B747 descends 150 ft below assigned altitude.

Attributes
ACN	347317
Time
Date	199609
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : jfk
State Reference	NY
Altitude	msl bound lower : 1350 msl bound upper : 1500
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : n90
Operator	common carrier : air carrier
Make Model Name	B747 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	descent other
Route In Use	arrival other enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : atp
Experience	flight time last 90 days : 280 flight time total : 13000 flight time type : 320
ASRS Report	347317
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 270 flight time total : 20000 flight time type : 5000
ASRS Report	347070
Events
Anomaly	altitude deviation : overshoot non adherence : far non adherence : clearance
Independent Detector	atc equipment other atc equipment : unspecified
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 descending and on relatively large intercept angle to localizer, we were at approximately 3500 ft. I was told to expedite our descent to 1500 ft. In the turn I greatly increased our descent rate. 250 ft prior to 1500 ft (on autoplt), I decreased autoplt selected descent rate to approximately 300 FPM. At 1500 ft I engaged altitude hold. The aircraft continued to descend approximately 125 ft below assigned altitude. We were given a 'low altitude alert check your altimeters' from approach. I disengaged the autoplt, flew back to assigned altitude and was cleared for approach and landed uneventfully. This incident in my opinion occurred because of the low altitude, expedite descent request in a turn and intercepting localizer. Also because of my inexperience with B747 and time required to 'stop the mass of the aircraft' while on autoplt. The dual role in B747 of both an altitude select and altitude hold position on the autoplt has been new to me and I do not like the system, however, recognize it will not be changed.

Original NASA ASRS Text

Title: B747 DSNDS 150 FT BELOW ASSIGNED ALT.

Narrative: WHILE DSNDING AND ON RELATIVELY LARGE INTERCEPT ANGLE TO LOC, WE WERE AT APPROX 3500 FT. I WAS TOLD TO EXPEDITE OUR DSCNT TO 1500 FT. IN THE TURN I GREATLY INCREASED OUR DSCNT RATE. 250 FT PRIOR TO 1500 FT (ON AUTOPLT), I DECREASED AUTOPLT SELECTED DSCNT RATE TO APPROX 300 FPM. AT 1500 FT I ENGAGED ALT HOLD. THE ACFT CONTINUED TO DSND APPROX 125 FT BELOW ASSIGNED ALT. WE WERE GIVEN A 'LOW ALT ALERT CHK YOUR ALTIMETERS' FROM APCH. I DISENGAGED THE AUTOPLT, FLEW BACK TO ASSIGNED ALT AND WAS CLRED FOR APCH AND LANDED UNEVENTFULLY. THIS INCIDENT IN MY OPINION OCCURRED BECAUSE OF THE LOW ALT, EXPEDITE DSCNT REQUEST IN A TURN AND INTERCEPTING LOC. ALSO BECAUSE OF MY INEXPERIENCE WITH B747 AND TIME REQUIRED TO 'STOP THE MASS OF THE ACFT' WHILE ON AUTOPLT. THE DUAL ROLE IN B747 OF BOTH AN ALT SELECT AND ALT HOLD POS ON THE AUTOPLT HAS BEEN NEW TO ME AND I DO NOT LIKE THE SYS, HOWEVER, RECOGNIZE IT WILL NOT BE CHANGED.

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.