A small aircraft fuselage is not a car in a different paint scheme. Before any water reaches it, three separate things have to be true. The aircraft's own documents have to permit the work. The surface and its openings have to be in a condition that can take heat and water. And the hangar has to be able to hold the machine, supply it, and carry the water away. Each of those questions has a different owner, and the person who returns the aircraft to service carries the consequences of any water that ends up inside a lap seam. Work through them in the order the constraints bind, starting with the ones that can veto the job, and finish with a written answer for each zone rather than a general yes.
Read what the aircraft's own paperwork already decides
Start with the documents, because they can end the discussion before it costs anything. Many owner's manuals and type maintenance documents name a cleaning fluid, describe a method, and warn against directing water at openings, instruments or control surfaces. Where the manual names a method, that is the boundary of the job. No cleaning contractor and no equipment supplier can talk past it, and a supplier who has never read your manual should not be asked to.
Where the manual is silent, treat the surface as unproven rather than as approved. Silence is not permission. It is a question, and questions belong on a test list rather than in a plan.
Then establish what the finish actually is. An original finish applied and cured by the manufacturer behaves differently from a shop respray, and a respray usually arrives with no published bake temperature, sometimes a different primer, and decals laid over somebody else's clear coat. Ask the paint shop for the coating system's continuous temperature limit. The comparison that decides your risk is between that limit and the temperature that reaches the skin, not the number printed on a machine's data sheet.
Add the modifications. Avionics, antennas, static ports, extra lighting and any repair that drilled new holes have each added a path for water. An approved kit fitted a decade ago may carry no cleaning instructions at all. List the penetrations zone by zone, because those are the places your evaluation is really about; the flat panels in between are the easy part.
The output of this step is one page: what the documents permit, what they prohibit, and what they do not address. Anything in the third column goes to a test, not into the schedule.
Walk the surface and grade its condition, zone by zone
Do the walk with a flashlight held low rather than under hangar lights. Raking light shows lifting edges, crazing and chalking that a ceiling light hides. Move zone by zone: the crown, leading edges, the lower skin, every decal and the registration numbers, lap seams and their sealant, bare-metal wear areas, access panels and fairings, windshield and window edges, door and baggage seals, hinge lines and control surface gaps, static ports and probes, drain holes, avionics cooling inlets and fuel vents.
Before anyone discusses heat, establish what the fuselage is made of. Metal skins, fabric covering with its dope or adhesive, glass and composite fairings, and a radome with its conductive coating respond differently to the same jet of steam. A fabric-covered airframe carries cleaning instructions from its covering manufacturer, and those are not the same document as the airframe manual. If you cannot say which of these you are about to heat, you are not evaluating; you are guessing.
One rule covers most of the risk. Anything held in place by adhesive, sealant or a gasket is heat-sensitive until that particular aircraft proves otherwise, and the proof includes a re-check once the surface has cooled. Lifted decal edges are rarely reported on the day they happen.
Grade what you find into three lists. Clean as it stands. Clean with restrictions, meaning dry mode, short dwell, masked openings, or a smaller area than you wanted. And repair first: paint already lifting, sealant gone brittle, a cracked window seal, a loose rivet head. Those are maintenance findings, and heat reveals a failing finish rather than improving it. A job that turns into a maintenance item was not cheap, however brief the washing appeared at the time. Note bare metal separately as well. Once the dirt and whatever was protecting it are gone, bare aluminium and steel can show corrosion within hours, which is why the check afterwards needs a time and a name attached to it.
Trace the water before it arrives
Steam condenses the moment it touches anything cooler than itself, so the wetted area is always larger than the area you sprayed. Decide where that water goes before the work starts, rather than while it is happening.
On an airframe it travels under rivet heads, into lap seams, behind decal edges, along hinge lines, into door and baggage gaps, under cowlings, into avionics bays, and down to the lowest points along the keel line and at the wing roots. Write those places down as inspection points before the first pass, so that afterwards you are checking a list instead of hoping you remember.
Confirm that the drain points named in the manual for the area you are working on are clear, and treat drying as a step of the job rather than as a hope. Decide what dry enough means in advance: dry enough to close cowlings, dry enough to refit inspection panels, dry enough for someone to sign the aircraft back into service.
That signature matters more than it sounds. The person who returns the aircraft to service should be the person who confirmed that no water entered the structure. That is a maintenance judgement, not a cleaning procedure, and it should be arranged before the job is booked rather than discovered at the end of a shift when the hangar is closing and the aircraft is still damp.
Mode selection is the practical control you have. Dry steam carries little free water and belongs near openings, avionics and anywhere you cannot actively dry. Wet steam carries more heat and more water, and is only acceptable where water can be seen, led away and dried within the same shift. A switch on a machine is not a plan; which surface uses which mode is. And if the aircraft goes back outside while temperatures are falling, add one question to the walk: what is still holding water somewhere it will freeze. Gaps, seams, drains and control surfaces are where that answer lives.
Evaluate the hangar as a work position
Fixed equipment works from one position while the job reaches out to it, so the position is the decision. Mark where the machine will stand on the hangar floor, then measure from that mark with a tape to the farthest surfaces you intend to clean: the far side of the crown, the keel line, the tail cone, the opposite side of the fuselage. Reach belongs to the position, not to the machine, and a floor plan will not tell you what a tape will.
The crown is the surface that usually fails this test. Decide now whether you will use a platform, a ladder, a second machine position, or a smaller area. Leaning on a thin panel to gain a metre leaves marks, and those marks belong in the record as much as any cleaning result does.
Then the services, each with a named person. A 6 kW load on 220 V is continuous heat, and whether the circuit is adequate is your electrician's call rather than the machine's: the protections listed on the product page protect the machine, not the hangar wiring, not the person holding the wand, and not the aircraft parked alongside. A tap-water inlet means water has to reach the position, and the tank is a buffer between walks to a tap rather than a supply. Where the runoff goes, whether it needs an interceptor and who decides that are questions for the airport operator and the local drainage authority, not for an equipment supplier. A closed hangar needs air movement while steam is being generated, and the inspection afterwards needs low-angle light, so plan for a lamp rather than relying on the ceiling fittings.
Check the route in, and the corner where the machine will live, against its real envelope rather than against a drawing. Machines rarely fail to arrive; they more often fail to fit the position they were bought for, and a hangar is a hard place to redesign after the delivery.
Read the data sheet for what it can and cannot answer
Reading a specification is a way of finding out whether a machine belongs in this hangar at all. The same discipline applies to any comparison of aircraft exterior cleaning equipment: the useful question is never how strong a machine is, but what it can be shown to do on your skin, at your distance, inside your window.
Take the Fixed Steam Cleaning Machine PowerDe HW02 page as a worked example. It lists a steam temperature of 160 degrees Celsius and a vapour pressure of 0.8 MPa. On a saturated machine those are two readings of one condition rather than two separate capabilities, and the question worth asking is whether they were measured at the boiler or at the nozzle, because a long hose changes what arrives at the surface. Steam gives up its energy when it condenses on something cooler than itself, so distance, pass speed and dwell decide the result more than any nameplate does.
The same page lists 6 kW of heating power and an 80 L tank in SUS304 stainless steel with a tap-water inlet. Read that as a work cycle and a walk to a tap, not as an area covered per hour. The dimensions, 1,200 x 800 x 1,200 mm including handles, describe the machine rather than the space it needs; add room to reach it for servicing and to walk around it with a hose in hand.
Hose and lance length, warm-up time, working time per tank and recovery after refilling are not published. Ask for them in writing before you fix the layout, because they are the numbers that decide whether a plan is honest or merely optimistic.
The HW02 page documents vehicle exterior washing at fuel stations and parking areas, interior vacuuming and disinfecting, and engine-compartment cleaning. Aircraft are not on that list. Every figure quoted above comes from a page written for vehicles, so treat airframe applicability as unconfirmed until a manufacturer answers in writing, and treat the unpublished numbers as questions rather than as gaps to fill in yourself.
Write down what clean means before the first pass
Agree the acceptance wording with whoever inspects the aircraft, in the language they already use. Phrases that hold up in practice: no lifted decal or registration edges under low-angle light; no blistering or softening at chipped bare-metal edges; no visible difference in gloss against the untreated half when viewed from the same angle; no change to sealant surfaces; no water standing in the seams and low points you marked; no flash rust on bare metal when you look later in the day.
Run the first pass as a test rather than as a job. Tape a line across a representative panel and treat one half, leaving the other as a reference, so that a poor result tells you whether the method failed or the crew did. Photograph fixed points from the same distance and angle, so the comparison does not depend on anyone's memory. Change one variable between passes, dwell before distance, and write down which one you changed.
Check twice, because the two failures that matter arrive late. Heat effects on adhesives and coatings often show up hours afterwards, and water hidden in a seam reports itself the next morning as a streak. Put both checks on a schedule with a time and a name against them, or they will not happen.
If a check fails, record it as what it is. A lifted decal edge is a maintenance finding, and the record should say so plainly. A crew that reports it has done its work; a crew that hides it has only moved the cost to the next inspection.
What the evaluation cannot settle
Some things stay outside the scope however carefully the earlier steps are done, and they are worth listing so that nobody assumes they were covered.
- Inside any system. Surface work does not open an engine, a control run, a fuel or hydraulic line, or an avionics stack. Those are separate tasks with separate approvals.
- Hygiene. The disinfecting language on equipment pages refers to vehicle interiors. A hygiene claim belongs to your own protocol and your own verification; heat leaving a machine is not a certificate.
- Inspection. Cleaning exposes lifted sealant, loose rivet heads and corrosion streaks in seams. Those findings belong to the licensed maintenance organisation, and a crew that finds them has helped rather than caused trouble.
- The documents. The manual's cleaning provisions and the coating manufacturer's limits are not negotiable by a supplier of cleaning services or of cleaning equipment.
One more item belongs in the record and is usually forgotten: the aircraft's own log. A wash carried out on an airframe is a fact about that airframe, and the next owner will want to know who did it, by what method, and what was found afterwards. An unrecorded wash is invisible at the next inspection and unforgivable if it caused something.
Turn the evaluation into one page and one test
The output of all of this is short. One page, zone by zone, saying proceed, proceed with restrictions, or defer, with a name against each answer and a list of the questions still open. That page is worth more than any specification you could have read first, because it is the only document that describes your aircraft, your hangar and your window.
Then run one panel on one aircraft, with the crew who will actually do the work, timed from the moment the machine is switched on to the moment the last inspection panel is refitted and the release is signed. Half a panel taped, fixed photographs, the acceptance wording written before the first pass. If the numbers do not fit the window you have, the honest options are a smaller area, a second machine position, or a different method. Asking the crew to work faster is not one of them.
Send that record to whoever you are talking to about equipment, together with the open questions: nozzle-end values with the hose length you would actually be given, lance length, warm-up time, runtime per tank, and a written answer on your covering type and coating system. A summary invites a brochure. A record invites answers.
The evaluation costs an afternoon and a tape measure. The alternative is a wash that becomes a maintenance item, discovered by the person who signs the aircraft. If you want written answers on the equipment side, the PowerDe cleaning equipment pages show what is specified today, and the contact page is how to reach the team who can answer for a particular airframe and hangar.
