B737 crew descended based on ILS DME. The ILS DME was unreliable.

Attributes
ACN	420988
Time
Date	199811
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : iubr airport : san
State Reference	CA
Altitude	msl bound lower : 1300 msl bound upper : 2000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : san
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : cfi pilot : atp
Experience	flight time last 90 days : 240 flight time total : 6700 flight time type : 500
ASRS Report	420988
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : crossing restriction not met conflict : ground critical non adherence : published procedure other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Supplementary
Primary Problem	Airport
Air Traffic Incident	Pilot Deviation
Situations
ATC Facility	procedure or policy : unspecified

Narrative:

On the morning of nov/xa/98, approaching san, we had ATIS winds 290 degrees at 5 KTS, visibility 4 mi broken, ceiling 1000 ft broken. Localizer runway 27 was in use. I was flying and the captain was the PNF. Captain told me he idented localizer. We were vectored for the approach and descended on profile. Descending through about 1500 ft the controller advised low altitude alert that we had descended below 2000 ft prior to reebo, FAF. We then climbed up to 1500 ft MSL and soon had VASI in sight and descended to the airport. We had ground contact and 4 NM forward visibility throughout the approach so safety was not compromised. However, it was a valuable lesson for the future. Contributing factors included: 1) unexpected failure of the ground based DME system. 2) approach controller not notifying pilots of failure. 3) notification of failure not on ATIS. 4) failure of DME not on NOTAMS. 5) failure of PNF to identify DME identify. 6) failure of PF to use timing and VOR xradials from poggi as stepdown fixes. The need to constantly xchk and not assume any navaids are accurate is essential. Complacency can lead to accidents.

Original NASA ASRS Text

Title: B737 CREW DSNDED BASED ON ILS DME. THE ILS DME WAS UNRELIABLE.

Narrative: ON THE MORNING OF NOV/XA/98, APCHING SAN, WE HAD ATIS WINDS 290 DEGS AT 5 KTS, VISIBILITY 4 MI BROKEN, CEILING 1000 FT BROKEN. LOC RWY 27 WAS IN USE. I WAS FLYING AND THE CAPT WAS THE PNF. CAPT TOLD ME HE IDENTED LOC. WE WERE VECTORED FOR THE APCH AND DSNDED ON PROFILE. DSNDING THROUGH ABOUT 1500 FT THE CTLR ADVISED LOW ALT ALERT THAT WE HAD DSNDED BELOW 2000 FT PRIOR TO REEBO, FAF. WE THEN CLBED UP TO 1500 FT MSL AND SOON HAD VASI IN SIGHT AND DSNDED TO THE ARPT. WE HAD GND CONTACT AND 4 NM FORWARD VISIBILITY THROUGHOUT THE APCH SO SAFETY WAS NOT COMPROMISED. HOWEVER, IT WAS A VALUABLE LESSON FOR THE FUTURE. CONTRIBUTING FACTORS INCLUDED: 1) UNEXPECTED FAILURE OF THE GND BASED DME SYS. 2) APCH CTLR NOT NOTIFYING PLTS OF FAILURE. 3) NOTIFICATION OF FAILURE NOT ON ATIS. 4) FAILURE OF DME NOT ON NOTAMS. 5) FAILURE OF PNF TO IDENT DME IDENT. 6) FAILURE OF PF TO USE TIMING AND VOR XRADIALS FROM POGGI AS STEPDOWN FIXES. THE NEED TO CONSTANTLY XCHK AND NOT ASSUME ANY NAVAIDS ARE ACCURATE IS ESSENTIAL. COMPLACENCY CAN LEAD TO ACCIDENTS.

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.