The first activity within the harrier jump jet lab series is the fault diagnosis. My group was assigned to examine the engine of the aircraft given the report from pilot that compressor surge occasionally happens during the last flight. Compressor surge is a condition where the engine is rumbling because of the air flow within the compressor was being pushed back towards the inlet of the engine. I will write further about this phenomena in this article.
Before the activity starts, we were given a mini lecture about aircraft engine and we have some trivia or quiz to challenge our knowledge on this area. I found out that the handout for the aircraft engine topic has been released several days before the activity and I took my chance to read through the handout a day prior the activity. This has helped me to be on advantage during the activity and I was capable to answer most of the trivia.
The mini lecture was very helpful towards completing the activity, we were given knowledge of how air flowing throughout the engine according to the engine cycle or as known as “suck squeeze bang blow”. The thing I learned is that an axial compressor stages are composed of stator and rotor. The rotor is connected to the engine shaft which is supplying energy from turbine in order to rotate the compressor blade. The Stator is not connected to the shaft, hence is not rotating. Throughout each stage, air is altered with different angles of the stator and rotor working in concept of convergent and divergent to vary its pressure and velocity in regards to the Bernoulli’s principle on fluid dynamics. In regards to the theory, the flow must be as smooth as possible to keep the pressure varying process on its optimum state. This is why the laminar flow is favoured throughout these stages because the flow is more predictable compared to the turbulent flow in order to define the angle for each compressor blade.
Compressor surge is a breakdown of flow in a turbine engine. The video above has a comprehensive explanation of compressor surge phenomenon and how the flight crews should react to it. The compressor surge is noticeable from lost of thrust, vibration and yaw depending on how severe it is. The most noticeable effect will be at low altitude and high power setting, hence compressor surge can be dangerous during take-off. There are a couple of factors that could cause compressor surge such as bird ingestion, pneumatic bleed malfunctions and internal engine failures. I would recommend to watch the video in order to see how the compressor surge could affect the engine.
According to the report from the pilot, the compressor surge happens occasionally and recoverable during flight, this leads to a conclusion that bird ingestion and pneumatic bleed malfunction is not the causing factor here. The compressor surge is also known as compressor stall as it can be caused by disruption to the airflow within the compressor. The remaining possible cause is a damaged compressor blade disrupting the laminar flow within the compressor and resulting a push back of air flow towards the front of the engine. The compressor section of a turbofan engine consists of low pressure and high pressure compressor. The low pressure is located closer to the front of the engine and high pressure compressor is closer to the combustion chamber. In the Harrier Jump Jet, the low pressure compressor is accessible from the front part of the engine while the high pressure compressor is not accessible.
If the fault is within the low pressure compressor, the maintenance crew can remove the damaged blade without removing the whole engine and repair it if possible. The blade is serviceable if the damages are not within the 30% of the blade length starting from the root. The figure below is an illustration of damaged blade.
The 30% length of the compressor blade is a critical area where the blade is considered as not serviceable because of the centrifugal force acting on it during the compressor rotation can tear the blade apart along its damaged location. The figure below is an illustration of centrifugal force.
Centrifugal force illustration (CalcTool 2008)
The high pressure compressor is located closer towards the combustion chamber and not accessible on Harrier Jumpjet. The procedure to examine the high pressure compressor require a borescope which is a very small camera capable of taking picture and record video in a less accessible area.
The aircraft has access for the borescope to reach the high pressure compressor through a small hole available for each high pressure compressor fan. This holes can be found by removing the forward thrust vector of the engine. The access holes can be seen on the figure below.
If there any damage found within the high pressure compressor blade, the available option is to swap the whole engine with the spare engine and then the depth maintenance needs to take place. This is because the aircraft will not be operable for up to five days during the maintenance and through aviation industry view, an inoperable aircraft is a big loss. The engine swap only takes a whole day and the aircraft can continue its operation, hence this is considered as the most efficient option.
List of Hazards Involved in This Activity
Examining Compressor Blades.
Before the compressor blade examination starts, we applied barrier cream to our hands in order to protect it from dangerous substances and then tool check need to be completed in the beginning to find out if there any missing tools within the toolbox. The tool check in the beginning is important to make sure that we have the same number of tools inside the toolbox after we have done with the activity. If there any difference between the check-out and check-in, it is possible that the tool is left behind somewhere within the aircraft and it is very dangerous for the aircraft. I was the person doing the tool check before and after the activity. I found out that several tool checks before have not specified the missing tools, It is important to specify the missing tools during the tool check in order to inform the next people doing the tool check which tools are missing.
Our team were divided into two groups for this activity, 2 persons working in front of the engine to examine the low pressure compressor and 2 others remove the forward thrust vector for the borescope. I was assigned to remove the forward thrust vector. To remove the forward thrust vector, there are two other panels need to be removed which can be seen below.
After the forward thrust vector removed, the borescope can be inserted to the four available holes for each compressor fan. To check each blade, the compressor was rotated manually from the panel on top of the engine. I took pictures of each blade with the borescope in order to investigate the damage on each blade.
This activity gives me a hands on experience of aircraft maintenance process. It taught me on how to make decision based on the condition and what matters the most. Although it is a military aircraft, it has the same philosophy with a civil aircraft it should worth the amount of money your company invested to it. Having an inoperable aircraft for a couple of days is considered as losing your investment.
From this activity, I learnt how to use a borescope properly and how it should be used on an aircraft especially during maintenance. One thing that i think could be improved is the quality of the borescope, It appears that the picture taken by the borescope is not really clear and detail. It is possible to identify a major damage to the blade with the borescope but it is also possible to miss small dent on the blades. By having a good initial analysis on compressor blades I can come up with a better decision whether engine swap is necessary or not.
I also learned that preparing before any lab activities will give you a boost during the activity, although as simple as reading through the handout. With a comprehensive knowledge on the subject you will also get the most out of the activities. It also works on a real life work of maintenance, most of the times you have to make a decision for every action you need to take during the maintenance. In conclusion, a good theoretical knowledge will result in a better decision and you could save your company from losing money or even a fatal accident.
Fault Diagnosis Laboratory Worksheet
CalcTool (2008) Centrifugal Force[online] available from <http://www.calctool.org/CALC/phys/newtonian/centrifugal> [4th March 2016]
tpub.com (n.d.) COMPRESSOR ROTOR BLADES[online] available from <http://aviationmiscmanuals.tpub.com/TM-55-2840-251-23/css/TM55-2840-251-23_219.htm> [4th March 2016]