A lear 35 flight crew descends 4000 ft below their assigned altitude, approximately 2000 ft below the MEA for that segment of the airway northwest of las, nv.

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Attributes
ACN	486114
Time
Date	200009
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : las.airport
State Reference	NV
Altitude	msl bound lower : 12000 msl bound upper : 16000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla.artcc
Operator	common carrier : charter
Make Model Name	Learjet 35
Operating Under FAR Part	Part 135
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : charter
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : commercial pilot : cfi pilot : instrument
Experience	flight time last 90 days : 60 flight time total : 1625 flight time type : 10
ASRS Report	486114
Person 2
Affiliation	company : charter
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence other other anomaly other other spatial deviation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance controller : issued advisory
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

In level flight, at FL290, en route from vancouver to las, the center controller cleared us to descend to what I believed was 12000 ft. I read back 'leaving two-niner-zero for one-two thousand,' to which there was no reply. The PF then descended to 12000 ft. Upon leveling at 12000 ft, I noticed terrain at our altitude. This terrain was approximately 20 mi ahead in the 12 O'clock position. As I was preparing to verify our altitude assignment, the controller asked for verification that we were level at 16000 ft. When I indicated we were at 12000 ft. He asked if we had the terrain in sight, and asked us to climb/maintain 14000 ft. The flight concluded normally. In this case a misunderstanding between ATC and the flight crew caused an error in vertical positioning of the aircraft. Although 4000 ft of error occurred, the system caught the mistake in time to prevent a loss of separation between the aircraft and terrain. Closer monitoring of readbacks by ATC, and closer monitoring of MSA's by the flight crew could prevent this type of problem in the future.

Original NASA ASRS Text

Title: A LEAR 35 FLC DSNDS 4000 FT BELOW THEIR ASSIGNED ALT, APPROX 2000 FT BELOW THE MEA FOR THAT SEGMENT OF THE AIRWAY NW OF LAS, NV.

Narrative: IN LEVEL FLT, AT FL290, ENRTE FROM VANCOUVER TO LAS, THE CTR CTLR CLRED US TO DSND TO WHAT I BELIEVED WAS 12000 FT. I READ BACK 'LEAVING TWO-NINER-ZERO FOR ONE-TWO THOUSAND,' TO WHICH THERE WAS NO REPLY. THE PF THEN DSNDED TO 12000 FT. UPON LEVELING AT 12000 FT, I NOTICED TERRAIN AT OUR ALT. THIS TERRAIN WAS APPROX 20 MI AHEAD IN THE 12 O'CLOCK POS. AS I WAS PREPARING TO VERIFY OUR ALT ASSIGNMENT, THE CTLR ASKED FOR VERIFICATION THAT WE WERE LEVEL AT 16000 FT. WHEN I INDICATED WE WERE AT 12000 FT. HE ASKED IF WE HAD THE TERRAIN IN SIGHT, AND ASKED US TO CLB/MAINTAIN 14000 FT. THE FLT CONCLUDED NORMALLY. IN THIS CASE A MISUNDERSTANDING BTWN ATC AND THE FLC CAUSED AN ERROR IN VERT POSITIONING OF THE ACFT. ALTHOUGH 4000 FT OF ERROR OCCURRED, THE SYS CAUGHT THE MISTAKE IN TIME TO PREVENT A LOSS OF SEPARATION BTWN THE ACFT AND TERRAIN. CLOSER MONITORING OF READBACKS BY ATC, AND CLOSER MONITORING OF MSA'S BY THE FLC COULD PREVENT THIS TYPE OF PROB IN THE FUTURE.

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.