ARTCC radar detected an altitude overshoot on medium large transport, flight crew notified of possible less than standard separation after landing.

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Attributes
ACN	86137
Time
Date	198804
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : bsr
State Reference	CA
Altitude	msl bound lower : 19400 msl bound upper : 20000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zoa
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport
Flight Phase	descent other
Route In Use	arrival : profile descent enroute airway : zoa
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : commercial pilot : instrument
Experience	flight time last 90 days : 150 flight time total : 15000 flight time type : 2500
ASRS Report	86137
Person 2
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation
Independent Detector	atc equipment other atc equipment : unspecified
Resolutory Action	none taken : anomaly accepted
Consequence	faa : reviewed incident with flight crew
Miss Distance	unspecified : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	other

Narrative:

Route lax to mry. Cleared to FL200 from FL280 south of big sur. Leveled off at FL200 proceeded to mry no further problems. However after landing at mry I was contacted by oak center and informed that we had triggered their separation alarm and they had read our altitude as FL194. According to our altimeters we had leveled at FL200. I checked our altimeters on the next leg xchking both altimeters against each other and checking with center several times as to our altitude readout. 200 to 300' was the biggest difference found between any instrument and readout from center in any mode of flight. I am baffled to say the least. We were descending at a fairly high rate to FL200. There was WX in the area. We were also over mountainous terrain. However I am not familiar enough with any of the equipment used for altitude to explain the difference in altitude readout. I am indebted to supervisor of oak center for informing me of the discrepancy for otherwise we would have been totally unaware of any conflict.

Original NASA ASRS Text

Title: ARTCC RADAR DETECTED AN ALT OVERSHOOT ON MLG, FLT CREW NOTIFIED OF POSSIBLE LESS THAN STANDARD SEPARATION AFTER LNDG.

Narrative: ROUTE LAX TO MRY. CLRED TO FL200 FROM FL280 SOUTH OF BIG SUR. LEVELED OFF AT FL200 PROCEEDED TO MRY NO FURTHER PROBLEMS. HOWEVER AFTER LNDG AT MRY I WAS CONTACTED BY OAK CENTER AND INFORMED THAT WE HAD TRIGGERED THEIR SEPARATION ALARM AND THEY HAD READ OUR ALT AS FL194. ACCORDING TO OUR ALTIMETERS WE HAD LEVELED AT FL200. I CHECKED OUR ALTIMETERS ON THE NEXT LEG XCHKING BOTH ALTIMETERS AGAINST EACH OTHER AND CHECKING WITH CENTER SEVERAL TIMES AS TO OUR ALT READOUT. 200 TO 300' WAS THE BIGGEST DIFFERENCE FOUND BETWEEN ANY INSTRUMENT AND READOUT FROM CENTER IN ANY MODE OF FLT. I AM BAFFLED TO SAY THE LEAST. WE WERE DESCENDING AT A FAIRLY HIGH RATE TO FL200. THERE WAS WX IN THE AREA. WE WERE ALSO OVER MOUNTAINOUS TERRAIN. HOWEVER I AM NOT FAMILIAR ENOUGH WITH ANY OF THE EQUIPMENT USED FOR ALT TO EXPLAIN THE DIFFERENCE IN ALT READOUT. I AM INDEBTED TO SUPVR OF OAK CENTER FOR INFORMING ME OF THE DISCREPANCY FOR OTHERWISE WE WOULD HAVE BEEN TOTALLY UNAWARE OF ANY CONFLICT.

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.