Aircraft System Lab: Depth Maintenance

Compressor Blade

This is the second part of the Harrier Jumpjet lab series which is the depth maintenance. Depth maintenance is one of the maintenance process which taken place off-aircraft and take a longer time than a daily maintenance. During the fault diagnosis I found out that damaged compressor blade is a possible cause for compressor surge. In this depth maintenance activity, my group were given three damaged compressor blade and we have been given a full authority and responsibility to do anything with the blades to complete the activity . The task was to find out which of the compressor blades are repairable or need to be replaced. We identified each blade by noting their production code.

1st Blade : NR 1424 UN

2nd Blade : NR 1513 VS

3rd Blade : QB 3734 VQ

Within the metrology lab, shadowgraph is a very useful tool to detect damage along the blade edge but there will be not enough time to finish the activity if we have to do a detailed check with shadowgraph for each blades. My team decided to do a visual check for each blades to see which of these blades are not repairable. There is a critical rule for a compressor blades, if there any damage within 30% length of the compressor blade from the root it is considered as not repairable. With the help of ruler, we measure each blade length and try to locate any damage along the blade’s edge. The result of our finding can be seen below:

1st Blade

Length : 20 cm

Visible damage at : 2 cm (10 %)

2nd Blade

Length : 16 cm

No visible damage along its 30 % length from the root

3rd Blade

Length : 20 cm

Visible damage at : 2.4 cm (12%) and 5.6 cm (28%)

Shadowgraph or also known as profile projector

Based on this result and in regards to the 30% length principle of compressor blades, my team decided that the 1st Blade and 3rd Blade are not repairable and must be replaced. For the 2nd blade we used shadowgraph to have a detailed view along the blade’s edges(leading and trailing edge).  A detailed view is needed to note any damage along its edges and to be reported for the repair process. Our finding on the 2nd blade with the shadowgraph can be found below.



From the shadowgraph result I found out that the 2nd blade is also not repairable because there is damage within 30 % of its length on the trailing edge. The damage was not visible when we do the initial check but we could see it clearly with the shadowgraph. We tried to improve the result by using the form tracer to check along the blade skin but we realized that this method is not effective because the shape of the blades itself is not completely flat and to be kept in mind that formtracer remove 10 microns thick of material from the blades. Within several repeated usage it could affect the surface smoothness of the blade and will also contribute to disrupt the airflow along the blade.

Formtracer result

Reflection :

This activity introduces me to the depth maintenance process in aerospace industry by giving a hands-on experience on the depth maintenance and also a full authority and responsibility. During this activity I had a chance to apply my knowledge and logical decision-making in order to complete the task. We were not given any procedure and have to complete the task based on our knowledge. Without any knowledge and experience about metrology I will not be able to know which tool should be used to complete this task, however I had a chance to learn about the metrology lab during the first semester. I have also developed my critical and logical thinking during this activity. If my team did not decide to have an initial check and try to examine all the blade with the shadowgraph, I believe we will not be able to complete the task within 2 hours. From this activity I also learned how to use a shadowgraph to have a detailed view of an object along its edges. Shadowgraph is a very useful tool to improve the result of a depth maintenance. For example, I was not able to find any visible damage along the 2nd blade’s edges but with the shadowgraph I can easily spot the dent.

As this is a group activity I am developing my teamworking and communication skills during the depth maintenance. It is very essential to be able to work efficiently with my team in order to have a satisfying result within the given time. With a good communication skill we will work more effectively. My team could complete the task without any serious problem because we have a very good teamwork, none of us acted as a weaklink to the team.

If i were to undertake this activity again I would like to improve my finding on the compressor blade with a NDT test. Using a Dye penetrant inpection technique I could check for any crack along the blade’s body which are not visible to the naked eye. I would also like to have more information related to the damage tolerance, because along the edges i found there are damages with different depth and width it would be really helpful to know the tolerance for these dents and would improve our decision and conclusion.

I am very satisfied with the way Coventry university giving a chance for every students to have a hands-on experience to some of the important maintenance process The critical thinking and the experience I have acquired during this activity will help me toward my further study on the coming year and also my future career.


Depth Maintenance Laboratory Worksheet

2-1 Sheet2-2 Sheet2-3 Sheet2-4 Sheet


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