Experts in jet engine maintenance
Pushing an engine to its limit requires engine maintenance of a comparable level. And it is; this is guaranteed by the work of seasoned specialists in jet engine technology, such as Hannu Paavilainen and Sales Manager Risto Marjomaa.
They compare flying a jet fighter to flying an airliner. Although both aircraft types are powered by jet engines, the strain put on an engine is directly proportional to the way in which it is used.
“The maximum strain on an engine occurs when a cold engine is powered up, then revved up to maximum power and then throttled back again. The engine components heat up and cool down during a single cycle. Some components of an F/A-18 Hornet multi-role fighter is subjected to up to eight such cycles per flight hour. An airliner on a ten-to-twelve-hour flight from Helsinki to Tokyo is subjected to only one. In other words, jet engines in military use are subjected to even a hundredfold strain compared to civilian engines,” Marjomaa and Paavilainen comment.
Engines have undergone numerous developmental leaps over the last 60 years. The first jet engines serviced by Patria were the Havilland Goblin engines of Vampire fighters, representing an early jet engine with a radial compressor and can type combustion chamber.
In the late 1950s, Patria was kept busy by the arrival of 80 Fouga Magister jet trainers and their 160 jet engines. These engines still had radial compressors, but represented a newer generation. New technology arrived with Folland Gnat interceptors, equipped with axial compressors.
The Tumansky engines of the Mig 21F fighters, which entered service with FINAF from 1964 onwards, turned a new page in engine maintenance, as the maintenance interval of the Tumansky engine was considerably shorter than that of western engines.
“At intervals of 200 flight training hours, each engine was completely disassembled, inspected, any defects found were repaired, components were replaced, and the engine was reassembled and put through a test run for the next 200-hour duty cycle,” recalls Paavilainen.
According to Marjomaa, the short maintenance interval of the Mig fighters and their engines was attributable to the fighter having been designed for mass production under wartime conditions, during which a long maintenance interval was deemed unnecessary. These engines also had a limited service life. The engines of the more modern Mig 21bis fighters underwent three maintenance operations at 300-hour intervals, to be scrapped after a service life of 1,200 hours.
Following developments in technology, the maintenance intervals of entire engines have been replaced by intervals targeting individual components.
“The engines of Hornet fighters are serviced when certain components, such as rotating parts that are subject to service life limitations, reach the end of their service life,” says Marjomaa.
Hornets undergo their first maintenance and component replacement operation after 700 to 1,000 flight hours.
“During such maintenance operations, the combustion chamber casings are inspected and the turbine rotor blades are replaced. Most rotor components are replaced after approximately 2,500 flight hours. After that, the engine is given one more overhaul before reaching the end of its service life of 5,000 flight hours,” Marjomaa says.
Not even this is necessarily the definitive end of the engine’s service life, as it can be elongated by replacing components. The final age of the engine is optimised in accordance with the total service life of the aircraft fleet and the decommissioning time.
Whereas repairs once mainly concerned the welding of cracks, special techniques such as the application of thermal coatings and electron beam welding are widely used today. A number of techniques developed by Patria are in demand on the export market.
“We have repaired Hornets’ components for RUAG, a Swiss company that repairs the engines of the Swiss Air Force’s Hornets, and for other customers,” says Marjomaa.
Turbomeca, an engine manufacturer, subcontracts the repair of NH90 helicopters to Patria. Demand is continuously increasing.
“We have been requested to develop new repair methods for a number of helicopter engine components that are prone to developing problems,” remarks Paavilainen.
The development and sale of expertise saves taxpayers’ money.
“We have built up our resources to meet the requirements of the Finnish Air Force FINAF. When we sell the same expertise onto other customers, the FINAF also benefits,” comments Marjomaa.