Cleared descend to 27000', thought 20000' and got to 26000'.

Attributes
ACN	147530
Time
Date	199006
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : fak
State Reference	VA
Altitude	msl bound lower : 26000 msl bound upper : 27000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc tower : day
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 150 flight time total : 13000 flight time type : 5000
ASRS Report	147530
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 158 flight time total : 3800 flight time type : 1260
ASRS Report	147251
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were cleared to cross 35 southwest richmond at FL270. Then we were reclred direct flat rock. We were then cleared to cross 95 south of flat rock at FL270, which I read back. The first officer (PF) thought he heard FL200, and reset the altitude alert to FL200, unbeknown to me. I was busy when he started the descent, and first noticed the aircraft leaving FL269. When I saw FL200 set in the altitude alert window I thought perhaps I had not heard a clearance the first officer had received. I queried ATC about the altitude by stating we were leaving FL260 for FL200. The controller replied that we were cleared only to FL270 and for us to climb to FL270, which we did. The problem arose by the first officer resetting the altitude alert west/O confirming with ATC or myself, and was complicated by my not being more aggressive in determining the correct altitude when I saw the different altitude set.

Original NASA ASRS Text

Title: CLEARED DESCEND TO 27000', THOUGHT 20000' AND GOT TO 26000'.

Narrative: WE WERE CLRED TO CROSS 35 SW RICHMOND AT FL270. THEN WE WERE RECLRED DIRECT FLAT ROCK. WE WERE THEN CLRED TO CROSS 95 S OF FLAT ROCK AT FL270, WHICH I READ BACK. THE F/O (PF) THOUGHT HE HEARD FL200, AND RESET THE ALT ALERT TO FL200, UNBEKNOWN TO ME. I WAS BUSY WHEN HE STARTED THE DSNT, AND FIRST NOTICED THE ACFT LEAVING FL269. WHEN I SAW FL200 SET IN THE ALT ALERT WINDOW I THOUGHT PERHAPS I HAD NOT HEARD A CLRNC THE F/O HAD RECEIVED. I QUERIED ATC ABOUT THE ALT BY STATING WE WERE LEAVING FL260 FOR FL200. THE CTLR REPLIED THAT WE WERE CLRED ONLY TO FL270 AND FOR US TO CLB TO FL270, WHICH WE DID. THE PROB AROSE BY THE F/O RESETTING THE ALT ALERT W/O CONFIRMING WITH ATC OR MYSELF, AND WAS COMPLICATED BY MY NOT BEING MORE AGGRESSIVE IN DETERMINING THE CORRECT ALT WHEN I SAW THE DIFFERENT ALT SET.

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.