HTP Blade replacement – through the eyes of the expert, Filip Stanisic

A320 CEOs and 737 NGs are the world’s most popular narrow-body airplanes. And these primary-choice aircraft are powered by CFM56-5B (together with V2500 engine from IAE, rough split 50/50 on A320 CEOs) and CFM56-7B engines (only engine option for the 737NG). With this in mind, it is safe to say that CFM56 family engines are the most popular engines in the world.

What are the ingredients of such popularity? First of all, their fuel consumption is very good, much better than earlier types. Second, CFM56-5B and CFM56-7B engines have shown much better reliability compared to earlier engine models and stay on-wing for a very long time. They are probably the most reliable engines built up to now (as LEAP and PW1000 have yet to prove their reliability). The 5B and 7B engines stay on-wing longer than any other engine type and are the most cost-effective choice for operators whose services are based on long operations between shop visits. In addition, the maintainability of these engines is very efficient as most of the minor problems can be fixed on-wing or with very simple, relatively cheap and fast repairs. Moreover, such small-scale repairs extend the time on-wing and until major shop visits – overhauls. All these pros of these engines are much appreciated by airlines and airplane manufacturers – and this is the reason for such impressive success of these engine models. But, like any other engine, these two models also have to be maintained when the time comes.

Engine maintenance is one of the biggest contributors to all maintenance costs

Engine maintenance accounts for approximately 40% of the total cost related to aircraft maintenance – it is one of the major points of airline spending. More specifically, a cost analysis of any core performance restoration of a CFM56-family engine shows that the cost of material has the most significant impact on overall cost.

And such material is usually split between Life Limited Part (LLP) material and non-LLP material. Let’s take a closer look at the costs: the cost of LLP material, in most cases, can be fairly accurately predicted as it is connected with engine-build standard and cycles remaining on LLPs that will be installed; if bought in advance, the cost of LLPs can be pretty well controlled.  On the other hand, the cost of non-LLP material can vary quite dramatically between core performance restoration shop visits (SVs) that have very similar – if not identical – work scopes. 

The reason for such variation is that hot-section material (HPT Nozzles, HPT Blades and LPT Nozzles Stg.1) is very expensive and, based on actual scrap rates, can vary significantly between the engines. On CFM56-5B and CFM56-7B engines, HPT Nozzles and LPT Nozzles Stg.1 have reasonable scrap rates and these can be predicted fairly well. However, it is worth pointing out that LPT Nozzles Stg.1 show a tendency to crack due to internal oxidation processes – however, this is likely to have been resolved with the introduction of new part number(s).

Challenges associated with HPT blade replacement

The situation with HPT blades is still not that simple and has to be tracked in detail for each engine. From the moment the problem with HPT Blades appeared, blades were liberating without obvious reason – but only up to the moment that OEM introduced technical solutions to significantly improve the reliability of HPT blades while in the engine. However, since such technical solutions were introduced, new challenges in repair have emerged from an engineering perspective, and this is potentially expensive from a commercial point of view.  

The new CFM56-5B/CFM56-7B-applicable HPT blades (Identical part numbers are applicable for both engine types) have a considerable list price, which increases annually. Replacing a full set of HPT blades at a performance restoration shop visit can raise the cost of a shop visit, since the cost of a new set of HPT blades is pretty substantial. Based on this commercial aspect, HPT blade replacement is a significant expense for operators and aircraft owners.

Technical improvements to HPT Blades and preventive actions recommended via Service Bulletins by OEMs have meant that HPT blades have become almost like Life Limited Parts – in some cases, even more complicated to technically follow and track. Additionally, OEMs have introduced certain SBs that limit the usage of certain HPT blades based on accumulated cycles since new. These SBs treat HPT blade management (limiting the life of the HPT blade in an engine) and BSI intervals (to prevent failures in specific geographical regions).

HPT blade management SBs define the remaining life of an HPT blade. Also, there are some SBs that define usage in general (benign) environment, as well as some that define specific usage in China. However, following such SBs is not mandatory (there are no ADs connected with them), but in the case these are not followed, there is a risk of in-flight shut down or aborted take-off, which would cause total damage to the engine, or at least an unscheduled engine removal and costly shop visit. This could create significant problems for an airline with the aviation authorities, insurance company and lessor – these all are good reasons for airlines to follow SBs that are limiting life and defining inspections after all.

Following these SBs would be fairly simple if HPT blades were always replaced in full sets and always installed new, as OEMs suggest, but due to cost implications, a lot of operators and owners try to use overhauled material for scrap replacement as much as possible. Based on this, there are blade sets installed in engines that are mixed from multiple sets of blades that previously operated in different engines, and in different regions, which makes it much more complicated. This results in the need to follow each HPT blade for its remaining life. SB related restrictions of HPT blade can change over time; it can easily happen that an airline will install used HPT blades into its engine with certain expectations, and later on, when these expectations are not fully met, potentially due to new SB restrictions, they would have to remove the engine sooner than hoped, or a lessor will not accept the engine back with these blades installed – furthermore, this can lead to a compensation request.

Regional implication effects on engine maintenance

It is important to underline that HPT blades are the second main driver of SVs for CFM56 engines after SVs driven by LLP replacement. Also, an additional complication has been introduced for certain regions; for example, there are specific SBs concerning the cycle limit of HPT blades for engines operated mostly in China.  There are also specific SBs that have introduced specific BSI limitations for engines operated in harsh environments, as well as for those operated specifically in China and India.

Issues arising from these SBs are as follows:

·        The list of countries considered to be harsh environments changes from time to time (some countries are sometimes in the list and sometimes not, depending on SB revision).

·        These SBs entail more strict BSI requirements on HPT blades than per MPD; this requirement holds even after HPT blades are sent for overhaul (so the additional maintenance burden remains because the engine must also be borescoped, even after overhaul, due to usage of these blades). 

When talking about China, the situation is a bit different: the interval is the same as per MPD but depending on specific findings, it can drop significantly. For HPT blades operated in China, there is no maintenance burden after overhaul with reduced BSI inspection.

·        There are additional inspections that are done on these blades at overhaul (after some cycles are accumulated) and due to these inspections, their yield drops.

Cases of HPT blades in engines that were operated in two out of three problematic areas (harsh environment, India and China) are not treated by these SBs; however, it’s not clear how to treat an engine that has not reached the limit of application of these SBs in any of these regions separately, but the combined utilization of these regions together is over the limit is some prorated values would be viewed.

SBs that treat HPT blades operated in a harsh environment or in India (due limitation of future life remaining even when they leave these areas) can be interpreted in different ways, and depending on the interpretation after leaving these areas, these SBs can be considered applicable or not. Such cases add a lot of uncertainty into the process of engine-repair planning and management and engineers who have vast experience on this topic should be involved in cases where such a blade set with a complicated history is being evaluated.

Conclusion

Given everything mentioned above, most operators decide to replace a whole set of HPT blades during a core performance restoration SV after operating in a harsh environment, India or China, as it becomes too expensive and complicated to reinstall the parts. Nobody wants to buy overhauled material previously operated in those areas for use in repair of their own engines as Used Serviceable Material (USM) because it will increase maintenance burden etc.

There were no such strict requirements to control HPT blade operation history in different regions at the moment when CFM56 engines began operation, but now there are often cases in which material comes off an engine with unknown history, simply because the operators who previously used it are long gone. Usage of USM (overhauled) HPT blades is becoming more and more complicated due to poor tracing of some parts from earlier years; sometimes tracing is even impossible and HPT blades are being replaced without a real technical reason, but rather due to lack of documentation. This drives the installation of new parts, which consequently drives the repair cost up. On the other hand, installation of new parts makes the engine more reliable and postpones potential operational problems (IFSD – In-flight shutdown or ATO – Aborted take-off) and/or postpones necessary scheduled engine maintenance. 

In the current circumstances with COVID-19, it makes the life of operators/leasing companies even worse as they have a dilemma between using new material at a high cost or using USM at a lower cost but with potential problems concerning its historical documentation. 

Like in any other case, to win a lot on reliability and time on-wing, there will be costs in replacing HPT blade.

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