Vibrational fault diagnosis includes the steps and strategies for detecting faults due to vibrations at initial stages and identifying the root cause of the fault. The paper by Christos SKLIROS[1], “A CASE STUDY OF VIBRATION FAULT DIAGNOSIS APPLIED AT ROLLS-ROYCE T-56 TURBOPROP ENGINE” describes various steps and approaches adopted to detect and isolate the root cause of the failure to start the engine. The steps, observations and decisions described in the case study have been summarized below.
The maintenance actions and decisions to be taken for fault diagnosis are suggested by the Fault Isolation Manual (FIM). After a certain maintenance action is carried out by the technician, the activity is recorded in the maintenance log(s). Moreover, engineering reasoning and judgement play a vital role in maintenance if the instructions provided by the manufacturer through the FIM do not solve the problem.
Instructions from FIM
The initial symptoms of excessive engine vibrations occurred while attempting to start the engine. The instruction mentioned in the FIM for “failure to start” was to replace the starter. The maintenance personnel followed the instructions, but the identical problem repeated and the previously suggested maintenance action to “replace the pneumatic starter” was performed again. The engine could not crank after five ignitions and after a visual inspection, one more fault was found; the flexible tube connecting the starter control valve with the starter was found to be broken. The FIM did not provide any guidance for this case, so the fault detection had to depend upon engineering reasoning and judgement.
Engineering Reasoning
Starter control valve is a device that regulates the pressure of bleed air coming from the Auxiliary Power Unit (APU) or another engine, which is to be transferred to the pneumatic starter through a flexible tube. Since the flexible tube was found to be broken, the starter control valve was suspected for not being able to regulate the bleed air pressure. To test the valve, dry motoring of the engine was conducted and the outlet pressure of the valve was measured. The results showed that the valve regulated the pressure correctly and dispatched the bleed air at appropriate pressures.
Since the underlying fault was not in the bleed air system, the structural integrity was suspected. Excessive vibrations are the next most probable cause of the repeated component failures. Such vibrations could be related to a fault in one of the main rotating modules: propeller, gearbox, torque meter shaft, compressor, and turbine. Based on the corresponding technical manuals, velocity transducers were installed to measure vibrations at three modules: gearbox (horizontal transducers), compressor (horizontal), and turbine (vertical). In addition to that, a velocity transducer was mounted horizontally at the front side of the gearbox to assure the dynamic balance of the propeller since an unbalanced propeller could be a source of excessive engine vibrations.
Judgement
After assuring the dynamic balance of the propeller the engine was run; the measurements from the vertical velocity transducer at the turbine were observed to be within the safe limit. However, both the measurements of the transducers at the gearbox and compressor were significantly above their respective safe limits. This was verified by repeating the same test three times under wide range of power outputs. Since the propeller was dynamically balanced and the turbine’s measurements were within the safe limit, the three remaining modules that could be responsible for the excessive vibrations were: gearbox, torque meter shaft, and compressor.
These modules consist of many parts that could be responsible for generating excessive vibrations. Therefore, it was necessary to isolate the fault to one of the main modules. For this, suspected modules needed to be serially replaced until one of them resulted in an engine free of excessive vibrations. Due to workshop logistics and available spare parts, torque meter shaft was replaced initially. When the vibration readings were collected using the same process used previously, all the measurements from the velocity transducers were found to be within the safe limits.
Based on this outcome, the torque meter shaft was identified as the faulty module. It can be seen that, even though the transducers mounted at the gearbox and compressor captured vibrations above the limit; neither the gearbox nor the compressor was responsible for the fault. When the engine was returned to service with a replaced torque meter shaft, none of the faults found previously was observed. This verified the successful diagnosis of the fault.
Importance of the maintenance logs
In an organization where maintenance is conducted continuously over day and night, and by different technicians, updated and well-organized maintenance logs help to capture fault repeatability. Fault repeatability is an aspect to be considered in order to avoid the endless loop of replacing the same component, as in the preceding case, it was possible to identify that repeated occurrence of the same fault was actually an indication of some underlying fault that was not addressed by FIM.
References:
[1] Skliros, Christos. “A CASE STUDY of VIBRATION FAULT DIAGNOSIS APPLIED at ROLLS-ROYCE T-56 TURBOPROP ENGINE.” Aviation, vol. 23, no. 3, 17 Jan. 2020, pp. 78–82, 10.3846/aviation.2019.11900. Accessed 28 June 2020.
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