Pilot of a piper PA28-180 discovered that his altimeter was giving an erroneous reading during pract...

Attributes
ACN	360048
Time
Date	199702
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : ccc airport : isp
State Reference	NY
Altitude	msl bound lower : 1300 msl bound upper : 3100
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zme
Operator	general aviation : personal
Make Model Name	PA-28 Cherokee/Archer II/Dakota/Pillan/Warrior
Operating Under FAR Part	Part 91
Flight Phase	climbout : intermediate altitude other
Flight Plan	None
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : instrument
ASRS Report	360048
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : critical other anomaly other other spatial deviation
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

While practicing holds northwest of the erick intersection, as aircraft approached the shoreline from the south, aircraft appeared to be lower than the altitude indicated by the altimeter. Based on pilots' visual perception of altitude, altimeter glass was 'tapped' and indicated altitude immediately changed from 3000 ft to 1300 ft. An immediate climb was initiated to 3000 ft and ATC was contacted to verify altitude reading of 3000 ft. ATC advised showing 3100 ft and provided current local altimeter setting. Resetting the altimeter to local barometric pressure confirmed aircraft and ATC altimeter readings to be the same. Flight was terminated immediately with a normal approach and landing at isp. Aircraft was grounded pending repair of altimeter. Pitot static system 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 flight following, a secondary altimeter, or mode C readout should be used if available to confirm indicated altitude, especially during operations while over water where visual ques can be mislea ding.

Original NASA ASRS Text

Title: 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.

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.

Pilot of a piper PA28-180 discovered that his altimeter was giving an erroneous reading during practice air holding. ATC helped him assess the actual altitude readout as opposed to the altimeter readout. The aircraft had a current altimeter pitot tube system test.