Brought to you by GE Aviation

by Jack Baldwin, Senior Propulsion Engineer, Product Support Engineering, GE Aviation

"In all affairs it's a healthy thing now and then to hang a question mark on the things you have long taken for granted.” - Bertrand Russell (1872-1970)

In the last installment, I discussed how to stagger your fleet by using minimum workscopes or quick turns. The use of these types of workscopes can expedite the return of engines to the operator.

As pointed out in the last article, using a minimum build or a quick turn means that the operator has taken the time to understand the engine health. By using the engine diagnostic data, one can get an idea of how well or poorly the engine is performing and can tailor the workscope accordingly.

What might this diagnostic data be? It consists of the basic items, such as:

  • Hot day EGTM (exhaust gas temperature margin): snapshot and last six months trend line
  • Delta fuel flow vs N2 rpm at cruise
  • Borescope Inspection (BSI) data (if available)
  • Vibration characteristics

Data speaks

By looking at this data, one can determine if the engine has a stable performance, identify any potential hardware issues (BSI data) and see if there are any other core engine issues that might be popping up (such as N2 vibrations).

With our example of the 20-engine fleet, we can rank the engine health based on the above parameters. There will be a distribution with several engines in the middle, and a few that are running really well and a few that will require a performance shop visit.

Part one of the stagger is to push out the shop visit for those engines running really well. That will start the spacing.

Next we look at those engines whose performance trend line indicates that performance is beginning to deteriorate, possibly to an unacceptable level. It is these engines that are candidates for a minimum build or quick turn type workscope.

Looking at the engine health data and the BSI images, if available, allows one to make a technical judgment on the amount of work required to up the performance of the engine. The goal would be to select an engine for a quick turn that only requires minor work in the hot gas flow path portion of the engine.

For example, a quick turn that replaces the high-pressure turbine (HPT) blade shrouds to get back to proper clearance may be all that is needed. Possibly replacement of the combustor hardware may be all that is needed. By identifying these engines, a minimum build workscope can be crafted that minimizes time in the shop. I created such a workscope for replacement of the combustor assembly (dome, liner and HPT stage 1 nozzles) on the CF34-8E engine.

By replacing those parts as a single assembly, the engine can be turned in three to four weeks rather than the standard turn time of seven weeks. That means an engine could be turned quickly enough to enable it to get back to service in time to be available when another engine is coming off wing.

The key to this process is making sure the engine health is properly understood. If the engine health is not truly known, then it is very likely that additional issues will be found that require additional teardown of the engine and thus a delay in when the engine is returned to service.

I cannot overemphasize the importance of evaluating what is happening with an engine in order to craft an effective workscope. Just looking at exhaust gas temperature hot day margin (EGTHDM) by itself can create unnecessary work.

As an example, every engine gets dirty and this can affect engine performance. The aerodynamic efficiency of the airfoils is reduced and the engine compensates by increasing the amount of fuel for a given RPM. If EGTHDM was the only information considered, a conclusion might be made that an engine needs to go to the shop.

In reality, it may be that the engine only requires a good waterwash. Waterwashes have been shown to increase EGTHDM by as much as 20˚C. Thus a good waterwash may be the way to push out engine shop visits. This is a good example of why one needs to look at all of the available engine data.

Your engine OEM can help you with this type of assessment. So if you are not sure, get with your OEM representative if you need support.

Engine health is one of two ways to create a fleet management stagger plan. Scheduling is another, and I will take a look at that in next month’s installment.