Date: 201308
Local time of day: 1201-1800
Location: DEN.Airport, CO
Flight conditions: Mixed
Light: Dusk
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B LDORA1
ATC / advisory: TRACON D01
Aircraft component: FMS/FMC
Component problem: Improperly Operated; Design
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Communication Breakdown; Human-Machine Interface; Confusion
Second reporter function: First Officer; Pilot Not Flying
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Critical; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure; Chart Or Publication
Primary problem: Procedure

Narrative: I will start by saying that DEN has 70 charts; including 10 for arrivals alone! We were cleared on the LDORA RNAV STAR. DEN Approach cleared us to descend via LDORA 1 Runway 34L. Inside of the transition (about 15 miles from LARKS); we were given the PEEKK two Runway 16R and a clearance for direct to LARKS descending on the PEEKK 2 Runway 16R. The FMC would not let me execute the PEEKK Arrival and we were now descending for LARKS between FL190 and 17;000 FT per the LDORA Arrival. I told the Controller we needed a heading and altitude as we were having trouble loading the new arrival into the FMC. He told us to fly present heading and descend to 17;000 FT. We reloaded the transition into the FMC and it finally let us execute the PEEKK Arrival. We told ATC that we now had the arrival loaded and he re cleared us to LARKS and to descend on [via?] the PEEKK 2. It was as we were descending though 19;500 FT that I noticed that you must cross LARKS on the PEEKK Arrival between FL220 and FL200!  First; they have to get rid of some of the procedures at DEN. The names are hard to understand and hard to find in the book. Then they have to stop giving us very complicated changes so late in the procedure. (We are going 300 KTS 15 miles from a fix and they expect us to know what all the silly named fixes and procedures are!) That close inside of the transitions [the only acceptable change to a descend via clearance] should be 'FLY HEADING XXX; MAINTAIN XXX; THIS WILL BE RADAR VECTORS FOR RWY XX.'Another BIG gotcha is the different crossing altitudes at LARKS!!!!One more thing; DEN changes runways way too many times. I know--it's in the mountains and the wind shifts a lot (I used to fly into Stapleton); but they seem to do it unnecessarily a lot of times.


Second narrative: The Captain was the pilot flying and I was the pilot monitoring. We were given a descent to FL350 with discretion to FL270. Captain used Altitude Intervention to initiate the descent. At the time; we were on the LDORA 1 Arrival into Denver. While in the descent; Denver Center changed our arrival to the PEEKK 2. The clearance was 'cleared direct to larks and the PEEKK 2 Arrival to Denver.'' The Captain; being the pilot flying; went about changing the arrival in the FMC and I started to get out the new arrival. He selected direct to LARKS and executed the selection; doing this; however; while the LDORA was still active. After getting the PEEKK 2 Arrival out; I noticed the Captain having trouble getting the new arrival loaded into the FMC. I offered a few suggestions as the Captain grew frustrated. He then noticed that the seatbelt sign was still off and suggested I turn it back on. I said goodbye to the passengers and turned the seatbelt sign back on. When I returned from that duty; the Captain was still having trouble with FMC. I believe we were given discretion to FL200 and were handed off to Denver Approach. The PEEKK 2 was still not loaded into the FMC. The Captain told Approach we were having trouble with the FMC and asked for vectors. We were told fly present heading and maintain 17;000 FT. This was my first chance to load the arrival into the FMC and; just as soon as the Captain asked for vectors; I notified him that I had the arrival loaded. The Captain then told ATC that we had the arrival ready and we were told 'cleared to Denver via direct LARKS and the PEEKK 2 descend via the PEEKK 2.' The problem was that we were below FL200 at this point (I believe we were at FL194) and the crossing restriction at LARKS on the PEEKK 2 is between FL220 and FL200. We were five to ten miles from LARKS at this point and we climbed back to FL200; crossed LARKS; and then continued a normal descent and arrival via the PEEKK 2. I don't feel this altitude deviation is an issue because we were descending as cleared by Approach to 17;000 FT. When we were recleared to the PEEKK 2 Arrival and we corrected our altitude. ATC never asked about our altitude or said anything about an altitude problem or traffic. In fact; the crossing altitude for LARKS on the LDORA Arrival is between 17;000 FT and FL190 so terrain or traffic could be an issue. The Captain still felt we should file a safety report.


Synopsis: Confusion ensued when ATC changed the B737-700's clearance from 'descend via the LDORA RNAV STAR to land north' to 'proceed direct LARKS (a fix on both assigned STARS); recleared to descend via the PEEKK RNAV STAR to land south.'