LR25B flight crew misunderstood clearance and descended below their assigned altitude of FL220.

Attributes
ACN	350280
Time
Date	199610
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : elp
State Reference	TX
Altitude	msl bound lower : 21400 msl bound upper : 22000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zab
Operator	common carrier : air taxi
Make Model Name	Learjet 25
Operating Under FAR Part	Part 135
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : first officer
Qualification	pilot : cfi pilot : atp pilot : commercial pilot : instrument
Experience	flight time last 90 days : 160 flight time total : 1980 flight time type : 700
ASRS Report	350280
Person 2
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : commercial pilot : atp
Experience	flight time last 90 days : 210 flight time total : 7000 flight time type : 2000
ASRS Report	350279
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	other controllera
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

During cruise at FL390, ZAB issued the following clearance: 'descend to 10000 ft, discretion out of FL290.' we started our descent to 10000 ft, and temporarily leveled at FL290. A few mins later, ZAB issued the following clearance: 'descend to FL220' with our altitude alerter still set at 10000 ft (no amendment issued). We started another descent and at FL215 we verified that we were still cleared to 10000 ft. The controller responded, 'negative, maintain FL220,' and we immediately climbed back to FL220. Communication was the problem, and we should have verified our descent clearance earlier, however, we were not told to amend our previously assigned altitude.

Original NASA ASRS Text

Title: LR25B FLC MISUNDERSTOOD CLRNC AND DSNDED BELOW THEIR ASSIGNED ALT OF FL220.

Narrative: DURING CRUISE AT FL390, ZAB ISSUED THE FOLLOWING CLRNC: 'DSND TO 10000 FT, DISCRETION OUT OF FL290.' WE STARTED OUR DSCNT TO 10000 FT, AND TEMPORARILY LEVELED AT FL290. A FEW MINS LATER, ZAB ISSUED THE FOLLOWING CLRNC: 'DSND TO FL220' WITH OUR ALT ALERTER STILL SET AT 10000 FT (NO AMENDMENT ISSUED). WE STARTED ANOTHER DSCNT AND AT FL215 WE VERIFIED THAT WE WERE STILL CLRED TO 10000 FT. THE CTLR RESPONDED, 'NEGATIVE, MAINTAIN FL220,' AND WE IMMEDIATELY CLBED BACK TO FL220. COM WAS THE PROB, AND WE SHOULD HAVE VERIFIED OUR DSCNT CLRNC EARLIER, HOWEVER, WE WERE NOT TOLD TO AMEND OUR PREVIOUSLY ASSIGNED ALT.

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.