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.
Last edited by Blacksheep; 7th April 2004 at 03:57.