As we know a crack in metal is started by surface damage. It can be corrosion, a scratch or a dent. Bearings are designed to have a minimal clearance to allow the oil film to separate the two bearing parts for lubrication and cooling.
However, if a hard metal particle, FOD, becomes lodged between the bearing and race it will take up this clearance and leave an indentation in the surface.
The indentation will stress the metal and start to fatigue the area. Spalling will occur and further damage the surface as material is peeling off from the spalled area. Spalling creates more stress, and cracks start to form as the gear and bearing material gets fatigued.
Photo: Spalling after a particle over-roll event. Spalled area grows A, B, C. Arrow shows rolling direction.
There are three requirements for fatigue to occur:
- Cyclic stress
- The stress must be tensile
- The local stress must exceed the metal’s yield strength
Fracture due to fatigue consists of three stages:
- Crack initiation. Microcracks less than 0.001 mm long form as the material cycles between the upper and lower stress. As the cycling continues the microcracks grow and coalesce, forming one or more larger cracks.
- Crack growth. The larger cracks grow into the material. As the cracks grow the nominal stress on the uncracked portion of the metal increases.
- Finally, when a critical amount of the cross section has cracked, the remaining uncracked material cannot bear the load and fractures by overload.
Photo: Fracture surface of a fatigue crack. Fatigue created by cycling stress.
FOD in the oil can origin from the engine itself in form of spalls from worn down or damaged bearings and gears. These foreign object debris are the ones you want to be alerted about by the chip detection and warning system. When a magnetic sensor picks up the FOD, a “chip light” is illuminated in the cockpit. The “chip light” can be a life saver if treated correct.
Read about “Why is it, that chip light warnings are ignored?”
Other FOD can come from opening turbine oil and hydraulic oil cans in an unapproved way using metal tools like screwdrivers and pliers. This will create metal cast-off from the oil can material and sometimes the tools itself.
Read about “The shocking inside of a turbine oil can opening.”
These are FOD you don’t want inside your engine. These foreign object debris will damage the engine parts and will also create “false chip lights”. Most chip detection and warning systems has magnetic sensors designed to attract magnetic metal you will also get indication from oil can material. The problem is that you can only tell the difference if you do a time-consuming spectrum analysis.
Photo: Cracked plant gear from Airbus Helicopters H225.
Normally a chip warning will be investigated by the mechanic. He will pull the chip plugs and check for debris, wipe the chip detector clean, reinstall and do a ground run to see if another warning comes on. If the light does not reappear – the helicopter flies again.
The issue is that valuable information is lost. You don’t know what type of metal triggered the chip warning. Was it T16NCD13 steel from which bearings and gears are made of, or just some oil can metal from poor oil maintenance practice?
Mechanics and pilots are getting too used to having the chip warnings due to contaminated fresh oil, that was opened with screw drivers and pliers. Pilots will “Zapp” and burn the debris away, and mechanics will wipe the chip plugs. All evidence is lost!
But what if you only get one chance to spot an eminent engine or gearbox failure?
Read NTSB report about Bell UH-1H helicopter accident in Chapelle, New Mexico: NTSB Report_CEN22FA317_105488_7_3_2024 UH1 The accident took 4 lives, and was caused by chip light improper handling.
When you see the NTSB reports with gearbox failures due to cracked gears, it is always easy to see what happened. Photos will show the surface of the crack, since the gear has split apart. You see the dark spot that is the origin of the crack – the dent formed by the over-roll event. You see the beachmark lines as the crack progresses and at the end there is the grainy part, when the gear no longer can take the load and it splits. The material is ripped apart and catastrophe happens.
You don’t want your helicopter to feature in the next NTSB report? USE PROPER OIL FILLING TECHNIQUES.
Here is what you need to do:
- Stop introducing FOD when opening turbine oil cans and hydraulic oil cans.
- Educate mechanics and pilots in the dangers of “invisible” FOD from oil cans.
- Use proper oil can handling procedures.
- Use proper FOD free oil can opening tools.
- Use proper oil fillers that creates a FOD free environment for the open oil can.
- Make clear guidelines for mechanics inspecting chip plugs.
- Send all metal particles for analysis.
- Treat every chip warning as it is the last one before failure.
Photo: Funnelcaps and Cirrus Aircraft logo printed TheCanKey FOD free turbine oil can opener.
Use TheCanKey turbine oil can opener to ensure not to introduce FOD to your oil system when servicing or filling oil. It is an easy, proper and effective way to prevent FOD. Using TheCanKey FOD free oil servicing tools will ultimately save costs and improve overall safety and performance of the gear box.
Read more about TheCanKey FOD free oil service tools here: www.TheCanKey.com
