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Delivery of NDT in the UK Military
Warrant Officer GeoffreyTipler,Non Destructive Testing Control Officer, Assistant Directorate Materials and Monitoring,
United Kingdom.
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| · Home · Table of Contents · Aerospace | Delivery of NDT in the UK MilitaryWarrant Officer GeoffreyTipler,Non Destructive Testing Control Officer, Assistant Directorate Materials and Monitoring, United Kingdom. Contact |
The paper outlines the fundamental changes to the logistics support of the UK Armed Forces and how those changes impacted on the provision of NDT support to the air environment. The role and responsibilities of the new NDT organisation are described together with some of the gains and efficiencies already achieved. A brief overview is provided of each of the NDT disciplines employed and an outline of future developments that could enhance the effectiveness of the application of NDT in the military.
Keywords: NDT, Military, UK, Organisation, Aircraft.
Until recently, the 3 Services of the UK's Ministry of Defence (MOD), consisting of the Royal Navy (RN), the Army and the Royal Air Force (RAF), had evolved along single Service lines with close and effective liaison between them for operational purposes. Consequently, similar support and logistic activities were triplicated across all three Services. In 1997 a new government initiated a 'Strategic Defence Review' (SDR) which had considerable impact on all 3 Services. The aim of the SDR was to modernise and re-shape the Armed Forces to meet the challenges of the 21st century and to give them a firm foundation upon which to plan for the long term. In particular, the SDR was to transform the way in which the Armed Forces do business in the support of equipment. In 1999 the post of Chief of Defence Logistics was created and his remit was to provide a unified defence-wide support organisation, the Defence Logistics Organisation (DLO), which would rationalise single Service support areas and maximise Cupertino between the Services. The 3 supporting pillars of the DLO are organised on an environmental basis (air, land and sea), with the Director General Equipment Support Air (DGES(Air)) responsible for supporting all air assets of the 3 Services. At the same time to facilitate the aim of more effective and efficient logistics, the support for each aircraft type was reorganised into Integrated Project Teams (IPTs) consisting of engineers, suppliers, financiers and contracts personnel to provide seamless whole life equipment support. The DLO is a young organisation and is still evolving to meet the difficult challenge of restricted defence budgets whilst still supporting ageing equipment fleets. The formation of DGES(Air) also brought together the individual Service technical support functions under Director Technical Air (DTech(Air)). Whilst for over 40 years the RAF and RN had their own separate NDT organisations to support their aircraft, with the RAF also supporting Army aircraft, assisted by 2 Army technicians, the drive towards rationalisation and providing cost-effective support resulted in the decision to amalgamate the single Service NDT organisations under one Assistant Director, who is responsible for both materials and monitoring.
Before the formation of DTech(Air) the RN`s NDT was controlled by the Naval Aircraft Materials Laboratory (NAML) located at Fleetlands, Gosport, Hampshire. RAF NDT was controlled from the NDT Squadron based at RAF St Athan in South Wales and the Sqn was part of the RAF Logistics Support Services. The Assistant Directorate Materials and Monitoring (AD M&M) was formed by the amalgamation of the NDT Sqn and NAML. AD M&M is commanded by a senior civilian scientist and the NDT Sqn, which now encompasses the RN NDT capabilities at Gosport, by a RAF aircraft engineer Squadron Leader. The amalgamation has provided opportunities to exploit the synergy from the creation of a single integrated community of materials scientists, NDT technique authors and the practitioners responsible for in-Service delivery of NDT. The gains achieved so far include the development of more sensitive and directed NDT techniques, the convergence of the individual Services` documentation into a common standard and the ability to deploy a complete solution to in-service equipment failures. The ultimate aim is to be the Tri-Service 'one stop shop' for all the DLO`s aerospace NDT requirements.
Fig 1:
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The Squadron is divided into 4 Flights:
Equipment & Training Flight.
The Equipment and Training Flight is commanded by a RAF aircraft engineer Flight Lieutenant and is sub-divided into 3 Sections which consist of:
Equipment Evaluation Specialists.
The Section provides technical expertise at all stages of the procurement cycle and the introduction of NDT equipment into service. The majority of the NDT equipment procured is Commercial Off The Shelf (COTS) and the Section provides "intelligent customer" capability, independent of the Original Equipment Manufacturers (OEMs), for the IPT responsible for purchasing NDT equipment. Moreover, the specialists act as a central information source for DGES (Air) on current equipments, equipment developments and related new technologies. The value of using NDT trained personnel with in-service experience is that any equipment is assessed against typical Service applications coupled with a knowledge of the range of environments in which the technician has to work and the skill level to be expected of the technician. This combination of factors increases the probability of an effective item of NDT equipment being delivered to the front-line NDT technicians.
Defence Procurement Agency Support Team.
The Defence Procurement Agency (DPA) is a MOD agency responsible for the introduction of major defence projects. NDT Sqn`s DPA Support Team was established 3 years ago to provide dedicated NDT technical support and advice to the various DPA IPTs. The principal aim of the Team is to provide NDT advice early in the procurement cycle that is independent of the OEM, to enable IPT`s to make informed decisions about equipment, applications and requirements with an accurate knowledge of in-service capability. The current focus of attention is directed at the new aircraft entering, or, due into service shortly such as Eurofighter (Typhoon), Nimrod MRA 4, C17, Apache and C130J. Future work will be directed to Joint Strike Fighter and the Future Offensive Air System.
The Tri-Service School of NDT.
With the formation of AD M&M, a rationalisation of NDT training was conducted and now all training is carried out at the Tri-Service NDT School, collocated with the NDT Sqn. The School is manned by 4 experienced NDT qualified RAF Senior Non Commissioned Officers (SNCOs) and provides courses for the basic training of NDT Technicians (Army, RN, RAF and Civilian), RN NDT Operators, specific equipment courses and NDT appreciation courses. The School is accredited to the British Institute of NDT and the NDT technician's course is comparable to the Institutes` PCN Level 2 for aerospace disciplines.
Technique Development Flight.
The Technique Development Flight is responsible for developing and writing all the NDT techniques (inspection procedures) used on MOD aircraft. The procedures are produced on behalf of each aircraft or equipment IPT and are published in the relevant aircraft or equipment publication. The Flight is split between 2 sites with Gosport producing Rotary Wing techniques and St Athan producing Fixed Wing techniques. The Flt is commanded by a civilian Higher Scientific Officer with practical experience and knowledge of materials science, and manned by 14 field experienced NDT technicians, drawn from all 3 Services, each with responsibility for a number of aircraft or equipment types. Each technique produced is validated on a reference standard and representative aircraft.
The Repair & Calibration Flt.
The Repair and Calibration Flt is commanded by a civilian Higher Professional & Technical Officer who manages 4 technicians and 3 Supply personnel. They are responsible for the repair, maintenance and calibration of the majority of the Aircraft Integrity Monitoring Equipment (AIME) within DGES(Air). The range of equipment includes NDT, vibration analysis, helicopter rotor track and balance and aeroengine debris testing. The Flt is also the supply depot for all of this equipment. The co-location of both functions has facilitated the development of a totally integrated and efficient service to the users with a rapid turnaround of equipment.
The Regional NDT Team Service
The Regional NDT Team (RNDTT) service delivers the NDT function to the Front Line Commands to ensure the continued airworthiness and structural integrity of the aircraft fleets whilst maximising aircraft availability. The day to day functional management is provided by the NDT Control Officer (NDTCO), a Warrant Officer who was previously employed as an NDT technician. His main function is to ensure the effective use of manpower and resources throughout the RNDTT service. The RNDTT service consists of 65 SNCO aircraft technicians trained and qualified in NDT located in 8 UK based teams, each responsible for NDT support directly to aircraft sqns within their geographical area. The teams are managed by either a Flight Sergeant or Chief Technician, depending on the size of the team, and the personnel establishment ranges from 4 to 13 men depending on the workload and the extent of the geographical area supported. The NDTCO also co-ordinates NDT support to all Out of Area Operations; currently support is provided to aircraft based in Saudi Arabia, Kuwait, Turkey and the Balkans, and to any other overseas aircraft detachments whether short notice or pre-planned. All the RNDTT personnel are prepared for a 24-hr standby-to-move in support of overseas operations. Aircraft types supported range from venerable Spitfires, Hawks of the Red Arrows Aerobatics Team, to the latest GR4 Tornados and the Hercules C130J. RAF St Athan is a 3rd line maintenance base for a large variety of aircraft and a 9 man civilian RNDT team has been established to provide on base support. To provide additional support to the Army Air Corps, 2 Army Royal Electrical and Mechanical Engineer technicians are established within the RNDTT service.
Following training by AD M&M, RN NDT Technicians are employed and managed completely independently of the Regional NDT Service. In the Tri-Service environment there are undoubtedly potential improvements in overall effectiveness and efficiency in having all NDT Teams integrated under one unified functional control with the adoption of common standards for certification, continuation training and technique recording. The closer integration of AD M&M and RN NDT activities is a goal that will be actively pursued with the aim of making the most efficient use of a common resource.
Due to the unsupervised nature and the fundamental importance of NDT tasking, the individual is the most important link in the application of any NDT procedure. Therefore, only SNCO technicians of the highest calibre are selected for training by means of a written exam and an intensive interview. The technician must be able to exhibit a high degree of personal integrity; be resourceful and imaginative; have a substantial engineering background; be decisive and precise; and be able to express himself competently, both orally and in writing. Following the 10-week technicians' course, the technician must then complete a minimum of 6 months programmed and supervised 'On the job' probation, to demonstrate his competence in all disciplines, before he can be authorised to work unsupervised by the award of a full NDT certificate as a 'Category A' technician. Before being considered suitable for employment at the NDT Sqn HQ a minimum of 3 years team experience is required.
The RNDTT technicians also train 'Category B' NDT operators on the units within their areas who are then qualified to carry out routine Dye Penetrant, Magnetic Particle and basic Eddy Current and Ultrasonic inspections. Category B personnel are re-examined and re-certified every 6-months by RNDTT personnel.
Some RN NDT requirements are fulfilled by a small number of Cat A technicians, mostly employed at the shore bases, whilst the majority of NDT tasking is undertaken by NDT Operators deployed on RN frigate/destroyer size vessels. A RN Operator is a supervisory Air Engineering Rating who has successfully completed a course at the NDT School on a particular NDT equipment and NDT discipline and who has been authorised to self certify in the relevant discipline on an aircraft or equipment type. With only one or 2 helicopters on a destroyer and the limited requirement for NDT the RN Operator provides flexibility, as NDT is complementary to his other engineering tasks. The use of NDT Operators may offer potential efficiencies to the operation of the RAF's Search and Rescue (SAR) helicopter flights, and AD M&M are therefore organising a trial whereby SAR aircraft technicians will undertake NDT training to RN Operator standard to explore the potential of this significant change to established RAF NDT practices.
The full range of NDT disciplines is utilised by AD M&M in order to detect and quantify faults, damage, corrosion and delamination to aircraft structures and components. However, in the technique development process the 'best' method is usually identified through a process of natural selection, depending on the component variables and accessibility to the inspection area. In addition, our objective is always to keep the technique as simple as possible, as this invariably provides the most cost effective and reliable solution. Moreover, the synergy resulting from the creation of a single integrated community of Materials Science, NDT technique authors and the Regional NDT Teams has resulted in a much more coherent and efficient response to investigating an in -Service failure. The resulting techniques are more directed and sensitive, with reduced spurious rejections thereby achieving greater confidence in the technique for the field teams and ultimately the customer IPTs. In this section a variety of disciplines are briefly described, addressed in increasing order of complexity. In addition, current limitations are described and potential developments offering improvements for the future are outlined.
The human eye is the most cost effective NDT method, and also the oldest, and the true value of the Mk 1 eyeball is that 'seeing is believing'. Even in the world of high technology, the eye is unmatched when examining, measuring and assessing the surface condition of a component. However, if the suspected fault is very small, or if access is restricted, then a wide range of equipment to aid the eye are available for use in the Services. These include basic magnifiers, Remote Viewing Aids - both flexible and rigid, and Flexible Probe Video Visual Aid (FPVVA). The current in-use FPVVA is a range of Welch Allen XL Video Probes with a liquid crystal screen located in the operating handle; however, for critical viewing, a high resolution Sony monitor is added. The use of FPVVA has increased dramatically throughout the Sevices, with considerable nugatory work avoided by the ability to find objects and investigate structure without extensive dismantling. The traditional use of RVA for engine inspection continues and in the near term we anticipate increased use of equipment to accurately measure damage to blades in situ. The equipment can be operated by 1st line operators and the consequent reduced aircraft down-time, savings in man-hours and components will provide significant financial savings for the aircraft IPTs.
The Penetrant Flaw Detection (PFD) method is still a useful basic means of finding fine surface breaking faults that could be invisible to the eye. PFD techniques are used extensively by Category 'B' trained operators both at 1st and 2nd line maintenance and on automated lines for component inspection. All new techniques employ the more sensitive fluorescent PFD system, in either oil or thixotropic form. There are currently 21 different PFD techniques used in service, which utilise a variety of penetrants, solvents and developing agents. Development work initiated at AD M&M is underway to quantify the performance of fluorescent PFD in ambient light conditions to make the method easier to use in field conditions.
Magnetic Particle Inspection (MPI) is another valuable basic NDT system used to detect surface and/or slightly sub-surface faults in ferro-magnetic materials. MPI is used predominately by Category B operators at 1st line and in component bays. MPI has the advantage that it is fast to apply and any indications can be viewed immediately. However, it can be messy and the component must be de-magnetised on completion. MPI is still a useful tool in our NDT armoury and we consider that our MPI benches and manual MPI equipment will remain in Service use for many years.
The majority of eddy current inspections are used to examine for surface breaking faults, however, they also have the ability to detect faults in the second layer or on the blind side of a component to a depth of 10-15mm, or to determine paint or cladding thickness on metal surfaces. As eddy currents are affected by changes in conductivity, Hocking Autosigma 2000 and 3000 instruments are used to check for heat damage following a fire. Eddy current inspections are one of the more sensitive methods available and are relatively quick and clean to carry out. In the UK MOD, most on-aircraft eddy current inspections are carried out by NDT technicians, except for simple aircraft wheel inspection techniques which use an automated test machine. There is a variety of eddy current equipment in-service including the Hocking Locator UH, a simple needle swing instrument, the Rototest rotary eddy current instrument and the Elotest B1 dual-purpose rotary and impedance plane instrument. The latter can interface with a PC-based scanning system called ANDSCAN to provide 'C'-Scan maps of structure, enabling a plan view of the structure to be stored, compared and/or printed. This is particularly useful for mapping corrosion in ac skins. The Hocking Locator 2, an impedance plane instrument, was recently procured to replace the Locator UH as the principal eddy current instrument. The Locator 2 is light, compact and easy to operate. In addition to the usual benefits of impedance plane presentation, it enables technique parameters to be saved to memory or uploaded from a PC. Other equipment being evaluated at present includes the 'JENTEK' Meandering Winding Magnetometer Array system. This equipment has the potential to accurately detect the onset of corrosion, multi site fatigue cracking and micro crack clusters in aircraft structure, and can monitor existing cracks and corrosion. The equipment supports permanently mounted sensors and conformable arrays which allow rapid inspection. Other technologies being examined include transient eddy currents and eddy current array equipment. Transient eddy currents have the potential to detect corrosion in 2nd and subsequent layers of structure, a prevalent problem in maritime aircraft. In certain applications, eg engine blade root inspections, arrays could eliminate the need for raster scanning and reduce inspection times considerably. In addition to increased speed, arrays offer repeatability, C-scanning and digital data storage, and can eliminate the need for complex mechanical scanners.
There is a broad selection of equipment in our inventory for carrying out ultrasonic inspections. NDT Technicians rather than operators carry out the great majority of these inspections. The Krautkramer DME/DL is used for simple thickness measuring, while our principal flaw detector is the Krautkramer USD 10, supplemented by the Panametrics Epoch III for applications where access is restricted or weight is a factor, eg working at height. The Krautkramer/DERA ANDSCAN system is employed to produce 'C' scan maps of areas of interest, particularly in composite structures. It can be used in conjunction with a wide range of in-service flaw detectors including the Elotest B1 eddy current flaw detector, the Krautkramer USD 15 ultrasonic flaw detector and the Staveley Bondmaster. These 'C' scan maps will show cracks, corrosion, disbond and delamination along with the ability to quantify any of the faults found such as the size and depth of delamination in a carbon fibre structure. Current ultrasonic projects include the evaluation of Boeing's MAUS IV (Mobile AUtomated Scanner) system and Andscan for the area scanning of Eurofighter. We are keen to practically evaluate phased array systems, which have the potential to reduce inspection time, and Full Waveform Capture, which could facilitate technique development and allow more detailed post-inspection analysis of C-scan images. Other methods within the ultrasonic sphere include Mechanical Impedance, Resonance and Pitch/Catch. Mechanical Impedance inspections are carried out using either the simple Woodpecker, which is an electronic 'toffee' hammer, or the Staveley Bondmaster. The Bondmaster is also frequently used in Resonance and Pitch/Catch modes to examine for disbonds in bonded structures and honeycomb sandwiches, for example the Hawk's rudders and ailerons.
After the concerns expressed over ageing aircraft as a result of the Aloha Airlines B737 experience, the RAF embarked on a Proof Pressure Testing (PPT) Programme of its VC10 fleet in Dec 1991. During the test the cabin was subjected to a pressure of 1.33 times its normal working pressure. Acoustic Emission (AE) monitoring was employed as a safety net for the PPT. AE would detect the onset of major damage in sufficient time to allow the pressure in the aircraft to be reduced before serious damage was caused to the structure. The AE test on the VC-10 fleet was one of the largest AE field tests ever conducted worldwide. A ramping cycle was used to increase the pressure to the maximum and during the pressure hold periods when the airframe had stopped moving; any AE signal produced that was not caused by an air leak was of significant interest. The programme ended in 1998 and the data gathered gave sufficient confidence in the structural airworthiness of the VC-10 to allow a life extension to the aircraft. Currently AE is under active consideration to monitor and determine the condition of composite pressure cylinders using a pressure test equal to, or just above, working pressure. In this case the existing revalidating proof pressure test of the cylinders involves testing at twice the working pressure which, in itself, may cause damage to the structure.
Radiography is the least preferred NDT method due to the Health & Safety implications generated by the hazards of radiation, the chemical processing of radiographs and the manual handling of heavy equipment. The safety of both the technician and other personnel are paramount whilst carrying out radiography. The in-Service equipment mainly comprises of 15 year old Andrex sets, the majority of which are 160kV. The RN operates 13 newer Sieffert 200kV CP sets. Shortly, the Equipment IPT will issue a specification to tender for a new buy of 35 lightweight Constant Potential sets to operate up to 160kV. All the RNDT teams process graphs using Kodak M6I automatic processors but they can revert to manual processing if required. For the future, we are carefully monitoring the advances made by industry in the development of filmless radiography and the digital storage of existing and new radiographs. Both methods offer significant advantages for military NDT once the sensitivity matches that of traditional wet film. AD M&M also have 2 X-TEC real-time Micro-Focus X-Ray System sets that offer a means of carrying out safe radiographic inspections and concurrent serviceability assessment on batches of small components. Because the images are digitised and the system is computer-controlled, faults can be dimensionally analysed. Additionally, as a further enhancement to the imaging of digital radiograph, the system has a computerised tomography capability, for C-Scan analysis of components. This is particularly useful in the assessment of engine blades and the internal geometry of components.
Developing technology has the potential to overcome many of the limitations of current equipments, offering improved detection capability and reducing the time needed for inspection.
The main drivers towards improved inspection capability are the need to support ageing aircraft beyond their initial design lives, and the need to inspect composite structures. Particular requirements for ageing aircraft include the detection of small faults deep within structure, detection of faults and corrosion beyond the 2nd layer, and detection of faults around installed fasteners. Composite structures will require inspection of large areas to detect disbonds, delamination, deterioration and other damage. Work is underway to detect corrosion within the 4th layer of a maritime aircraft's metallic structure using transient eddy currents. Initial results appear promising, with the potential to detect approx 20% material loss within each layer, representing 5% loss of the total thickness.
With the introduction of the Eurofighter Typhoon into service in 2003, NDT Sqn have been evaluating the options for large area scanning of the aircraft both on delivery and during scheduled maintenance.
Work so far indicates the need for an ultrasonic phased array mounted on a fully- or semi-automatic scanning system in order to complete the inspection within time limits and without excessive manpower requirements. Full-waveform capture is attractive, as storage of the complete ultrasonic waveform greatly facilitates post-scan analysis and could allow through-life condition monitoring of the structure.
Eddy current arrays have the potential to reduce the time taken to inspect complex geometries such as gas turbine blade roots, and an evaluation program is due to start shortly at RAF St Athan.
Laser Shearography (Speckle Pattern Interferometry) is being successfully used to identify defects on the E3D Sentry's 50m2 Rotodome. Shearography is sensitive to delaminations in the composite skin and disbonds between the skin and the honeycomb, and does not require coupling to the structure. It is also quick and efficient, acquiring and locking each image within seconds; inspection time is reduced compared to the hand-held Woodpecker. However, there is still some uncertainty over the precise quantification of damage detected. In-plane laser strain mapping, a sub-set of laser shearography, could have wide applications on composite structures, and NDT Sqn is monitoring developments carefully.
UK MOD has only limited NDT thermography capability. Passive systems are used to detect leaks in air conditioning systems on Hercules aircraft. Other airforces have used passive thermography to good effect to detect the ingress of water into honeycomb structures. Active thermography shows potential for application on thin composite structures, but quickly loses sensitivity on thicker structures; this capability has not been acquired.
This paper provides a very brief overview of the application of NDT in the UK military air environment. The recent fundamental changes to the support of aerospace equipment are described which resulted in the formation of the DLO, IPTs and the rebrigading of single service NDT. A description was provided of the NDT organisation and the varied roles, responsibilities and tasks conducted with brief details of some of the gains and efficiencies already achieved by the new organisation. A brief review of each of the NDT disciplines employed, together with the future developments that are being actively pursued to increase the effectiveness of NDT and support the new generation of aircraft. The 'Delivery of NDT in the UK Military' will continue to be critically evaluated to ensure it efficiently and effectively responds to its customers' needs and continues to contribute to the drive to reduce output costs.
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