A GLASAIR S4-2R PLT RPTS THE ALTIMETER READING 200 FT HIGHER THAN ACTUAL ALT. TESTS PROVED CERTIFIED PMA PITOT STATIC TUBE RPTING A LOWER THEN AMBIENT PRESSURE.

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AFTER RECEIVING CLRNC AND ALTIMETER SETTING ENTERING CLASS D AIRSPACE AT ZZZ FOR A LOW LEVEL PHOTO SHOOT OF AN ADJACENT PARK; DESCENDED TO 1500 FT MSL CALCULATED TO BE ADEQUATE MINIMUM CLRNC AGL FOR A SAFE APCH. THE PHOTO TARGET IS IN THE FOOTHILLS OF A LCL MOUNTAIN CHAIN. IT SOON BECAME APPARENT THAT THE ACTUAL ALT AGL WAS SURPRISINGLY LOW AND WOULD NOT BE CONSIDERED SAFE OR LEGAL. AFTER DEPARTING THE AREA AN APPROX CHECK SHOWED A SUBSTANTIAL ALT RPTING ERROR INDICATING SUBSTANTIALLY ABOVE ACTUAL ALT. SUBSEQUENT TEST FLT AT A LOCAL ARPT AT THAT HEIGHT INDICATED APPROX 200 FT HIGHER THAN ACTUAL ALT. FURTHER CHKING AND TESTING REVEALED THAT THE STANDARD CERTIFIED (PMA) PITOT STATIC TUBE WAS RPTING A FAULTY STATIC READING IE; LOWER THAN AMBIENT. THIS REQUIRED PLACEMENT OF AN ADJUSTING RING AFT OF THE STATIC PORTS AS SUGGESTED BY ENGINEER OF BELLANCA; WHICH USES AN IDENTICAL PROBE. AS FOR MANY SIMILAR EXPERIMENTAL CATEGORY AIRPLANES PROBABLY ARE FLYING AND RPTING VIA MODE C ERRONEOUS ACTUAL ALTS THAT VARY BY LEVEL OF INDICATED AIRSPEED. THEY PROBABLY MAY BE UNAWARE OF THIS POTENTIAL HAZARD AND SHOULD IMMEDIATELY CHK FOR PROPER STATIC PORT CALIBRATION AND RPTING ALT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED WHEN THE ERROR WAS DISCOVERED A CALL WAS MADE TO A LIGHT ACFT MANUFACTURER THAT USES THE SAME PITOT STATIC PROBE AND WAS ADVISED A 'CHEATER RING' WAS REQUIRED. THE RPTR SAID THE RING WAS PLACED ON THE PITOT TUBE EITHER FORWARD OR AFT OF THE STATIC PORTS AND ADJUSTED TO ELIMINATE THE ERROR. THE RPTR STATED IN THIS AIRPLANE THE ADJUSTMENT RING WAS POSITIONED AFT OF THE STATIC PORTS AND BROUGHT THE ERROR DEPENDING ON AIRSPEED AND ANGLE OF ATTACK TO WITHIN 50 TO 60 FT. THE RPTR STATED MANY EXPERIMENTAL CATEGORY AIRPLANES ARE FLYING AND MAY BE RPTING MODE C ALT IN ERROR AND BEING UNAWARE OF THE POTENTIAL HAZARD. THE RPTR STATED THE PROB IS GREATER WHEN REDUCED VERTICAL SEPARATION MINIMUMS GO INTO EFFECT.