Fault Diagnosis

20th February 2018: Fault Diagnosis (Engine Task)

This activity, led by the formidable Alex Harrison, was  undertaken to give us students first hand insight into how the aerospace industry operates regarding how technical diagnosis and repairs are carried out on aircraft.

To my understanding, this was very identical to how certain faults or issues are observed and processed in industry, right from the initial suspicions of the technicians, pilots and ground crew, to the analysis of evidence, to the justifiable  diagnosis with a detailed course of action on how to deal with the issue, while minimising time consumed (translating to “how much will it cost the company?”).

Side View of the Harrier T4 at Coventry University (Coventry Labs)
At the the beginning of this activity, we were debriefed by the “pilot” on the the conditions and events leading up to his initial suspicions that something might be wrong with the aircraft.  The aircraft in question is a Hawker-Siddely Harrier T4, a decommissioned twin seater aircraft purchased and owned by the university.  The facts as presented by him:
  • 25 minutes after takeoff, he observed “vibration and rumbling noise” from under seats throughout the aircraft
  • At this point, he had been on a low level bombing run, descending from Flight Level 300 (3000 ft) to Level 30 (300 ft)
  • Prior to this, he had initiated a ‘slam accel
  • He carried on with the bombing run (ascended to Level 300) after carrying out Standard Operating Procedures for a suspected compressor surge.
  • With no further issues, he continued with the sortie and returned to land.
Front view of the Harrier, with the engine intake visible from behind the cockpit (Fighter Sweep, 2018)
We were then provided an opportunity to ask the pilot questions, to ascertain what evidence to look for, and what other situations surrounded the short-lived sortie:
  • How many of you were flying? Formation flying or solo?
  • Were you informed of any defects or fault with the aircraft or its components prior to the flight?
  • Did you encounter a bird strike?
  • Has this happened before?
  • Did you get any warnings or indications on your panel?
We reached a decision to investigate a compressor surge, after we had made a justifiable conclusion that this wasn’t a mass event affecting multiple aircraft (a cause to ground a whole fleet if necessary), but a confined occurrence. This was after eliminating other possibilities (no indication that it was a bird strike, he was flying alone, no warnings or alarms in the cockpit). Due to the size of the cohort, we had to be split to smaller units to foster an efficient working environment (“too many cooks in the kitchen…”); I was fortunate enough to be on the group working on the engine.

Before any work could be done, we were implored to prioritise “safety first!” This might seem like a cliché phrase tossed around, but safety is of paramount importance especially in an industrial environment such as this (encouraged to act as we would in a “normal” job where there are responsibilities and real consequences if safety is not adhered to).

Safety Provisions
Personal Protective Equipment including coveralls, safety goggles & steel toe capped boots were worn as a preventative measure against any potential hazards.
A ‘tool check’ was conducted to ensure the tool kit was complete and signed for,  and the necessary tools needed for the task were present. This is crucial at the beginning and end of any maintenance task for two reasons: to verify the correct tools are present and accounted for and, who is to say a tool hasn’t been left on the aircraft somewhere by the previous technician who worked on it? That is a solid cause to ground the aircraft until the issue is rectified, possibly for longer than planned thereby costing the company.
One other vital component of safety was to “ground” the aircraft, ensuring electrical safety.
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Risk Assessment
Other than initial visual assessment of the aircraft surroundings for danger i.e. spills, sharp edges, stray tools, active equipment, other technicians on the aircraft we were briefed on how to use some equipment.
The borescope, never before used by me, was one that caught my eye with how so simple in operation, yet so beneficial it was to someone in a trade like this. We were shown how to use it, its uses, benefits and drawbacks were explained to us.
Orientation to other equipment such as wheeled step ladder, some of the tools we were later to use.
(Click here for the risk assessment sheet)
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Learning Objective
To my understanding of the task, the learning objective was to foster an engineering mindset of keeping an eye to detail, using previously gained knowledge, while upholding health and safety of all concerned, to diagnose a fault on an aircraft part or component for it to be fixed.
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Background Information: The Logic Behind It

This Harrier model is powered by a lone Pegasus Turbofan engine, capable of providing directional thrust for Vertical Takeoff & Landing (VTOL), a desirable feature in attack aircraft operations. The engine is located in the fuselage behind the seats. For the sake of this task we went ahead dealing with the engine as a Turbojet.

The engine blades are made from high titanium aluminium, giving the blades a benefit of high resistance to creep; with an engine bypass ratio of 4.3:1. This low ratio is useful for the role of the aircraft, to compromise between combat requirements and fuel economy.

Cutaway of the Pegasus, (FLIGHT, 1972)
A turbojet engine consists of a compressor, combustion chamber and a turbine, it operates similar y to the piston engine, starting with air intake, the air gets compressed from low pressure to high pressure. Once the air is compressed, it flows into the combustion chamber, where the started engine will be ignited. Once combustion happens, compressed air flows through another turbine from high pressure coming down to low pressure then out through the exhaust.
Simple operation of a Turbojet engine (Jeff Dahl, 2018)
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The Turbofan engine works in a similar fashion to the turbojet (the same four stages from intake to exhaust), but only connects to a large inlet fan. It produces an air-stream of large volume with low pressure that can bypass the engine. Turbofan engines, due to their design, are typically larger in size  and are more efficient with energy consumption.
Simple operation of a Turbofan engine ( K. Aainsqatsi, 2018)
Compressor blades, due to their unique shapes, behave like an aerofoil under laminar flow. In this engine case we were dealing with an axial flow compressor. This type produces a continuous flow of compressed gas, with a high mass flow rate () and is highly efficient.
In an engine like this, a rotor comprises of multiple blades, a ‘stage’ is made of the rotor and stator, and multiple stages in the aircraft make a spool.
Air flow in a ‘zig-zag’ motion across the stator and rotor (Jeff Dahl, 2018)
Due to the multiple number of stages (with different rotational speeds for each) that the compressor has, there is progressive pressure ratio reduction throughout the whole system. Assuming that each stage in a compressor has the same temperature change (Δt), at entry point the temperature to each stage (denoted T_stage)increases steadily throughout the compressor and the ratio (Δt/T_stage) entry decreases, thereby expressing a steady reduction in stage pressure throughout the compressor.
This means each stage will have a noticeably lower pressure than the previous. That at the end of the spool, the last stage will have a lower pressure ratio than the first.
A compressor surge is when the exiting air pressure from compressor is higher than what the compressor can maintain. This is more likely to occcur in turbulent flow. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. Of course one of the noticeable characteristics of an aerofoil stall is “buffeting”. Now imagine multiple blades experiencing a stall at the same time, that would be what the pilot was experiencing when he mentioned “vibration and rumbling”.
Diagnosis
Excerpt of the maintenance manual regarding a compressor surge (Hawker-Siddeley, 1969)
At this point, after consulting the maintenance manual,  we had determined there were a few possible reasons why the compressor may have experienced a surge:
  • The variable inlet guide blades may be out of adjustment, or
  • The blow-of valves are closed in the low RPM region
Both these possibilities had a course of action dedicated to fix the issue. Also from the manual and further research, we spotted a few other possibilities:
  • Defective variable inlet guide vanes
  • Deformation to the blades due to FOD, poor maintenance, bird strike, or precipitation (e.g. ice and rain); some of these were eliminated as there wasn’t sufficient evidence to suggest so.
  • The compressor bypass valve is not integrated into the intake network between the compressor outlet and throttle body
  • The outlet for the bypass valve may be too restrictive
Analysis: Investigation & Findings
We set about looking for a way to ascertain the main cause of the issue, while trying to minimise the time spent achieving the learning outcome (remember, time is money! A grounded aircraft is not useful to anyone except the technicians!). One thing learnt in this activity was the ability to carry out fault diagnosis in an evidential, methodical and logical manner. Simply put, it won’t be an effective diagnosis if there is no evidence to back it up, no recorded process, or if it makes no sense to whoever gets to review your work as an engineer.
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One of the simplest and easiest methods of diagnosis is the age old visual inspection. A good eye can spot a fault without the need for tools. It doesn’t take expensive equipment to figure out there was a bird strike if you can clearly see dried feathers and blood streaks along the side of the engine. Visual inspection also saves you the stress of taking apart a whole aircraft, possibly rendering some components useless in the process, just to find out repair was unnecessary.
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We went about visually inspecting; looking and “feeling” for any noticeable defects or clues all around the front and sides of the engine. This way up close with the engine we could spot any dents, cuts, bends, smears or even missing parts. This did not reveal any distinct evidence of damage.
Using the naked eye enabled us to make a decision to go one step further in the diagnostic process; a hands on inspection of the compressor fan blades (seeing nothing visible that would have contributed significantly to a compressor surge). This would require a further process of panel  removal for access to the engine.
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The next step in the investigation was to make use of equipment that would advance our ability to “see” the more intricate parts of the engine. That is where the borescope comes in to effect. The borescope, as the name suggests, is a very practical tool used to inspect the inside of an otherwise inaccessible structure without destroying it. It operates through a camera connected to a handheld screen via a relay tube, which may be flexible for use in curvy orifices or rigid.
A boroscope in use
The next process involved some teamwork. This was a point where communication was implemented. One team member held the camera and tube part of the borescope, one held the screen, two were mounted atop the airicraft, for access to the top of the fan blades.
Of the ones on the aircraft, one of them used a turning tool to “spin” the blades, while the other supported with access. While the blades were being turned, the person with the borescope screen would give the signal / shout to “stop!”, so the blade in view could be thoroughly inspected. When inspected to a satisfactory standard by the borescope team, there would be the call for “spin!”, then a “stop!”. The process is repeated until all the blades have been sufficiently covered.
Fan blade captured by the borescope camera
A visible dent on a different fan blade

After thoroughly looking through the blades, we were pleased to move on to conclude that the turbine blades were not damaged due to FOD (also using the “bottom third” rule), and there was no further need to take the engine apart. As per the SOP in the maintenance manual, the next logical step was to test the Acceleration Control Unit, and the respective valves.

If after testing these, there are any noted faults, or the certain margin of  error has been crossed, those components would be repaired or replaced. Regarding one of the questions asked to the pilot during the debrief, about any known previous occurrence like this, there were no recorded significant issues like this. This was the final step in our collective decision to “clear” the aircraft for reintroduction to service.

Conclusions & Evaluation

With the learning objectives met, this activity was an enlightening process. It helped me understand and appreciate the value of teamwork and effective communication in a work environment. Everyone looked out for each other, to avoid critical mistakes or to point out  any errors in judgement. As a group, the importance of safety was not lost on us. It was ingrained in us that safety is not an individual responsibility, but it involves everyone in a vigilant state. Safety being of high importance, paired with effective communication among group members could mean the difference between a smooth operation, or losing some fingers to an aircraft engine.

Another small part of safety that is often overlooked is tool safety. Personnel accountability is vital to the smooth running of a work space like this. A missing tool could ground an aircraft for hours, days, or even weeks.

I have also gained an increased understanding of how important record keeping is in the aerospace sphere. This helps multiple parts to be tracked and compared, saving aerospace companies time, resources and money.

If this activity was conducted again, I would spend more time on gaining knowledge of the aircraft, its operational capabilities and how this affects its performance. This could aid a faster route to diagnosis and help an easier gauge of any other potential underlying issues.

 


References

 

 

 

 


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