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Most people don't think about air quality on a plane. You get on, you sit down, you breathe. You assume the air is fine.
But it doesn't just come from nowhere. On most jets, cabin air is bleed air – air tapped from the engine compressor. That air is hot, and it can carry trace amounts of ozone, carbon monoxide, volatile organic compounds, even oil fumes if something's leaking.
So the plane has to clean it before it reaches your seat. That's where catalytic converters come in. Not big ones like under a car. Small ones. Tucked into the environmental control system.
We've built substrates for a few of those applications. And for APU exhaust treatment too. Here's what we learned.
Two Different Jobs, One Substrate
Aircraft air treatment falls into two categories.
First, cabin air purification. The ECS – environmental control system – pulls bleed air, runs it through a catalyst, then sends it into the cabin. The catalyst has to remove ozone and CO. Ozone is the big one at altitude. Without a catalyst, you'd smell it. With a bad catalyst, you'd still smell it.
Second, APU exhaust treatment. The auxiliary power unit runs on the ground. Its exhaust has CO and hydrocarbons. Airports don't like that. So the APU needs a small catalytic converter to clean up before the exhaust hits the tarmac.
Same substrate technology. Different requirements.
Cabin Air Purification – Fast and Clean
For cabin air, the flow is high. Hundreds of pounds per hour. The catalyst has to work at low temperature too – bleed air is hot, but it can cool down before it hits the converter if the system is designed poorly.
Ozone destruction is easy. Ozone is unstable. A little heat, a little catalyst surface, and it falls apart. CO is harder. Needs more contact time.
For these applications, we recommend relatively high cell density – 400 cpsi or even 600. More surface area for the same small package. The substrate is small anyway – maybe 3 or 4 inches across. You want as much catalyst as you can fit.
Foil thickness can be standard 0.05 mm or even thinner. No heavy vibration in the ECS compartment. Weight matters, so thin foil helps.
Material? Stainless. Always. Because bleed air is hot, and there's no reason to risk aluminum corrosion.
We built a batch for a business jet ECS a few years back. 600 cpsi, 0.04 mm stainless, just 80mm diameter, 50mm long. Weighed almost nothing. The customer said it cut ozone from 400 ppb to below 10 ppb. That's good enough for any flight.
APU Exhaust Treatment – Hotter and Rougher
APU exhaust is a different animal. The gas temperature is high – often 600 degrees or more. There's vibration from the APU itself. And the converter is mounted near the tail, where space is tight.
For APUs, we go with lower cell density – 300 cpsi. Helps with backpressure. The APU already has to push exhaust through a long pipe. You don't want to choke it.
Foil thickness: 0.08 mm stainless. Thicker than ECS. The vibration could crack thin foil over time.
Material: 304 or 316. APU exhaust doesn't get as hot as engine exhaust, so no need for Inconel.
Mounting mat has to be heavy‑duty. APU shakes. We've seen mats loosen up on cheaper substrates, then the honeycomb rattles and cracks.
One regional airline had a fleet of jets with APU converters failing every 1,000 hours. The substrate was cracking right at the brazed joints. We looked at their data. The vibration level was higher than they thought. We built a substrate with 0.1 mm stainless and a denser mat. Next set went 3,000 hours with no failures.
Efficiency Is About Surface Area and Flow
High efficiency means converting most of the bad stuff into harmless gas. That takes surface area – more cells – and proper flow distribution.
If the gas flows unevenly through the substrate, some cells get overloaded, some cells do nothing. The average conversion looks good on paper, but real‑world peaks and valleys mean you fail at high flow.
We test for flow distribution using a light test and a flow bench. Dark spots in the light test mean uneven cells. Those parts get scrapped.
Also, the inlet cone of the converter matters. The gas has to spread out evenly before it hits the substrate. We've seen customers with bad inlet designs blame the substrate. Then they fix the inlet, and our part works fine.
Space Constraints – Make It Small but Effective
An aircraft ECS compartment has no spare room. The converter is shoehorned between ducts and valves.
That means the substrate has to be short. Sometimes 50mm or 60mm long. A short substrate has less residence time – the gas spends less time in contact with the catalyst. So you need higher cell density to compensate.
We've done substrates as short as 40mm. At that length, you have to be very careful with brazing. No voids, no incomplete bonds. A weak spot will cause the whole thing to crack.
We use a slower braze cycle for short substrates. More time for the filler to flow into every joint. It adds cost, but it's necessary.
Weight – Every Gram Counts
Aircraft customers weigh everything. We've had buyers ask for the weight of the substrate before and after coating. They log it.
To save weight, we use thin foil. 0.04 mm instead of 0.05 mm. That cuts mass by 20%. For a small ECS substrate, that might be only 50 grams. But 50 grams times 100 aircraft is 5 kg. They care.
We also use lightweight mounting mats. Not as dense as industrial mats. Still holds the substrate, just with less material.
One customer asked us to laser‑etch part numbers instead of using ink. The ink added measurable weight. No, I'm not joking.
Testing for Aircraft Applications
We don't just ship these parts. We put them through tests that mimic aircraft conditions.
Ozone conversion test. Flow air with a known ozone concentration through the substrate at temperature. Measure outlet ozone. Needs to be under 10 ppb.
CO conversion test. Same setup. CO needs to drop to near zero.
Thermal cycle test. Heat to 500 degrees, cool to ambient, repeat 200 times. Check for cracks.
Vibration test. Shake at frequencies from 10 to 500 Hz. Check for loose mat, cracked brazing.
Pressure drop test. Measure backpressure at room temperature and again at 500 degrees. The difference should be small.
We've had ECS customers send us their own test rig results. Our substrates passed every time.
A Real Example
We made a substrate for a turboprop commuter plane. The ECS used bleed air from the engine. The customer needed to remove CO and ozone. Space was extremely tight – the converter had to fit inside a round cavity only 70mm deep.
We built a 400 cpsi, 0.04 mm stainless substrate, 70mm diameter, 60mm long. Weight was 130 grams. They tested it. Ozone removal was 99%. CO dropped to undetectable levels.
They ordered 500 pieces for their fleet. Then another 500 for their next model.
High‑efficiency catalytic substrate for aircraft cabin air and APU exhaust are not magic. They're just carefully engineered metal honeycomb.
For cabin air: high cell density, thin foil, stainless, short length, lightweight mat.
For APU exhaust: lower cell density, thicker foil, heavy‑duty mat, good vibration tolerance.
Test for ozone, CO, thermal cycling, vibration, and pressure drop.
We've built these for business jets, regional turboprops, and even some military platforms. It's not our biggest business, but it's some of the most interesting work.
If you need a substrate that fits in a tiny space, weighs next to nothing, and still cleans the air, we can do it. Bring us your dimensions and your performance targets. We'll build you a sample. You test it. That's how it works.