Airframe loads and usage monitoring of the CH-47D "Chinook" helicopter of the Royal Netherlands Air Force

Abstract

Prompted by severe structural maintenance issues, the Royal Netherlands Air Force has tasked the National Aerospace Laboratory NLR to develop an airframe loads & usage monitoring programme for their CH-47D helicopter fleet. After an initial pilot phase during which the technical and operational possibilities were explored, a routine programme named "CHAMP" (CHinook Airframe Monitoring Programme) was started in 2007. In addition to a fleet wide installation of a Cockpit Voice & Flight Data Recorder for the collection of the relevant parameters from the digital avionics data bus, two airframes have been equipped with a state-of-the-art data acquisition system and nine strain gauges each, which are recorded at a high sample rate. All data processing is performed off-board; no onboard data reduction is done. This has led to a vast and ever-growing database that can be used to conduct analyses that go beyond those traditionally performed within a loads & usage monitoring programme. This paper gives an overview of CHAMP and the underlying structural integrity concept that has been dubbed the "stethoscope method". This method centres around the development of Artificial Neural Networks that use the recorded data bus parameters to predict internal loads at the strain gauge locations. After the creation of such a "virtual strain gauge", the actual strain gauge can be relocated to monitor other key structural locations. Successive relocation of strain gauges finally results in a usage monitoring system that, in the long run, will be invaluable for structural life cycle management. Attention is paid to the acquisition and the off-board storage of the large sets of collected data, and an explanation is given of the analysis tools and methods that have been developed, and the results that have been achieved so far.