Why the MRO heavy-maintenance facility is its own HVAC discipline
The phrase "aviation HVAC" gets used loosely. In a passenger terminal it means cooling 50,000 m² of marble concourse to 23 °C and handling the heat plume from 30,000 daily passengers. In a tower or a radar shelter it means a precision low-RH environment for electronics. On the airfield apron it means almost nothing — that volume is the sky. The Maintenance, Repair and Overhaul heavy-maintenance facility is none of those problems. The MRO hangar is where the aircraft comes apart, where its fuel tanks are vented, where its skin gets stripped and repainted, where its composite structure gets ground and re-laid, and where its engines come out and go onto a test cell stand to be run up to maximum power. Every one of those processes generates a hazardous airborne contaminant — solvent vapour, fuel vapour, isocyanate paint mist, hex-chrome aerosol, carbon fibre dust, oil mist, hot exhaust gas — and the duct that captures, conveys and discharges those contaminants is the most safety-critical and the most material-sensitive ductwork on any aviation site.
The MRO ductwork problem is also a scale problem. A wide-body hangar designed to take an A330 or a 777 routinely has 30,000 m² of floor area at a clear ceiling height of 18–25 m. The volume is 30,000–50,000 m³. At 6 air changes per hour the facility moves 50–84 m³/s of conditioned air through main supply trunks that are 1,000–1,800 mm in diameter. The mainline trunk in those buildings is rarely made on the same machine that makes a residential air-handler riser. The mainline trunk for an MRO hangar is spiral, almost always — and the machine that produces it at scale is a heavy-duty spiral tubeformer of the kind SBKJ ships under the SBTF-2020 designation. Downstream of the spiral trunk, the contaminant zones — paint shop, depaint cell, composite room, engine test cell — branch off in rectangular and round duct made of 304 or 316L stainless. That stainless duct is formed coil-to-duct on a stainless-configured auto duct line of the SBAL-V family, with longitudinal seams TIG-welded, pickled and passivated.
This guide walks the specification from the regulatory framework outward to the machine bill of materials. It assumes the reader is the consulting engineer, the facility owner's project lead, the MRO operations manager or the duct fabricator who will be tendering on the work. It is not a treatise on aircraft fire suppression and it is not a heating, cooling or psychrometric design textbook. It is a duct specification guide. Specifically, it answers the questions: what material, what gauge, what joint, what machine origin, what audit trail.
Regulatory framework: who actually writes the rules
Five regulatory streams converge on an MRO facility. They overlap and they reinforce each other but they have different audit dates and different inspectors. An MRO facility designed to pass only one of the five will fail the others.
FAA Advisory Circular AC 43.13
The US Federal Aviation Administration's Advisory Circular 43.13-1B and 43.13-2B are the granddaddies of MRO practice. They are formally only "guidance" in the US system but they are referenced by transport-category aircraft maintenance organisations worldwide as the international precedent for acceptable practices, methods and techniques. AC 43.13-1B covers airframe inspection and repair; AC 43.13-2B covers alterations and major repairs. For HVAC duct purposes the most relevant elements are the airframe corrosion-control chapters and the painting and finishes guidance, which set the expectation that an MRO facility shall provide a paint cell with controlled airflow, contamination-free supply air, and a means of solvent vapour evacuation. AC 43.13 does not specify the duct material. It does specify the outcome — and the outcome cannot be achieved in degraded galvanised duct that is shedding zinc carbonate particles into the supply air.
CASA Civil Aviation Regulations (CASR)
In Australia, the Civil Aviation Safety Authority issues the Civil Aviation Safety Regulations 1998 (CASR). Part 145 of the CASR — Approved Maintenance Organisations — is the operative regulation for an Australian MRO. CASR Part 145 mandates that the maintenance organisation hold a current exposition (the MOE, Maintenance Organisation Exposition) describing its facilities and procedures, including the environmental controls of its workshops and hangars. The CASA surveillance auditor will compare the as-installed HVAC and the exposition. If the exposition references AS 1668.2 outdoor air rates, NFPA 33 fully ducted spray booth and a chromate stripping HEPA discharge, the auditor will go look at the duct material, the filter pressure differential gauges and the discharge stack. CASR Part 145 is the document that puts a CASA inspector in front of your ductwork.
EASA Part-145 maintenance organisation approval
For any Australian MRO that services European-registered transport aircraft — or that provides component overhaul to European Part-21 design organisations — the European Union Aviation Safety Agency's Part-145 approval is also required, under a bilateral aviation safety agreement. EASA Part-145 is similar in structure to CASR Part 145 but its inspectors are EU-based and its expositions are scrutinised against the EASA Acceptable Means of Compliance (AMC) for Part-145. The AMC explicitly mentions environmental control of work in confined spaces (fuel tank entry), paint and composite work, and the need for verified outdoor air supply to those workstations. An MRO facility holding both CASR Part 145 and EASA Part-145 will be audited twice a year. The HVAC ductwork has to satisfy both.
AS 1668.1 and AS 1668.2 ventilation
Australian Standard 1668.1 covers fire and smoke control aspects of mechanical ventilation, and AS 1668.2 covers the ventilation design for indoor air quality. Inside the MRO hangar the relevant section of AS 1668.2 is the outdoor air rate for the floor area and occupant count, and the supplementary contaminant-source rates for fuel system maintenance, paint application and welding. AS 1668.1 applies to the duct penetrations through fire compartmentation, the fire and smoke dampers, the actuators and the control matrix. The HVAC duct designer cannot route a 1,800 mm spiral through a fire wall without an AS 1668.1-compliant damper. The penetration detail is one of the things the building certifier will check before issuing the occupation permit.
NFPA 410, NFPA 33 and NFPA 409
The US National Fire Protection Association issues three codes that flow into MRO design even outside the United States, because they are adopted by underwriters worldwide and because they are the most operationally specific texts available on aircraft maintenance fire safety. NFPA 410 Standard on Aircraft Maintenance covers maintenance operations broadly: fuel-system work, fuel-cell repair, electrical work and welding in the hangar bay. NFPA 33 Standard for Spray Application Using Flammable or Combustible Materials covers the paint cell — the standard that drives the "fully ducted spray booth" requirement, the downdraft preference, the face velocity target and the booth-discharge HEPA or carbon bed. NFPA 409 Standard on Aircraft Hangars covers the hangar shell itself: the AFFF foam suppression layout (or fluorine-free transition to F3, where the operator has moved away from AFFF), the floor drains, the egress and the smoke control. The HVAC duct designer crosses paths with NFPA 409 mostly at the foam suppression piping coordination and at the duct penetrations through the hangar shell.
ASHRAE 62.1 outdoor air
ASHRAE 62.1 Ventilation for Acceptable Indoor Air Quality is the international reference for outdoor air rates. In the MRO context the ASHRAE 62.1 rates are typically used as a sanity check on the AS 1668.2 calculation — they will agree within 5–10 % for occupied maintenance space at typical occupant density and contaminant load. Where they diverge is in the high-contaminant areas (paint shop, depaint cell, composite grinding) where the contaminant capture velocity dominates the outdoor air calculation, and both standards defer to the industrial hygiene face-velocity target.
Hangar bay HVAC: the big-volume problem
The hangar bay is the largest contiguous volume on the MRO site and it is the volume that drives the duct machine selection. A narrow-body hangar (737, A320, E-Jet) is typically 15,000–25,000 m³. A wide-body hangar (787, A330, 777, 747) is typically 30,000–50,000 m³, with some triple-wide bays exceeding 80,000 m³. The clear height under the highest crane runway is 18–25 m and the tail clearance on a 777 is 20 m, so the supply ductwork has to be routed above the tail level or it has to enter from the floor through pedestal diffusers.
Air change rate
The design air change rate depends on the operations classification. For occupied maintenance with no live fuel work and no spray application, 3–6 ACH is typical and consistent with AS 1668.2 and ASHRAE 62.1. For fuel-system maintenance, defuelling, draining or open-tank entry, the rate climbs to 10–20 ACH locally over the work zone, often by deployment of high-volume floor-mounted spot ventilation that ties into the main return system. For solvent washdown between coats the rate climbs again, and operators frequently temporarily isolate the hangar from the rest of the building and exhaust to atmosphere through the contaminant-zone duct.
A 30,000 m³ hangar at 6 ACH moves 50 m³/s of air. At a typical main-trunk velocity of 12 m/s the duct cross-sectional area is 4.2 m² and the spiral diameter is 2,300 mm. A 50,000 m³ hangar at 6 ACH moves 84 m³/s — and the same arithmetic gives a 3,000 mm trunk. Few facilities route a single 3,000 mm trunk; the practice is two or three 1,500–1,800 mm spirals in parallel, which both reduces the headroom envelope and provides redundancy if one fan or one duct run goes down. These are the diameters that the SBKJ SBTF-2020 spiral tubeformer was designed to produce: 80 mm to 2,000 mm in steel, stainless or aluminium, with a continuous Pittsburgh-lock spiral seam.
Heating and cooling load
The thermal load on the hangar bay is dominated by the door operations. Every time the hangar doors open for an aircraft tow-in, a 50,000 m³ volume loses or gains heat with the outside air. The HVAC duct system is typically sized for the steady-state load (people, lights, small power, solar gain through the high windows) plus a recovery margin for door cycles. Most MRO operators do not design the system to maintain temperature during a door open event; they design for recovery within 30–60 minutes after the door closes. This is one reason the duct system is sized aggressively — recovery requires the ability to move the full air volume in less than an hour.
Foam fire suppression integration
NFPA 409 hangar fire protection has historically been delivered through Aqueous Film Forming Foam (AFFF) overhead and pop-up systems, with deluge release on detection of pool fire. The HVAC duct routing has to coordinate against the AFFF piping, the foam concentrate tanks, the proportioner and the deluge valves. In Australia, in line with the Commonwealth PFAS National Environmental Management Plan, MRO operators are progressively transitioning away from AFFF (which contains per- and polyfluoroalkyl substances) to fluorine-free foam (F3) systems. The transition concern for the duct designer is that the F3 systems often require higher application rates and may need additional overhead piping, which can crowd the duct routing path. The duct contractor should request the latest fire protection layout before finalising the high-level supply trunk path.
Duct material selection for the hangar bay
The hangar bay floor and roof envelope is benign from a corrosion standpoint. The contaminants present at low concentration — JP-8 vapour, hydraulic mist, dust — do not attack galvanised duct in the way the contaminants of the paint or chrome bays do. Mainline hangar bay supply and return duct is therefore generally specified as galvanised, gauge per SMACNA or AS 4254, with TDF or slip-and-drive joints. Hangar bay duct is the canonical SBAL-V product: 0.5–1.5 mm galvanised coil, into a coil-to-duct line that produces flanged rectangular sections at typically 18–30 m per hour.
The exception is the duct serving the fuel system maintenance area inside the hangar bay. Where fuel-cell entry or open-tank work is performed, the local supply and exhaust are specified in 304 stainless minimum, because the fuel vapour and the solvent flush carry into the duct and condense, and galvanised duct in that service has shown corrosion failure inside 5 years. The boundary between galvanised mainline and stainless local duct is usually a transition piece with an isolation damper, fabricated to the same SBAL-V tolerance as the mainline duct.
Aircraft paint shop: NFPA 33 fully ducted spray booth
The paint shop is the single most safety-critical and material-sensitive zone in the MRO facility. An aircraft repaint involves the application of solvent-borne primers, intermediate coats and topcoats — typically polyurethane systems with isocyanate hardeners — over a 24–72 hour cycle. The booth must contain the overspray, capture the solvent vapour, maintain a contamination-free supply air to deliver an acceptable finish, and isolate ignition sources from the flammable atmosphere. NFPA 33 is the operative code.
The economics of getting the paint shop wrong are punishing. A wide-body strip-and-repaint job sells at AUD 450,000 to AUD 1.2 million depending on the airline livery complexity. A reject for surface contamination — a single fisheye, a rust freckle from a shedding duct, a piece of paint flake from a peeling galvanised joint — can write off the topcoat for that aircraft side and force a re-strip-and-recoat. The duct specification is the upstream lever that controls that reject rate. MRO operators who have lived through a galvanised paint duct failure invariably specify stainless on the replacement, and they audit it on the next facility build.
Downdraft versus cross-draft
NFPA 33 permits both downdraft and cross-draft booth configurations but downdraft is strongly preferred for aircraft paint application, for three reasons: it carries overspray away from the work surface and away from the operator simultaneously, it does not require the operator to work against the air pattern, and it accommodates the geometry of an aircraft (long, thin, and with surfaces at every orientation) more gracefully than a cross-draft. The downdraft booth has supply plenums in the ceiling running the full booth length, perforated to deliver 0.3–0.5 m/s laminar downflow, and floor grating with extract plenums beneath.
The booth working plane face velocity target is 0.5 m/s minimum, with industrial practice running 0.6–0.8 m/s during application. The booth volumetric flow follows directly from the booth plan area and the face velocity. A 50 m × 30 m wide-body booth at 0.6 m/s moves 900 m³/s of air — and this is the design volume for the supply fan, the recirculation circuit (where used) and the discharge path. Most MRO paint booths in Australia are single-pass to atmosphere, with no recirculation, because the solvent recovery economics rarely justify the activated carbon plant against single-pass operation.
Duct material and joint specification
NFPA 33 fully ducted spray booth means every metre of duct from the booth extract to the discharge stack is enclosed, no plenum recirculation through the building, no shared return path with other zones. The duct material specification is non-trivial. The solvents in aircraft topcoats — methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), toluene, xylene, butyl acetate — and the alkaline detergents used in the booth washdown between coats both attack the zinc layer on galvanised duct. Field experience inside 18–36 months of operation is consumption of the zinc, then flash rust on the base steel, then iron oxide flakes shedding into the next paint job. The remedy is to specify the booth discharge duct in 304 minimum, and to specify 316L for the duct downstream of the filter bank where any liquid condensate concentrates. The booth supply plenum and ceiling diffuser are also typically 304 — the supply air is filtered to a high standard and there is no contaminant load, but the booth washdown wets the supply plenum and the zinc washes off galvanised over time.
The joint specification follows the duct material. Stainless duct for paint service is seam-welded TIG along the longitudinal joint and at every transverse joint, pickled and passivated to remove the heat-affected oxide and restore the passive chromium oxide layer. The TIG procedure is qualified to AS 1554.6 (stainless steel welding) or ASME IX, the welder is qualified to the procedure, and the welder qualification record is held in the QA file against the duct material certificate. Mainline aviation MRO clients of SBKJ specify "316L, TIG seam-welded, pickled and passivated, with mill certificates and weld procedure on file" as the boilerplate for paint shop duct, and they audit it.
Filter banks and HEPA discharge
The paint booth discharge typically passes through a three-stage filter set before atmospheric release: a coarse paint arrestor (paper or fibreglass roll), an intermediate bag filter to MERV 13, and a final HEPA H13 or H14 stage if the booth is performing chromated primer or other listed hazardous coating work. Some operators add an activated carbon bed for solvent vapour scrubbing where local air-quality permits demand it. The duct between the booth and the filter bank is 304 stainless; the duct between the filter bank and the discharge stack is 316L stainless, sized for the stack discharge velocity (typically 12–15 m/s to give plume rise sufficient to clear the building wake), and earthed at the stack base.
Depaint stripping cell: chromate, laser, dry-ice
Aircraft depaint is the process of removing the old topcoat, intermediate and primer system back to bare aluminium or composite skin before the new paint cycle. There are three technologies in current use, and each has a different duct implication.
Chemical chromate stripping
The traditional process is chemical stripper, typically a methylene chloride or benzyl alcohol formulation with an alkaline activator, applied in a brushed or sprayed coat and allowed to dwell. The stripper softens the paint film, which is then peeled or pressure-washed off. Where the original primer is a chromated primer (now phased out for new applications but still present on legacy airframes), the stripped paint flake contains hexavalent chromium and the airborne aerosol from the stripping operation is hexavalent chromium-bearing. Hexavalent chromium is a known human carcinogen and an Australian Workplace Exposure Standard exists at very low concentrations (8-hour TWA 0.05 mg/m³ for chromium VI compounds).
The capture and duct strategy is a downdraft or slot hood, sized at 0.5 m/s minimum face velocity over the work area, ducted in 316L stainless, routed to a HEPA filter bank (H13 minimum, H14 preferred), and discharged through a stack tall enough to clear the building wake. The HEPA filter is upstream of the fan, in a negative-pressure configuration, so a filter failure does not discharge contaminant into the building. 316L is the correct material because the strippers themselves are aggressive on 304 and on galvanised, and the solution gets onto the inside of the duct at every flush cycle.
Laser ablation
Newer depaint technology uses high-power pulsed laser to ablate the paint film off the substrate. The advantages are no chemistry and a tighter spatial control on the depaint zone. The disadvantage is that the ablated paint generates a fine aerosol and an LCSP (laser-generated air contaminant) plume that is captured by a slot hood directly behind the laser head and ducted to a HEPA-and-carbon bed. The duct is still 316L stainless, because the ablation generates trace chromate from chromated primer if it is present, and because the duct serves the same operation through the lifecycle.
Dry-ice / sponge-jet abrasion
The third option is dry-ice (carbon dioxide pellet) or sodium bicarbonate sponge-jet abrasion. Both work mechanically — kinetic impact of the abrasive media on the paint film — and both generate a particulate aerosol of paint flake and spent media. The duct strategy is downdraft hood, HEPA filter, atmospheric discharge. 316L stainless duct because the abrasion zone is repeatedly washed down and the duct interior sees standing alkaline water between cycles.
Hex-chrome plating and stripping
Some MRO facilities have on-site hard-chrome plating capability for landing-gear and undercarriage refurbishment. This is a hex-chrome process. The plating tanks generate an acid mist of chromic acid that is captured by a push-pull slot hood across the tank and ducted in 316L through a chrome mist eliminator (typically a packed-bed scrubber with mesh demisters), then through HEPA, then to discharge. The duct material here is absolutely non-negotiable — chromic acid mist consumes galvanised duct in under 12 months and consumes 304 stainless within 5 years. SBKJ specifies 316L stainless for every metre of plating-shop and stripping-shop duct, formed on the SBAL-V auto duct line configured for 1.0–1.5 mm stainless coil.
Composite repair bay and autoclave room
Modern transport aircraft — the 787, the A350, the A320neo wingtips, the A380 centre fuselage panels, and the F-35 wing skins — are extensively composite. MRO facilities now run composite repair bays that are physically and procedurally separated from the metallic structures bays. Composite repair involves three operations: damage assessment, layup, and cure. Each has its own ventilation requirement.
Composite layup room
The layup room is a clean environment, conditioned for temperature (typically 21 ± 2 °C) and humidity (typically 50 ± 10 % RH), and ventilated to remove the solvent component of the epoxy or BMI prepreg adhesives. The duct system is conventional galvanised with HEPA-filtered supply, similar in approach to a Class 100,000 cleanroom. The contamination concern is particulate ingress onto the prepreg lay, so the supply diffusers are HEPA terminal filters and the room is held at positive pressure to the surrounding facility.
Autoclave room
The autoclave is a pressure vessel that cures the composite layup at controlled temperature and pressure. Standard cure cycles for aerospace epoxies run 120–180 °C at 6–7 bar. Bismaleimide (BMI) and cyanate ester systems run higher — up to 230 °C at 7 bar. The autoclave room is typically conditioned with isolated supply and return, because the autoclave vents through its own dedicated stack on every cycle and that vent contains residual solvent and resin volatiles. The autoclave room is also designed to handle the radiant heat from the autoclave skin during the cure cycle, which can be 60 °C or more on the outer shell despite the insulation. Duct in the autoclave room is generally 304 stainless because the operating temperature and the periodic vent excursions exceed the safe service envelope for galvanised. SBKJ supplies the autoclave room duct in 304 stainless on the SBAL-V line and the autoclave vent stack itself in 316L because the vent condenses acidic species.
Composite grinding and machining dust capture
The most safety-critical part of the composite operation is the grinding and machining of cured carbon fibre laminates for damage trim-out and scarf preparation. Cured carbon fibre dust is a combustible dust under NFPA 484 Standard for Combustible Metals (the standard covers conductive combustible dusts more generally), electrically conductive (so it bridges energised circuits and can be a fire ignition risk if it lodges on hot electrical surfaces), and finely sized (much of it sub-75 micron, which means it stays airborne for hours).
The capture strategy is the downdraft table for hand-grinding or a built-in machine extraction for CNC trim, ducted to a cartridge dust collector with explosion isolation valves between the duct and the collector, and a deflagration vent on the collector body. The duct material is 316L stainless — never plastic, because plastic duct can build static charge and ignite the dust cloud, and never galvanised, because the rough zinc surface can hold deposit and become a kindling site. The duct is continuously bonded across every flange — each flange pair has an earth bond strap — and the dust collector and its filter media are bonded to the building earth ring. SBKJ supplies the composite-shop duct in 316L with welded TIG seams, flanged with conductive gaskets, and with bonding lugs cast into every flange.
Engine test cell
An engine test cell is a hardened, sound-attenuated, dedicated facility for running a gas turbine engine through its acceptance test cycle after overhaul. The engine is mounted on a thrust frame, fuelled from the test cell fuel system, started, and run from idle up to maximum continuous and through transient profiles. The HVAC ventilation problem in the test cell is the highest mass-flow ventilation problem in the entire MRO facility.
Engine test cells are not common in the wider Australian MRO landscape — most engine overhaul is performed in dedicated specialist shops with their own test cells, including the StandardAero Australia facility in Brisbane and several offshore partner facilities. Where a domestic test cell is in use, it is most often a small-to-mid-thrust cell for regional turboprops (PW100 series, GE H80, Honeywell TPE331) or for rotary-wing turboshafts (T700, RTM322). The duct package for these cells is correspondingly smaller than the figures discussed below for the wide-body turbofans.
Mass flow
A small commercial gas turbine engine (CFM56, V2500, PW1100G) ingests 100–200 kg/s of air at maximum thrust and exhausts the products of combustion at temperatures up to 600 °C. A large engine (Trent 1000, GEnx, PW1500G) can ingest 1,200–1,500 kg/s. The test cell ventilation has to provide that ingestion air through an inlet stack with bellmouth and silencer, capture the engine exhaust through an augmenter and ejector, mix it with secondary cooling air, and discharge the diluted exhaust through an exhaust stack with secondary silencer. The supply air mass flow including bypass cooling is typically 1.5 to 2 times the engine intake mass flow.
Duct considerations
The supply and exhaust paths in a test cell are not strictly "duct" in the SBAL-V or SBTF-2020 sense — they are large field-fabricated steel structures sized in the multiple-metre range. The ductwork that the duct fabricator typically supplies into a test cell project is the secondary HVAC: the test cell control room ventilation, the engine pedestal local extraction (oil mist capture, fuel vent capture for the start-up and shut-down phases), the fuel-vapour purge for the engine fuel-system bench, and the building-services duct around the cell. These are conventional sizes — 300–800 mm rectangular — in galvanised for the office and control room runs and in 304 or 316L stainless for the engine pedestal and the fuel zones. Sound attenuation is built into the supply and exhaust paths through perforated-plate splitter silencers, which are integrated into the duct fabrication scope.
Fuel vapour capture
During engine start-up and shut-down, the fuel system purge releases jet fuel vapour into the test cell. The capture strategy is a slot hood at the engine pedestal level ducted in 316L stainless to a vapour-recovery condenser or to a thermal oxidiser. The duct is gas-tight, leak-tested to 250 Pa positive pressure, and earthed continuously. SBKJ delivers this duct as a TIG-welded 316L assembly with bolt-pattern transitions to the OEM thermal oxidiser inlet flange.
Materials selection: why galvanised fails outside the hangar bay
The single most consequential decision in MRO duct specification is when to leave galvanised and step up to 304 or 316L stainless. Get this wrong on the up-front design and the duct life is 3–7 years instead of the 25–40 year design life. The decision matrix below summarises field experience.
Galvanised (G350 or Z275 to AS 1397): hangar bay general supply and return; office and control room; cleanroom composite layup supply (with HEPA terminal); building-services HVAC. Service life 25–40 years.
304 stainless (1.4301 / S30400): paint shop supply plenum and ceiling diffuser; paint shop discharge duct upstream of the filter bank; autoclave room supply and return; fuel-system maintenance local exhaust; engine pedestal supply; composite grinding extract upstream of the dust collector. Service life 25–40 years.
316L stainless (1.4404 / S31603): paint shop discharge duct downstream of the filter bank to the stack; depaint cell extract (chemical stripper, laser ablation, dry-ice); hex-chrome plating and stripping extract; composite dust collector discharge to atmosphere; engine fuel-vapour capture duct to thermal oxidiser; chromic acid scrubber inlet and outlet duct. Service life 30–50 years.
Aluminium duct is occasionally specified in the cleanroom layup space but is not used in any contaminant-zone duct in MRO because aluminium dust is itself a combustible dust hazard, and because aluminium oxide is a heat-resistant deposit that fouls duct interiors over time. PVC and FRP plastic duct is not permitted in NFPA 33 spray booth discharge service and is strongly discouraged in composite dust service for the static-charge reason described above.
The galvanised failure pattern
The failure mode of galvanised duct in MRO paint or depaint service is well documented in the field. The zinc layer is consumed first at the weld seams and the screw penetrations, then at the underside of horizontal runs where condensate pools, then along the full interior surface. Underneath the consumed zinc the base steel flash-rusts. The rust scale builds, then dislodges in flakes during high-velocity transients (a damper closure, a fan start). Those flakes get into the supply air and end up on the next aircraft to be painted. A six-figure repaint job is rejected because the topcoat has rust freckles in it. The MRO operator's commercial response is to switch the duct out for stainless on the next maintenance window, at three to four times the per-metre cost they would have paid to specify stainless on day one. This is the calculation that drives SBKJ to recommend 304/316L stainless from the original specification in all paint, depaint, composite and engine-fuel zones, even where the up-front cost difference is real.
Australian MRO landscape
The Australian aviation MRO sector is concentrated around a small number of major operators and a long tail of specialist component overhaul shops. The list below is representative of the facility types that drive the HVAC duct specifications discussed above.
Civilian heavy maintenance
Qantas Engineering operates heavy-maintenance facilities in Brisbane (the principal heavy-maintenance base, with approximately 700 engineering staff supporting the wide-body fleet), Sydney and Melbourne. Brisbane handles A330 and 787 heavy checks; Sydney and Melbourne handle narrow-body. Qantas Engineering's Brisbane facility is a textbook example of a mixed-discipline MRO site: heavy maintenance hangar, paint shop, composite repair bay and engine shop on the same campus.
Hawker Pacific at Bankstown Sydney is the long-established general aviation and business jet MRO, also providing line and base maintenance for regional aircraft.
Toll Aviation operates aeromedical and rotary fleets with associated MRO capability.
Australian Aerospace, the former Eurocopter Australia entity based at Brisbane, provides helicopter MRO and was historically a major component repair hub.
Cobham Aviation in Adelaide provides fixed-wing surveillance and special-mission MRO.
StandardAero Australia (formerly RUAG Australia) at Brisbane is a major engine and component MRO, focused on regional turboprop and small turbofan platforms.
Hatch Australia is a helicopter MRO specialist working across multiple civil rotorcraft platforms.
Aerolinc Aviation is one of several specialist component overhaul shops operating into the wider Australian MRO supply chain.
Brisbane Airport Corporation's MRO precinct, anchored by Qantas Engineering and a cluster of specialist tenants, is one of the densest concentrations of aviation MRO infrastructure in the southern hemisphere and is the single largest customer-base concentration for paint shop and composite shop duct in Australia.
Defence MRO and through-life support
Boeing Defence Australia provides through-life support for RAAF Boeing platforms, with its principal MRO and engineering footprint at Williamtown NSW.
BAE Systems Australia operates at multiple sites including Williamtown NSW (where it supports the F-35 sustainment alongside the OEM and the RAAF) and Salisbury SA (the historic Edinburgh Defence Precinct, supporting electronic systems and complex platforms).
Lockheed Martin Australia is the F-35 OEM and is the prime sustainment contractor for the Australian F-35 fleet, working out of Williamtown alongside BAE and the RAAF.
Northrop Grumman Australia has expanded its Australian MRO footprint at Newcastle (Williamtown adjacent) and at Adelaide, supporting MQ-4C Triton sustainment among other platforms.
Marand at Moorabbin VIC is best known as a tier-one F-35 components supplier (vertical tails and other major assemblies) and is also a defence MRO and engineering services provider.
The defence-aviation footprint that drives this MRO demand is concentrated at four RAAF bases and one Navy facility. RAAF Base Williamtown NSW is the home of the F-35A fleet, with the legacy F/A-18A/B Hornet phase-out continuing through the operational conversion role; the base has the largest single concentration of fighter MRO activity in Australia. RAAF Base Amberley in southeast Queensland is the home of the F/A-18F Super Hornet, EA-18G Growler and C-17 Globemaster fleets, with significant heavy MRO infrastructure on the base. RAAF Base Edinburgh in South Australia hosts the P-8A Poseidon maritime patrol fleet and the MQ-9 Reaper remotely piloted aircraft, along with the AP-3C Orion legacy support. HMAS Albatross at Nowra NSW is the Royal Australian Navy's primary air station, supporting the MH-60R Romeo naval combat helicopter fleet and the MRH-90 Taipan multirole helicopter (the latter being progressively replaced by the MH-60R and the UH-60M Black Hawk). Each of these bases has hangar, paint shop and component overhaul facilities that are subject to the same NFPA, AS 1668 and CASR Part 145 design framework as the civilian MRO operators.
Composite manufacturing co-location
Quickstep Holdings operates composite manufacturing facilities at Geelong VIC and at Bankstown NSW. The Geelong site is one of the Australian primes' principal composite production sites; the Bankstown site produces F-35 vertical tail assemblies under contract to Lockheed Martin. These facilities co-locate composite layup, autoclave cure and finished-assembly paint application — and they drive the same duct material specifications as the MRO repair bays.
Audit trail and documentation
The single output of the MRO duct package that the CASA or EASA inspector will ask to see is not the physical duct — it is the documentation. The acceptance criteria below are the documentation pack that the duct contractor should commit to deliver at handover, and that the MRO operator should require in the tender.
Material certificates. Mill certificate (3.1 to EN 10204 or equivalent) for every coil, against every duct section. The certificate identifies the heat number, the chemical analysis, and the mechanical properties. For 316L stainless the certificate must show the carbon content below 0.030 % and the chromium-nickel-molybdenum balance within the 1.4404 specification. For galvanised coil the certificate must show the coating mass (Z275 for the standard MRO duty cycle) and the base steel grade.
Welding procedure specifications and welder qualifications. Each stainless seam-welded duct run is welded to a qualified procedure, by a welder who is qualified to that procedure. The procedure specification (WPS) and the procedure qualification record (PQR) are held in the QA pack, along with the welder qualification record (WQR) for every welder who worked on the package. Qualifications are typically to AS 1554.6 Part 1 or ASME IX, expire after the standard validation interval, and are revalidated by the qualification body.
Pickling and passivation records. Stainless duct after seam welding has a heat-affected zone where the chromium oxide passive layer has been disrupted by welding heat. Pickling removes the heat tint and the disrupted layer with a nitric-and-hydrofluoric acid mix, and passivation restores the passive layer with a nitric acid soak. The records show the bath chemistry, the immersion time and the rinse procedure. Without pickling and passivation, the welded seam corrodes preferentially and the duct life is shortened by an order of magnitude in service.
Pressure and leakage test reports. Each closed duct run is pressure-tested per AS 4254 or SMACNA HVAC Air Duct Leakage Test Manual to the specified class. For contaminant-zone duct the class is typically Class 6 (low leakage) or better, tested at the design positive or negative pressure. The test report identifies the duct run, the test pressure, the measured leakage and the acceptance result.
As-installed drawings and penetration register. The as-built drawing set shows the final duct routing, the fire damper locations, the access panel positions and the test point locations. The penetration register records each duct penetration through a fire compartment wall, the damper type, the certification reference (AS 1682 or UL 555), and the fire-stop seal detail.
The CASA or EASA surveillance audit interval is typically 12 months for an active maintenance organisation, and the documentation pack above is the package the auditor reviews. Operators who cannot find the WPS or the pickling record three years after installation have to either commission third-party verification of the existing duct or replace the run. SBKJ supplies the documentation pack with every contaminant-zone duct package, indexed by duct section number and held in PDF and physical-file form.
SBKJ machine configuration for an MRO duct package
The duct fabricator tendering on an MRO package typically needs three machine capabilities: a coil-to-duct auto line that can switch between galvanised and stainless, a spiral tubeformer for the mainline trunk, and a TIG seam-welder for the stainless service.
SBAL-V auto duct line, stainless-configured
The SBAL-V is SBKJ's auto duct production line. In its base specification it runs 0.5–1.5 mm galvanised coil and produces flanged rectangular sections at 18–30 m per hour, with TDF flanging, integrated longitudinal seaming, plasma or laser cross-cut, and downstream Pittsburgh-lock. For the MRO duty cycle the line is specified in the stainless-capable configuration: tool steel rollers hardened to 60 HRC, stainless-rated guides, drag-chain protected coil cradle, and a TIG seam welder option in place of the Pittsburgh-lock for the contaminant-zone duct. The line can switch between galvanised production for the hangar bay and stainless production for the paint and composite zones with a tooling changeover of approximately 90 minutes.
SBTF-2020 spiral tubeformer
The SBTF-2020 is SBKJ's heavy-duty spiral tubeformer, producing 80–2,000 mm diameter spiral duct in galvanised, stainless or aluminium at line speeds appropriate to the diameter. For MRO hangar bay applications the machine is typically run at 1,000–1,800 mm diameter, with galvanised coil for the occupied-bay supply and return, and with a separate run on 304 stainless coil for the fuel-system maintenance area extract trunks. The line produces a continuous Pittsburgh-lock spiral seam suitable for the design pressure in hangar mainline service (typically 750–1,500 Pa).
TIG seam welder
The contaminant-zone duct (paint, depaint, composite, hex-chrome, engine fuel) is specified TIG seam-welded along the longitudinal joint. SBKJ supplies a TIG seam welder station that operates in line with the SBAL-V or as a standalone bench, qualified to AS 1554.6 Part 1 / ASME IX with welder qualification records held against the procedure. The output is pickled and passivated to remove the heat-affected oxide and restore the chromium oxide passive layer on the stainless. The pickling line is part of the package and produces the duct ready for installation without further surface preparation on site.
Tooling, consumables and changeover
A typical MRO duct package contract is staged: the galvanised hangar bay duct is produced first on the SBAL-V and the SBTF-2020 in their default configurations; the stainless contaminant-zone duct is produced second, after a tooling changeover on the SBAL-V and a coil change on the SBTF-2020. The total package for a wide-body hangar with attached paint shop, composite bay and engine test cell typically runs 8–15 km of duct in galvanised and 2–4 km in stainless, produced over 14–20 weeks of plant time with a single shift on each line.
Cross-references and adjacent guides
This guide concentrates on the MRO heavy-maintenance facility. Several adjacent SBKJ guides cover related but distinct aviation and industrial domains:
Closing: the duct that survives the audit
The MRO heavy-maintenance facility runs on a 25–40 year asset life. The HVAC duct in that facility is rebuilt only at major refurbishment or after a failure. The duct that survives the asset life is the duct that was specified for the hazard, formed in the right material on the right machine, welded by qualified procedures and installed against an audit trail that the CASA or EASA inspector can read. The duct that fails the asset life is the duct that was specified for cost only, formed in galvanised across the contaminant zones, joined by self-tapping screws and gasket, and installed against a paper trail that nobody can find when the rust shows up in the next topcoat.
SBKJ Group, headquartered at Box Hill North VIC, specialises in the duct machinery — the auto duct line, the spiral tubeformer, the TIG seam welder — that produces the durable end of that distribution. Our engineering team works with consulting engineers, MRO operators and duct contractors on the specifications above, and we ship into the defence and civilian MRO supply chain across Australia and into the export markets that recognise the same NFPA, EASA and ASHRAE framework.
If you are scoping an MRO duct package and want a line-item bill of materials against the framework in this guide — galvanised tonnage, stainless tonnage, weld procedure scope, machine-time estimate — contact the SBKJ Engineering Team for a no-obligation review.
