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DO-160 Ingress and the Breathing Box: Why a Sealed Enclosure Needs a Vent
A sealed avionics box that passes its bench leak check can still arrive at a heavy check full of water, and the culprit is usually not a bad gasket. It is sealing the box completely without giving it a way to breathe. Every climb and descent swings the pressure differential, and a fully sealed box answers by drawing humid air in and condensing it inside. This article walks ingress qualitatively against the DO-160 and IEC 60529 framing and why a vent, not a tighter seal, is the fix.
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The box that pumped in its own condensation
A sealed avionics enclosure that passes its bench leak check can still arrive at a heavy check full of water, and the culprit is usually not a bad gasket. It is the decision to seal the box completely without giving it a way to breathe. Every climb and descent swings the pressure differential across the enclosure wall, and a fully sealed box answers that swing by drawing air, and the moisture in it, in and out through whatever leak path exists, condensing it inside on the cold descent.
The box pumped in its own weather. This article walks ingress qualitatively against the RTCA DO-160 and IEC 60529 framing, why altitude turns a sealed box into a pump, and why a vent membrane is the fix rather than a tighter seal, all by designation.
What DO-160 and the IP code actually frame
Two framings describe ingress, and they answer different questions. IEC 60529 gives the IP code, a two-digit shorthand for protection against solids and liquids tested under defined conditions, useful as a target but defined at ground conditions. RTCA DO-160 frames the airborne environment more completely, with sections covering humidity, fluids, sand and dust, waterproofness, and crucially the altitude and temperature-altitude cycling that an aircraft box actually sees.
The qualitative point for a seal designer is that an IP rating proven on the bench does not by itself describe how the box behaves over an altitude cycle, and the DO-160 environment, not the static IP test, is where a sealed box reveals whether it breathes safely or pumps. The rating is the box builder's to demonstrate; the seal and vent are what make it achievable.
Why altitude turns a sealed box into a pump
The mechanism is pressure differential. On the ground the inside and outside of a sealed box are at the same pressure. As the aircraft climbs, the outside pressure falls and the higher-pressure air inside pushes outward through any leak path; as it descends, the outside pressure rises and air, now humid ambient air, is drawn back in. Repeat that cycle every flight and a fully sealed box with even a small leak path becomes a slow pump that accumulates moisture inside, where it condenses on the cold descent and collects on the coldest internal surface.
A tighter gasket does not fix this; it only changes which leak path the pump uses. The real fix is to stop the box from having to equalize through its seals at all.
The vent membrane, and the three other ingress failures
The cure for the breathing box is a vent, and the parent page catalogs four more ingress failures around it.
Give the box a vent that breathes but blocks water
A Gore ePTFE venting membrane equalizes the pressure differential across the altitude cycle while blocking liquid water, so the box breathes through the vent instead of pumping through its seals. The membrane is microporous: air molecules pass, liquid water does not. With a vent in place the primary seal is no longer cycled by the pressure differential, which also extends its life.
Put the vent where wash fluid does not sit
A vent installed at a low point where wash-down fluid pools is a vent that gets a column of water pushed against it. The fix is placement: the vent goes where fluid drains away, not where it collects, which is a drawing decision about orientation, not a material one.
Do not let a thin lid bow open midspan
A thin lid bows between fasteners under the pressure differential and opens a midspan gap the corner closure force never closes. The fix is a perimeter seal sized to the worst-case unsupported span: a softer, higher-deflection BISCO RS or kSil V-0 grade that the available force can fully seat midspan, per its ASTM D1056 compression-force-deflection class.
Do not seal with an open-cell pad
An open-cell foam wicks water; a closed-cell sponge blocks it. Specifying an open-cell pad where a closed-cell perimeter seal belongs lets water straight in, no matter the IP target. The perimeter seal is closed-cell silicone sponge or PORON; reticulated open-cell foam belongs at the cooling inlet, filtering air, never at the seal line.
Five Ingress Failures, and the Drawing Decision That Prevents Each
| Field symptom | Mechanism | Framing | The drawing decision |
|---|---|---|---|
| Box full of water at the check | Sealed box pumps over altitude cycle | DO-160 altitude | Add an ePTFE vent membrane |
| Vent itself lets water in | Vent at a fluid-pooling low point | DO-160 fluids | Place the vent where fluid drains |
| Leak midspan on a thin lid | Lid bows between fasteners | IEC 60529 + CFD | Seal sized to the unsupported span |
| Water through the seal line | Open-cell pad where closed-cell belongs | IEC 60529 | Closed-cell sponge at the seal |
| Antenna loses signal | Aperture gasket attenuates the band | MIL-DTL-83528 | RF-transparent or correct conductive seal |
Illustrative and relative only; confirm each grade on its current technical data sheet. The DO-160 qualification and IP rating belong to the box builder; H-O converts the seal, vent, and filter the design specifies.
The pattern: a sealing problem is often a venting problem
The condensation failure on the parent page is the clearest case of a counterintuitive rule: the way to keep water out of an altitude-cycled box is often to let air out, through a controlled vent, rather than to seal harder. A tighter gasket on a fully sealed breathing box only moves the leak path; a vent removes the pressure differential that drives the pump.
The cure is a system decision, sealed-and-vented with the right cell structure and lid support, framed by DO-160 and IEC 60529. The qualification is the box builder's; what H-O converts is the closed-cell perimeter seal, the ePTFE vent membrane, and the filter that execute the decision.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer. We take closed-cell silicone sponge, microcellular urethane, ePTFE venting membrane, and reticulated filter foam stock and die-cut, kiss-cut on liner, laminate, and kit it to your drawing, including the perimeter gasket, the vent patch, and the inlet filter as one kitted part number where that helps, with material traceability and lot-code TDS records under our ISO 9001:2015 certified quality management system in Winsted, Connecticut.
The sealed-versus-vented decision, the vent placement, the lid-stiffness analysis, and the DO-160 qualification stay with the box builder; H-O converts the parts that execute the design.
Frequently asked questions
Why does a fully sealed avionics box collect water inside?
Because it pumps. On every climb the outside pressure falls and the higher-pressure air inside pushes out through any leak path; on descent the outside pressure rises and humid ambient air is drawn back in, condensing on the cold internal surfaces. Over many flights a fully sealed box with even a small leak path accumulates moisture this way. A tighter gasket does not fix it, it only changes which leak path the pump uses.
The fix is a vent membrane that equalizes the pressure differential so the box no longer has to breathe through its seals.
What is RTCA DO-160 and what does it mean for gasket selection?
RTCA DO-160 is the airborne environmental test framing for civil avionics, with sections covering humidity, fluids, sand and dust, waterproofness, and the altitude and temperature-altitude cycling an aircraft box actually sees. For gasket selection it means the static IP test is not the whole story: a seal proven at ground conditions may still let a sealed box pump moisture over the DO-160 altitude cycle. The seal and any vent are chosen so the box can pass its DO-160 sections, but the qualification itself is the box builder's, not the converter's.
Should the vent be a membrane patch or a machined vent assembly?
For most avionics enclosures a die-cut ePTFE membrane patch is the simplest effective vent: it equalizes the altitude pressure differential while blocking liquid water, and it adds no machined hardware. A machined vent assembly with a replaceable element suits larger boxes, higher flow, or where the program standardizes on a serviceable vent.
Either way the function is the same, equalize pressure, block water, and the placement rule is identical: put it where wash fluid drains away, not where it pools. H-O die-cuts the membrane patch and laminates it into the seal kit.
Can one die-cut part combine the gasket, vent, and filter?
Often yes, and it reduces the part count and the chance of a missing element at assembly. H-O can deliver the closed-cell perimeter gasket, the ePTFE vent patch, and the reticulated inlet filter as one kitted part number, die-cut and laminated to the drawing. The seal, the vent, and the filter are different materials doing different jobs, a closed-cell sponge to seal, a microporous membrane to breathe, an open-cell foam to filter, so the kit keeps them distinct while shipping them together.
Does H-O qualify the enclosure to DO-160 or certify the IP rating?
No. The sealed-versus-vented decision, the vent placement, the lid-stiffness analysis, the DO-160 qualification, and the IEC 60529 IP-rating certification belong to the box builder and its test houses. H-O converts the parts that execute the design: the closed-cell perimeter seal sized to the lid's unsupported span, the ePTFE vent membrane, and the inlet filter, to your drawing, with material traceability and lot-code TDS records. Grade values are confirmed against each maker's current TDS at quote.
Have an enclosure seal-and-vent system reviewed
Send the drawing with the ingress severity, the altitude behavior, the lid stiffness and fastener pattern, and the wash-down exposure, and we will recommend a closed-cell seal, an ePTFE vent, and a filter and quote die-cut parts.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- RTCA DO-160 (airborne environmental and ingress framing including altitude and temperature-altitude cycling; qualification belongs to the box builder).
- IEC 60529 (the IP ingress-protection code, a ground-condition target, cited qualitatively).
- ASTM D1056 (compression-force-deflection classes for the perimeter seal across the unsupported lid span).
- MIL-DTL-83528 (conductive elastomer framing for shielded aperture seals, by designation).
- Manufacturer technical data sheets for the closed-cell silicone sponge, microcellular urethane, ePTFE venting membrane, and reticulated filter foam named on the parent application page.
This article summarizes enclosure sealing and venting material-selection principles for avionics and electronics using the families and published test methods cited on the parent application page. Comparisons are qualitative; per-grade values quoted are maker-TDS figures and must be confirmed on the manufacturer's current technical data sheet at your closure force, altitude, and installed condition. Standards (RTCA DO-160, IEC, ASTM, UL, MIL-DTL) are cited by designation for context, not as a claim of qualification.
H-O is a precision die-cut converter, not a raw-material producer, and does not qualify enclosures to DO-160, certify ingress ratings, or hold platform certifications; those belong to the box builder and its test houses.