A glasair S4-2R pilot reports the altimeter reading 200 ft higher than actual altitude. Tests proved...

Attributes
ACN	599581
Time
Date	200311
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl bound lower : 1300 msl bound upper : 1500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : zzz.tower tower : atl.tower
Operator	general aviation : personal
Make Model Name	Experimental
Operating Under FAR Part	Part 91
Flight Phase	descent : approach
Route In Use	approach : visual
Flight Plan	None
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : private
Experience	flight time last 90 days : 25 flight time total : 2500 flight time type : 2000
ASRS Report	599581
Person 2
Affiliation	government : faa
Function	controller : local
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	other other : person 1
Resolutory Action	none taken : detected after the fact
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

After receiving clearance 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 clearance AGL for a safe approach. The photo target is in the foothills of a local mountain chain. It soon became apparent that the actual altitude AGL was surprisingly low and would not be considered safe or legal. After departing the area an approximately check showed a substantial altitude reporting error indicating substantially above actual altitude. Subsequent test flight at a local airport at that height indicated approximately 200 ft higher than actual altitude. Further checking and testing revealed that the standard certified (pma) pitot static tube was reporting 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 reporting via mode C erroneous actual altitudes that vary by level of indicated airspeed. They probably may be unaware of this potential hazard and should immediately check for proper static port calibration and reporting altitude. Callback conversation with reporter revealed the following information: the reporter stated when the error was discovered a call was made to a light aircraft manufacturer that uses the same pitot static probe and was advised a 'cheater ring' was required. The reporter said the ring was placed on the pitot tube either forward or aft of the static ports and adjusted to eliminate the error. The reporter 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 reporter stated many experimental category airplanes are flying and may be reporting mode C altitude in error and being unaware of the potential hazard. The reporter stated the problem is greater when reduced vertical separation minimums go into effect.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

A glasair S4-2R pilot reports the altimeter reading 200 ft higher than actual altitude. Tests proved certified pma pitot static tube reporting a lower then ambient pressure.