Reliance on aging military platforms has become a global concern and military aircraft today are expected to remain in service for longer than their original life cycles. This is partly as a result of a post-Cold War slowdown in purchasing of new material in the late 1990s, and to cuts in defense spending. Indeed, the current USAF fleet is the oldest in its history, with the average age of aircraft being 26 years.
Moreover, the rising cost of new weapon systems, juxtaposed with the need to ensure mission capability and effectiveness, has made the maintenance cycles of aging military fleets an issue of critical importance. To solve this, the military must extensively source, remanufacture and upgrade components, not only to maintain availability and reliability, but also to improve their mission-capability and superiority.
Extending Service Life Of Aging Military Fleets
The useful life expectancy of aging military platforms may be extended significantly using customized high-performance electronic sensors to enhance existing systems or to replace obsolete items with those manufactured embodying the latest sensor technology. Theoretically, engineers could sustain aircraft almost indefinitely through modernization and maintenance. The iconic B-52 operated by the U.S. Air Force (USAF) for example, first flew in 1952 and entered military service in 1954, while the F-5's initial flight took place in 1963. Both have undergone extensive retrofits and continue to fly missions today.
In 2005, the USAF initiated a four-year program to upgrade the B-52's communications system, its first major upgrade since the Kennedy administration (see fig. 1). The upgrades included software and hardware, such as the ACR-210 Warrior (beyond-line-of-sight software compatible with radio and able to transmit voice) and LINK-16, a high-speed digital data link for transmitting targeting and Intelligence, Surveillance, and Reconnaissance (ISR) information.
Fig. 1: An upgraded B-52 bomber.
Upgrading Platform Performance With Electronic Sensors
The military employ defense applications where situational awareness is critical. Sensors provide critical information to enable the systems into which they are incorporated to take the most appropriate action. Today, sensors play a crucial role in enabling this situational awareness and, as a result, there has been a huge increase in the volume of information becoming available to military personnel.
The scope of sensor applications for avionics in defense aircraft is extensive, and all demand the highest levels of precision, repeatability, ruggedness, and reliability. Indeed, the need to consistently deliver measurement precision and repeatability continues to drive the need for further customization of sensors. Often these devices must function in the harshest operating conditions, and frequently in space restricted areas; thus sensors are constantly evolving in order to fully address the specific requirements of the application.
The Royal Air Force's (RAF's) interdictor strike (IDS) aircraft, the Tornado, for example, has been the principal strike weapon employed by the UK, Germany, and Italy for over three decades. It has an expected life-span of 40 years. The Mid-Life Update (MLU) program that took place between 1998 and 2003 has been vital to ensuring its longevity, upgrading 142 Tornados to a new variant, designated Tornado GR4/4A, with advances in systems, stealth technology, and avionics at a cost of £943 million.
Fig.2: An upgraded RAF Tornado
Compared to the Tornado GR1, the GR4 has Forward-Looking Infra-Red (FLIR), a wide angle Heads-Up Display (HUD), improved cockpit displays, Night-Vision Goggle (NVG) compatibility, new avionics and weapons systems and updated computer software. The GR4's upgraded navigation systems include a Global Positioning System (GPS), BAE Systems Terprom digital terrain mapping system and a Honeywell H-764G laser Inertial Navigation System (INS).
Sourcing Spare Parts Brings Unique Challenges
Militaries increasingly rely on sustaining and modernizing aging aircraft that form the bulk of their fleets but this comes with two key challenges. Firstly, they must confront the issue of how to source essential parts which have become obsolete. Secondly, maintaining these aging planes is increasingly diverting funds that could be used to design a new generation of aircraft for the costs of maintenance, repair, and operations (MRO) services for the legacy fleet.
A common issue is that the original strategy for sustainment and replacement of sensors, based on the original projected life, becomes redundant. To combat this, each aircraft requires extensive repair and remanufacture, component by component, in order to maintain airworthiness, mission capability and effectiveness. Subsequently, there are unique issues for sourcing parts that both fit the aging platform model and conform to contemporary quality standards – for example, attempting to integrate a digital system into a platform built in the analogue era.
Often the desired spare parts are out of production as the original manufacturers may have become bankrupt, closed down, or been absorbed into a larger organization. More often than not, the low demand is simply not commercially viable. Cannibalizing parts from other aircraft, either permanently grounding the aircraft or rendering them no longer mission capable, may be the only option available.
Sustainable Maintenance Strategies Are Urgently Required
While replacing obsolete parts does extend service life and upgrade performance, the life cycle of a commercial off-the-shelf (COTS) part may only be about 18 months. An aircraft's service life is measured in decades. As such, in the long-term, it is vital that this approach, dictated by the short refresh cycle of technology advancements, is managed effectively, otherwise it can exacerbate the problem.
Military equipment ages in two basic ways: redundant hardware or software that renders the equipment insupportable; and inadequate performance that renders the equipment unable to fulfil its mission. There are also two distinct types of aging: chronological; and cyclic. The former is driven by factors such as system obsolescence, corrosion, environmental damage and general wear. The latter is determined by the way the aircraft, vehicle, or vessel is operated and includes fatigue, thermal and stress damage.
Both chronological and cyclic events affect the rising cost of maintaining an aging fleet. Aging can cause flaws to develop earlier than predicted in the original strategy for replacement of parts and extended usage can accelerate their growth. Aggressive environments can also accelerate the development of flaws faster than what was initially predicted.
Maintenance cycles based on the fatigue life of structures or the mean time between failures (MTBF) are determined through rigorous testing. This is to determine when failures become prevalent, or the function of the system becomes compromised. Maintenance cycle inspections are, therefore, timetabled regularly to ensure safety and parts are replaced or repaired accordingly.
The USAF implements two major strategies for its maintenance work: Condition Based Maintenance + Prognostics (CBM+), and Reliability Centred Maintenance (RCM). The former performs maintenance when there is evidence from sensor data, or from off-line trend monitoring. The latter uses reliability tools and techniques to schedule maintenance to balance safety, schedule, and risk by considering the probability of parts failure.
The Path Forward
Sensors are a crucial component in many significant defense applications including fire control systems, naval communications, and vehicle systems. Mission capability drives this trend because personnel need to have ever more precise and diverse information about the environments in which they operate.
Given the rapid pace of development in sensor technologies, it is essential that any replacement sensors are form, fit, and function compatible with the original product. They should also be manufactured by a high quality organization that understands the requirements of military systems, and by one that has the relevant approvals.
As a result, the sensors industry will continue to further develop sensor technologies so legacy aircraft can adapt to ever more stringent military requirements. In most cases, this pathway will be more cost effective than developing, testing, and fielding a new technological solution, a process which typically takes many years. Precise, robust and environmentally protected sensors therefore offer core capabilities which can meet operational demands. They are one crucial solution to the challenge of ensuring legacy fleets are mission capable and effective for present and future service.
About the Author
Mike Baker is the Managing Director of Sherborne Sensors. He began his career with an apprenticeship at the Royal Aircraft Establishment in Farnborough, and a further four years with them as mechanical design engineer. Mike entered sensor design and development and by 1973 was Chief Engineer with Kulite Sensors, working in the USA prior to setting up their manufacturing operations in the UK. He then founded and built his own transducer business, Maywood Instruments Limited, which traded successfully for over 20 years. Following the sale of the business, Mike spent a period in specialist instrumentation and sensor consultancy, in the UK, USA, and Sweden before becoming General Manager of Schaevitz Sensors, part of Measurement Specialties (UK). Mike led a management buyout to form Sherborne Sensors in July 2002.