Narrative:

A student and myself departed pao in an small aircraft for a club proficiency check. I decided to head west over the hills to the coast as the WX at pao was 1000' overcast with 20 mi visibility. Although rain showers were forecasted it looked good toward the coast. After reaching the intended practice area over the coast I noticed rain to the south of our position. I assumed it was moving in a southerly direction as the wind was out of the north at pao. After doing several maneuvers I decided to practice a few lndgs at half moon bay airport. The wind at half moon bay was out of the south and we were using runway 12. Departing half moon bay to the south to return to pao we encountered rain about 6-7 mi south that dropped the visibility to about 3 mi. We were flying at about 2000' MSL and wanted to cross the hills south of woodside VOR back to pao. The low visibility made climbing to a safe altitude impossible. The tops of the ridgeline weren't visible either. We reversed course back to half moon bay to where the visibility was better and circled until the ridge tops were more visible. The ridgeline is lower in this area and is safe to cross at about 2500' MSL. When we crossed the ridgeline the visibility again lowered and we dropped to about 2000' MSL paralleling highway 280 southeast to pao. Upon crossing the ridgeline I should've contacted san carlos tower to request permission to transition their air traffic area to pao. However the intercom in the airplane malfunctioned and I could not communicate with my student nor use the radios. The low visibility made it difficult to figure out the problem as I had to constantly yell at my student to get him to head where I wanted, as he didn't know what I was doing since he couldn't hear me on the intercom. (The low visibility made it difficult for the student to know where he was as I had the map.) I gave up on fixing the intercom and concentrated on navigation and remaining in VFR conditions. I used the aircraft microphone and speaker to contact san carlos tower, but we had already passed through the west side of the air traffic area and were told to contact pao. It's my opinion that I should've been more aware of the rain when I first noticed it at the practice area. I also made a wrong assumption on the wind direction over the coast. (It was southerly not northerly.) the low visibility in the rain was expected, but I was surprised at the extent of the rainfall. I was expecting showers and had planned to fly around any if necessary. The fact that I had to divert my intended route into an area that I'm not too familiar with in marginal conditions compounded the confusion when the intercom quit. As a result I did not contact san carlos tower before entering the air traffic area. I did not discover this error until I looked at the chart on the ground at the end of the flight.

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Original NASA ASRS Text

Title: ATA PENETRATION.

Narrative: A STUDENT AND MYSELF DEPARTED PAO IN AN SMA FOR A CLUB PROFICIENCY CHK. I DECIDED TO HEAD W OVER THE HILLS TO THE COAST AS THE WX AT PAO WAS 1000' OVCST WITH 20 MI VISIBILITY. ALTHOUGH RAIN SHOWERS WERE FORECASTED IT LOOKED GOOD TOWARD THE COAST. AFTER REACHING THE INTENDED PRACTICE AREA OVER THE COAST I NOTICED RAIN TO THE S OF OUR POS. I ASSUMED IT WAS MOVING IN A SOUTHERLY DIRECTION AS THE WIND WAS OUT OF THE N AT PAO. AFTER DOING SEVERAL MANEUVERS I DECIDED TO PRACTICE A FEW LNDGS AT HALF MOON BAY ARPT. THE WIND AT HALF MOON BAY WAS OUT OF THE S AND WE WERE USING RWY 12. DEPARTING HALF MOON BAY TO THE S TO RETURN TO PAO WE ENCOUNTERED RAIN ABOUT 6-7 MI S THAT DROPPED THE VISIBILITY TO ABOUT 3 MI. WE WERE FLYING AT ABOUT 2000' MSL AND WANTED TO CROSS THE HILLS S OF WOODSIDE VOR BACK TO PAO. THE LOW VISIBILITY MADE CLBING TO A SAFE ALT IMPOSSIBLE. THE TOPS OF THE RIDGELINE WEREN'T VISIBLE EITHER. WE REVERSED COURSE BACK TO HALF MOON BAY TO WHERE THE VISIBILITY WAS BETTER AND CIRCLED UNTIL THE RIDGE TOPS WERE MORE VISIBLE. THE RIDGELINE IS LOWER IN THIS AREA AND IS SAFE TO CROSS AT ABOUT 2500' MSL. WHEN WE CROSSED THE RIDGELINE THE VISIBILITY AGAIN LOWERED AND WE DROPPED TO ABOUT 2000' MSL PARALLELING HWY 280 SE TO PAO. UPON XING THE RIDGELINE I SHOULD'VE CONTACTED SAN CARLOS TWR TO REQUEST PERMISSION TO TRANSITION THEIR ATA TO PAO. HOWEVER THE INTERCOM IN THE AIRPLANE MALFUNCTIONED AND I COULD NOT COMMUNICATE WITH MY STUDENT NOR USE THE RADIOS. THE LOW VISIBILITY MADE IT DIFFICULT TO FIGURE OUT THE PROB AS I HAD TO CONSTANTLY YELL AT MY STUDENT TO GET HIM TO HEAD WHERE I WANTED, AS HE DIDN'T KNOW WHAT I WAS DOING SINCE HE COULDN'T HEAR ME ON THE INTERCOM. (THE LOW VISIBILITY MADE IT DIFFICULT FOR THE STUDENT TO KNOW WHERE HE WAS AS I HAD THE MAP.) I GAVE UP ON FIXING THE INTERCOM AND CONCENTRATED ON NAV AND REMAINING IN VFR CONDITIONS. I USED THE ACFT MIC AND SPEAKER TO CONTACT SAN CARLOS TWR, BUT WE HAD ALREADY PASSED THROUGH THE W SIDE OF THE ATA AND WERE TOLD TO CONTACT PAO. IT'S MY OPINION THAT I SHOULD'VE BEEN MORE AWARE OF THE RAIN WHEN I FIRST NOTICED IT AT THE PRACTICE AREA. I ALSO MADE A WRONG ASSUMPTION ON THE WIND DIRECTION OVER THE COAST. (IT WAS SOUTHERLY NOT NORTHERLY.) THE LOW VISIBILITY IN THE RAIN WAS EXPECTED, BUT I WAS SURPRISED AT THE EXTENT OF THE RAINFALL. I WAS EXPECTING SHOWERS AND HAD PLANNED TO FLY AROUND ANY IF NECESSARY. THE FACT THAT I HAD TO DIVERT MY INTENDED ROUTE INTO AN AREA THAT I'M NOT TOO FAMILIAR WITH IN MARGINAL CONDITIONS COMPOUNDED THE CONFUSION WHEN THE INTERCOM QUIT. AS A RESULT I DID NOT CONTACT SAN CARLOS TWR BEFORE ENTERING THE ATA. I DID NOT DISCOVER THIS ERROR UNTIL I LOOKED AT THE CHART ON THE GND AT THE END OF THE FLT.

Data retrieved from NASA's ASRS site as of August 2007 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.