Narrative:

First flight after 6.5 months for new avionics modification; paint; interior; and phase 1 through 4 inspection. All preflight and run-up was completed. The owner (pilot) was in the left seat. On climb out using the checklist we verified that we were pressurizing. Climbing through FL270 we received a 'cabin altitude advisory' and a notice a 5.0 psi and a cabin altitude of 10;000 ft. I then placed the environmental control from low to normal and noticed a little movement of the needles. I then asked center for a lower altitude and started a descent. The oxygen masks in the passenger compartment deployed and I noticed a 'cabin altitude hi' warning light and we lost pressurization at about FL250. I then grabbed my oxygen mask and could not put it on due to my headset. I took off the headset; put on my mask; turned on the mic switch and told center that we needed to take it down to 10;000 ft. Not hearing a reply because I could not find the speaker switch on the new avionics panel; I put back on my headset and holding my mask; I started flying the plane. Still trying to focus on procedures I then turned on the passenger oxygen. I was instructed to contact approach at or about FL187 and they asked if we needed any assistance. We asked to return to the departure airport and the rest of the flight was uneventful. Back at our base I was told that one of the mechanics passed out; but once oxygen was given came around quickly. The maintenance facility found two lines crossed at the controller. The flight the next day was normal.

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Original NASA ASRS Text

Title: A BE300 on a test flight lost pressurization at FL270 because two sense lines on the pressure controller had been reversed during extended maintenance. The pilot had communications problems because of headset/oxygen mask incompatability.

Narrative: First flight after 6.5 months for new avionics modification; paint; interior; and phase 1 through 4 inspection. All preflight and run-up was completed. The owner (pilot) was in the left seat. On climb out using the checklist we verified that we were pressurizing. Climbing through FL270 we received a 'Cabin Altitude Advisory' and a notice a 5.0 PSI and a cabin altitude of 10;000 FT. I then placed the environmental control from low to normal and noticed a little movement of the needles. I then asked Center for a lower altitude and started a descent. The oxygen masks in the passenger compartment deployed and I noticed a 'CABIN ALT HI' warning light and we lost pressurization at about FL250. I then grabbed my oxygen mask and could not put it on due to my headset. I took off the headset; put on my mask; turned on the mic switch and told Center that we needed to take it down to 10;000 FT. Not hearing a reply because I could not find the speaker switch on the new avionics panel; I put back on my headset and holding my mask; I started flying the plane. Still trying to focus on procedures I then turned on the passenger oxygen. I was instructed to contact Approach at or about FL187 and they asked if we needed any assistance. We asked to return to the departure airport and the rest of the flight was uneventful. Back at our base I was told that one of the Mechanics passed out; but once oxygen was given came around quickly. The maintenance facility found two lines crossed at the controller. The flight the next day was normal.

Data retrieved from NASA's ASRS site as of July 2013 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.