Air carrier descends below assigned altitude.

Attributes
ACN	224139
Time
Date	199210
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : lga
State Reference	NY
Altitude	msl bound lower : 2500 msl bound upper : 3000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	tracon : n90 tower : phl
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent : approach
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi pilot : instrument pilot : flight engineer
Experience	flight time last 90 days : 240 flight time total : 20000 flight time type : 4000
ASRS Report	224139
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other spatial deviation
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While being vectored for an lda-a approach to lga runway 2L, we were cleared for approach to cross 'casle' intersection at 3000 ft. When cleared for approach we were flying at 250 KTS 90 degree intercept, navigating on both lda and INS. We flew through the localizer and while correcting the first officer deleted the waypoints from the FMC (removed casle and cohop) and put in direct runway 22. At this point I did not know he position of the aircraft from casle and while trying to retrieve the approach data on the FMC the aircraft descended to 2500 ft. Approach questioned us and we returned to 3000 ft and corrected back to localizer. I later briefed the first officer to never touch the FMC unless directed to while maneuvering for approach. This incident occurred because we were too fast (not enough warning from controller) and the first officer putting in direct to runway 22 removing needed data (casle). This resulted in my being distracted and losing 500 ft altitude. In this case there was no conflict and no problem as it was VFR.

Original NASA ASRS Text

Title: ACR DSNDS BELOW ASSIGNED ALT.

Narrative: WHILE BEING VECTORED FOR AN LDA-A APCH TO LGA RWY 2L, WE WERE CLRED FOR APCH TO CROSS 'CASLE' INTXN AT 3000 FT. WHEN CLRED FOR APCH WE WERE FLYING AT 250 KTS 90 DEG INTERCEPT, NAVIGATING ON BOTH LDA AND INS. WE FLEW THROUGH THE LOC AND WHILE CORRECTING THE FO DELETED THE WAYPOINTS FROM THE FMC (REMOVED CASLE AND COHOP) AND PUT IN DIRECT RWY 22. AT THIS POINT I DID NOT KNOW HE POS OF THE ACFT FROM CASLE AND WHILE TRYING TO RETRIEVE THE APCH DATA ON THE FMC THE ACFT DSNDED TO 2500 FT. APCH QUESTIONED US AND WE RETURNED TO 3000 FT AND CORRECTED BACK TO LOC. I LATER BRIEFED THE FO TO NEVER TOUCH THE FMC UNLESS DIRECTED TO WHILE MANEUVERING FOR APCH. THIS INCIDENT OCCURRED BECAUSE WE WERE TOO FAST (NOT ENOUGH WARNING FROM CTLR) AND THE FO PUTTING IN DIRECT TO RWY 22 REMOVING NEEDED DATA (CASLE). THIS RESULTED IN MY BEING DISTRACTED AND LOSING 500 FT ALT. IN THIS CASE THERE WAS NO CONFLICT AND NO PROBLEM AS IT WAS VFR.

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.