With unreliable runway 27 DME signal, B737-300 flight crew began descent early resulting in a low al...

Attributes
ACN	421304
Time
Date	199811
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : san
State Reference	CA
Altitude	msl bound lower : 1500 msl bound upper : 2000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Operator	common carrier : air carrier
Navigation In Use	ils localizer & glide slope : iubr 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 : captain oversight : pic
Qualification	pilot : flight engineer
Experience	flight time last 90 days : 190 flight time total : 10000 flight time type : 2000
ASRS Report	421304
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : crossing restriction not met non adherence : published procedure non adherence : far other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera other flight crewa
Resolutory Action	other other
Consequence	Other
Supplementary
Air Traffic Incident	Pilot Deviation

Narrative:

San runway 27 localizer DME had been notamed OTS several hours earlier, however, appeared to be back up and working fine. Socal approach vectored us for the localizer 27 approach and mentioned the DME is OTS. Since it was up and working, I assumed he didn't know it was back up. The first officer's navigation control head we discovered was stuck in the navigation/IRS position and he could not select a VOR or localizer frequency. During the localizer 27 approach I used the DME to identify the letdown fixes vydda, swatt, and reebo. At approximately 5 DME at approximately 1500 ft MSL, socal approach said they were receiving a low level or low altitude alert on us. We immediately began a climb and announced a missed approach. On the second approach, socal approach used the words 'DME unreliable' which has a whole different meaning. We then used FMC position to identify the letdown fixes and all went well. As it turns out the DME was in error about 2 mi. Had the controller used the word 'unreliable' instead of 'OTS' we would never have used it. The DME indicated accurately at vydda and swatt. Due to the DME error we had started our descent to MDA prior to reebo. We learned to consider OTS and unreliable as one and the same. With the first officer's VOR receiver control head stuck in LNAV, we could not use the xradials from poggi to identify the fixes vydda, swatt, and reebo. We didn't discover the control head problem until we were inbound on the runway 27 localizer and the first officer moved his selector switch from navigation to VOR ILS.

Original NASA ASRS Text

Title: WITH UNRELIABLE RWY 27 DME SIGNAL, B737-300 FLC BEGAN DSCNT EARLY RESULTING IN A LOW ALT ALERT AND ENSUING MISSED APCH AT SAN, LINDBERGH FIELD.

Narrative: SAN RWY 27 LOC DME HAD BEEN NOTAMED OTS SEVERAL HRS EARLIER, HOWEVER, APPEARED TO BE BACK UP AND WORKING FINE. SOCAL APCH VECTORED US FOR THE LOC 27 APCH AND MENTIONED THE DME IS OTS. SINCE IT WAS UP AND WORKING, I ASSUMED HE DIDN'T KNOW IT WAS BACK UP. THE FO'S NAV CTL HEAD WE DISCOVERED WAS STUCK IN THE NAV/IRS POS AND HE COULD NOT SELECT A VOR OR LOC FREQ. DURING THE LOC 27 APCH I USED THE DME TO IDENT THE LETDOWN FIXES VYDDA, SWATT, AND REEBO. AT APPROX 5 DME AT APPROX 1500 FT MSL, SOCAL APCH SAID THEY WERE RECEIVING A LOW LEVEL OR LOW ALT ALERT ON US. WE IMMEDIATELY BEGAN A CLB AND ANNOUNCED A MISSED APCH. ON THE SECOND APCH, SOCAL APCH USED THE WORDS 'DME UNRELIABLE' WHICH HAS A WHOLE DIFFERENT MEANING. WE THEN USED FMC POS TO IDENT THE LETDOWN FIXES AND ALL WENT WELL. AS IT TURNS OUT THE DME WAS IN ERROR ABOUT 2 MI. HAD THE CTLR USED THE WORD 'UNRELIABLE' INSTEAD OF 'OTS' WE WOULD NEVER HAVE USED IT. THE DME INDICATED ACCURATELY AT VYDDA AND SWATT. DUE TO THE DME ERROR WE HAD STARTED OUR DSCNT TO MDA PRIOR TO REEBO. WE LEARNED TO CONSIDER OTS AND UNRELIABLE AS ONE AND THE SAME. WITH THE FO'S VOR RECEIVER CTL HEAD STUCK IN LNAV, WE COULD NOT USE THE XRADIALS FROM POGGI TO IDENT THE FIXES VYDDA, SWATT, AND REEBO. WE DIDN'T DISCOVER THE CTL HEAD PROB UNTIL WE WERE INBOUND ON THE RWY 27 LOC AND THE FO MOVED HIS SELECTOR SWITCH FROM NAV TO VOR ILS.

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.

With unreliable runway 27 DME signal, B737-300 flight crew began descent early resulting in a low altitude alert and ensuing missed approach at san, lindbergh field.