Altitude deviation. Overshoot on descent.

Attributes
ACN	146901
Time
Date	199005
Day	Tue
Local Time Of Day	0001 To 0600
Place
Locale Reference	airport : bwi
State Reference	MD
Altitude	msl bound lower : 2300 msl bound upper : 2500
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : bwi
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 descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 158 flight time total : 6400
ASRS Report	146901
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : published procedure
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

We were descending to 2500' which we were to maintain until ellicott NDB, then cleared the ILS 15R approach to bwi. We were flying an medium large transport. Our last clearance was direct to ellicott, which we were initially navigating with the LNAV button depressed and lighted. As I remember, we approached ellicott on a course of 110-130 degrees to the station. During the descent we were below G/south indications received with our VOR/ILS/navigation switches in VOR/ILS. As we approached to within 7 mi or so of ellicott as indicated on the CDU distance, and with the track to ellicott well established and locked on, I took the aircraft out of LNAV and pushed approach. Shortly after this, as we were descending through about 2650' MSL, with the MCP altitude set at 2500', the aircraft locked onto what must have been a spurious and false G/south signal, diving the aircraft in an abnormally rapid fashion to approximately 2300' MSL. As the aircraft began its dive, I immediately disconnected the autoplt, putting considerable back pressure on the yoke to arrest the sink, and then returned to our 2500' altitude. Approximately 30 seconds to 1 min thereafter, we visually sighted the runway and landed west/O incident.

Original NASA ASRS Text

Title: ALT DEVIATION. OVERSHOOT ON DESCENT.

Narrative: WE WERE DSNDING TO 2500' WHICH WE WERE TO MAINTAIN UNTIL ELLICOTT NDB, THEN CLRED THE ILS 15R APCH TO BWI. WE WERE FLYING AN MLG. OUR LAST CLRNC WAS DIRECT TO ELLICOTT, WHICH WE WERE INITIALLY NAVIGATING WITH THE LNAV BUTTON DEPRESSED AND LIGHTED. AS I REMEMBER, WE APCHED ELLICOTT ON A COURSE OF 110-130 DEGS TO THE STATION. DURING THE DSNT WE WERE BELOW G/S INDICATIONS RECEIVED WITH OUR VOR/ILS/NAV SWITCHES IN VOR/ILS. AS WE APCHED TO WITHIN 7 MI OR SO OF ELLICOTT AS INDICATED ON THE CDU DISTANCE, AND WITH THE TRACK TO ELLICOTT WELL ESTABLISHED AND LOCKED ON, I TOOK THE ACFT OUT OF LNAV AND PUSHED APCH. SHORTLY AFTER THIS, AS WE WERE DSNDING THROUGH ABOUT 2650' MSL, WITH THE MCP ALT SET AT 2500', THE ACFT LOCKED ONTO WHAT MUST HAVE BEEN A SPURIOUS AND FALSE G/S SIGNAL, DIVING THE ACFT IN AN ABNORMALLY RAPID FASHION TO APPROX 2300' MSL. AS THE ACFT BEGAN ITS DIVE, I IMMEDIATELY DISCONNECTED THE AUTOPLT, PUTTING CONSIDERABLE BACK PRESSURE ON THE YOKE TO ARREST THE SINK, AND THEN RETURNED TO OUR 2500' ALT. APPROX 30 SECS TO 1 MIN THEREAFTER, WE VISUALLY SIGHTED THE RWY AND LANDED W/O INCIDENT.

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.