B737-300 flight crew has altitude deviation.

Attributes
ACN	557181
Time
Date	200208
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : gbn.vortac
State Reference	AZ
Altitude	msl bound lower : 24500 msl bound upper : 25000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zab.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Experience	flight time last 90 days : 150 flight time total : 8000 flight time type : 300
ASRS Report	557181
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	controller : issued new clearance flight crew : returned to assigned altitude
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While en route to phx, ATC gave us a clearance to cross mohak at FL250. My captain entered FL250 in the MCP, which I verified, and I also checked the FMC display to ensure FL250 was entered for mohak. Upon doing so, I vacated FL270 at the proper time in order to make the requested restr. During the descent, I was monitoring the aircraft's approach to FL250 and I decided to reduce the rate of descent by manually applying rearward pressure to the control column. After attaining the desired rate of descent, I allowed the aircraft to continue its descent without manual input. I then diverted my attention to the FMS display, and while doing so, I heard the altitude alerter tone. Immediately noticed the aircraft descending through FL250 and I reacted by pulling on the control column and disengaging the autoplt. I was able to level off at FL245. Immediately upon doing so, ATC cleared me to FL240 which I did without further incident. (ATC never queried the incident nor commented on it.) I was not aware of the fact that a manual control column input can cause the autoplt to not capture an altitude. I also should not have diverted my attention during the leveloff.

Original NASA ASRS Text

Title: B737-300 FLC HAS ALTDEV.

Narrative: WHILE ENRTE TO PHX, ATC GAVE US A CLRNC TO CROSS MOHAK AT FL250. MY CAPT ENTERED FL250 IN THE MCP, WHICH I VERIFIED, AND I ALSO CHKED THE FMC DISPLAY TO ENSURE FL250 WAS ENTERED FOR MOHAK. UPON DOING SO, I VACATED FL270 AT THE PROPER TIME IN ORDER TO MAKE THE REQUESTED RESTR. DURING THE DSCNT, I WAS MONITORING THE ACFT'S APCH TO FL250 AND I DECIDED TO REDUCE THE RATE OF DSCNT BY MANUALLY APPLYING REARWARD PRESSURE TO THE CTL COLUMN. AFTER ATTAINING THE DESIRED RATE OF DESCENT, I ALLOWED THE ACFT TO CONTINUE ITS DESCENT WITHOUT MANUAL INPUT. I THEN DIVERTED MY ATTN TO THE FMS DISPLAY, AND WHILE DOING SO, I HEARD THE ALT ALERTER TONE. IMMEDIATELY NOTICED THE ACFT DSNDING THROUGH FL250 AND I REACTED BY PULLING ON THE CTL COLUMN AND DISENGAGING THE AUTOPLT. I WAS ABLE TO LEVEL OFF AT FL245. IMMEDIATELY UPON DOING SO, ATC CLRED ME TO FL240 WHICH I DID WITHOUT FURTHER INCIDENT. (ATC NEVER QUERIED THE INCIDENT NOR COMMENTED ON IT.) I WAS NOT AWARE OF THE FACT THAT A MANUAL CTL COLUMN INPUT CAN CAUSE THE AUTOPLT TO NOT CAPTURE AN ALT. I ALSO SHOULD NOT HAVE DIVERTED MY ATTN DURING THE LEVELOFF.

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.