Narrative:

Event happened on the ramp. We were preparing for our flight to ZZZ1 and entered the cockpit to start our preflight. It was cold so we did the fire test and started the APU. We then turned the packs on and it instantly started to fill with what I initially thought was condensation but within seconds it became very thick and I realized I was having a hard time breathing at which point I quickly shut-off the packs and told the first officer to get out of the cockpit. The cockpit had filled with smoke (hydraulic vapor or burning hydraulic fluid that was pushed into the pack system). We waited for the smoke/fumes to dissipate and then we re-entered the cockpit; opened the windows; wrote up the problem in the maintenance log and called maintenance control to let them know what happened. At this point we didn't yet realize that the smoke/vapor was in fact hydraulic fluid coming through the pack system. Maintenance came to the airplane; we explained what happened and we stepped off the plane while they trouble shot the problem. I re-approached maintenance while they were walking around the pack and wheel well area at which time it was explained to me that what had happened is earlier in the morning; maintenance had serviced the hydraulic fluid; only it was severely over serviced; well beyond the full mark which then pushed fluid into the pack area; so when we turned on the packs it vaporised/burned the hydraulic fluid and pushed it through the entire system and into the cockpit ventilation. Maintenance then wiped and cleaned up as much hydraulic fluid in the wheel well and pack area and then turned the packs on to try and burn off any residual hydraulic fluid in the system with the hope of getting the plane back into service. I watched from a company van 50+ feet away while maintenance ran the packs with all doors and windows open while smoke/vapor billowed out of the doors and windows. After approximately :20 min they told us they thought they had removed everything from the system and would we take a look and see what we thought. We still had all our personal items in the cockpit so we re entered the cockpit; I turned off the packs; waited a couple minutes and then turned the packs back on at which time we started to get the fumes and vapor/smoke again at which point I immediately turned the packs back off and told maintenance there was no way we were going to be able to use the airplane. I called operations and informed them of everything and that we were not going to be able to take the plane and asked if chief pilot or anyone from management was in the office; I was told no one was going to be in for a few hours. During the course of everything; I noticed I was starting to have an irritation to my eyes; throat and a skin irritation on my forehead. After cancelling the flight and heading back to the company building; the irritations became more noticeable and I contacted operations asking for the crew to be released and that I needed to go take a shower and clean up. I then contacted the safety department telling them what had happened and that I would be submitting a report later in the day after I went back to my apartment for a shower; dispose of my contacts and take a nap because I was having a headache. Event occurred due to maintenance overfilling the hydraulic fluid which then reached the pack system and entered the ventilation system into the cockpit; according to our maintenance personnel. Check airman. Airline transport pilot.

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Original NASA ASRS Text

Title: A Check Pilot reports that he and the First Officer started a B737 (2nd generation) APU on a cold night then turned on the Air Conditioning packs. The cockpit and cabin immediately filled with very thick smoke/fumes thought to be hydraulic vapor or burning hydraulic fluid. Same situation occurred when Maintenance and Flight crew ran the packs two more times with APU on. Captain experienced eyes; nose; throat; forehead irritations and headaches. Flight cancelled.

Narrative: Event happened on the ramp. We were preparing for our flight to ZZZ1 and entered the cockpit to start our preflight. It was cold so we did the fire test and started the APU. We then turned the packs on and it instantly started to fill with what I initially thought was condensation but within seconds it became very thick and I realized I was having a hard time breathing at which point I quickly shut-off the packs and told the FO to get out of the cockpit. The cockpit had filled with smoke (Hydraulic vapor or burning hydraulic fluid that was pushed into the pack system). We waited for the smoke/fumes to dissipate and then we re-entered the cockpit; opened the windows; wrote up the problem in the Maintenance log and called Maintenance Control to let them know what happened. At this point we didn't yet realize that the smoke/vapor was in fact hydraulic fluid coming through the pack system. Maintenance came to the airplane; we explained what happened and we stepped off the plane while they trouble shot the problem. I re-approached Maintenance while they were walking around the pack and wheel well area at which time it was explained to me that what had happened is earlier in the morning; maintenance had serviced the hydraulic fluid; only it was severely over serviced; well beyond the full mark which then pushed fluid into the pack area; so when we turned on the packs it vaporised/burned the hydraulic fluid and pushed it through the entire system and into the cockpit ventilation. Maintenance then wiped and cleaned up as much hydraulic fluid in the wheel well and pack area and then turned the packs on to try and burn off any residual hydraulic fluid in the system with the hope of getting the plane back into service. I watched from a company van 50+ feet away while Maintenance ran the packs with all doors and windows open while smoke/vapor billowed out of the doors and windows. After approximately :20 min they told us they thought they had removed everything from the system and would we take a look and see what we thought. We still had all our personal items in the cockpit so we re entered the cockpit; I turned off the packs; waited a couple minutes and then turned the packs back on at which time we started to get the fumes and vapor/smoke again at which point I immediately turned the packs back off and told Maintenance there was no way we were going to be able to use the airplane. I called Operations and informed them of everything and that we were not going to be able to take the plane and asked if chief pilot or anyone from management was in the office; I was told no one was going to be in for a few hours. During the course of everything; I noticed I was starting to have an irritation to my eyes; throat and a skin irritation on my forehead. After cancelling the flight and heading back to the company building; the irritations became more noticeable and I contacted Operations asking for the crew to be released and that I needed to go take a shower and clean up. I then contacted the safety department telling them what had happened and that I would be submitting a report later in the day after I went back to my apartment for a shower; dispose of my contacts and take a nap because I was having a headache. Event occurred due to maintenance overfilling the hydraulic fluid which then reached the pack system and entered the ventilation system into the cockpit; according to our maintenance personnel. Check Airman. Airline Transport Pilot.

Data retrieved from NASA's ASRS site 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.