PLT OF A PIPER PA28-180 DISCOVERED THAT HIS ALTIMETER WAS GIVING AN ERRONEOUS READING DURING PRACTICE AIR HOLDING. ATC HELPED HIM ASSESS THE ACTUAL ALT READOUT AS OPPOSED TO THE ALTIMETER READOUT. THE ACFT HAD A CURRENT ALTIMETER PITOT TUBE SYS TEST.

Narrative

WHILE PRACTICING HOLDS NW OF THE ERICK INTXN; AS ACFT APCHED THE SHORELINE FROM THE S; ACFT APPEARED TO BE LOWER THAN THE ALT INDICATED BY THE ALTIMETER. BASED ON PLTS' VISUAL PERCEPTION OF ALT; ALTIMETER GLASS WAS 'TAPPED' AND INDICATED ALT IMMEDIATELY CHANGED FROM 3000 FT TO 1300 FT. AN IMMEDIATE CLB WAS INITIATED TO 3000 FT AND ATC WAS CONTACTED TO VERIFY ALT READING OF 3000 FT. ATC ADVISED SHOWING 3100 FT AND PROVIDED CURRENT LCL ALTIMETER SETTING. RESETTING THE ALTIMETER TO LCL BAROMETRIC PRESSURE CONFIRMED ACFT AND ATC ALTIMETER READINGS TO BE THE SAME. FLT WAS TERMINATED IMMEDIATELY WITH A NORMAL APCH AND LNDG AT ISP. ACFT WAS GNDED PENDING REPAIR OF ALTIMETER. PITOT STATIC SYS WAS CERTIFIED ON JUL/XX/96 AND WAS OPERATED NORMALLY FOR 31.57 SINCE THAT DATE. TO PREVENT RECURRENCE; OTHER MEANS SUCH AS THE USE OF FLT FOLLOWING; A SECONDARY ALTIMETER; OR MODE C READOUT SHOULD BE USED IF AVAILABLE TO CONFIRM INDICATED ALT; ESPECIALLY DURING OPS WHILE OVER WATER WHERE VISUAL QUES CAN BE MISLEA DING.