AN MD80 CAPT FLYING AT 2000 FT 4 MI FINAL APCH ON GS NOTED ACFT TOO LOW. FO'S GS INDICATOR 1 DOT LOW. NO WARNING FLAGS OR COMPARATOR ALERT.

Narrative

DURING THE APCH TO RWY 17R IN AUS WITH THE ILS TUNED AND IDENTED; WE NOTICED THAT MY GS INDICATION WAS SHOWING R ON GLIDE PATH WHILE THE FO'S WAS 1 DOT LOW. WE WERE VISUAL AT THE TIME AND AT APPROX 4 MI OUT ON FINAL. I THOUGHT THAT I WAS TOO LOW TO BE THIS FAR OUT AND CHKED THE FO'S SIDE. WE THEN RE-TUNED THE ILS AND ALSO TRIED SWITCHING THE 'RAD NAV' SWITCH. I ATTEMPTED TO TRY MANUAL ILS BY TURNING OFF THE FMS. NOTHING WORKED AND THE ILS CONTINUED TO SHOW FALSE ON GLIDE PATH INFO WITH NO WARNING OR COMPARATOR LIGHTS. THIS IS A FORMER ACR Y BIRD THAT HAS BEEN MODIFIED. WE MIGHT WANT TO CHK AND SEE IF THE WIRING HAS A PROB WITH THIS JET AND OTHERS LIKE IT. THIS COULD BE A POTENTIAL PROB AND CAUSE NUMEROUS MISSED APCHS DUE TO ERRONEOUS INFO WITH NO WARNING FLAPS. A GOOD XCHK SHOULD CATCH IT; BUT WE SHOULD TRY AND CORRECT THIS QUICKLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID; 'MY GS INDICATOR WAS CTRED; BUT LOOKING OUTSIDE THE AIRPLANE IT APPEARED TOO LOW ON PROFILE.' THE FO AGREED AND STATED THE R GS INDICATOR WAS 1 DOT LOW WHILE THE CAPT'S INDICATOR DISPLAYED EXACTLY ON GS. THERE WAS NO WARNING FLAG OR COMPARATOR ALERT. ALL OTHER SYS OPERATED NORMALLY. THE RPTR STATED THIS AIRPLANE WAS A BUYOUT AIRPLANE AND HAS BEEN MODIFIED TO SOMEWHAT LIKE THE ACQUIRING CARRIER'S FLEET. THIS DIFFERENCE IS OF GREAT CONCERN AND IS A POTENTIAL PROB. MAINT CLRED THE RPT WITH REMOVED AND REPLACED. NO OTHER DETAILS. CALLBACK CONVERSATION WITH RPTR ACN 638509 REVEALED THE FOLLOWING INFO: THE RPTR STATE THAT ON FINAL APCH THE CAPT REMARKED THAT HE SEEMED LOW ON PROFILE AND HE WAS R 1 DOT LOW ON THE R GS INDICATOR. ALL NAVAIDS WERE PROPERLY TUNED AND BOTH SYS ON CORRECT FREQ. NO WARNING OF ANY KIND ALERTED THIS DIFFERENCE. THE LOGBOOK WAS CLRED AS RE-RACKED UNITS.