B737-700
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 301; Flight Crew Type 8500
Situational Awareness
Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Human Factors
At FL370 on the KORRY 4 Arrival into LGA (shortly after passing PXT) we were given 'cross RIDGY at FL270 and 300 knots.' As pilot flying; I was in the middle of a manual fuel calculation to update the FMC required by an MEL. I stopped the fuel calculation; set the MCP to 270; and began to load the FMC. I entered the speed on the LEGS page at RIDGY; but the FMC would not accept it. I realized I needed to load the speed on the descent page first. I accomplished that then loaded the speed on the LEGS page. I confirmed with the pilot monitoring the RIDGY restrictions and we both agreed. We checked that the top of descent was a number of miles ahead. I returned to the manual fuel calculation for valid speeds during the approach.Upon a frequency change the pilot monitoring reported 'FL370 to cross RIDGY at FL270 and 300 knots.' The Controller responded 'How are you going to do that?' With panic (of course) we looked at the PFD; we still had 30 miles until the top of descent; but we were now just 1 mile prior to RIDGY. Then we both realized that the FMC still had RIDGY at FL270 or above. We apologized and began our descent and asked what the Controller needed us to do. He said to 'just descend via the KORRY4.' There appeared to be no traffic conflict. The remainder of the flight was uneventful.Upon debriefing the event we realized that I focused on solving the problem of entering the speed. 'FL270A' was already loaded on the LEGS page as normal part of the arrival. Once I solved the issue with loading the speed; my mind went to 'problem solved' mode; and moved on to the fuel calculation issue. Neither the pilot monitoring nor I realized that we never changed the 'at or above' restriction to an 'at' restriction. On the previous leg we had just discussed the importance of doing a manual calculation of descent angles to insure the result of the FMC was correct. Trust; but verify. But I didn't in this instance because I re-focused on the fuel calculation. Clearly this was a pure clerical oversight on my part. We were simply lucky that the issue didn't create a conflict. We also focused on the top of descent point; rather than the individual points on the arrival.Checking the individual points versus just focusing on the top of descent would have revealed our error sooner. In the future I will focus on the detail of the FMC loading; complete solving one issue before moving on to the next (or previous); and always; always do a manual descent calculation.
B737-700 Captain reported failing to comply with a crossing restriction that has been modified by ATC on the KORRY 4 arrival into LGA.