A B737-300 descended below the assigned altitude on approach to ewr.

Attributes
ACN	451540
Time
Date	199910
Day	Sat
Local Time Of Day	1201 To 1800
Place
State Reference	NJ
Altitude	msl single value : 2179
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : ewr.tower
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 22l other
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : multi engine pilot : commercial pilot : instrument pilot : atp
Experience	flight time last 90 days : 200 flight time total : 10000 flight time type : 3500
ASRS Report	451540
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : commercial pilot : instrument
Events
Anomaly	altitude deviation : crossing restriction not met non adherence : clearance
Independent Detector	other flight crewa other flight crewb
Resolutory Action	none taken : anomaly accepted
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

The problem arose while on approach to ewr (ILS runway 22L). Approach control cleared us to maintain 3000 ft till established, cleared ILS runway 22L. Autoplt coupled, approach mode engaged. As GS intercept occurred, while continuing to configure for landing and attempting to locate traffic, autoplt initiated descent. Aircraft crossed gimme intersection at 2179 ft MSL as indicated by progress page. Corrective action: none taken since aircraft was already past the point of the altitude restr. Contributing factors: the main contributing factor is the procedure to clear aircraft to intercept the ILS from an altitude of 3000 ft. Once established on localizer, because we have been cleared for the full approach, GS will be followed to the missed approach. If there is to be an altitude restr while inside GS capture, the altitude at which one is cleared to start the approach should be adjusted either to the altitude of the restr or an altitude that will allow descent via the GS without descending below the restr prior to the appropriate time/location. I would also like to see a change in wording for the approach clearance -- more precise, inclusive.

Original NASA ASRS Text

Title: A B737-300 DSNDED BELOW THE ASSIGNED ALT ON APCH TO EWR.

Narrative: THE PROB AROSE WHILE ON APCH TO EWR (ILS RWY 22L). APCH CTL CLRED US TO MAINTAIN 3000 FT TILL ESTABLISHED, CLRED ILS RWY 22L. AUTOPLT COUPLED, APCH MODE ENGAGED. AS GS INTERCEPT OCCURRED, WHILE CONTINUING TO CONFIGURE FOR LNDG AND ATTEMPTING TO LOCATE TFC, AUTOPLT INITIATED DSCNT. ACFT CROSSED GIMME INTXN AT 2179 FT MSL AS INDICATED BY PROGRESS PAGE. CORRECTIVE ACTION: NONE TAKEN SINCE ACFT WAS ALREADY PAST THE POINT OF THE ALT RESTR. CONTRIBUTING FACTORS: THE MAIN CONTRIBUTING FACTOR IS THE PROC TO CLR ACFT TO INTERCEPT THE ILS FROM AN ALT OF 3000 FT. ONCE ESTABLISHED ON LOC, BECAUSE WE HAVE BEEN CLRED FOR THE FULL APCH, GS WILL BE FOLLOWED TO THE MISSED APCH. IF THERE IS TO BE AN ALT RESTR WHILE INSIDE GS CAPTURE, THE ALT AT WHICH ONE IS CLRED TO START THE APCH SHOULD BE ADJUSTED EITHER TO THE ALT OF THE RESTR OR AN ALT THAT WILL ALLOW DSCNT VIA THE GS WITHOUT DSNDING BELOW THE RESTR PRIOR TO THE APPROPRIATE TIME/LOCATION. I WOULD ALSO LIKE TO SEE A CHANGE IN WORDING FOR THE APCH CLRNC -- MORE PRECISE, INCLUSIVE.

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.