A DA20 ATX FO DSNDS ON A FALSE GS WHEN RETURNING TO FSD; ND; AT NIGHT. RETURN MANDATED BY A PRESSURIZATION PROB DURING DEP CLB. ACFT WAS BELOW GS WHEN ATC ALERTED THE FLC.

Narrative

WE HAD JUST DEPARTED FROM FSD RWY 15 BOUND FOR IND AT NIGHT IN IFR AND WERE CLRED TO FL230 BY ATC. IT WAS MY LEG TO FLY. AT 10000 FT I CALLED OUT; '10000 FT.' CAPT PERFORMED THE 10000 FT CHKS; TURNED OFF SOME EXTERIOR LIGHTS; CHKED THE PRESSURIZATION AND AIRSPD; AND CALLED OUT; '10000 FT CHKS COMPLETED.' WHEN WE WERE ABOUT TO REACH 11000 FT; A RED CABIN WARNING LIGHT CAME ON; ON THE MASTER FAULT ANNUNCIATOR PANEL. I DECIDED TO MAINTAIN 10000 FT BEFORE CAPT TOLD ME NOT TO DSND; I TOLD CAPT TO CONTACT ATC. BUT CAPT WAS TRYING TO FIND OUT WHY THE PRESSURIZATION WAS NOT FUNCTIONING WHEN ATC ASKED US; 'ARE YOU CLBING?' THEN CAPT TOLD ATC THAT WE WERE EXPERIENCING A CABIN PRESSURIZATION PROB AND REQUESTED TO RETURN TO FSD FOR LNDG. ATC VECTORED US TO ILS RWY 21 AT SIOUX FALLS. WITH THE LAST ASSIGNED HDG OF 240 DEGS; MAINTAINING AN ALT OF 3100 FT MSL; ATC CLRED US FOR ILS RWY 21 APCH. AS THE LOC WAS ALIVE; I BEGAN A SHALLOW BANK TO THE L AND CAPTURED THE LOC. ABOUT THAT TIME; THE RED GS FLAG WAS RETRACTED AND WAS INDICATING 'ON GS' CONDITION. SO; I CALLED OUT FOR; 'ON GS; AIRSPD 200 KTS; FLAP 15 DEGS.' AFTER THAT I CALLED FOR FLAP 25 DEGS; GEAR DOWN AND BEFORE LNDG CHKLIST; THEN CAPT ASKED ME WHY I WANTED TO CONFIGURE THE ACFT SO EARLY. I EXPLAINED TO CAPT BECAUSE WE WERE ON GS AND ON COURSE. CAPT CHKED MY NAV INSTS TO CONFIRM MY REASONING; NO RED FLAGS AND ON GS INDICATIONS ON BOTH ADI AND HSI. AFTER HE CONCURRED AND I ALSO CALLED FOR FLAP 40 DEGS AS I WAS DSNDING ON GS. AS WE WERE ABOUT TO PASS THROUGH 2100 FT MSL (APPROX 500 FT AGL); ATC INFORMED THAT WE WERE WELL BELOW THE GS PATH. IMMEDIATELY; WE CLBED BACK TO 3100 FT AND FOUND OUT THAT BOTH OF THE R SIDE GS INSTS WERE GIVING FALSE INDICATIONS. SUPPLEMENTAL INFO FROM ACN 393799: THE FO WHO WAS FLYING CALLED OUT; '10000 FT CHKS.' I COMPLETED THE CHKS WHICH ARE: LIGHTS; AIRSPD AND PRESSURIZATION. I THEN RESPONDED; '10000 FT CHKS ARE COMPLETE.' THE LIGHTS FOR THE CABIN PRESSURIZATION WERE TURNED WAY DOWN AND I MISTOOK THE CABIN ALT FOR DIFFERENTIAL. LESS THAN 1 MIN LATER THE CABIN ALT LIGHT AND HORN CAME ON. THE FO CALLED FOR FLAPS 25 DEGS AND I HESITATED AND RESPONDED THE GS IS NOT ALIVE YET. HE THEN SAID; 'I'M SHOWING ON GS.' I LOOKED OVER AND SURE ENOUGH THE GS WAS CTRED AND THERE WAS NO FLAG. I SELECTED FLAPS 25 DEGS; HE CALLED FOR GEAR DOWN AND THE BEFORE LNDG CHKLIST. I LOWERED THE GEAR BUT DID NOT DO THE CHKLIST. THINGS DID NOT LOOK NORMAL. I STARTED CHKING FREQS AND CONFIRMED THEY WERE BOTH THE SAME FREQ; TURNED ON AND THEN NOTICED THE GPS SAID WE WERE STILL 14 MI FROM THE ARPT. SO I GRABBED THE YOKE CALLED OUT 'MY AIRPLANE' AND STARTED TO CLB FROM 2100 FT BACK TO 3100 FT. AT THAT MOMENT THE TWR CALLED AND SAID; 'LOW ALT ALERT; WHAT IS YOUR ALT?' THE GS RECEIVER WAS MELED AND SO WAS THE PRESSURIZATION PROB AND THE FLT CONTINUED 1 1/2 HRS LATER TO IND WITHOUT ANY PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THE SETTING OF THE PRESSURIZATION WAS INCORRECT AND WAS HIS FAULT. THE RPTR STATED THE GS FAILURE WAS A RECEIVER FAULT AND WAS NOT IMMEDIATELY REPAIRED BUT DEFERRED.