Date: 201205
Local time of day: 0001-0600
Location: DEN.Airport, CO
Flight conditions: IMC
Weather elements / visibility: 4
Light: Daylight
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Initial Approach
Airspace: Class B DEN
ATC / advisory: TRACON D01
Aircraft component: FMS/FMC
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Confusion
Second reporter function: First Officer; Pilot Flying
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: On radar vectors to ILS 35L DEN. LOC ID on both sides by pilot not flying. Given heading to intercept LOC; maintain 11;000 FT until established; cleared for ILS 35L. Our intercept point was just outside CRUUP. As LOC intercepted; pilot flying announced there was a right cross track error reference extended centerline in FMC. Pilot not flying stated established on LOC. We both verified the LOC frequency; course; and ID correct. Lateral raw data correct. Glide slope started to come down off max low position indicating we were approaching CRUUP. Pilot flying set 10;000 FT in MCP and started V/S descent. Pilot flying announced we would make CHOLA 10;000 FT restriction and set MCP to 9;000 FT. At 9;600 FT ATC asked if we were on glide slope. Pilot not flying advised no we were not on glide slope. Pilot not flying (Captain) engaged ALT HOLD at 9;500 FT. Aircraft leveled off. At this point; both pilot flying/pilot not flying identified we both lost our situational awareness as to distance from runway. ATC advised we need to meet crossing restrictions. Pilot not flying asked if there was a problem. ATC said possibly. We maintained 9;500 FT and recaptured the profile; rest of approach and landing normal.1. Both pilots briefly lost distance situational awareness with the lateral cross track distraction.2. At no time was separation; terrain clearance or any safety compromised. 3. This incident was a pure human error (mistake). No safety compromised. Once raw data verified; stick with it. I should have taken control of plane while First Officer got his situational awareness back. Instead I was distracted instead of staying on a sure thing.


Second narrative: Vectored to final approach for the ILS 35L Denver; weather was IMC. ATC gave us a vector to intercept the LOC; maintain 11;000 FT to CRUUP; cleared the ILS approach. I was the pilot flying; autopilot was engaged. Aircraft was on the vector initiated a turn onto the final approach course. I had earlier extended the centerline on the FMS. I selected PERF page 2 to crosscheck the cross track error. The FMS showed the aircraft offset to the right of track. Both pilots immediately crosschecked our raw data/LOC frequencies and Idents; and determined that we were in fact on the course. At this point; I momentarily lost situational awareness of our exact distance to the field. I thought we were closer to the field than we actually were; and I initiated a descent to comply with the altitude restrictions on the approach chart. Aircraft descended down through 10;000 FT; and as we were passing 9;700 FT MSL; Denver Approach Controller asked us if we were following the glide slope. The pilot not flying stated no. The Controller then stated we needed to comply with the altitudes on the approach chart. The Captain immediately selected Altitude Hold; and we reassessed our exact position. We then continued the descent per the approach chart. The Captain asked the Controller if there was a problem; he responded 'I don't know.' We continued the approach to a landing. Upon taxi in; Ground Control provided a phone number to contact Approach Control about a possible violation. We were never told if we had an altitude violation. My momentary loss of situational awareness resulted in me making a human error in misreading my instruments.The potential lateral deviation upon intercepting final distracted me enough to cause me to believe I was behind on the approach; causing me to rush the descent. I should have directed the pilot not flying to immediately notify ATC and have the Controller verify our position or cancel the approach clearance until we were confident in our exact aircraft position.


Synopsis: B737 flight crew reports descending early during the ILS 35L at DEN in IMC. While troubleshooting a discrepancy between FMC position and raw data LOC; situational awareness is lost. ATC notices the early descent and questions the crew.