Date: 201701
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: Mixed
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: MCP
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Diverted
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: MCP altitude selector failed.


Second narrative: We took off turned left to 280 and to climb to 5000'. We were climbing and everything indicated proper. They gave us a higher altitude. I dialed it in to the MCP altitude selector; and the window showed 4700'; and would not change from that. On the Ground; we had already selected 5000'. The MCP altitude froze at 4700'. I asked Approach to level off at 5000'; and told them the reason for it. Approach started delay vectoring us. He wanted to know if we had an emergency; and I said no. It is not like that (to that effect). I got out the QRH Checklist on this; and the closest thing was a MCP Lockup Checklist. We did the MCP Lockup Checklist; and it worked for barely a second. I tried to dial up a higher number (First Officer was hand flying); and it froze now at 9800'. We told Approach we were still working on it and we got lots of vectors around at 5000'. I immediately sent Dispatch a message that we needed to talk and we needed to talk to Maintenance Control. I told Dispatch the problem we were having (he told Maintenance for us). We got a hold of Dispatch; told them the latest. He asked if we did the checklist on QRH: 4:1 I said; 'We did; it did not help.' We were evaluating what are our options. If we went back to ZZZ immediately; which would be a major overweight landing (12;000 pounds). I said to them; that would be an emergency thing; and this is not an emergency. A few minutes later; the Chief Pilot at Dispatch told Dispatch and us; we would be approved to land overweight. I ran the numbers and the computer said it was doable. But I was not a fan of such a heavy overweight landing. We discussed holding for four hours would take forever. Then the idea to go to ZZZ1 came up. Dispatch ran the numbers (took a while.) We were still getting vectored around; getting in cumulus build ups with lots of moderate bumps. I asked to climb to 10;000'. We cautiously climbed to 10;000'; and I had the First Officer use the autopilot and Vertical Speed and when we got to 10;000'; and then used altitude hold. Finally Dispatch got back with us to head to ZZZ1 and we got the data; route and revision to a new destination to ZZZ1 with Dispatcher initials and time in Zulu. We got weather updates; forecasts; NOTAMs; etc.; even looked up the weather in route on the EFB; to see if this lower altitude was doable. I briefed the Flight Attendants and the Passengers multiple times during all of this; what was going on. We then climbed to 20;000' and went on to ZZZ1. There we wrote the aircraft up and they replaced the MCP. We were given another aircraft. The MCP locked up at least four times. After long durations; all of a sudden; it would wake up and be usable. It was the strangest thing. I have had the MCP lockup once before in a 737-300. I also had the Altitude KNOB mechanism freeze up above 9700' in a 737-700 years ago. Both of those situations helped me to know what to do with this flight a lot. The First Officer did a marvelous job throughout this event; great job by him.We worked very hard to do this right. Sadly; as our aircraft get older; this kind of stuff is going to happen more and more.


Synopsis: B737-700 flight crew experienced a frozen MCP altitude window during initial climbout. After discussions with Maintenance Control and the Chief Pilot the crew elected to fly below RVSM airspace to an intermediate airport. After an aircraft swap the flight continues to destination.