Narrative:

I performed the run-up checks prior to flight. All systems indicated correctly; I don't remember specifically checking to see if both vacuum buttons were pushed in but I did see the left pushed in when I started the left engine. I forgot to look when I started the right one. I ran up the engines to 1;700 RPM and the right engine; left magneto was running rough. This distracted me while I tried to figure out how to clear the magneto. This caused me to forget to check the vacuum gauge during the run-up. When I shut down the engines I shut down each individually and watched the vacuum buttons to check the valve between them. By the time I looked at the buttons I had already pulled back the left engine's mixture and saw the button (on the left) pop out as it should. I did the same with the right. Nothing seemed out of the ordinary. We loaded the passengers and I took off under IFR clearance. On the takeoff roll I noticed the attitude indicator was indicating a steady 3 degree turn to the left. I figured it needed more time to cage so I continued with the takeoff. As I climbed through 200 ft it was no better. At 800 ft after I finished the climb flow I glanced down at the heading indicator to confirm wings level and found it to be turning also. In addition; it was indicating I was passing through a 090 heading (after departing with no turns yet) that's when I realized something was very wrong. By the time I concluded there was a real problem we were at 1;000 ft and were beginning to enter the fragmented bottoms of the clouds. I contacted approach and told them I was having gyro issues and intended to return for landing. I did not declare an emergency; hoping that if I completed a 180 degree turn I could just land on the runway I had just departed; opposite direction. I had stopped climbing at about 1;400 ft and was in the clouds; so I descended slightly as I continued turning. As I was turning I looked at the vacuum gauge and saw that the left button was fully out and the right was half out; trembling between in and out. I did not descend below 1;200 ft and when I had turned for approximately 1 minute I started to see buildings. When I saw outside I realized how wrong my instruments really were. I leveled the wings and flew on for a few more seconds until suddenly I saw the airport poking out above a fog layer. I canceled my flight plan and landed without incident. After landing I contacted maintenance who suggested that the left vacuum pump failed and due to the heavy rain the right pump could not suck in enough air to support the vacuum instruments. This is the only explanation I have heard to justify a dual simultaneous vacuum pump failure.

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Original NASA ASRS Text

Title: C402 pilot experiences a dual vacuum pump failure during takeoff and initial climb. When instrument failures are confirmed the reporter descends back into VMC and returns to departure airport.

Narrative: I performed the run-up checks prior to flight. All systems indicated correctly; I don't remember specifically checking to see if BOTH vacuum buttons were pushed in but I did see the left pushed in when I started the left engine. I forgot to look when I started the right one. I ran up the engines to 1;700 RPM and the right engine; left magneto was running rough. This distracted me while I tried to figure out how to clear the magneto. This caused me to forget to check the vacuum gauge during the run-up. When I shut down the engines I shut down each individually and watched the vacuum buttons to check the valve between them. By the time I looked at the buttons I had already pulled back the left engine's mixture and saw the button (on the left) pop out as it should. I did the same with the right. Nothing seemed out of the ordinary. We loaded the passengers and I took off under IFR clearance. On the takeoff roll I noticed the attitude indicator was indicating a steady 3 degree turn to the left. I figured it needed more time to cage so I continued with the takeoff. As I climbed through 200 FT it was no better. At 800 FT after I finished the climb flow I glanced down at the heading indicator to confirm wings level and found it to be turning also. In addition; it was indicating I was passing through a 090 heading (after departing with no turns yet) that's when I realized something was very wrong. By the time I concluded there was a real problem we were at 1;000 FT and were beginning to enter the fragmented bottoms of the clouds. I contacted Approach and told them I was having gyro issues and intended to return for landing. I did not declare an emergency; hoping that if I completed a 180 degree turn I could just land on the runway I had just departed; opposite direction. I had stopped climbing at about 1;400 FT and was in the clouds; so I descended slightly as I continued turning. As I was turning I looked at the vacuum gauge and saw that the left button was fully out and the right was half out; trembling between in and out. I did not descend below 1;200 FT and when I had turned for approximately 1 minute I started to see buildings. When I saw outside I realized how wrong my instruments really were. I leveled the wings and flew on for a few more seconds until suddenly I saw the airport poking out above a fog layer. I canceled my flight plan and landed without incident. After landing I contacted Maintenance who suggested that the left vacuum pump failed and due to the heavy rain the right pump could not suck in enough air to support the vacuum instruments. This is the only explanation I have heard to justify a dual simultaneous vacuum pump failure.

Data retrieved from NASA's ASRS site as of April 2012 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.