A B737-400 first officer reports noted windshields frosting over. Found window heat switches in 'off...

Attributes
ACN	701363
Time
Date	200606
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	agl single value : 0
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B737-400
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	701363
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	aircraft equipment problem : critical maintenance problem : improper maintenance non adherence : published procedure non adherence : far
Independent Detector	other flight crewa other flight crewb
Resolutory Action	none taken : detected after the fact
Consequence	other other
Factors
Maintenance	contributing factor : schedule pressure performance deficiency : testing performance deficiency : non compliance with legal requirements
Supplementary
Problem Areas	Aircraft Chart Or Publication Environmental Factor Flight Crew Human Performance Maintenance Human Performance
Primary Problem	Maintenance Human Performance

Narrative:

In cruise from ZZZ1 to ZZZ we noticed that the windshields were frosting over so we checked the window heat switches and found them all off. This was our third leg in this plane and we know they were on for the first leg and that neither of us had turned them off at any time. Between the first and second legs while on the ground in ZZZ; mechanics came into the cockpit to work on various squawks. These included re-lamping one of the pitot heat off lamps near the window heat switches; jumpseat oxygen mask/smoke goggles and also having the windows cleaned. They must have turned the window heat switches off to clean the windows. They never turned them back on; and we never thought about them touching the switches for any reason. Our through-flight procedures don't require rechking window heat and we didn't see it. In my mind; the mechanics did nothing that would affect our system so I had no reason to redo the complete cockpit flow and checklist. I hope the mechanics are reminded to not change cockpit confign on through-flts. This is potentially very dangerous.

Original NASA ASRS Text

Title: A B737-400 FO RPTS NOTED WINDSHIELDS FROSTING OVER. FOUND WINDOW HEAT SWITCHES IN 'OFF' POS. BELIEVES WINDOW HEAT TURNED OFF DURING MAINT PERFORMED BEFORE SECOND LEG.

Narrative: IN CRUISE FROM ZZZ1 TO ZZZ WE NOTICED THAT THE WINDSHIELDS WERE FROSTING OVER SO WE CHKED THE WINDOW HEAT SWITCHES AND FOUND THEM ALL OFF. THIS WAS OUR THIRD LEG IN THIS PLANE AND WE KNOW THEY WERE ON FOR THE FIRST LEG AND THAT NEITHER OF US HAD TURNED THEM OFF AT ANY TIME. BTWN THE FIRST AND SECOND LEGS WHILE ON THE GND IN ZZZ; MECHS CAME INTO THE COCKPIT TO WORK ON VARIOUS SQUAWKS. THESE INCLUDED RE-LAMPING ONE OF THE PITOT HEAT OFF LAMPS NEAR THE WINDOW HEAT SWITCHES; JUMPSEAT OXYGEN MASK/SMOKE GOGGLES AND ALSO HAVING THE WINDOWS CLEANED. THEY MUST HAVE TURNED THE WINDOW HEAT SWITCHES OFF TO CLEAN THE WINDOWS. THEY NEVER TURNED THEM BACK ON; AND WE NEVER THOUGHT ABOUT THEM TOUCHING THE SWITCHES FOR ANY REASON. OUR THROUGH-FLT PROCS DON'T REQUIRE RECHKING WINDOW HEAT AND WE DIDN'T SEE IT. IN MY MIND; THE MECHS DID NOTHING THAT WOULD AFFECT OUR SYS SO I HAD NO REASON TO REDO THE COMPLETE COCKPIT FLOW AND CHKLIST. I HOPE THE MECHS ARE REMINDED TO NOT CHANGE COCKPIT CONFIGN ON THROUGH-FLTS. THIS IS POTENTIALLY VERY DANGEROUS.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

A B737-400 first officer reports noted windshields frosting over. Found window heat switches in 'off' position. Believes window heat turned off during maintenance performed before second leg.