to the level of involvement explained above, approval of estimates may require many revisions to the original cost. Some customers prefer new items on certain parts instead of reworking the current ones. Others prefer the purchase of a serviceable part. This can be driven by factors such as cycle-to-hour rates during operation or the frequency of use. Customer service representatives must possess a great deal of patience, as customer preferences are constantly-moving targets! Once the cost estimates are approved by the customer, the engine is released into work.


During the rework phase, individual parts are sent out from the cart on the earlier described routers. Rework of items includes (but is not limited to) welding, plasma spray, brazing and plating. Most of these processes are followed by machining operations to bring the part into its final dimensional requirements. Heat treatment may also be required for parts to ensure mechanical properties are within tolerance, avoiding premature removal of engine or module. Turbine nozzles are checked for flow area, while turbine wheel blade tip paths are restored to optimal dimensions. In many cases, overhaul facilities have a tighter set of internal specifications than the OEM manual dictates. These closer tolerances do not only return them to an acceptable state but are designed to optimize performance. These internal specifications are compiled using years of experience and test cell power data. Upon completion of repair to a part, the part is returned to the cart by production control. Once all rework is complete, the cart with all components is moved to the assembly area for final assembly.



Reassembly of engines and modules is conducted by qualified individuals. These individuals are trained to the sub-component level. Therefore, one individual may be the most qualified for assembly of a particular module. Gearboxes, compressors, turbines and then the final “stack” of an engine are carried out at segregated work stations with applicable tooling. Almost all individuals are trained at stacking or final assembly of completed modules, allowing work to flow through the shop at a faster pace. If a unit arrived as a complete engine, it will be tested as an engine. Modules that arrived individually in most cases are placed onto a slave unit to be tested individually. Turbines and compressors are assembled using an alignment process to ensure internal dimensions are maintained. Turbine models are the most common items to be sent in individually and most are tested on a slave engine before being returned to service. Once assembly is complete, the unit is sent to inspection to ensure the required work scope has been completed, serial numbers are correct and the configuration is correct prior to testing. 



Once arriving at the test prep area, the engine is “dressed.” This involves installation of items such as the starter, exhaust stacks and vibration pickups. The unit is then mounted on a dynamometer, which simulates the load the engine would see on an aircraft. Many dynamometers use water to create the resistance that simulates the airframe transmission, while others use air. The fuel lines are attached and the engine is started and put through the required testing. Performance is measured by the amount of shaft horsepower produced by an engine at a given temperature of operation. A low-power engine would be considered running hot if it would exceed its temperature limit point to produce a given horsepower.


In some cases such as a gearbox module test, performance is not applicable but vibration parameters must be met. Vibration is measured on the gearbox, compressor and turbine. If the vibration limit is exceeded, a frequency analyzer can be used to try and determine the source of the vibration. This allows the engineer to help determine what parts need to be reinspected. Once the test run is complete, the engine is shut down and the exhaust is watched for any smoke. The oil level is checked again to ensure that consumption limits are within limits. Chip detectors and oil filters are examined to confirm that the engine generated no metal during operation. If the unit is a complete engine, it’s preserved for shipping and storage. This process removes any fuel from the engine for shipment and replaces it with a light oil to preserve and protect certain components. The engine is then removed from the test cell and returned to the shop.

Shipment Back to the Customer  

At this point, individual modules are removed from the slave and units are prepared for shipment. This includes paperwork reviews and cleaning. Torque striping and standard safety wire are applied where required. Once this is completed, the final inspector creates an 8130 for the unit and sends the work package to the records department. The records analyst then makes log card entries and returns the logs to customer service. This is the cue for the customer service representative to generate shipping paperwork. The representative creates this paperwork based on customer needs (shipping speeds), then attaches it to the logs and reunites it with the unit. The paperwork and unit is then sent to shipping and receiving. There, engines are placed back in their cans and modules are packaged accordingly for transport. Some customers pick up their units themselves while others are shipped via truck. In some cases such as an AOG situation, air shipment or custom freight methods are used. The final step of reinstallation on the aircraft is handled by the customer, or by field service from the overhaul facility coordinated by customer service. This cycle is repeated many times per month to ensure the rotorcraft industry is ready to meet the needs of its operators around the world.


Wow, I am impressed! All that work was done to my engine so I can fly again! A few more days and it will be here and reinstalled and I will be airworthy again. My mechanics really take good care of me. I am glad I asked what was happening to my engine. I learned quite a bit and I hope you did too.


Jason Giebler has been employed with Premier Turbines for 14 years. His roles have varied from customer service to program management, leading up to his current position as plant manager. His turbine engine experience includes the Honeywell TFE731, GE M601, GE J85/CJ610/CF700, and Premier’s current centerpiece, the Rolls Royce M250.  When not at work, Giebler enjoys spending time with his family, farming, hunting, fishing and vacationing at Yellowstone National Park.