Patria’s chief expert on the GE engine, Timo Nurmi, holds up a metal component that fits into the palm of his hand.
“This is a blade of high-pressure turbine of a Hornet fighter that is beyond its recommended lifespan of 2,800 hours. This blade, of which there are a total of 64 in each engine, has been used for an extended period of 3,162 hours,” says Mr Nurmi.
It is still in good condition, although mechanical stress and thermal cycles have caused it to crack. During flight, the metal can heat up to almost a thousand degrees. If the cracks become too big, they may break the blade and cause serious engine damage.
The manufacturer, General Electric (GE) has defined lifespans for all the main components of the engine, such as the wings and blade, based originally on US navy experiences. Lifespans and maintenance interval recommendations are reviewed as experiences are obtained from different countries, to correspond to each country’s specific conditions.
High pressure turbine (HPT blades/HPT disk)
How do cracks develop?
The Joint Systems Centre of the Defence Forces Logistics Establishment, which is in charge of the lifespan planning, maintenance and usability of the Finnish Air Force’s fleet, and its strategic partner Patria, which carries out repairs and maintenance for the former, decided to find out together whether the blades’ lifespan can be extended to that of the disks. The objective was to be able to use the blades until the end of the disks’ lifespan and thereby save in spare part costs.
“During two decades we have during accumulated plenty of information on how our Nordic conditions and how we use the planes contribute to the condition of the engines,” says Major (Eng.) Jukka Taattola from the Defence Forces Logistics Establishment.
“Finns fly as a rule in very different conditions from their American counterparts, who often operate in desert conditions. The heat to which American planes are subjected, occasionally combined with sand, puts more stress on the turbine head than the cold and clear air in Finland. The distance to the training area affects the amount of stress placed on the engine. Our flight time from airfield to training area is just five minutes – in the United States, transit flight times are long. Flight times are relatively short and effectively used in Finland, but having the engines running while on ground consume some of the hour-based lifespan of the turbine blades. Even use on ground with a low level of thrust does not, however, stress the engines as much as flying, which means that the need for increasing the blades’ lifespan is understandable,” says Major Taattola.
It was therefore necessary to determine how cracks develop, in order to redefine the service life of the blades.
Tampere University of Technology charted hundreds of cracks of micrometer-scale. VTT studied the microstructures of the metal, discovering that the cracks either stop lengthening or lengthen very slowly. Stress caused by temperature of centrifugal force was analysed. VTT also studied the softening of the material’s microstructure. The blades were in good condition except for some overheated areas in the vicinity of the trailing edge in a few blades.
VTT was in favour of extending the service life of the blades from 2,800 to 3,100 hours – the same length as the disks. These results were also sent to the manufacturer (GE) that reached the same conclusion and subsequently increased the blades’ lifespan recommendation by ten per cent. The maintenance interval could be extended. Thanks to this, there was no need to buy a new set of blades for each engine.
This decision will generate savings of around EUR 2.7 million during the service life of the planes.
“Owing to a variety of development action, the total savings during the engines’ lifespan are estimated at 60 million euros,” says Timo Nurmi of Patria.
Jukka Taattola (left) holding a damaged high-pressure turbine blade, Timo Malmi with the next object of study, i.e. a low pressure- turbine blade, and Juhani Rantala with a high pressure turbine blade which has 3,162 hours of flight time.
Importance of expert network emphasised during crisis
“We combined various kinds of VTT expertise for the Defence Forces assignment: strength technology, material technology, microscopy and coating expertise. Information about the blades’ lifespan was sent to Patria to be assessed by VVT repair development engineer Jani Simelius,” says VTT Senior Scientist Juhani Rantala.
VTT has been a partner of the Finnish Air Force for around 15 years. VTT’s first tasks had to do with determining the condition of the Hawk fighter’s engine turbines.
“From there, the work continued with the analysis of the Hornet’s high pressure turbine blades in 2009. We have not only examined the blades but also other parts, such as the combustion chamber flange material,” recalls Mr Rantala.
“The analysis of the combustion chamber cracks referred to by Mr Rantala has provided us with valuable information we can use in our boroscopic inspections and to provide grounds for extending the planes’ flight times,” continues Timo Nurmi.
Alongside domestic partners, the Defence Forces makes use of experiences of other Hornet users from various countries. As well as Patria and VTT, the research network of partners in Finland includes Aalto University, Tampere University of Technology, Finflo, Milldyne and Emmecon.
This is based on a decision, taken by the Finnish Air Force in the early 1990s, to build, invest in and finance a national expert network meeting the needs of aircraft lifespan management.
“Hornet fighters must fully operational at all times. Any crisis situation must also be prepared for. It is important that we have a proper expert network enabling us to manage by domestic means as far as possible. So in effect this cooperation is also about developing security of supply in a crisis situation in Finland,” says Jukka Taattola.
Molten ash close to the cooling holes after a training flight. The holes must not become blocked because otherwise the cooling air will not get through.
Repairs in an orderly and rational manner
The Defence Forces Logistics Establishment orders factory maintenance level repair and maintenance services primarily from Patria. Any other than minor maintenance operations require the engine to be taken out, and this is always done at Patria’s plant in Linnavuori, Nokia, where MiG 21s and Dragens also used to be maintained. The Air Force handles operational and routine maintenance itself.
“When we at Patria identify a problem, we react immediately. Developing a new repair procedure here is often much faster than when there are participants from many countries,” says Mr Nurmi.
“Finns are regarded in terms of Hornet engines as innovative, highly-skilled experts who can come up with and implement new solutions, and quite quickly too, if necessary, says Mr Taattola.
The design of Hornets began in the 60s and they were given their maiden flights in the United States in 1978, moving into operational use in the 80s. All the engines and spare engines of the Air Force’s one-seater fighters were assembled in Linnavuori between 1995 and 2000. This provided valuable experience and expertise in engine maintenance.
Finland has been using these planes for 20 years, but they still have plenty of years ahead, although the purchase of new fighters are already being debated.
Eyjafjallajökull brought European flights to a halt
Air traffic was grounded in April 2010 in Northern Europe during the eruption of the Eyjafjallajökull volcano in Iceland. This raised a question: what constitutes a safe ash concentration from the point of view of aviation?
Lapland Air Command Hornets on a training flight were exposed to a cloud of ash and were afterwards found to have volcanic ash in the engines. This is how the Finnish Air Force became the top aviation story.
The Air Force, Patria and VTT examined swiftly how congested the engines really were. Despite the ash, the planes were found to be flightworthy.
Later, the Defence Forces gave VTT the task of determining whether volcanic ash corrodes turbine blades.
”I ordered a bag of fine-grained, volcanic ash collected from the ground in Iceland. Molten ash was injected on the surface of high-pressure turbine blades, and the blades were then annealed in an oven. We could find no signs of corrosion. Despite the fact that volcanic eruptions are not particularly rare, this was a new discovery,” recalls Juha Rantala of VTT.