The power supply had passed every test on the ground.
Thermal testing at +70°C: pass. Humidity: pass. Vibration: pass. Functional test at full load: pass. The qualification binder was complete. The unit shipped into a commercial avionics application and was installed in an unpressurised equipment bay behind the cockpit bulkhead.
At 38,000 feet, it arced internally between two conductors spaced 3.2mm apart. The same 3.2mm gap that had never caused a problem in any ground-level test.
The failure was not a manufacturing defect. It was a design oversight rooted in a physical law that every electrical engineer learns in university and most forget to apply at altitude: Paschen's Law, which describes how the dielectric breakdown voltage of a gas varies with pressure and gap distance. At sea level, 3.2mm in air is safe at the operating voltage. At 80 kPa — the equivalent of 2,000 metres — the breakdown voltage drops. At 26 kPa — 35,000 feet — it drops substantially further, and a gap that was conservative at ground level becomes a conduction path.
The altitude test exists to find exactly this failure. The power supply program had not run one.
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What altitude testing actually simulates
An altitude test chamber — also called a low-pressure chamber or hypobaric chamber — reduces the air pressure inside its workspace to simulate the atmospheric conditions at elevation.
At sea level, atmospheric pressure is approximately 101.3 kPa (1 atmosphere, 1013 mbar, 14.7 psi — the number varies by unit but the condition is the same). As altitude increases, pressure drops:
- 2,000 m (6,500 ft): ~80 kPa — mountain environments, unpressurised light aircraft - 8,000 ft cabin altitude: ~75 kPa — commercial aircraft pressurised cabin equivalent - 15,000 m (49,000 ft): ~12 kPa — high-altitude surveillance aircraft - 20,000 m (65,000 ft): ~5.5 kPa — stratospheric balloon payloads - Near-vacuum: <1 kPa — orbital and suborbital applications
The chamber achieves these conditions using a vacuum pump connected to the workspace. A control valve regulates the pump's draw to maintain a setpoint pressure. The system can hold any intermediate pressure between ambient and near-vacuum, and transition between pressure levels at controlled rates — both the depressurisation rate and the repressurisation rate can be programmed to simulate ascent and descent profiles.
More capable chambers combine altitude control with temperature control — achieving the cold, low-pressure conditions of high-altitude flight simultaneously, rather than applying each stress independently. At 15,000 metres, ambient temperature is approximately -55°C. A product that is thermally stressed only at sea-level temperature has not been tested in the combined condition it will actually experience.
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The three failure modes altitude testing is designed to find
Every altitude test is hunting for one or more of three specific failure mechanisms. Understanding which mechanism your product is susceptible to determines how the test should be designed — the pressure level, the temperature, the functional monitoring, and the acceptance criteria.
Failure mode 1: Dielectric breakdown — the Paschen curve problem
The dielectric strength of air — its resistance to electrical breakdown — varies with pressure. At reduced pressure, the mean free path of gas molecules increases. Electrons accelerate further between collisions, carrying more energy into each collision, which ionises more molecules and eventually produces a conduction path through what should be an insulating gap.
The Paschen curve, first published by Friedrich Paschen in 1889 and well-established in applied physics ever since, describes this relationship quantitatively. For air, the breakdown voltage as a function of pressure × gap distance (pd) has a minimum — the Paschen minimum — at approximately pd = 0.75 kPa·mm, where breakdown occurs at only 327 volts.
For a designer specifying a clearance gap at sea level, the calculation is straightforward: maintain enough gap distance that the operating voltage stays well below the Paschen breakdown voltage at 101.3 kPa. That calculation, done correctly at sea level, can be completely wrong at altitude, where the same gap distance at a different pressure may sit on the wrong side of the Paschen curve.
The practical consequence: every high-voltage electronics design that will operate at altitude — power supplies, motor drives, inverters, high-voltage signal conditioning — requires a clearance analysis that accounts for the minimum operating pressure, not just sea-level conditions. IEC 60664-1, the coordination of insulation standard for low-voltage equipment, addresses altitude correction for clearance distances, requiring increased gaps at elevated altitudes. Altitude test chambers verify that the design analysis was correct.
Failure mode 2: Thermal runaway from convective cooling failure
At sea level, convective cooling removes heat from component surfaces through air circulation — natural or forced. The thermal resistance of the air-to-surface convective path depends on air density. As pressure drops, air density drops proportionally, and the convective heat transfer coefficient decreases.
At 35,000 feet (26 kPa), air density is approximately 26% of sea-level density. A heatsink that dissipates 50W at sea level with a 15°C temperature rise above ambient may produce a 45–60°C rise at the same altitude and the same airflow velocity — because the same volume of air at that altitude carries 26% of the thermal capacity of the same volume at sea level.
Electronics that rely on convective cooling without accounting for altitude derating will overheat at altitude even when the ambient temperature is lower than at sea level. The combination of lower air density and lower conductive air velocity is sufficient to push junction temperatures above rated limits for components operating near their thermal ceiling.
Altitude testing with simultaneous functional load and temperature monitoring identifies this failure mode before it occurs in the field. The test should run the product at full operating power, at the maximum ambient temperature expected at the operating altitude, with all airflow paths representative of the installed configuration. A product that passes ground-level thermal testing and altitude testing at no load has not been tested for convective cooling failure at altitude.
Failure mode 3: Seal integrity and outgassing from pressure differentials
Sealed enclosures — hermetic packages, pressurised housings, sealed optical assemblies, sealed battery cells — experience a pressure differential between their internal environment and the external atmosphere at altitude. An enclosure sealed at sea level at 101.3 kPa internal pressure experiences an outward pressure differential of 75 kPa at 35,000 feet. That differential loads every seal, every gasket, every O-ring, and every hermetic feedthrough.
Seals that hold at ground-level hydrostatic test pressures may not hold at the differential pressures of altitude. Gasket materials that are compliant and sealing at sea-level compression may extrude slightly under the outward pressure, breaking the seal at altitude. O-rings designed with the correct cross-section for sea-level face seal applications may not maintain adequate contact force when the pressure differential acts outward against them.
Outgassing is a related concern. Some polymers, adhesives, and potting compounds absorb gas at atmospheric pressure and release it at reduced pressure — either as dissolved gas coming out of solution (similar to nitrogen bubbles in a scuba diver's blood during rapid ascent) or as volatile compounds that boil at pressures below their vapour pressure. In sealed optical assemblies, outgassing can deposit contamination on optical surfaces. In sealed electrical assemblies, outgassing can produce conductive deposits on PCBs or alter the dielectric environment around high-voltage conductors.
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The standards that govern altitude testing
MIL-STD-810 Method 500 — Low pressure (altitude)
The primary altitude test standard for US military and defense equipment. Method 500 includes four procedures:
- Procedure I: Storage/airborne — 15,240 m (50,000 ft), -55°C, 72 hours unpowered. Tests seal integrity and material behaviour under sustained low-pressure cold storage. - Procedure II: Operational — altitude appropriate to the deployment environment, operational temperature, product powered at full load. Tests the three failure modes described above under realistic operating conditions. - Procedure III: Rapid decompression — simulates the sudden cabin depressurisation event in pressurised aircraft at cruise altitude. Product transitions from cabin pressure (75 kPa) to ambient at cruise altitude (26 kPa) in 15 seconds. Tests seal integrity and mechanical response to rapid pressure change. - Procedure IV: Explosive decompression — an even more severe rapid decompression, used for equipment in pressurised aircraft hulls where explosive structural failure could expose the equipment to near-instantaneous pressure loss.
Each procedure tests a different failure mechanism. Method 500 Procedure II is the most relevant for electronics that will operate at altitude. Procedures III and IV are specific to pressurised aircraft applications.
DO-160 Section 4 — Altitude
The avionics altitude test standard, published by RTCA and accepted by both the FAA and EASA for commercial aviation equipment qualification. Section 4 defines equipment categories (A through F) based on the maximum altitude the equipment will experience and whether the installation is pressurised or unpressurised. Category F equipment — unpressurised installations at cruise altitude — is tested to 21,300 m (70,000 ft), which is the highest altitude category in DO-160.
DO-160 Section 4 also specifies decompression testing for pressurised installations, with requirements closely related to MIL-STD-810 Method 500 Procedure III.
IEC 60068-2-13 — Low air pressure
The IEC standard for low-pressure testing of electrical and electronic equipment. Less prescriptive than MIL-STD-810 in terms of procedures, but widely referenced in European product standards that require altitude capability verification.
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What an altitude test chamber looks like
The hardware is simpler than a climatic chamber in one respect — it needs only a vacuum pump and pressure control system, not a precision humidity or refrigeration system.
A standard altitude chamber consists of:
The pressure vessel. A chamber rated for the minimum operating pressure required — typically 1 kPa or below for high-altitude applications. The vessel must maintain structural integrity at the pressure differential between its workspace and the atmosphere. For large walk-in altitude chambers, the structural loading is substantial: a 3m × 3m wall area at 100 kPa differential experiences 900 kN of atmospheric load — nearly 90 tonnes.
The vacuum pump system. One or more vacuum pumps of sufficient capacity to achieve the target pressure within the required time and maintain it against leakage. Oil-sealed rotary vane pumps are standard for pressures down to 1 kPa. Below that, multi-stage pumps or turbomolecular pumps are required.
The pressure control system. A proportional control valve and pressure transducer that regulate pump draw to hold setpoint pressure, and that control the rate of pressure change during simulated ascent and descent profiles.
The thermal conditioning system (for combined altitude/temperature chambers). A separate temperature control system that conditions the workspace air before it enters the low-pressure environment. Achieving -55°C at 5 kPa requires both a capable refrigeration system and careful management of the interaction between pressure and the refrigeration system's performance.
DUT monitoring and power passthroughs. Cable penetrations sealed against the pressure differential, allowing product power supply, signal monitoring, and thermocouple instrumentation to pass through the chamber wall without breaking the pressure seal.
Weiss Technik and Cincinnati Sub-Zero both manufacture combined altitude/temperature chambers used in aerospace and defense qualification programs — the CSZ Dynavac line covers vacuum and altitude chamber configurations specifically.
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The rapid decompression test: a failure mode most engineers haven't thought about
Rapid decompression testing exposes a fundamentally different failure mode from sustained altitude testing, and it deserves specific attention because the failure mechanisms are counterintuitive.
When a pressurised aircraft cabin loses pressurisation — through a seal failure, a blown window, or structural damage — the pressure drops from cabin altitude (~75 kPa) to ambient at cruise altitude (~26 kPa) in seconds. Equipment mounted in that environment experiences a rapid outward pressure load on every sealed surface, a sudden temperature drop as expanding gas cools, and a mechanical shock from the pressure wave.
The specific failure modes in rapid decompression include:
Seal extrusion and blowout. O-rings and gaskets that are compressed into their grooves at cabin pressure may extrude past their groove edges under the outward pressure differential of rapid decompression. A seal that holds indefinitely under sustained low pressure may fail in the first second of a rapid decompression event.
Pressurised internal volumes. Any component that contains a sealed gas volume at cabin pressure — certain electrolytic capacitors, sealed potted assemblies with trapped air voids, battery cells — will experience internal overpressure relative to the surrounding atmosphere during decompression. A potted assembly with an air void at 75 kPa internal pressure, suddenly surrounded by 26 kPa air, has a 49 kPa net outward pressure on its casing. Enough to crack inadequately designed housings.
Cooling fluid flashing. Liquid cooling systems in aircraft equipment that use low-boiling-point coolants can flash to vapour if the rapid pressure drop reduces the system pressure below the coolant's vapour pressure at operating temperature. The resulting vapour lock disrupts thermal management and can cause localised overheating within seconds.
DO-160 Section 4.6 specifies the rapid decompression test conditions and procedures for avionics equipment in pressurised aircraft installations. MIL-STD-810 Method 500 Procedure III covers the equivalent military application.
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The altitude test that most programs skip — and shouldn't
The most informative altitude test for electronics is one that nobody likes to schedule: powered functional testing at full load, at minimum pressure, at maximum ambient temperature, with continuous electrical monitoring throughout.
It is the test nobody likes because it requires the product to be powered inside a vacuum-rated chamber, which means extensive passthrough instrumentation, a safety protocol for powered equipment at reduced pressure (where arc risk is elevated), and a test duration long enough for the thermal state to stabilise.
Most programs run depressurisation only, with the product unpowered, and then measure electrical parameters after repressurisation. That sequence finds seal failures and outgassing effects. It does not find dielectric breakdown at altitude or convective cooling failure at altitude — because neither failure mode manifests when the product is unpowered and the chamber is being repressurised before measurement.
The power supply that arced at 38,000 feet in this post's opening would have passed the unpowered depressurisation test. It would have arced during the powered functional test at altitude, if anyone had run one.
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Connecting altitude to the full environmental programme
Altitude testing rarely drives a complete environmental qualification programme on its own. It sits alongside temperature, humidity, vibration, and shock testing in the sequence defined by the relevant standard — DO-160, MIL-STD-810, or IEC 60068 for equipment deployed at altitude.
The types of environmental test chambers post covers altitude chambers in the context of the full chamber landscape. The environmental testing standards post covers how MIL-STD-810 and DO-160 relate to each other and how to determine which standard governs your application. The vibration test chambers post covers the six-DOF and single-axis vibration testing that typically accompanies altitude qualification in aerospace programs.
For the insulation coordination question — determining the correct clearance distances for high-voltage electronics at altitude — IEC 60664-1 is the governing standard, and it deserves a design review before the altitude test is scheduled rather than after the arc report comes back.
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The Paschen calculation nobody ran
The power supply that arced at 38,000 feet was not a badly designed product. It was a ground-level-designed product deployed into an altitude environment without anyone having asked the altitude-specific design questions.
What is the minimum pressure this product will operate at? What are the highest operating voltages in the design? What are the minimum clearance distances between those conductors? Where do those clearance distances fall on the Paschen curve at the minimum operating pressure?
Those four questions, asked in the design review, produce a design that passes the altitude test. Skipped, they produce an arc report from 38,000 feet.
The altitude test chamber exists to catch the gap between what was designed at sea level and what works at altitude. It catches it reliably — but only when the test is powered, loaded, and run at conditions that actually excite the failure modes it was designed to find.
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Next in this series: IP Ingress Testing: The Chamber That Reveals Every Weak Seal in Your Design · Xenon Arc vs. Fluorescent UV: Choosing the Right Weathering Chamber for Your Material
Related reading: IEC, MIL-STD, ASTM, ISO: The Environmental Testing Standards Map Every Engineer Needs · Vibration Test Chambers: Single-Axis vs. Six-DOF and Why the Difference Is Everything · Not All Environmental Test Chambers Are Equal · The Top 10 Environmental Test Chamber Manufacturers
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