Altimeter malfunction results in flight crew leveling aircraft 300' low.

Attributes
ACN	96761
Time
Date	198810
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : ord
State Reference	IL
Altitude	msl bound lower : 6700 msl bound upper : 7300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent : approach
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 8500 flight time type : 600
ASRS Report	96761
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : flight engineer pilot : instrument pilot : atp pilot : cfi
Experience	flight time last 90 days : 200 flight time total : 16000 flight time type : 1500
ASRS Report	96765
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : undershoot non adherence : clearance
Independent Detector	other controllera
Resolutory Action	none taken : detected after the fact
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Vectors for approach at ord. We were told to descend to 7000' MSL. 7000' was set in the altitude set window and flight level change was activated on the AFDS panel. After leveling at 7000' we noticed the center altimeter read about 7300'. We rechked the altimeter settings, verified they were correct and decided that for some reason the computer had leveled us off 300' high. We then descended to 7000' MSL on the center altimeter. The altimeters in front of each pilot had previously been set to QFE as per SOP. Shortly thereafter, approach control advised us that we were at 6800' MSL and to check our altitude. I replied to them that our altimeter said about 6950'. We then decided the center altimeter was in error and that the computer had actually leveled us off at the correct altitude the first time. We then climbed back up to 7300' MSL indicated on the center altimeter. On the ground all 3 altimeters checked out well within tolerances, but a maintenance discrepancy was entered in the aircraft log book. We should have asked approach control what they showed before we descended, but it's pretty hard to squeeze a word in when working ord approach.

Original NASA ASRS Text

Title: ALTIMETER MALFUNCTION RESULTS IN FLT CREW LEVELING ACFT 300' LOW.

Narrative: VECTORS FOR APCH AT ORD. WE WERE TOLD TO DSND TO 7000' MSL. 7000' WAS SET IN THE ALT SET WINDOW AND FLT LEVEL CHANGE WAS ACTIVATED ON THE AFDS PANEL. AFTER LEVELING AT 7000' WE NOTICED THE CENTER ALTIMETER READ ABOUT 7300'. WE RECHKED THE ALTIMETER SETTINGS, VERIFIED THEY WERE CORRECT AND DECIDED THAT FOR SOME REASON THE COMPUTER HAD LEVELED US OFF 300' HIGH. WE THEN DSNDED TO 7000' MSL ON THE CENTER ALTIMETER. THE ALTIMETERS IN FRONT OF EACH PLT HAD PREVIOUSLY BEEN SET TO QFE AS PER SOP. SHORTLY THEREAFTER, APCH CTL ADVISED US THAT WE WERE AT 6800' MSL AND TO CHK OUR ALT. I REPLIED TO THEM THAT OUR ALTIMETER SAID ABOUT 6950'. WE THEN DECIDED THE CENTER ALTIMETER WAS IN ERROR AND THAT THE COMPUTER HAD ACTUALLY LEVELED US OFF AT THE CORRECT ALT THE FIRST TIME. WE THEN CLBED BACK UP TO 7300' MSL INDICATED ON THE CENTER ALTIMETER. ON THE GND ALL 3 ALTIMETERS CHKED OUT WELL WITHIN TOLERANCES, BUT A MAINT DISCREPANCY WAS ENTERED IN THE ACFT LOG BOOK. WE SHOULD HAVE ASKED APCH CTL WHAT THEY SHOWED BEFORE WE DSNDED, BUT IT'S PRETTY HARD TO SQUEEZE A WORD IN WHEN WORKING ORD APCH.

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.