Oil Filters Are Never 100% Safe

If you trust your oil filter to catch every contaminant in the oil system, you are in for a surprise.

To properly filter engine oil, you want the finest filter media possible. But filters are a trade-off: they must remove contaminants without restricting oil flow. The smaller the holes in the filter media, the smaller the particles it captures—but the harder it is to maintain sufficient oil flow. In other words, if you reduced micron size to zero, nothing would pass through.

Some P&W PT6 engines are equipped with a 15-micron filter to allow for optimal filtration during normal operation. A secondary filter, located within the main filter, is 40 microns. So, what is this secondary filter for? It captures contamination during oil filter bypass.

Bypass occurs when the oil filter differential valve opens to protect the filter from collapsing due to high system pressure before the filter and low pressure after it. The differential pressure is typically limited to around 20 psi to keep the filter safe.

Filter bypass usually happens during engine start-up and operation with cold oil, when the oil is too thick to flow through the fine media of the main filter. As differential pressure builds, the bypass valve opens to prevent filter collapse. Fine-filtered oil is traded for higher micron oil in case you have a secondary filter installed. In less sophisticated oil systems without a secondary filter, completely unfiltered oil is bypassed.

Momentary bypass can also occur during rapid changes in oil flow—such as during engine power transients—which can challenge the fine filter media and trigger a bypass event.

Bypass duration can last from a split second to several minutes and goes unseen. There is no bypass warning or indication; it all happens inside the filter housing. Oil pressure may be observed to decrease depending on the degree of bypass and how closely the oil pressure is monitored.

Proper engine and gearbox run-up to operating oil temperature is essential to minimise the duration of oil filter bypasses.

Understanding Micron Sizes

To put it in perspective: you can feel 5-micron particles as very fine grit with your fingers, and you can see 40-micron particles with the naked eye. For reference, human hair is about 50 to 180 microns.

Damage to bearings and gear teeth can occur from hard particles as small as 5 microns. The larger the debris, the greater the potential damage.

Filter Efficiency

Another factor is the capture efficiency of oil filters. Filters with the same micron rating can perform very differently depending on the test method used to certify them. Three terms attempt to define filter performance: nominal, absolute, and beta (β) rating.

Nominal rating often means around 50% capture efficiency at the stated micron size. For example, ~50% efficiency at 10 microns and 95% efficiency at 20 microns.
Absolute 10-micron filters typically achieve 98–99% efficiency at their rated micron size. Better than nominal, but still not perfect.
Beta rating filters are tested under the ISO 16889 multipass test. Particle counts are measured upstream and downstream in real time. The beta ratio is calculated as the number of particles upstream ÷ downstream. For example: β10 = 200 → 199 out of 200 particles ≥10 microns are captured, providing 99.5% efficiency.

However, these tests are conducted under ideal laboratory conditions with steady flow, fixed viscosity, no vibration, clean oil, and controlled contamination. Real-world conditions in aircraft oil systems—fluctuating pressures, vibration, moisture, and variable contamination—are far harsher. Even beta ratings should be treated as guidelines, not guarantees.

Handling Practices Matter

Recognising that oil and filter handling practices can introduce risks to oil system cleanliness is a big step toward safer and more cost-effective engine and gearbox operation.