As part of our Aircraft Systems module, we undertook two labs, namely fault diagnosis, and depth maintenance. The former involved a brief refresher on jet propulsion theory followed by a technical skills instructor lead ‘signposted’ diagnosis of the problem; whereby we were asked questions to help form our understanding of the situation, the task, and the technical background.


Risk Assessment & Training

As with all Harrier labs, personal protective equipment (PPE) such as coveralls and steel toe-capped boots were required. Additionally, before commencing work the typical tool checks with signature were performed, and the basic university lab safety practices were adhered to. For example, one person supported another when using a step ladder. All of these risk management practices were taught in first-year lab sessions and are upheld throughout our time at the university.

The vital importance of tool checks before and after conducting work alone cannot be understated. Tools unaccounted for may well be inside the aircraft on which work is conducted and in the event they’re not located, and the aircraft is released to service, can lead to fatal consequences. As such, shadow board toolkits and tool checks are required to prevent latent errors helping to reduce the likelihood of an incident due to human factors. As is required by law the university has an obligation to perform risk assessments for the safety of academic staff as employees and the student body as persons likely to be affected by any unsafe work (HSE n.d.). Further training was also required on this occasion for use of the boroscope as well as in the theoretical understanding of compressor surges. A copy of the task risk assessment can be found here.


Background, Planning & Tasking

Initially those present were split into two groups with my group’s tasking focusing on an engine issue. In this case, due to the University owning a decommissioned Hawker-Siddeley Harrier T4, the engine was a Rolls-Royce Pegasus 103. Although categorised as a high-bypass turbojet, for simplicity’s sake it can be thought of as a turbofan engine. A turbofan being a form of jet engine where a fan, driven by the core turbojet, is used to accelerate a large volume of air around the outside of the core. The bypass air produces roughly 75% of the thrust of the engine (King 2010), with more and more engine manufacturers looking to increase the bypass ratio from the current average of approximately 8:1, to double-digit figures as demonstrated by the Rolls-Royce Trent 1000, with a bypass ratio greater than 10:1 (Rolls-Royce 2017). This reduction in the core airflow comes about by improvements in combustion efficiency, among other things, which in turn helps make the engine quieter by having a larger bypass air layer interact with the exhaust gas stream. Although typically seen in commercial use, the Harrier was a unique aircraft which utilised this propulsion method making it capable of Vertical Take-Off and Landing (VTOL), earning its nickname the ‘Jump Jet’.

A typical commercial aviation turbofan arrangement showing the core turbojet, and the fan bypass (Rolls-Royce n.d.).

As a brief synopsis of the situation put to us, on ground handover the aircrew of the Harrier reported that during their low-level bombing sortie when descending from FL300 to FL30 after the initial ‘slam accel’ dive, a vibration and rumbling noise could be felt and heard throughout the airframe. The pilot paused the sortie, returned to FL300 and performed the compressor surge Standard Operating Procedure (SOP) checks with no further occurrence, deciding to continue the sortie before returning to the airfield. Initial questions therefore asked by myself and my colleagues included:

  • May you have suffered a birdstrike?
  • Did the CWP (Centralised Warning Panel) provide any warnings?
  • Was there any other action/defect which may have had an influence?
  • Was this a solo sortie or a two-ship exercise?


Although some of these questions would be apparent had the situation been real-world, it must be remembered that when working in a lab environment even the situation based obvious parameters may need to be clarified. As an example, birdstrikes would leave very clear evidence on the airframe which would have been spotted by the ground crew on the recovery of the aircraft, and other factors such as solo or two-ship exercise status would likely be known from the operations board; something often present in nearly all aircraft despatch offices.


Having satisfied ourselves that the incident was isolated, and was not caused by any immediately obvious occurrence, further investigation was required which started with reviewing compressor surge characteristics and causal factors to allow us to outline a suitable process to follow. Since by their very nature compressors compress air, it is known from Bernoulli’s principle that if the pressure of a fluid is increased its velocity must reduce, this is based on the fundamental principles of energy conservation. However, since axial flow compressors such as those used in turbofan engines have a rotational velocity component, kinetic energy is imparted on the airflow thereby allowing for an increase in pressure and, when passing through rotors, velocity as well. Over the course of the spool the pressure is increased, however, the velocity of the flow is kept fairly constant since when it passes through the stators no kinetic energy is imparted, and the velocity must therefore reduce.


As all compressor blades are aerofoils they are only able to perform optimally when the airflow on which they act is laminar. In the event of turbulence being created in the airflow the useful energy of the air is diminished, and this results in the velocity reducing to such an extent it becomes near stagnant (when compared with the free-stream or inlet velocity). More air is introduced and compressed until a point at which the pressure behind the rotor is so great that a reversal of airflow becomes possible, in which case the pressure is instantaneously released before being built up again by the continual input. This cycle of build-up and release occurs around 60-80 times per second, resulting in the rotor assembly on which it affects being violently shaken, which is what causes the rumbling and airframe vibration noted by the pilot.


The question then becomes, for what reason did the compressed air become turbulent? Returning to the fact that the compressor blades are aerofoils it follows that the angle of attack exceeding the critical angle is a likely cause of turbulent air downstream of the rotor. Some areas which are likely to be overlooked by the uninitiated, such as myself and my colleagues, were highlighted by the engine maintenance manual which points us to two potential causes from devices not initially known to exist to us. The factors affecting compressor blade angle of attack could therefore in this case include:

  • Blade distortion caused by:
    – Foreign Object Damage (FOD)
    – Icing
  • Blade setting angle being incorrect due to malfunctioning Variable Inlet Guide Vanes (V-IGVs)

And some additional areas of interest from the manual included:

  • Compressor pressure bleed valves
  • Acceleration Control Unit

The Pegasus 103 Maintenance Manual outlining potential causes of compressor surge, and the rectification actions required.

Having identified potential causes, the next part of the process was to determine a systematic approach to identify the root cause. Aircraft by their nature are of course designed to fly, therefore the longer an aircraft is grounded either, the lower the operational effectiveness of military applications, or the greater the profit loss in commercial applications. Aircraft maintenance fault diagnosis must therefore be:

  • Logical
  • Sequential
  • Evidence-driven

The quickest and most inexpensive action to undertake in most situations is a visual inspection. Major abnormalities and defects can often be seen without the need for specialist, time-consuming equipment and processes; in this case, it may determine if FOD is apparent, or any other visible deformities in the compressor blades. It will also provide you with a ‘heads-up’ for any other work that might need carrying out, for example, if it becomes apparent that a blade has failed and was further ingested, especially causing damage in the turbine assembly, the quickest and most cost-effective option is to perform an engine change whilst the damaged one is repaired where possible.


In performing the task, we initially examined the fan blades and compressor stages as well as the exhaust turbine assembly by torchlight inspection through the inlet and exhaust ducts. The fan blades could then be inspected for obvious signs of damage and the rear of the turbine, if the engine had ingested any pieces of broken blade, would usually show the remnants of molten metal which had solidified on exit. Neither inspection produced any discernible results, therefore inspection further into the heart of the engine, namely the less visible main turbine assembly was necessary by use of a boroscope. This process was even more time-consuming, but having been satisfied by no evidence of damage in the previous examinations the next logical step in the sequence was to perform this investigation. Fortunately, modern boroscopes are often LED camera based making use of an LCD screen on the control unit for a compact and lightweight tool much more manageable than older versions.

A typical still image captured by the boroscope camera of a turbine blade tip.

Satisfied that there was no damage in the turbine assembly the compressor surge did not appear at this point in time to be FOD related. The next stage would be to perform ground runs testing the Acceleration Control Unit, V-IGVs, and blow-off valves. Particular care would need to be taken as outlined at the rear of the lab sheet found here. Should any work be required it would be undertaken and once all is serviceable and within tolerance, then assuming there was no known recent history of a similar incident, the aircraft could have been released to service. These later parts of the process were impossible to undertake due to the airframe used, and the environment in which it is stored.


As future managers, it was worth our time undertaking this task to gain an appreciation of the extent of work required to successfully and thoroughly diagnose the apparent problem. That said, such activities can only really be learned on-the-job through real-world experience, however, the work conducted by myself and my peers lead us to a reasonable understanding of the general process followed by aircraft maintenance technicians, the sort of timescales required for diagnostic issues, and ultimately the various pressures induced by the market’s dynamic. The old saying ‘time is money’ can certainly be applied to the aerospace industry, and although safety is always a top priority, a balance needs to be struck in a modern world between thorough to excess and faster yet less safety-borne processes.



Health and Safety Executive (n.d.) Risk assessments [online] available from <> [14 March 2018]


King, P. (2010) How to Build… – A Jumbo Jet Engine [online] available from <> [14 March 2018]


Rolls-Royce (2017) Trent 1000 Infographic [online] available from <> [13 March 2018]


Rolls-Royce (n.d.) How does a jet engine work? Infographic [online] available from <> [14 March 2018]

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