Air carrier medium large transport altimeter static source problem.

Attributes
ACN	101554
Time
Date	198901
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : pmm airport : azo
State Reference	MI
Altitude	msl bound lower : 15000 msl bound upper : 14500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau tracon : mer
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 125 flight time total : 13074 flight time type : 7205
ASRS Report	101554
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : undershoot
Independent Detector	other flight crewa
Resolutory Action	aircraft : equipment problem dissipated
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

During descent into azo, captain's altimeter hung up briefly at 15000', the unwound rapidly for about 1 second and stopped at 12600'. Clearance altitude was 13000'. First officer's instruments indicated 14500' and 1000 FPM rate of descent which corresponded with actual aircraft attitude and location. Aircraft was immediately leveled off with autoplt altitude hold. In level flight, first officer's altimeter indicated 14500', and captain's altimeter and vsi indicated 1000 FPM rate of climb as captain's altimeter slowly climbed back to 14100'. Even though autoplt was in altitude hold mode, autoplt attempted to descend aircraft and consequently was disconnected. 12000' traffic was noted at our 1 O'clock position at about 2 mi and about 2000' below our altitude. In accordance with far, the variable altimeter discrepancy and corresponding autoplt problem was entered into aircraft log book upon arrival at azo. Company maintenance personnel discovered leak in aircraft static system. Previous aircraft history of #1 altimeter problems seems to have been a static system leak which occurred intermittently between 16000 and 12000'. At 10000' and below and above 16000', there was no noticeable altimeter discrepancy. Review of maintenance log prior to flight helped to determine faulty altimeter.

Original NASA ASRS Text

Title: ACR MLG ALTIMETER STATIC SOURCE PROBLEM.

Narrative: DURING DSCNT INTO AZO, CAPT'S ALTIMETER HUNG UP BRIEFLY AT 15000', THE UNWOUND RAPIDLY FOR ABOUT 1 SEC AND STOPPED AT 12600'. CLRNC ALT WAS 13000'. F/O'S INSTRUMENTS INDICATED 14500' AND 1000 FPM RATE OF DSCNT WHICH CORRESPONDED WITH ACTUAL ACFT ATTITUDE AND LOCATION. ACFT WAS IMMEDIATELY LEVELED OFF WITH AUTOPLT ALT HOLD. IN LEVEL FLT, F/O'S ALTIMETER INDICATED 14500', AND CAPT'S ALTIMETER AND VSI INDICATED 1000 FPM RATE OF CLB AS CAPT'S ALTIMETER SLOWLY CLBED BACK TO 14100'. EVEN THOUGH AUTOPLT WAS IN ALT HOLD MODE, AUTOPLT ATTEMPTED TO DSND ACFT AND CONSEQUENTLY WAS DISCONNECTED. 12000' TFC WAS NOTED AT OUR 1 O'CLOCK POS AT ABOUT 2 MI AND ABOUT 2000' BELOW OUR ALT. IN ACCORDANCE WITH FAR, THE VARIABLE ALTIMETER DISCREPANCY AND CORRESPONDING AUTOPLT PROB WAS ENTERED INTO ACFT LOG BOOK UPON ARR AT AZO. COMPANY MAINT PERSONNEL DISCOVERED LEAK IN ACFT STATIC SYS. PREVIOUS ACFT HISTORY OF #1 ALTIMETER PROBS SEEMS TO HAVE BEEN A STATIC SYS LEAK WHICH OCCURRED INTERMITTENTLY BTWN 16000 AND 12000'. AT 10000' AND BELOW AND ABOVE 16000', THERE WAS NO NOTICEABLE ALTIMETER DISCREPANCY. REVIEW OF MAINT LOG PRIOR TO FLT HELPED TO DETERMINE FAULTY ALTIMETER.

Data retrieved from NASA's ASRS site as of August 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.