Air carrier crew misinterprets clearance, descends below assigned altitude.

Attributes
ACN	149135
Time
Date	199006
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : yng
State Reference	OH
Altitude	msl bound lower : 15000 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : pit
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Route In Use	enroute airway : zob
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 10000
ASRS Report	149135
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Experience	flight time last 90 days : 100 flight time total : 7000
ASRS Report	149401
Events
Anomaly	altitude deviation : overshoot non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Flight was cleared from FL350 to FL240 at 15 mi east of the youngstown VOR while westbound on a routing from ewr to cle. As the captain, I worked the radios and the first officer was flying the aircraft. This aircraft is an medium large transport with an automated MCP and inertial reference system navigation (IRS). I acknowledged the clearance and the first officer began a 6000 FPM descent, as we were in moderate turbulence and desired the smoother altitude below. I diverted my attention to the approach plate book for cle information and reference. The problem in this situation was that the first officer placed 15000' in the altitude alerter on he MCP instead of 24000' as required by the clearance. I didn't notice this and hwile my attention was diverted away from the first officer's actions, he descended through FL240 and ZOB noted this when he reached FL210. We were then cleared to FL220 with a 20 degree heading change to the right. No further problems arose. In retrospect, in my opinion, the first officer heard the 15 mi east of yng, but programmed 15000' in his thoughts instead of the 24000'. I should have observed and x-chked his actions, but failed to do so by attending to other nonessential details at the time. It cannot be stressed enough to be always vigilant--'copy, confirm and x-chk.'

Original NASA ASRS Text

Title: ACR CREW MISINTERPRETS CLRNC, DESCENDS BELOW ASSIGNED ALT.

Narrative: FLT WAS CLRED FROM FL350 TO FL240 AT 15 MI E OF THE YOUNGSTOWN VOR WHILE WBND ON A RTING FROM EWR TO CLE. AS THE CAPT, I WORKED THE RADIOS AND THE F/O WAS FLYING THE ACFT. THIS ACFT IS AN MLG WITH AN AUTOMATED MCP AND INERTIAL REF SYS NAV (IRS). I ACKNOWLEDGED THE CLRNC AND THE F/O BEGAN A 6000 FPM DSNT, AS WE WERE IN MODERATE TURB AND DESIRED THE SMOOTHER ALT BELOW. I DIVERTED MY ATTN TO THE APCH PLATE BOOK FOR CLE INFO AND REF. THE PROB IN THIS SITUATION WAS THAT THE F/O PLACED 15000' IN THE ALT ALERTER ON HE MCP INSTEAD OF 24000' AS REQUIRED BY THE CLRNC. I DIDN'T NOTICE THIS AND HWILE MY ATTN WAS DIVERTED AWAY FROM THE F/O'S ACTIONS, HE DSNDED THROUGH FL240 AND ZOB NOTED THIS WHEN HE REACHED FL210. WE WERE THEN CLRED TO FL220 WITH A 20 DEG HDG CHANGE TO THE RIGHT. NO FURTHER PROBS AROSE. IN RETROSPECT, IN MY OPINION, THE F/O HEARD THE 15 MI E OF YNG, BUT PROGRAMMED 15000' IN HIS THOUGHTS INSTEAD OF THE 24000'. I SHOULD HAVE OBSERVED AND X-CHKED HIS ACTIONS, BUT FAILED TO DO SO BY ATTENDING TO OTHER NONESSENTIAL DETAILS AT THE TIME. IT CANNOT BE STRESSED ENOUGH TO BE ALWAYS VIGILANT--'COPY, CONFIRM AND X-CHK.'

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.