Optimising the engine size to the multi-engine helicopter's needs is a difficult process which has traditionally been carried out through the mechanism of the engine rating structure. However, the rating structure has many limitations and does not allow the best potential use of the engine to be realised in service. The introduction of the micro-processor based Engine Monitoring System permits a re-evaluation of the rating structure and also the presentation of limitations to the pilots. By using EMS it should be possible to achieve a better relationship between the demonstrated capability of the engine as shown in the Qualification Programme and the authorised release for in-service use. This may be regarded as a first step to be followed later by a change of the qualification test to a more representative form with the EMS giving a more tangible link between bench test and customer operation. This should allow better use of the engine to be made for the short time/high power requirement thereby allowing a more efficient engine performance at cruise conditions.