Only the best of the best have the skills to repair damage on military aircraft

Text: Matti Remes Photos: Patria

Patria is a strategic partner of the Finnish Defence Forces in providing lifecycle services for military aviation systems. Damage repairs of the aircraft fleet are an important part of this service.

Ari Kivistö from the Logistics Establishment of the Finnish Defence Forces heads up this sector. He says that damage repairs involve defects in aircraft structures and systems.

“Such damage is relatively rare, but it does happen when aircraft are used. For instance, a fighter tyre can burst on landing – flinging pieces into the structures and damaging them,” says Kivistö.

Damage can also be caused by a midair collision with a bird, a tanker crashing into a wing during slippery conditions on the airfield, or landing gear damage when the aircraft drifts to the edge of the runway.

“Aircraft can also be damaged by hard use, which can lead to metal structure fatigue, for instance,” says Patria’s Mika Keinonen, MSc (aeronautical engineering).

National expertise is vital

Janne Linna, Engineering Manager responsible for damage repairs of Hornet fighters, emphasises that the purpose of all damage repairs is to keep equipment in operational condition and maintain military performance.

In addition, from the perspective of public finances, it’s important to ensure that aircraft bought with tax revenues remain flightworthy and that any damages can be repaired at a reasonable cost.

“We primarily seek to repair damage without resorting to major part replacements,” says Linna.

Markus Wallin, a structural analyst specialising in composite structures, points out that skilled damage repairs become increasingly important as the fleet ages.

“As aircraft age, the availability of spare parts and to some extent even materials weakens.

For that reason, it’s important that we have the capability to carry out even very demanding repairs locally.

It’s crucial to ensure that we can rely on national expertise in exceptional circumstances, when it would be difficult to source spare parts from abroad,” says Wallin.

Ari Kivistö also emphasises the importance of maintaining and developing national expertise in damage repairs during both peacetime and times of crisis.

“During crises, enemy activity also poses a threat. For instance, we must be able to rapidly repair fragment damage to aircraft.

“Speed is especially important for a small country like ours. Finland has to make do with 64 fighters. We have to repair the damage and get them back into service fast in order to maintain our defence capability,” says Kivistö.

The broad scope of the work poses a challenge

Patria’s lifecycle services cover the Defence Forces’ F/A-18 Hornet fighters, Hawk Mk51/66 jet trainers, NH90 helicopters, CASA 295M transport aircraft and Grob G115E trainers.

Janne Linna believes that one of the greatest challenges that damage mechanics face is the broad scope of the work. The types of damage are very different and may affect any part of the aircraft.

“A damage repair engineer must know as much about the original parts as their designer. On top of that, you need robust expertise in repair technology and methods,” says Linna.

He points out that in order to carry out damage repairs, you need education in aeronautics– but in order to become a top repair engineer, you also need to rack up years of work experience. The same is true for aircraft mechanics who perform the actual  repair.

A precisely defined repair process

The repair process always begins with an assessment of the nature and scope of the damage. Non-destructive methods are generally used for this – above all, visual inspections, ultrasound scans and eddy-current inspections of metal structures.

Once the nature and extent of the damage have been determined, a decision is made on whether to repair or replace the part.

“To make this decision, you need a lot of design data and the ability to interpret it. This calls for traditional engineering expertise and strong knowhow in the strength of materials, for instance,” says Mika Keinonen.

Patria is authorised to approve smaller-scale repairs and modifications on its own. Approval from the aviation authority is required for larger repairs, including ones that affect the characteristics of the aircraft.

After the repair decision has been taken, the planning of the repair process begins. Patria has long carried out this process with 3D CAD software. In addition, designers also employ state-of-the-art analysis tools, such as finite element method in order to simulate the stresses that structures are subjected to and to calculate the necessary strengths.

3D printing speeds up the process

Model parts produced on a 3D printer are used to check that the part geometry is correct before the actual part is made from materials such as high-strength aluminium or composites.

3D printing saves a lot of time – you can print the parts overnight and then check their fit the next day. Earlier, models had to be commissioned from a machinist or metals workshop.

“We only manufacture the parts that will be installed once we’ve verified their suitability,” says Keinonen.

Once the repair plan is finished, the work plan is drafted. It defines aspects such as the work stages, schedules, raw materials to be procured and the skills needed for the work.

“The task of the supervisors is to see to it that the work is carried out according to the instructions. Once the work is done, a separate inspector or quality assurance engineer performs a final inspection. The aircraft is then handed over to the customer,” says Keinonen.

Ari Kivistö points out that as the aircraft have become more advanced, damage repairs have become increasingly demanding.

“In the past, the structures were often made of aluminium, and could be repaired by riveting on a patch. These structures are now often carbon fibre – bonded repairs require precise conditions and processes.

“Patria’s damage repair technicians know their job. We’ve worked together to develop operations and keep the equipment in good condition.”