16:00   Structural Health and Structural Loads Monitoring II
Chair: Michel Guillaume
30 mins
Timothy Fallon, Devinder Mahal, Iain Hebden
Abstract: The three variants of the Joint Strike Fighter aircraft pose unique problems in designing and implementing and operating a structural health management system that will ensure structural integrity of the aircraft while allowing it to reach its design goals for service life. Airworthiness requirements from the US Air Force, NAVAIR and our partner countries have to be considered when designing the system. The JSF will have a state of the art onboard and off board system to facilitate structural prognostics and health management of the JSF fleets. This presentation will provide an overview of the system and will discuss how the design challenges are being addressed. Structural health management of a fleet is the embodiment of bridging the gap between theory and operational practice. New onboard sensors, more data collection than ever before, latest fatigue damage and crack growth algorithms, corrosion monitoring and prediction, unprecedented interface with operational and maintenance data - what does it all mean to the maintenance officer trying to keep his jets in the air, or to the fleet manager in planning future deployment of the "best" jets (from a structural life perspective). This presentation will discuss what the JSF program is doing to address structural health management and from these perspectives, and how prognostics will aid a Performance Based Logistics contract.
30 mins
Stephen Willis
Abstract: OLM: A hands on approach With increasing costs, and shrinking budgets, there is considerable economic pressure to get the most out of existing aircraft, extending their in-service life if possible and to do so in a manner that does not compromise flight safety. One of the methods to achieve this is Operational Load Monitoring (OLM). OLM involves the capture, analysis and reporting of measured strain data, or derived loads, plus associated flight parameters from a sample of instrumented in-service aircraft within a fleet. This is especially important for military aircraft as the same aircraft type can experience different flight envelopes depending on their use (transport, surveillance, army etc). Monitoring the fatigue of individual military aircraft is necessary to quantify the loads experienced by each aircraft, such that the total fatigue experienced by the airframe can be determined and informed decision can be made regarding safety and maintenance. Initial design calculations and predicted flight fatigue can be compared with the actual fatigue to ensure that the aircraft can be flown safely, and with adequate flight clearance for the airframe. This paper presents an overview of military aircraft OLM programs, the typical parameters they measure and how they store and analyze the data. The research utilizes case studies of four aircraft – a military trainer, a military AWACS aircraft, a military transporter and a civil aircraft modified for military usage – for which ACRA CONTROL provided the data acquisition systems. The paper focuses on the data acquisition technology used to measure the flight parameters, the data processing and storage and how this data is interpreted on the ground. Finally, some of the technology and other improvements that have allowed newer OLM systems to provide a better fatigue profile of the aircraft are described. Such improvements facilitate the acquisition of greater numbers of channels of data, with greater accuracy, and the recording of significantly higher volumes of data.
30 mins
Kate Lucas, Michael Duffield
Abstract: The VC10 has been in continuous service with the Royal Air Force for over 40 years and is required to remain in service for several years to come until the planned replacement by a new fleet of modern tanker/transport aircraft. This requirement dictates that structural usage monitoring of the fleet is essential which itself relies on a proper understanding of the service histories of the individual aircraft since entry into service. Therefore, the basis for structural usage monitoring is a comprehensive record of flying data for each aircraft. These records may be used to generate estimates of fatigue loads spectra which can then be analysed in terms of fatigue and damage tolerance criteria. There are currently three versions of the VC10 in service with the RAF and each version has a distinctly different typical history. The CMk1K aircraft were built as multi-role transport aircraft for the RAF and were upgraded to an additional tanker capability during the mid-1990s; the KMk3 aircraft served with a commercial operator during the 1970s before being converted to 3-point tankers for the RAF and entering military service during the early 1980s; the KMk4 aircraft served with another commercial operator for many years and were eventually converted to 3-point tankers for the RAF, albeit to a different overall configuration to the KMk3, during the mid-1990s. Whilst the service records from 1991 onwards of all aircraft are complete and available in an easily interrogated database, the earlier service histories are limited to basic and, usually, incomplete information. This means that it is not possible to easily or directly determine the total value of the service histories of the aircraft in terms of structural usage. As reported in the UK National Review at the 2007 ICAF Conference a structural monitor using the NzW method (where Nz = normal acceleration or ‘g’ and W = nominal aircraft mass) was developed for the CMk1K. This methodology has since been extended to the KMk3 and KMk4 aircraft. However, in order to quantify the historic usage of the aircraft, additional work was needed to generate a baseline for the monitoring programme. This involved a comprehensive analysis of the available information for the different variants, including the KMk2 aircraft which had been retired from service during the late 1990s. The KMk2 aircraft were of particular interest because they had been used to provide a large number of samples taken from the wing structure for the purposes of teardown inspection – as described in the UK National Reviews at the 2001 and 2003 ICAF conferences. The results from those inspections were combined with the analysis of the service histories of the donor airframes in order to validate the fatigue calculations for the other variants of the aircraft remaining in service. The purpose of this paper is to demonstrate how the various elements of the theoretical and practical assessments of the aircraft in terms of historic and current flight profiles, analysis of ‘g’ spectra, teardown findings and fatigue and damage tolerance calculations were combined to provide a credible basis for structural usage monitoring of a varied fleet of large, ageing aircraft. It will be shown how the various assumptions underpinning the assessments were validated and how the results have been used to demonstrate that the structural integrity of the critical joint features on the VC10 wing can be assured until the planned retirement from service at an average age of over 40 years.