Narrative:

The problem arose when I mis-selected the heading mode on the HSI flight director. I was descending into a 2000 ft undercast in a right hand turn to 145 degree to fly the dca runway 18 lda DME approach. I should have selected the GS mode. I also misunderstood the clearance. The clearance was to descend to 2000 ft from 3000 once established on the approach. I was descending before the localizer was declared 'alive' by the captain. By the time I corrected, I was on the localizer and began descending referencing the command bars on the HSI. As I looked at the command bars, it seemed as if I had the GS 'wired.' of course, they were only displaying course correction. The first time I referenced the 'raw data' GS display, it was pegged at the top of the case. I had flown through the GS and was now more than 2 dots low. I could not reconcile this with what seemed a 'right on' GS displayed by the erroneously selected heading mode on the flight director. At 6.5 mi out, with visual references becoming more distinct, I was at 1280 ft AGL. At this point I should have been at 1770 ft AGL. As the captain began asking me about this discrepancy (he assumed I was picking up visual reference on the river approach), the approach controller alertly declared a low altitude warning, and the captain commanded I level off immediately and re- establish the GS. A bit shaken, I flew the last 5 mi of the approach referencing raw ILS data only, and made an uneventful touch down. The captain debriefed me, but I was unable to explain my confusion. It was not until the drive home I realized my error. I had recently been using technique of placing the flight director to the heading mode while on a vector from ATC. This served to remind me that the aircraft was no longer coupled to the ONS, but on vectors. I selected heading instead of GS on the approach due to the recent regularity I had been turning that selector to the heading position. I did not cross check raw data quickly enough and should have immediately said something about a GS display that indicated at least a 2 DOT low flight parameter. Supplemental information from acn 204277: I'm PNF, first officer is PF - only has 150 hours as a pilot in this aircraft in the last 6 mos, only 150 total actually. Military fighter backgnd military fgts, limited use of flight directors.

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Original NASA ASRS Text

Title: ACR FO INADVERTENTLY DSCNDED BELOW THE PRESCRIBED ALT DURING AN INST APCH DUE TO AN IMPROPER SETTING OF THE ACFT'S FLT DIRECTOR.

Narrative: THE PROBLEM AROSE WHEN I MIS-SELECTED THE HDG MODE ON THE HSI FLT DIRECTOR. I WAS DSNDING INTO A 2000 FT UNDERCAST IN A R HAND TURN TO 145 DEG TO FLY THE DCA RWY 18 LDA DME APCH. I SHOULD HAVE SELECTED THE GS MODE. I ALSO MISUNDERSTOOD THE CLRNC. THE CLRNC WAS TO DSND TO 2000 FT FROM 3000 ONCE ESTABLISHED ON THE APCH. I WAS DSNDING BEFORE THE LOC WAS DECLARED 'ALIVE' BY THE CAPT. BY THE TIME I CORRECTED, I WAS ON THE LOC AND BEGAN DSNDING REFING THE COMMAND BARS ON THE HSI. AS I LOOKED AT THE COMMAND BARS, IT SEEMED AS IF I HAD THE GS 'WIRED.' OF COURSE, THEY WERE ONLY DISPLAYING COURSE CORRECTION. THE FIRST TIME I REFED THE 'RAW DATA' GS DISPLAY, IT WAS PEGGED AT THE TOP OF THE CASE. I HAD FLOWN THROUGH THE GS AND WAS NOW MORE THAN 2 DOTS LOW. I COULD NOT RECONCILE THIS WITH WHAT SEEMED A 'RIGHT ON' GS DISPLAYED BY THE ERRONEOUSLY SELECTED HDG MODE ON THE FLT DIRECTOR. AT 6.5 MI OUT, WITH VISUAL REFS BECOMING MORE DISTINCT, I WAS AT 1280 FT AGL. AT THIS POINT I SHOULD HAVE BEEN AT 1770 FT AGL. AS THE CAPT BEGAN ASKING ME ABOUT THIS DISCREPANCY (HE ASSUMED I WAS PICKING UP VISUAL REF ON THE RIVER APCH), THE APCH CTLR ALERTLY DECLARED A LOW ALT WARNING, AND THE CAPT COMMANDED I LEVEL OFF IMMEDIATELY AND RE- ESTABLISH THE GS. A BIT SHAKEN, I FLEW THE LAST 5 MI OF THE APCH REFING RAW ILS DATA ONLY, AND MADE AN UNEVENTFUL TOUCH DOWN. THE CAPT DEBRIEFED ME, BUT I WAS UNABLE TO EXPLAIN MY CONFUSION. IT WAS NOT UNTIL THE DRIVE HOME I REALIZED MY ERROR. I HAD RECENTLY BEEN USING TECHNIQUE OF PLACING THE FLT DIRECTOR TO THE HDG MODE WHILE ON A VECTOR FROM ATC. THIS SERVED TO REMIND ME THAT THE ACFT WAS NO LONGER COUPLED TO THE ONS, BUT ON VECTORS. I SELECTED HDG INSTEAD OF GS ON THE APCH DUE TO THE RECENT REGULARITY I HAD BEEN TURNING THAT SELECTOR TO THE HDG POS. I DID NOT CROSS CHK RAW DATA QUICKLY ENOUGH AND SHOULD HAVE IMMEDIATELY SAID SOMETHING ABOUT A GS DISPLAY THAT INDICATED AT LEAST A 2 DOT LOW FLT PARAMETER. SUPPLEMENTAL INFO FROM ACN 204277: I'M PNF, FO IS PF - ONLY HAS 150 HRS AS A PLT IN THIS ACFT IN THE LAST 6 MOS, ONLY 150 TOTAL ACTUALLY. MIL FIGHTER BACKGND MIL FGTS, LIMITED USE OF FLT DIRECTORS.

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.