PA28 INSTRUCTOR PLT DID NOT CLOSE A VFR FLT PLAN.

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COMMENCED ON A DUAL XCOUNTRY TRAINING FLT WITH A STUDENT PLT AT APPROX XA00 LCL TIME. A WX BRIEFING WAS OBTAINED BOTH BY THE STUDENT AND MYSELF; AND A VFR FLT PLAN FILED TO ALM. THE FLT PLAN SHOULD HAVE BEEN FILED AS A 'ROUND ROBIN' ELP-ALM-ELP; BUT I DIDN'T CATCH THIS MISTAKE UNTIL LATER. THE ESTIMATED FLT TIME WAS 90 MINS; INCLUDING TIME IN PATTERN FOR A TOUCH-AND-GO AT ALM AND A FULL-STOP LNDG AT ELP. THE VFR FLT PLAN WAS OPENED WITH ABQ RADIO IMMEDIATELY PRIOR TO DEPARTING ELP. TEMP AT THE TIME OF DEP WAS 37 DEGS C AND WE WERE EXPERIENCING THE USUAL TURB EXPERIENCED IN THIS AREA IN THE SUMMER FROM THERMALS AND DUST DEVILS. THE FLT PROCEEDED AS PLANNED; USING ELP DEP AND HMN APCH FOR RADAR ADVISORIES ALONG THE RTE UNTIL AFTER DEP FROM ALM; AT WHICH POINT WE ELECTED NOT TO USE HMN APCH BUT RATHER WAIT UNTIL HALF-WAY BACK TO ELP AND REQUEST RA'S FROM THAT POINT; WHICH WE DID. THE ENTIRE FLT WAS VERY HOT AND BUMPY. BOTH THE STUDENT AND I FOUND OURSELVES JUST WISHING WE WERE ON THE GND AND IN A COOL PLACE. IN SPITE OF US BOTH HAVING CONSUMED PLENTY OF FLUIDS BEFORE THE FLT (I HAD ALSO TAKEN SOMETHING TO DRINK ALONG) AND THE SHORT DURATION OF THE FLT; ARR BACK AT ELP FOUND US BOTH FEELING SOMEWHAT TIRED AND BATTERED BY THE BUMPS. IN RETROSPECT; I BELIEVE BOTH OF US WERE PROBABLY SOMEWHAT DEHYDRATED; ALSO. THE BEST DESCRIPTION I CAN PLACE ON MY SENSE OF PHYSICAL WELL-BEING AT THE TIME IS 'BRAIN NOT WORKING RIGHT.' AS A RESULT OF THE FATIGUE INDUCED BY THE HEAT AND BUMPS AND; IN MY OPINION; STATE OF DEHYDRATION; I DID NOT FOLLOW MY USUAL ROUTINE IN THE HANDLING OF THE FLT PLAN AND FAILED TO CLOSE IT. THIS IS PROBABLY ALSO COMPOUNDED BY THE FACT THAT THIS WAS MY THIRD TRAINING FLT OF THE DAY. IN SUMMARY: FATIGUE INDUCED BY HEAT; TURB; AND DEHYDRATION LED TO A FAILURE TO PROPERLY CLOSE A VFR FLT PLAN. POTENTIAL PREVENTIVE ACTIONS FOR THE FUTURE: ENSURE ENOUGH FLUIDS ARE CONSUMED BOTH BEFORE AND DURING THE FLT TO MAINTAIN PROPER HYDRATION; AVOID FLYING IN EXTREME HEAT.