scottco189 Posted July 8, 2008 Share Posted July 8, 2008 Got a call from a fellow maintainer in Iraq asking for help with a pressurization problem. The 130 pressurizes fine in auto and manual until underfloor heat is turned on. When it is turned on, the guage climbs 1000 feet, which is normal. However, it fluctuates 700-1000 feet the whole flight. This problem has came down over 4 times and almost the whole system has been swapped out. Any suggestions on where they should start? Quote Link to comment Share on other sites More sharing options...
SEFEGeorge Posted July 8, 2008 Share Posted July 8, 2008 You have to forgive an old brain from over 30 years ago but one of my 1st overwater flights I had a similar event happening. The underfloor heat was cycling. Can\'t remember the actual fix though. Maybe one of these smart maintainers came give you a clue to the actual unit. Quote Link to comment Share on other sites More sharing options...
Jake Posted July 9, 2008 Share Posted July 9, 2008 There are two thermostats that operate the underfloor heat valve. One (the overheat, set to 180 degrees if memory serves) will close the valve and subsequently open it when the temp has reduced (do not remember what temp). This will cause the cabin pressure to cycle. I suspect you have changed this as your post says you have changed most of the components. Quote Link to comment Share on other sites More sharing options...
donwon Posted July 9, 2008 Share Posted July 9, 2008 It\'s been over 40 years for me. Did the A model have the under floor heat? Quote Link to comment Share on other sites More sharing options...
casey Posted July 9, 2008 Share Posted July 9, 2008 No under floor heat on A\'s. Quote Link to comment Share on other sites More sharing options...
Steve1300 Posted July 9, 2008 Share Posted July 9, 2008 Jake wrote: There are two thermostats that operate the underfloor heat valve. One (the overheat, set to 180 degrees if memory serves) will close the valve and subsequently open it when the temp has reduced (do not remember what temp). This will cause the cabin pressure to cycle. I suspect you have changed this as your post says you have changed most of the components. If the cycling is from the overheat stat, the control thermostat would have to be bad as well, wouldn\'t it? Sounds like two bad parts to me. Quote Link to comment Share on other sites More sharing options...
donwon Posted July 9, 2008 Share Posted July 9, 2008 I knew there was a reson I never walked barefoot on that floor while we were inflight. Quote Link to comment Share on other sites More sharing options...
SEFEGeorge Posted July 9, 2008 Share Posted July 9, 2008 Hated the underfloor heat fan on pre-72\'s (I think it was). So damn loud when I had to go in back. Quote Link to comment Share on other sites More sharing options...
Steve1300 Posted July 10, 2008 Share Posted July 10, 2008 Thinking back on it, the problem as explained - does the pressurization never stop increasing? You say that it stays between 700 and 1000 feet the whole time? It really sounds like the underfloor heat is working OK, but they can go back and feel the floor if they think the overheat switch is cycling. It will surely be hot to the touch. Quote Link to comment Share on other sites More sharing options...
GVS Posted July 10, 2008 Share Posted July 10, 2008 donwon wrote: It\'s been over 40 years for me. Did the A model have the under floor heat?Under floor heat?Most times A models didn\'t have much over floor heat. Quote Link to comment Share on other sites More sharing options...
NATOPS1 Posted July 10, 2008 Share Posted July 10, 2008 You have a temp control box (continuous more heat signal)or motor driven UFH temp control valve stuck open(more likely). Most people forget about the temp control box for UFH.... The UFH temp control box is under the floor. It senses the temp and opens and closes the motor driven valve to maintain 75F. If the valve stays open the overheat T-stat (180F) will cause the air actuated valve to close (this portion is working) as well as drive the motor driven valve closed. This will cause the pressurization to cycle... normal operation is for the motor valve to be open and then close more (less heat signal) to limit the temp... that’s why you don’t see large swings in pressurization... sounds like your overheat T-stat is actuating (working) and causing at least your air actuated valve to close, thus causing large press swings... In the OFF position the motor driven valve should be closed, check and see if it is... If not the valve is allowing bleed air to flow freely until the over-temp T-stat is closing the other valve. Quote Link to comment Share on other sites More sharing options...
Zaherk Posted November 12, 2009 Share Posted November 12, 2009 Any feedback on this snag with the underfloor heat would be appreciated as we've got a similar problem on our Baby. Quote Link to comment Share on other sites More sharing options...
TC001 Posted November 16, 2009 Share Posted November 16, 2009 What model aircraft is it? Didn't see that in the post anywhere. Quote Link to comment Share on other sites More sharing options...
Dan Wilson Posted November 16, 2009 Share Posted November 16, 2009 If memory serves (and more often than not it don't) cycling underfloor heat was usually attributable to the low limit thermostat being bad and forcing it to cycle off the high limit thermostat (140F ?). Dan Quote Link to comment Share on other sites More sharing options...
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