Air carrier large transport overshot altitude on descent. Less than standard separation.

Attributes
ACN	97850
Time
Date	198810
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : fim
State Reference	CA
Altitude	msl bound lower : 23550 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	enroute airway : j7
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 8500
ASRS Report	97850
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	altitude deviation : overshoot conflict : airborne less severe non adherence : clearance
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	faa : investigated
Miss Distance	horizontal : 21000 vertical : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were at FL270 descending to FL240 and I gave control of the ATC radio to the first officer and went to pick up bur ATIS and talk to company. While I was copying ATIS I heard the altitude alert sound. I looked up and noticed we were 300' low and that the first officer had disconnected the autoplt. He arrested the descent at 23550' and climbed back to FL240. During this the controller questioned our altitude and the first officer said we were going back to FL240. Later on the phone ATC said we had come within 3 1/2 mi and 400' of another airplane. I later learned that the first officer thought we had been cleared to 14000'. He was also asked to increase speed from 260 to 290 KTS. To accomplish this the first officer pressed the autoplt pitch down selector. When this is done close to an altitude to be captured it disengages the altitude capture. This is why the autoplt didn't level us at FL240.

Original NASA ASRS Text

Title: ACR LGT OVERSHOT ALT ON DESCENT. LESS THAN STANDARD SEPARATION.

Narrative: WE WERE AT FL270 DSNDING TO FL240 AND I GAVE CTL OF THE ATC RADIO TO THE F/O AND WENT TO PICK UP BUR ATIS AND TALK TO COMPANY. WHILE I WAS COPYING ATIS I HEARD THE ALT ALERT SOUND. I LOOKED UP AND NOTICED WE WERE 300' LOW AND THAT THE F/O HAD DISCONNECTED THE AUTOPLT. HE ARRESTED THE DSCNT AT 23550' AND CLBED BACK TO FL240. DURING THIS THE CTLR QUESTIONED OUR ALT AND THE F/O SAID WE WERE GOING BACK TO FL240. LATER ON THE PHONE ATC SAID WE HAD COME WITHIN 3 1/2 MI AND 400' OF ANOTHER AIRPLANE. I LATER LEARNED THAT THE F/O THOUGHT WE HAD BEEN CLRED TO 14000'. HE WAS ALSO ASKED TO INCREASE SPD FROM 260 TO 290 KTS. TO ACCOMPLISH THIS THE F/O PRESSED THE AUTOPLT PITCH DOWN SELECTOR. WHEN THIS IS DONE CLOSE TO AN ALT TO BE CAPTURED IT DISENGAGES THE ALT CAPTURE. THIS IS WHY THE AUTOPLT DIDN'T LEVEL US AT FL240.

Data retrieved from NASA's ASRS site as of August 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.