A Health and Usage Monitoring System (HUMS) records the status of critical systems and components on helicopters so that the early detection of progressive defects, or indications of them, is possible and thus rectification can be achieved before they have an immediate effect on operational safety. The on-board equipment stores data on a PCMCIA Card. For analysis, the card is downloaded after flight and maintenance analysis can then be performed on a ground-based computer. These systems were first deployed in the early 1990s as a response to the relatively poor continuing airworthiness record and their introduction led to, and continues to support, significant improvements in both safety and reliability.
A typical HUMS system uses sensors, distributed throughout the airframe and its components, which are linked to a central computer unit with a data recording and storage system. Monitoring trends in the recorded data is particularly important - it allows system specialists to determine whether the aircraft has developed (or is likely to develop) faults that require rectification.
The extent of HUMS data capture varies considerably. A basic system collects some usage parameters such as take-offs, landings, engine starts and winch lifts as well as a small subset of engine and transmission health data. The most modern systems monitor the health of all significant vibrating and spinning parts - engines, gearboxes, shafts, fans, rotor systems - and other components. The operational context of events is recorded so that the trends can be fully analysed and maintenance crews are thus able to proactively perform condition-based maintenance. The latest equipment allows the data acquired to be processed onboard the aircraft or at a ground station - and some systems allow it to be transmitted, whilst the helicopter is in flight, via satellite communications to operator maintenance control units so that subsequent maintenance downtime can be minimised by pre-planning. These systems can also be configured to automatically report urgent or emergency conditions to the operator and manufacturer from anywhere in the world. An example of a typical HUMS system is briefly described in the reference below.