With an electrical system connecting all the equipment with power, the control system controlling all the actions of the system, a monitoring system is needed to log the actions, performance and status of the components in these systems.
Monitoring systems technologies log the actions, performance an status of the components in the electrical and control systems. They collect data from various components, sub-systems or systems, which is then used to draw certain conclusions, based on algorithms programmed into the collection system. The monitoring collection system can be ground-based, while monitoring systems flying in the air, floating on the sea, or generating electricity on another continent.
All monitoring systems work on the simple basis of:
Sense - Acquire - Transfer - Analyse - Act.
The aim of a monitoring system is to maximise reliability and availability. A monitoring system will not stop a system from malfunctioning, but will log system data from which system characteristics can be deduced. This provides our customers with advanced data on the status of their system and data to plan maintenance schedules around.
The monitoring system collects data from all over the system and provides feedback at a specific location, this location can either be the control room on a ship or the control room on land receiving feedback signals from a fleet of aircraft, which are currently flying, or the generator set situated in on an oilrig.
The monitoring systems provides us with the freedom of knowing what our system is doing, how well it is doing it and will help predict how it will react next time we run it.
Rolls-Royce is improving its capability continuously to improve its monitoring capability and providing its customers with a higher level of in-service support.
Rolls Royce uses Engine Health Management (EHM) to track the health of thousands of engines operating worldwide, using onboard sensors and live satellite feeds.
A corporate EHM team covers Civil, Defence, Marine and Energy which enables the Group to develop technologies and best practice across all business sectors. In the Civil market for example, the Trent family of engines is supported by a comprehensive Rolls-Royce EHM capability operated in conjunction with Controls and Data Services (CDS), a Rolls-Royce company, and accessible as appropriate by the airlines involved.
EHM is a pro-active technique for predicting when something might go wrong and averting a potential threat before it has a chance to develop into a real problem. It is especially useful in industries such as aerospace where the results of a technical failure could prove very costly. EHM covers the assessment of an engine’s state of health in real time or post-flight and how the data is used reflects the nature of the relevant service contracts. Essentially, EHM is about making more informed decisions regarding operating an engine fleet through acting on the best information available.
The evolution of EHM and the revolution in its use has significantly reduced costs by preventing or delaying maintenance, as well as flagging potentially costly technical problems. New assets will incorporate EHM capability, and techniques will, where possible, be retrofitted to existing equipment. Broader engineering disciplines can benefit from the growing reservoir of supporting data. As operational profiles of technical performance are revealed in ever more detail – from individual components to whole engines – so engineers can develop more thorough and cost-effective maintenance schedules, and designers can feed higher reliability features into the engine products of the future.
EHM uses a range of sensors strategically positioned throughout the engine to record key technical parameters several times each flight. The EHM sensors in aero engines monitor numerous critical engine characteristics such as temperatures, pressures, speeds, flows and vibration levels to ensure they are within known tolerances and to highlight when they are not. In the most extreme cases air crew could be contacted, but far more often the action will lie with the operator’s own maintenance personnel or a Rolls-Royce service representative in the field to manage a special service inspection.
The Trent engine can be fitted permanently with about 25 sensors. The figure below shows the typical parameters measured for EHM.
Many of these are multi-purpose as they are used to control the engine and provide indication of engine operation to the pilot as well as being used by the EHM system. These are selected to make the system as flexible as possible.
The main engine parameters – shaft speeds and turbine gas temperature (TGT) – are used to give a clear view of the overall health of the engine. A number of pressure and temperature sensors are fitted through the gas path of the engine to enable the performance of each of the main modules (including the fan, the intermediate and high pressure compressors, and the high, intermediate and low pressure turbines) to be calculated. These sensors are fitted between each module, except where the temperature is too high for reliable measurements to be made.
Vibration sensors provide valuable information on the condition of all the rotating components. An electric magnetic chip detector is fitted to trap any debris in the oil system that may be caused by unusual wear to bearings or gears. Other sensors are used to assess the health of the fuel system (pump, metering valve, filter); the oil system (pump and filter); the cooling air system and the nacelle ventilation (nacelle is the cover housing – separate from the fuselage that holds engines, fuel, or equipment on aircraft). As engine operation can vary significantly between flights (due to day temperature or pilot selection of reduced thrust), data from the aircraft to provide thrust setting, ambient conditions and bleed extraction status is also used.
Most modern large civil aircraft use an Aircraft Condition Monitoring System (ACMS) to acquire the data for EHM. This captures three types of reports:
The first are snapshots, where the sensor data listed above is captured and collected into a small report. This is carried out during take-off, during climb and once the aircraft is in cruise.
The second type is triggered by unusual engine conditions. Examples might be if an engine exceeded its TGT (Turbine Gas Temperature) limits during a take-off. These reports contain a short time-history of key parameters to enable rapid and effective trouble-shooting of the problem.
The final type is a summary, which is produced at the end of the flight. This captures information such as maximum conditions experienced during the flight, and power reductions selected during take-off and climb.
The Trent 900 is the first engine to be fitted with a dedicated Engine Monitoring Unit as well as the ACMS. This engine-mounted system places a powerful signal processing and analysis capability onto the engine. A fan -mounted EMU is shown below:
This is used to look in more detail at the vibration spectrum, which helps to pick up problems with bearings or rotating components. It also provides a flexible computing platform so new EHM software can be rapidly deployed to detect specific problems.
A critical aspect of the EHM system is the transfer of data from aircraft to ground. Aircraft Communications Addressing and Reporting System (ACARS) digital data-link systems are used as the primary method of communication. This transmits the Aircraft Condition Monitoring System (ACMS ) reports via a VHF radio or satellite link whilst the aircraft is in-flight.
A worldwide ground network then transfers this data to the intended destination. The positive aspect of this system is its robust nature and ability to distribute information worldwide. On the other hand, the Airplane Condition Monitoring Function (ACMF) reports are limited to 3kB, hence the acquisition systems need to work within this limitation. Future systems are being deployed to increase data volumes through wireless data transmission as the aircraft approaches the gate after landing. This will enable more data to be analysed, but will not be as immediate as ACARS, where data can be assessed well before the aircraft lands again.
In the Defence business, the transfer of data is controlled by the service requirement. Some EHM data requires a rapid in-theatre response; some, such as fleet trends, has a more long-term aspect. Some of the longer-term information can wait until the engines return to the UK, although Rolls-Royce can still provide 24/7 support through a combination of deployed service engineers and the Operations Centre in Bristol.
As soon as the individual reports arrive at the specialist EHM analysts - Controls and Data Services (CDS) they are processed automatically. The data is checked for validity and corrections applied to normalise them. The snapshot data is always ‘trended’, so that subtle changes in condition from one flight to another can be detected. Automated algorithms based on neural networks are used to do this, and multiple sensor information is fused to provide the most sensitive detection capability.
When abnormal behaviour is detected, this is confirmed by a CDS analyst based in the Operations Centre, before being sent to the aircraft operator and logged by the Rolls-Royce Technical Help Desk. Manual oversight is still an important part of the process, as false alerts can cause unnecessary maintenance actions to be taken by airlines and these need to be avoided. Trended data, and data from the other types of ACMS report, are also uploaded onto the Rolls-Royce Aeromanager website, so that plane operators can easily view the health of their fleet of engines.
The EHM signature will typically highlight a change in an engine characteristic. Expert knowledge is then used to turn this symptom into a diagnosis and usually a prognosis. This is done by using the skills of Rolls-Royce engineers working with the Controls and Data Services (CDS) analysts, to assess the most likely physical cause of a particular signature, how an operator can confirm this and how urgently this needs to be carried out. For an engine that is showing gradual deterioration, for example, an inspection in several weeks time may be appropriate.
If a step change in performance has been observed, inspection within the next 2-3 flights might be recommended. The Technical Help desk will discuss the recommendations with the operator (to manage the best fit with their planned operation) and will then regularly liaise with them until the problem is understood and any risk to their service mitigated.
For more information on engine services offered by Rolls-Royce, please go to: http://www.rolls-royce.com/services