Low time GA pilot descended too soon on instrument approach. Reporter cites failure of DME and marker as contributory to being unable to identify left outer marker. Error caught as pilot utilized ADF to identify location and as low altitude alert was issued by tower controller.

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Attributes
ACN	130970
Time
Date	198912
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : bfi
State Reference	WA
Altitude	msl bound lower : 1700 msl bound upper : 2200
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	tower : bfi
Operator	general aviation : personal
Make Model Name	Small Aircraft, Low Wing, 1 Eng, Retractable Gear
Flight Phase	descent : approach
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : private pilot : instrument
Experience	flight time last 90 days : 94 flight time total : 440 flight time type : 204
ASRS Report	130970
Person 2
Affiliation	government : faa
Function	controller : local
Qualification	controller : non radar
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : crossing restriction not met conflict : ground less severe non adherence : clearance non adherence : published procedure
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Ambiguous
Air Traffic Incident	Pilot Deviation other

Narrative:

IFR, cleared for approach at bfi. Upon intercepting localizer, was waiting for G/south, OM signal and DME to locate OM (nolla). DME gave intermittent mileage which appeared correct for OM distance, then failed. Called tower and started descent. Upon realizing that DME, marker receiver and G/south all had failed, pilot used ADF and realized that altitude was low (1700' MSL). Tower notified pilot that they had altitude alert. Pilot informed tower that problem was discovered and corrective action had been underway. Climbed back to 2200' on localizer, and at same time intercept nolla on ADF. Informed tower that landing could be completed on localizer. Cleared to land with no problem resulting. Factors affecting performance: subsequent repair of instruments suggests that time creates weakness in older instruments. Possibly there should be time limits on certain essential and critical IFR instruments.

Original NASA ASRS Text

Title: LOW TIME GA PLT DESCENDED TOO SOON ON INSTRUMENT APCH. REPORTER CITES FAILURE OF DME AND MARKER AS CONTRIBUTORY TO BEING UNABLE TO IDENTIFY LEFT OUTER MARKER. ERROR CAUGHT AS PLT UTILIZED ADF TO IDENTIFY LOCATION AND AS LOW ALT ALERT WAS ISSUED BY TWR CTLR.

Narrative: IFR, CLRED FOR APCH AT BFI. UPON INTERCEPTING LOC, WAS WAITING FOR G/S, OM SIGNAL AND DME TO LOCATE OM (NOLLA). DME GAVE INTERMITTENT MILEAGE WHICH APPEARED CORRECT FOR OM DISTANCE, THEN FAILED. CALLED TWR AND STARTED DESCENT. UPON REALIZING THAT DME, MARKER RECEIVER AND G/S ALL HAD FAILED, PLT USED ADF AND REALIZED THAT ALT WAS LOW (1700' MSL). TWR NOTIFIED PLT THAT THEY HAD ALT ALERT. PLT INFORMED TWR THAT PROB WAS DISCOVERED AND CORRECTIVE ACTION HAD BEEN UNDERWAY. CLBED BACK TO 2200' ON LOC, AND AT SAME TIME INTERCEPT NOLLA ON ADF. INFORMED TWR THAT LNDG COULD BE COMPLETED ON LOC. CLRED TO LAND WITH NO PROB RESULTING. FACTORS AFFECTING PERFORMANCE: SUBSEQUENT REPAIR OF INSTRUMENTS SUGGESTS THAT TIME CREATES WEAKNESS IN OLDER INSTRUMENTS. POSSIBLY THERE SHOULD BE TIME LIMITS ON CERTAIN ESSENTIAL AND CRITICAL IFR INSTRUMENTS.

Data retrieved from NASA's ASRS site as of August 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.