For years, I have written about one of the simplest and most overlooked sources of FOD in aircraft lubrication systems: the way we open a turbine oil can.
We worry about contamination in workshops. We inspect funnels and servicing equipment. We protect open oil containers. We analyze oil samples, inspect filters and install increasingly sensitive chip detection systems.
But one very basic question is rarely asked: what happens at the exact moment somebody punches a hole in a turbine oil can with a screwdriver?
The Test: One Screwdriver Punch, Sent to the Lab
In an earlier test, I punched 16 holes into oil can material with a flat screwdriver and collected the debris on a sheet of white paper. You could see it with the naked eye. This time, instead of showing it, I wanted to measure it.
A turbine oil can was punctured once with a 5 mm screwdriver. The particles from that single punch were collected, and TheCanKey commissioned Spectro | Jet-Care in the UK to carry out an independent SEM/EDX debris analysis. Spectro | Jet-Care is the laboratory many operators rely on to monitor wear metal in their engines. The report identifies the sample simply as: “Turbine oil can. One punch with 5mm screwdriver.”
What the Lab Found
More than 1,000 particles from one punch
Spectro identified a shiny machine curl of approximately 0.1 × 0.05 mm, about 100 × 50 microns.
More importantly, the laboratory reported more than 1,000 brown and orange particles measuring up to 30 microns.
Not from opening cans for a year. Not from an engine with an internal failure. Not from a deteriorating bearing or gear.
One punch.
There is an important detail in the wording: particles “up to” 30 microns. Thirty microns is the upper size of that population, not the size of every particle. The SEM image in the report shows a wide variation in size, with many particles much smaller than the largest ones. You can read the full Spectro | Jet-Care report here (PDF).
That matters when we start thinking about filtration. We are not dealing with a few large pieces of metal that a filter can be expected to catch. We are looking at a population of microscopic contamination, created in a fraction of a second.
The chemistry points straight back to the can
The principal metallic particle analyzed was dominated by iron: 95.69% by weight, with 3.62% aluminium and smaller amounts of manganese, sulfur and silicon. Spectro’s nearest material match was a low-alloy carbon steel, such as mild steel.
Two other elements in the report are worth noting: tin and oxygen.
The population of brown and orange particles contained approximately 26.0% iron, 0.5% tin, 33.6% oxygen and 37.9% carbon, with smaller quantities of other elements.
Tin is the interesting one. Traditional metal cans are made from thin steel with a protective coating. Tinplate, steel sheet coated with tin, is where the old expressions “tin can” and, in our industry, “oil tin” come from.
A screwdriver forced through that material does not make a clean hole. It penetrates, tears, shears and rubs against the coated steel. So iron together with tin is consistent with debris coming from the can itself.
These are not wear particles produced after thousands of engine hours. They were created before the oil ever entered the aircraft.
Why so many of the particles are brown
Microscopic iron particles have a very large surface area compared with their mass. Freshly exposed to oxygen and moisture, they oxidize quickly. The high oxygen content in the analysis is consistent with that. In simple terms, freshly torn steel starts to rust.
What Happens When That Oil Goes Into the Aircraft
Particles stay suspended in the oil
Very small particles can stay suspended in oil for many hours. How they behave depends on their size, shape and density, and on the viscosity of the oil.
Then we pick up the can and start pouring. Particles that were floating or slowly settling are carried along with the oil during the top-up.
That makes the timing of the top-up worth a closer look.
Pre-flight or post-flight oil top-up?
Many aircraft, engine and gearbox manufacturers require the oil level to be checked within a set time after shutdown, on some installations around 30 minutes. Oil can drain internally after shutdown, so waiting too long can give a wrong reading and lead to overfilling.
So post-flight checking already makes sense for reasons the manufacturer defines. From a FOD prevention perspective, there may be an extra benefit.
Top-up immediately before flight: the can is opened, the oil is disturbed, contamination is suspended, the oil goes in, and minutes later the engine starts. The particles have had almost no time to settle. It is also the situation where top-ups are most often rushed, sometimes by pilots without the right tools.
Top-up after the previous flight, following the manufacturer’s procedure: the aircraft may then stand overnight. Many hours pass before the next start.
That does not make contamination harmless or guarantee that every particle settles. But it gives suspended particles far more time to settle or adhere to surfaces before the engine runs again. The aircraft or engine manufacturer’s approved procedures must always govern servicing.
Cold engine starts and filter bypass
Cold oil is more viscous, so the pressure difference across the oil filter can be higher than at normal operating temperature.
Lubrication systems must keep oil flowing to critical components, which is why they include a filter bypass valve. If the pressure difference becomes high enough, the valve opens. Keeping lubrication going matters more than forcing the oil through a filter that cannot pass enough flow.
From a contamination perspective, that creates an obvious concern. Depending on the system design, oil can briefly reach downstream components without full filtration. I explain how bypass works, and why nobody sees it happen, in Oil Filters Are Never 100% Safe.
Now combine it with a pre-flight top-up:
Freshly opened can → microscopic particles → particles suspended in the oil → immediate cold start → high-viscosity oil → higher filter pressure → possible bypass.
That does not mean every cold start causes a bypass event. It depends on the system design, filter condition, oil temperature, flow and operating conditions, for example low oil temperature combined with high engine power. But if foreign material never enters the oil, we do not have to rely on the filter to solve a problem we created ourselves.
Why the Filter Will Not Catch It All
“The filter will catch it”
This is probably the most common argument about contamination from opening oil cans. But will it catch everything?
The report found more than 1,000 particles up to 30 microns. Visually, the SEM image appears to include many particles below about 15 microns, and smaller still. That is relevant in lubrication systems using filtration around the 15-micron range.
A filter rated at “15 micron” is also not an absolute wall. Its performance depends on its specification, whether the rating is nominal or absolute, and its efficiency at different particle sizes.
The point is simple: the debris includes particles small enough that we cannot assume the filter will remove them all. And small does not mean harmless.
Even a 5-micron particle can matter to a bearing
Five microns is 0.005 mm. You cannot see a single particle that size with the naked eye.
But a turbine engine bearing does not work on the scale of the human eye. Polished balls or rollers and their raceways run with an extremely thin oil film between them. What matters is not only the particle’s size compared with the filter rating, but its size and hardness compared with that film.
Under the enormous local pressure in a rolling bearing, a hard particle can be over-rolled and leave a microscopic dent. Material pushed up around the dent creates raised edges. Every following roller stresses that spot again. Over millions of contacts, this can contribute to surface distress, fatigue cracking and eventually spalling. More on this in Why FOD Shortens Turbine Engine Bearing Life.
Not every 5-micron particle will damage a bearing. But “too small for us to see” is not the same as “too small to matter.”
What It Means for Condition Monitoring
What does a chip detector think?
Magnetic chip detectors collect ferromagnetic debris. Electronic debris monitoring and chip counting systems go further, helping maintenance teams spot developing mechanical problems early.
But a detector cannot ask a particle: “Did you come from a deteriorating bearing, or did somebody put you here with a screwdriver?”
Invest in increasingly sensitive detection, then introduce iron particles during routine servicing, and the system simply sees metal. Someone then has to work out where it came from. A bearing? A gear? Or did it arrive with the oil? I first raised this in Sensitive Chip Detector Sensors or Contaminated Oil System?. The real danger is what happens when crews get used to it, as I wrote in Why Is It That Chip Light Warnings Are Ignored?.
In this test, we know exactly where the contamination came from. The maintenance action itself created it.
Oil analysis has the same problem
Oil analysis is one of the most valuable tools for monitoring turbine engines and gearboxes. Changes in iron, chromium, molybdenum and other elements can reveal component wear.
But it works best when the baseline is understood and external contamination is kept out. If metallic debris is repeatedly added during servicing, some of what appears in later samples may not come from internal wear at all. I go deeper into this in M50 Bearing Steel, Oil Analysis and FOD Prevention.
It makes little sense to use sophisticated laboratory analysis to understand what an engine is producing, and then add an uncontrolled source of iron just before the oil goes in.
Condition monitoring works best when the oil going into the system is clean.
Prevention Comes First
Filters are the last defense, not the first
Oil filters, chip detectors, oil analysis and condition monitoring are all essential. But they all work after contamination has entered the system.
Filtration is contamination control. FOD prevention is contamination avoidance. The second should happen first.
A filter should protect an engine from unexpected contamination. It should not become the excuse for creating avoidable contamination during routine servicing. It is one of the most overlooked FOD risks in oil handling, precisely because it happens during normal maintenance.
We invest in detecting particles. Why create them ourselves?
Aviation spends enormous sums looking for microscopic evidence of component deterioration: oil analysis, filter inspections, magnetic chip detectors, electronic debris monitoring, vibration monitoring, HUMS, borescope inspections.
Then somebody walks to the aircraft with a perfectly clean can of approved turbine oil and punches a hole through the lid with a screwdriver.
In this test, that single action produced more than 1,000 particles up to 30 microns, plus a larger metallic curl. Some will be caught by the filter. Some will settle. Some will stay suspended. Some may be small enough to pass efficient filtration, and some may reach highly loaded bearing and gear contacts.
The technology downstream can be extraordinarily sophisticated. The contamination event upstream is extraordinarily simple. And unlike normal component wear, it is preventable.
One screwdriver. One 5 mm punch. More than 1,000 particles.
Perhaps the cheapest particle to detect is still the particle we never put into the engine.
About this test: the Spectro | Jet-Care report notes that its results relate only to the item tested, and that interpretations depend on the sample information supplied. This single-punch test does not establish that every screwdriver puncture produces the same number, size or composition of particles. It demonstrates what one ordinary maintenance action, lasting less than a second, can put into the oil before it ever reaches the aircraft. The full report is available here (PDF).
Want to keep particles out of the oil from the start? Download our free FOD Prevention Guide, or see the certified FOD-free TheCanKey opener, already used by operators like NetJets.
