Dehumidifying to improve Avionic Reliability

[Aeronut]

Does anyone know if airlines dehumidify their aircraft avionics on the ground to improve reliability?
I am looking at doing a project on this and could do with a couple of steers as to who does it and if so how successfully.

I think it would only apply to long haul that stay on the deck a little more than a low cost airline 20 minute turnround?

Any info would be a great help, thanks.

[Golden Rivet]

Never heard of that one before. I would imagine that humid air would be better for heat transfer, latent heat of evoparation and all that....

Some aircraft do however refrigerate the equipment cooling air for improved reliability

[STC]

All required avionics equipment are tested and qualified for specific operating conditions. Those that qualify for installation in humid conditions or conditions that promote condensation, are protected by various means, most common of which is applying a protective coating.

Using humidity control is far too complex and cost prohibitive. Besides, aircraft air is generally very dry. The main problem with humidity and avionics comes in the form of condensation when cold avionics components are exposed to relatively humid warm air as the aircraft is started. After a few minutes of operation, the equipment temperature rises above the dew point and further "humidity" problems are no more.

[Milt]

High Humidity and Avionics

Anyone who has been involved with tropical trials of aircraft at places like Darwin, Oz will know that you can pour water from a recently flown black box.

We.RAAF, had lots of trouble with avionics in Mirage and F-111 despite conformal coating of circuit boards.

Water seperators in the ECS help but potted circiuts, conformal coatings and sealed connectors add up to greater reliability.

Aircrew suffer also as the ECS systems are prioritised to look after the avionics before the crew!!

[Blacksheep]

Despite twenty three years of maintenance in Borneo where the ambient temperature is a fairly steady 32 degrees celsius and the typical relative humidity value lies between 90 and 100 per cent I have never known any avionics equipment to be a source of water such as Milt describes. Perhaps he is referring to military aircraft ? The operating conditions in the cabin of a typical airliner are very dry, with relative humidity values below 40 per cent. The equipment racks are within the pressurised zone of the aircraft and share the same controlled temperature and low humidity environment. On the ground, the air conditioning system removes water in the water seperators, keeping the humidity down to comfortable levels. In the rare event that an aircraft remains on the ground without air conditioning for any length of time, the R.H in the aircraft rises to match the ambient levels but the relatively high equipment temperature prevents any condensation in the equipment racks.

I don't believe that de-humidifiers are justified in any airliner that I know of, in fact the opposite is true. There is a strong case for installing humidifiers for passenger comfort reasons.

There is however a strong case for storing electronic equipment in non-airconditioned but de-humidified storage conditions. Here in the tropics, it is very common to experience heavy condensation in avionic equipment just drawn from storage.

[mono]

My current airline is trialing a dehumidifier system. This is however not for the avionics but to prevent condensation around the a/c doors and crown skin area.

It uses a rotating drum of silica gel crystals to dry the air which is then ducted to the skin area around the doors and a smaller amount to the crown skin area.

We are testing it as our airbusses (A321) have suffered from moisture pooling around the #2 and #3 door areas as a result of condensation. This has lead to carpet staining and insulation blanket deterioration and pax complaining of the cold as a result.

Not sure how effective it is but the carpet stains seem to be less!!

[Kanga767]

I know a Bizjet that whenever hangared (in the tropics of Australia) always has a dehumidifier hooked up to it and the aircraft kept closed.

I was told it helped out with avionics reliability.

K

[oldships]

Aeronut,

I use DH on Warships in preservation. I understand from the manufacturers that it is used, with considerable benefit to avionics, by the Swedish Air Force.

[Carl Kendrick]

hi,

We Currently use DH on 216 Sqn RAF Tristars for our Avionics Equipment, plus we also used them on Nimrods.

[Blacksheep]

Apart from major checks, proper airliners fly most of their lives, with short breaks for changing the SLF, baggage and crew together with refuelling and tyre kicking by maintenance. An aircraft may spend less than two hours on the ground at main base and no more than an hour at a time anywhere else. Power stays on throughout a turn-round and the avionics kit remains constantly powered for weeks at a time; in these circumstances avionic equipment doesn't get cold enough for condensation to occur, even when the equipment cooling system air supply is at a high relative humidity level. On the other hand, military machines like the RAF Tri-Stars and many GA aircraft may spend days on end parked in the cold with ground power applied intermittently. Thats' why I qualified my earlier response.

When I left military aviation for civil life I was initially astonished by the hours racked up by airliners. In the RAF our VC10s had around 8,000 hours on them after 12 years in service. In contrast I encountered Big Airways B747s racking up 6,000 hours in a single year, with most of our B707s and VC10s having more than 60,000 hours on them. De-humidification would have little effect on the avionics kit of any aircraft in which the equipment seldom, if ever, gets cold, although a case could certainly be made for de-humidification to reduce crown and belly skin corrosion caused by condensation of moist air on the cold skin.

Early model B747-100 'Classics' were equipped with humidifiers that operated above 30,000 feet where the aircraft spend most of their lives. These were intended to improve passenger/crew comfort but were first de-activated then eliminated on the basis of the effects of increased condensation on the aircraft structure. I know of no significant improvement in avionic reliability that resulted from eliminating these humidifiers. As I said, airliner avionics kit seldom gets cold enough to be bothered by humidity. Dust is a much bigger problem and I for one would be interested in more efficient dust filtration in equipment cooling systems. Air entering the cabin may be at almost 'Clean Room' standard but by the time it gets to the equipment cooling system it is contaminated by dust from the cabin which eventually clogs the equipment filters, reducing cooling efficiency and leading to equipment overheat.

[Aeronut]

Thanks for the info everyone. :ok:

The kind of use I imagined is best described by a couple of posts relating to aircraft such as the RAF TriStar and Nimrod. Unlike a commercial airliner I imagine they spend a fair amount of time waiting on the deck in a more humid environment than at
36, 000ft. So I suppose that there are no airlines that dehumidify on the ground for reliability reasons.

The question is does it actually make the Nimrods or Tristars more reliable in practice? Less liable to avionics problems on start-up? Carl, any views?

[Carl Kendrick]

Aeronut,

Being a "lecky" (who has yet to do a Avionics Multi Skill Course) i plus new to Tristar, i have absouloutly no idea what difference it makes, all i know is the avionics guys just tend to "take box out - put box in" when they have a fault?


sorry!:ouch:

[Aeronut]

Thanks Carl.

I wonder how much dehumidifying prevents the need to change those black boxes which cost a small fortune every time they are removed. If it saves just a few then it is a good investment. Nice to see the airforce protecting taxpayers investment in avionics. Seems they are the only aircraft fleet that is applying this idea to improve in use reliability as opposed to storage.

The theory is that if a nice warm circuit board is switched off at ground level (High relative humidity) and then cools, condensation forms and this leads to corrosion of conductors or short circuits. If, as soon as possible after shutdown, dry air is pumped in this can be avoided. First few moments after shutdown are critical when hot, then maintain the dry atmosphere until just before next start.
I suspect that the airforce Nimrods have especial need operating over/near the oggin.

Overall, I suspect limited current application by airlines but there may be reliability potential, which is exactly what I hope to study for a project for a course I am taking. I just need to find an airline with experience or willing to experiment!!


I am still keen to hear if any airline dehumidifies avionics for reliability purposes- possibly long haul jets that may spend longer on the deck than short haul??

[Nineiron]

The effect of humidity on avionics reliability was researched in the 1960s by the Marconi company. They produced a range of avionics where each box was filled with dry nitrogen and sealed.The advance in component technology (especially capacitors), sealing of printed boards etc. subsequently outweighed any advantage in doing this. I have cautiously put power on old aircraft that have stood outside for up to a year and dripping with condensation inside. A few problems occur, mainly with corrosion or filming on mating connector surfaces. A bit of heat, ventilation and use, usually sorts everthing out.

[Blacksheep]

I used to work on VC10s at Brize Norton. The aircraft would often spend a couple of days between filghts and every de-brief brought us a dozen defects [or snags as they were called in the RAF]. Then came the Turkish invasion of Cyprus. We had all spare aircraft flying round-the-clock evacuating civilian, stopping only for fuel and the occasional wheel change. [This is of course how they were designed to be used}. We were dreading the workload when they eventually finished the evacuation. Surprise, surprise! - when the evacuation ended we had only three snags to clear across the lot of them. Airliners like to fly and as long as they keep flying, the avionics kit gives very few problems.

[Aeronut]

Thanks 9 and Blacksheep, I think you are right and this is probably why the RAF have decided to use DH on their under utilised fleets. Still can't find an airline that uses it, probably none do??!?!?

[vector4fun]

Just a bit of barely related trivia. At many FAA ATC facilities in the drier areas of the U.S., Humidifiers have had to be installed in the equipment rooms. Seems the conditioned air in some facilities became soooo dry, the equipment was prone to problems caused by static discharges. At many places, we have had to use humidifiers, anti-static mats and carpets, and re-done the grounding.

Used to be able to knock out a controller's entire communications by sliding your feet on the carpet a few times, and then touching his com panel....:uhoh:

[lame]

Well McDonnell Douglas used to think so. ;)

At Southern Cross (Compass 2) we were having quite a lot of avionics problems with the MD80s when overnight here in Brisbane. :(

After investigation into the problem by McDonnell Douglas, their only advice was to leave the Aircraft air conditioned all night. :rolleyes:

[insty66]

De humidifiers are currently de rigeur on the Tornado fleet after a GAF study found that they achieved an approximatley 30% reduction in Avionics/Electronic failure. This means less money on spares etc, which the bean counters like.

As a slight aside there is a study currently being undertaken to determine if a/c left out side are more or less serviceable than those protected from the elements. Wonder what the conclusions will be.

[JetFixer]

We operated some BAe 146s in Mexico prior to us recieving them they had been stored in the desert.

During the first few months after delivery we had various electrical avionic glitches. Dust and lint was found in various PCBs and relay supports once this had been cleaned off we had few problems.

In my opinion the dust and crap helps to retain the condensation once a cold soaked air conditioned aircraft lands and the doors are opened.

I suggest that a good vac and brush out of these areas follwed by thorough cleaning of affected components with brushes and contact cleaner will do the trick.

Obviously its not a good idea to break down systems to do this. We thoroughly cleaned system after they were reported by the crews. By attritition all of the systems became good over time.

Another thing we found out is that pitot /static covers hold moisture in the lines, we had a pretty high turnover of servo/alts. I dont think that any manufacturer would suggest you leave these off. The operator I am with at the moment always leaves pitot static covers off and we have few problems.

[tonkatechie]

Like insty said, DH is pretty much the done thing on Tornados now. Bear in mind that the avionics bays will have conditioned air bled from the engines throughout flight, so fitting a DH as soon as possible replicates the longhaul jet characteristics you mention. Also, the German airforce study that was mentioned went as far as dehumidifying the engines whilst on the ground by using specially adapted intake blanks. The photos comparing a stripped down DH engine to a 'normal' one of the same use showed a marked difference in wear and corrosion. The corrosion avoidance benefits would also apply to the rest of the airframe surrounding the avionics kit that is susceptible to creating condensation.
Hope this helps, and have you tried contacting the GAF about their study? You know how ruthlessly efficient those Germans are...!:ok:

[Aeronut]

Well, thanks for the leads. In case anyone is interested, I am well into the project now and it seems that for aircraft that remain on the ground a fair deal, dehumidifying has a massive positive effect on the life of your avionics. We are talking almost doubling their life in many cases.

I got hold of reports from the German Air Force, Swedish Air Force , US Marine Corps trials of dehumidification the results are quite impressive.

In most cases the investment paid off in under a year. The only downside I can see is the effort to appply the dehumidifier and convince those whose responsibility it is that it is so worthwhile.

I would love to highlight fleets other than air forces that might respond to dehumidification. Anyone know any routes that see an aircraft locked up and left for a while at the end of a flight. Most airlines turn around so fast that dehumidifiying is pointless but I would like to highlight an airline that, because of wind conditions say, delays its return trip leaving it on the ground for more than 12 hours and doesn't air condition throughout. Maybe a long haul route. Anyone know of any such operation?


This would give my project an airline angle, which I am supposed to be focusing on.

[UNCTUOUS]

Aeronaut

Please see your PM's

UNC

[winglit]

Blacksheep, I too used to work on VC 10s at Brize, but unlike you have had an experience where condesation cause repeated failures in the autopilot system, mainly lat and long amps. We were playing around in Malaysia with the tankers doing 4 or 5 hour sectors. Then on the ground overnight. Come the following morning during crew-in, AP would fail to engage. Replacement on long or lat amps would fix it, but when taking those bloody heavy boxes out from the radio racks, there was water coming out them! Relative humidity was 100% and it was monsoon season and very wet and hot. We later found that the main cause of this problem was the use of airconditioning carts plugged into the AC system. This must have cause the radio rack cooling to get too cold and consequently condensation when the cart was removed and the aircraft downpowered for the night.

On the subject of dehumidifying aircraft, my airline is to trail a system on our B757s and A320s. 757s are notorious for "plane rain" and the usual cause for TCAS and enhanced GPWS failures due to moisture ingress in the antenna connectors. However this is not the motivating factor to trail this mod. The expected result is weight saving of over 500kgs of water saturated in the insulation.

[goferthehammer]

In the SH fleet we have been using DH equipment for a while now and have found a reduction in first flight of day failure since starting to DH.