An air carrier medium large transport flight crew flew below its assigned altitude.

Attributes
ACN	258025
Time
Date	199311
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : sjn
State Reference	AZ
Altitude	msl bound lower : 27850 msl bound upper : 28000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zab
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
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 : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 6000 flight time type : 1800
ASRS Report	258025
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : undershoot non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Miss Distance	vertical : 750
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Capts flying leg while using 'perf,' a computer to control flight of airplane. It has a tolerance of plus or minus 150 either side of level flight. This was a long day for both captain and first officer, approximately 8 hours of flying plus a commute in early in the morning. ZAB advised traffic 11-12 O'clock. We had him on TCASII and we both saw and reported oncoming traffic in sight. As air carrier Y came closer, our TCASII was telling us to climb but we had traffic in sight and seemed no conflict of 3 mi away, but we showed him 700 ft below us. Performance might have been past its tolerance or he might have been a little high. We had a visual on him and no conflict with our separation.

Original NASA ASRS Text

Title: AN ACR MLG FLC FLEW BELOW ITS ASSIGNED ALT.

Narrative: CAPTS FLYING LEG WHILE USING 'PERF,' A COMPUTER TO CTL FLT OF AIRPLANE. IT HAS A TOLERANCE OF PLUS OR MINUS 150 EITHER SIDE OF LEVEL FLT. THIS WAS A LONG DAY FOR BOTH CAPT AND FO, APPROX 8 HRS OF FLYING PLUS A COMMUTE IN EARLY IN THE MORNING. ZAB ADVISED TFC 11-12 O'CLOCK. WE HAD HIM ON TCASII AND WE BOTH SAW AND RPTED ONCOMING TFC IN SIGHT. AS ACR Y CAME CLOSER, OUR TCASII WAS TELLING US TO CLB BUT WE HAD TFC IN SIGHT AND SEEMED NO CONFLICT OF 3 MI AWAY, BUT WE SHOWED HIM 700 FT BELOW US. PERFORMANCE MIGHT HAVE BEEN PAST ITS TOLERANCE OR HE MIGHT HAVE BEEN A LITTLE HIGH. WE HAD A VISUAL ON HIM AND NO CONFLICT WITH OUR SEPARATION.

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.