Air carrier X unauthorized descent through assigned altitude. Altitude deviation.

Attributes
ACN	294392
Time
Date	199501
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : unz
State Reference	GU
Altitude	msl bound lower : 3500 msl bound upper : 4000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : gum artcc : czqx
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	descent other
Route In Use	enroute : direct
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 : 11000 flight time type : 3000
ASRS Report	294392
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 10000 flight time type : 300
ASRS Report	294390
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 18000 vertical : 800
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Air carrier X had been cleared to descend to 2500 ft, which was set in altitude alerter. During descent, a TA for a short was issued. Upon passing 3800 ft MSL, I acquired visual contact corresponding to an aircraft displayed on the TCASII. Concurrently, the controller queried our altitude, saying that we were assigned 4000 ft. At this moment, the shot was passing at what appeared to be minimal, if not compromised lateral and vertical separation. The crew felt confident that the clearance and readback from us was for an assignment of 2500 ft and that the controller was in error believing he had assigned descent to 4000 ft.

Original NASA ASRS Text

Title: ACR X UNAUTH DSCNT THROUGH ASSIGNED ALT. ALTDEV.

Narrative: ACR X HAD BEEN CLRED TO DSND TO 2500 FT, WHICH WAS SET IN ALT ALERTER. DURING DSCNT, A TA FOR A SHORT WAS ISSUED. UPON PASSING 3800 FT MSL, I ACQUIRED VISUAL CONTACT CORRESPONDING TO AN ACFT DISPLAYED ON THE TCASII. CONCURRENTLY, THE CTLR QUERIED OUR ALT, SAYING THAT WE WERE ASSIGNED 4000 FT. AT THIS MOMENT, THE SHOT WAS PASSING AT WHAT APPEARED TO BE MINIMAL, IF NOT COMPROMISED LATERAL AND VERT SEPARATION. THE CREW FELT CONFIDENT THAT THE CLRNC AND READBACK FROM US WAS FOR AN ASSIGNMENT OF 2500 FT AND THAT THE CTLR WAS IN ERROR BELIEVING HE HAD ASSIGNED DSCNT TO 4000 FT.

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.