AN INSTRUCTOR PLT DISABLED THE LNDG GEAR WARNING SYS.

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I WAS TEACHING MY STUDENT (200 PLUS HRS PLT WORKING ON COMMERCIAL RATING) SHORT AND SOFT FIELD TKOFS AND LNDG TECHNIQUES. WE WERE CONDUCTING OUR SECOND SHORT-FIELD LNDG. WE BOTH MISSED THE FACT THAT THE GEAR HAD NOT BEEN LOWERED. WE WERE FLYING IN A PA28R-200 WITH THE AUTO- EXTEND GEAR FEATURE. THE PROC TAUGHT AT THE SCHOOL FOR SHORT-FIELD TKOFS INCLUDED OVERRIDING THE AUTO EXTENSION FEATURE SO THAT THE ACFT COULD BE 'CLEANED' UP AT A SLOWER AIRSPD TO ACHIEVE BETTER CLB PERFORMANCE. THE OVERRIDE WAS INADVERTENTLY LEFT ON AFTER TKOF AND; THEREFORE; THE GEAR DID NOT AUTO-EXTEND. THE ACFT IS ALSO EQUIPPED WITH A GEAR UNSAFE LIGHT AND HORN. WITH THE AUTO EXTENSION FEATURE OVERRIDDEN; THERE IS NO WARNING UNTIL THE THROTTLE IS RETARDED TO IDLE POS. BECAUSE OF FAIRLY STRONG WINDS ON THE DAY IN QUESTION; ENG PWR WAS USED UNTIL NEAR TOUCHDOWN; AND NO WARNING HORN SOUNDED. EXTREME CARE SHOULD BE TAKEN WHEN THE AUTO-EXTEND FEATURE IS OVERRIDDEN. PLT FATIGUE MAY HAVE BEEN A FACTOR ON MY PART BECAUSE; AT THE TIME OF THE INCIDENT; I WAS WORKING 2 JOBS AT APPROX 70 HRS WK. I BELIEVE THAT PLT FATIGUE SHOULD BE OF GREAT CONCERN TO THE FAA; ESPECIALLY IN REGARD TO FLT INSTRUCTORS. FOR FINANCIAL REASONS; MANY FLT INSTRUCTORS ARE FORCED TO WORK LONG HRS OR EVEN 2 JOBS. SUPPLEMENTAL INFO FROM ACN 262971: THE ACFT WAS EQUIPPED WITH AN AUTOMATIC GEAR EXTENSION SYS WHICH HAD BEEN DEACTIVATED BY REQUEST OF THE INSTRUCTOR. I NEGLECTED TO PERFORM THE PRE LNDG CHKLIST.