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.