The Indian Air Force (IAF) adopted a multi-pronged approach to customise the Su-30MKI to the IAF’s qualitative requirements and enhance its operational reliability and serviceability, especially once it became clear that Russia was perfectly willing to incorporate systems and sub-systems of non-Russia origin. One of the main areas of thrust was predictive maintenance through HUMS.
For acquiring this capability, the IAF joined forces with South Africa’s Aerospace Monitoring And Systems (Pty) Ltd (AMS), a high-technology electronics engineering company that designs, develops, manufactures and supports specialised proprietary Aircraft Monitoring and Data Recording Systems. AMS has serviced this specific niche of the global aerospace and defence market since 1984. Predictive maintenance means the on- and off-board processing of aircraft sub-systems data, resulting in an accurate, conclusive indication of the health and usage status of various airborne systems. The heart of any health-and-usage monitoring system (HUMS) is a Data Acquisition Unit (DAU), capable of handling hundreds of input signals, supported by powerful processing hardware and software. The HUMS not only has the capability to monitor almost every aircraft system and sub-system, including the avionics sub-systems, it can also act as an engineering data recorder. For the Su-30MKI, AMS was contracted for providing total HUMS solutions, starting with the definition of the IAF’s qualitative requirements, followed by the provision (development and implementation), integration and support phases. AMS’ total HUMS package, as installed on the Su-30MKI, includes the following:
• The Crash Survivable Memory Unit (CSMU), whose prime mission is to save aircraft data and cockpit voice information in a crash-protected, non-volatile memory for post-incident/accident investigations. The CSMU receives aircraft flight data from a DAU on an ARINC 717 Harvard Bi-phase communications channel, and stores the data in a crash-protected memory module in the CSMU. The CSMU also records two channels of cockpit voice information, and stores the information is the crash-protected memory module in the CSMU.
• The DAU, whose prime mission is to acquire, process and store aircraft flight data and cockpit voice data, and forward the aircraft data to the CSMU on an ARINC 717 FDR channel. Recorded aircraft flight data and cockpit voice data is stored in non-volatile flash disk memory in the DAU for post-flight analysis of Su-30MKI component health and usage, and debriefing.
• The airborne Flight Data Recorder (FDR) system, which consists of a DAU, CSMU, and a Maintenance Panel Unit (MPU). The FDR ground support system consists of a Flightline System (FLS) and a CSMU Control Unit (CCU). The prime mission of the FDR system is to record and process aircraft data and cockpit voice information. A recorded sub-set of flight and voice data is stored in crash protected memory for post incident/accident investigations. Recorded flight data in the DAU is used for post flight analysis of aircraft component health and usage. The prime mission of the FLS is to download data from the on-board DAU and retrieve modify the DAU configuration data. The FLS is a ruggedised notebook computer that can be transported to the flight line and operated under harsh exposed conditions. All inputs in the field are via a touch-sensitive screen using graphic-based MMI screens. The FLS can download selected flight and voice data from the DAU. The FLS also has an external keyboard and mouse that allow easier use under office conditions using a special interface loom.
Fatigue in metallic structures is associated with cyclic loading, and can occur at low stress levels which would not otherwise cause failure when applied as a single event. Repeated loading causes cumulative damage. A structure will absorb cumulative damage from fluctuating loads, which eventually leads to the formation of small, detectable cracks. This is termed the life-to-crack initiation of the aircraft and in certain circumstances the airframe life is considered to have been expended when these first cracks start to appear. Nevertheless, aircraft can be operated past this time, into the so-called crack-growth phase, albeit with different maintenance and inspection intervals and procedures. With modern fracture mechanics techniques, it is possible to predict and predict/correct crack growth in almost any geometrical configuration. This in turn allows one to extend the life of an airframe beyond the threshold of crack initiation. This approach, along with damage tolerant repair procedures, can safely extend the life of an airframe beyond its previously-anticipated phasing out date. Finally, the cracks start to grow rapidly, leading to fracture. Tolerance to cyclic loading varies widely for different materials, with certain fracture-tough steels exhibiting excellent fatigue properties. However, the choice of material is often dictated more by the weight for static strength.
Even with the ever-increasing employment of composite materials, aircraft structures are still predominantly made up of aluminium and titanium alloys, and this brings up the issue of structural fatigue caused by fluctuating loads. For fracture-tough steel structures, designing for low mean- and fluctuating stress levels can give a near-infinite life. Unlike these steels, high-strength aluminium alloy structures cannot be designed for infinite life without considerable costs in structure weight. Any fluctuating stress imposes cumulative damage, which will eventually lead to fracture and failure. Weight efficient aircraft structures therefore have a finite life. The basic aims for the Su-30MKI’s aircraft fatigue monitoring and management system are generally:
• Verification of the structural integrity and longevity of new aircraft.
• Life monitoring, maintenance, and safe life extension of aging aircraft.
Interest in the monitoring and managing of aircraft structural fatigue has greatly increased globally over the last two decades. Fleet operators who are considering the implementation of a HUMS programme should take cognisance of why this is so. Notwithstanding finite structural life, the much higher rate of advance in avionics technology, relative to aerodynamic, airframe and engine technologies, has meant that aircraft “obsolescence” is now determined more by the avionics fit than the performance and efficiency of the airframe/engine combination. This is particularly the case for military aircraft. Because of rapid ageing of avionics technology, avionics upgrades to in-service aircraft have become the norm. These can take the form of frequent small upgrades, or major refits once or twice during its service life. A new engine and airframe can account for 40-50% of the total cost of a new combat aircraft, while adding little of consequence to its operational capabilities. Increasingly, cost-conscious air forces and other fleet operators are opting to retain existing airframes and engines and to spend their money on capability-enhancing equipment. An extreme example is the B-52H aircraft, which entered service in the late 1950s and is currently expected to remain in service up to the year 2040, a life of 80 years!
In light of the above it is clear that the airframe is the limiting factor in aircraft longevity, and this in turn determines the economic viability, or otherwise, of any upgrade programmes that might be envisaged. The remaining airframe life is a decisive factor in aircraft upgrade decisions. Therefore there is an increasing demand for accurate tracking of life consumption. Many operators acquire used rather than new aircraft as a matter of policy, and the availability of accurate remaining life estimates can greatly increase the resale value of an aircraft fleet. As part of the initial design process, an aircraft manufacturer will assume a flight loading spectrum to which the aircraft will be subjected to during service, and design for a specified life on the basis of that spectrum. However, changing roles and missions may, to a greater or lesser degree, invalidate the initial assumptions. In the case of military (and particularly combat) aircraft it is quite possible that the roles they are eventually called upon to fulfil, and missions they are then required to carry out, will be very different to those originally envisaged. Initial estimates of fatigue life are then invalid, and a great deal of uncertainty is introduced into fleet management. The objective of fatigue monitoring in HUMS is to reduce these uncertainties. There are cases where the initial life estimates have been extended by 50% as a result of more accurate measurement and monitoring programs implemented in service.
In Output-Based Contracting (OBC) an aircraft supplier is no longer paid for a once-off delivery of systems, but instead is contracted to provide a service, for example flying hours. There is currently a marked trend towards this type of contracting, which brings greater contract risk while at the same time also holding out the prospect of higher financial returns. Minimising risks, while increasing operating efficiencies and maximising the life of the aircraft, poses great challenges. The aircraft provider can meet these challenges by including fatigue monitoring as part of general health- and usage monitoring. Fatigue monitoring can protect the service provider from the consequences of unduly severe usage, by providing a record of such usage and the impact thereof on the structural life of the aircraft. Output-based contracting exposes the aircraft service provider to higher levels of risk. One major risk is that the client’s usage of the aircraft will not correspond to that assumed by the contractor at the time of initial costing. For example, it is quite possible that different missions with more demanding mission profiles could be imposed during the life of the aircraft. Furthermore, flight limitations may be exceeded more frequently during an given mission. These eventualities would impact negatively on the cost of providing the service contracted for. Monitoring will provide clear indications as to whether the aircraft is being operated within its contractual limitations and provide recourse to the supplier.
The aircraft operator/end-user can use health-and-usage monitoring to minimise contractual risk. But after this, aircraft maintenance operations can be managed so as to improve profitability. If costs are estimated conservatively at the time of bidding, and if realised costs can be minimised during the service period by intelligent management, the higher reward commensurate with higher risk will be obtained. A more accurate knowledge of the probable condition of the aircraft can allow on-condition maintenance, as opposed to unnecessarily frequent and therefore more expensive scheduled teardowns and inspections. OBC requires that a given number of flying hours are to be provided per month. Aircraft availability is therefore a major factor. Anticipation of problems via fatigue monitoring (and general systems condition monitoring) allows early corrective action to be taken, with beneficial effects on availability. Less aircraft are then required to fulfil the contractual obligations.
Initially at the OEM the focus is on methods for fatigue and fracture mechanics technology in the design of durable, damage-tolerant aircraft structures. Aircraft manufacturers generate and maintain Fatigue and Stress Corrosion Manuals. Once the aircraft has been in service for some time, the operator will focus on the extended safe use of aging aircraft: The applicable technologies have to do with the basic fatigue and fracture behaviour of structural metallic materials. For the Su-30MKI, methods have been developed by the IAF for the following:
• Fatigue loading spectra;
• Fatigue analysis methods;
• Material fatigue behaviour;
• Fracture mechanics;
• Damage tolerance analysis and testing of redundant metallic aircraft structures;
• Fatigue crack growth analysis;
• Crack growth, residual strength analyses, and aircraft structural integrity programmes;
• Ageing aircraft issues.—Prasun K. Sengupta
Within 72 hours of the fatal crash of an IAF Su-30MKI on April 30, it was theoretically deduced by experimental test pilots attached to Russia’s Sukhoi OKB that only a catastrophic failure of the aircraft’s digital redundant fly-by-wire flight control system (FBW-FCS) would have caused loss of flight control and the consequent crash. But what could not be explained until recently was under exactly what circumstances would a redundant FBW-FCS—designed to be a fail-safe system—not deliver as advertised. And the answer could not have come from Sukhoi OKB simply because it, in case of the Su-30MKI, was responsible for only prescribing the flight control envelopes of the aircraft for both conventional flight profiles as well as for those involving supermanoeuvrability. What the OKB could not monitor or enforce was the operational flight regime of the Su-30MKI. And if initial findings of the IAF-instituted Board of Inquiry (BoI) are to be believed, the crashed Su-30MKI’s loss of flight control can now be pinned down, with a high degree of probability, to the FBW-FCS (see the block diagram above) suffering catastrophic failure as a result of non-prescribed usage of the aircraft’s FBW-FCS regime, which in turn had probably caused loss of the aircraft’s directional control (attached above is a brief explanation of the Su-30MKI’s FBW-FCS as appearing in the aircraft’s technical manual).
