GRUMMAN AA-5A INSTRUCTOR AND STUDENT INADVERTENTLY ENTER IMC ON A VFR FLT.

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VFR TRAINING MISSION WITH CFI S OF HOUSTON HOBBY AREA. PLANNED TO CONDUCT COMMERCIAL MANEUVERS. RECEIVED WX BRIEFING BEFORE TKOF; RPTED CEILING 2200 FT AT 5 MI VISIBILITY WITH NO LAYER BELOW IN THE AREA SSW OF HOBBY. IFR CONDITIONS FORECASTED TO START 8:00 PM LCL TIME; 2 HRS AFTER OUR FLT WAS TO END. HOUSTON HOBBY ATIS SIMILAR RPT. WE DEPARTED S; HIT SCATTERED CLOUDS AT 1500 FT MSL ABOUT 10 MI S OF ARPT. PROCEEDED 15 MI SSW OF HOBBY TO PLANNED TRAINING AREA; SCATTERED CLOUDS DROPPED TO 1300 FT MSL WITH BROKEN LAYER ABOVE AT APPROX 2000 FT MSL. CIRCLED IN AREA CLR OF CLOUDS TO CONTACT HOUSTON APCH AND GET CLRNC BACK INTO HOBBY DUE TO UNEXPECTED CLOUDS. RECEIVED CLRNC INTO CLASS B AIRSPACE WITH RADAR CONTACT; PUT ON INITIAL VECTOR ON 270 DEG HDG FOR SEQUENCING. ABOUT 1 MIN LATER; ATC RPTED TFC ALERT FOR APCHING TFC ON 090 DEG HDG AT 1300 FT MSL AT 3 MI. WE COULD NOT LOCATE THE TFC; BUT SUSPECTED IT HAD DEPARTED AXH ON THE RWY 090 WAS GOING E BELOW THE CLOUDS. ABOUT 30 SECONDS LATER; ATC INSTRUCTED US TO CONDUCT IMMEDIATE CLBING TURN TO 360 DEG HDG TO 2000 FT MSL FOR TFC AVOIDANCE. WE COMPLIED; AND ENTERED LARGE CLOUD THAT COULD NOT BE AVOIDED AND WAS IN IMC FOR ABOUT 30 SECONDS. BROKE OUT ABOVE THE SCATTERED LAYER; ATC ASKED US TO VECTOR DIRECT TO HOUSTON ASTRODOME FOR SEQUENCE TO LNDG (TYPICAL VISUAL REF POINT). WE COULD NOT SEE DOME DUE TO CLOUDS; AND ASKED FOR IFR CLRNC INTO HOBBY. WE RECEIVED A CLRNC; CLBED TO 3000 FT FOR NORMAL APCH/LNDG INTO HOBBY RWY 12R. WE WERE BOTH INST RATED WITH CURRENT APCH PLATES FOR ARPT. THE AIRPLANE RENTED FROM THE FLT SCHOOL WAS FOR VFR FLTS BUT EQUIP WITH THE IFR REQUIRED INSTS. WE DID NOT CHK THE VOR TO SEE IF IT HAD BEEN TESTED IN THE PAST 30 DAYS BEFORE FLT. AFTER REVIEWING THE INCIDENT; A FEW ITEMS TO LEARN FROM: FIRST; BEFORE THE FLT; I QUESTIONED MY CFI IF WE SHOULD CANCEL DUE TO THE CLOUDS. THE LOWER CLOUDS LIMITED THE TRAINING WE HAD PLANNED; BUT WE AGREED TO GO AFTER THE WX BRIEF. I HAVE CANCELED MANY FLTS BEFORE WHEN IN DOUBT; AND PROBABLY SHOULD HAVE THIS TIME. SECOND; THE MORE APPROPRIATE ACTION WE SHOULD HAVE TAKEN TO AVOID THE TFC ALERT BUT KEEP PROPER CLOUD CLRNC WOULD HAVE BEEN TO TURN R TO HDG 360 DEGS BUT DSND DOWN TO 1000 FT MSL. WE WERE FAMILIAR WITH TOWERS IN THE AREA AND WERE IN A SAFE AREA. THERE WERE SEVERAL TALL TOWERS UP TO 2050 FT MSL NW OF OUR POS 3-4 MI. WE ALSO HAD THE OPTION TO LAND AT PEARLAND REGIONAL; THE FIRST ARPT WHEN THE LOWER CLOUDS WERE ENCOUNTERED. WE COULD HAVE LANDED AND RECEIVED AN IFR CLRNC BACK INTO HOBBY.