Date: 202107
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: MD-11
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Initial Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Air Data Computer
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: After takeoff; during a turn to a heading of 270 degrees; we received an ALT and IAS cross-side miss-comparison. The autopilot also would not engage when commanded and I hand flew up to 5;000 ft. After leveling off; and receiving instructions to climb; I called for the autopilot again and it connected. During the climb; we kept receiving the IAS and ALT notices and leveled off at 16;000 ft.; FL260; and FL320. We also noted a 200 ft. discrepancy between altimeters at one point. We leveled off at these altitudes because of the miss-comparison. After leveling off; for a minute the miss-compare would go away and we would continue the climb. During the climb and at one of the level-offs; we tried using our alternate static sources. This seemed to work on the Captain's side but the miss-compare came back when we started to climb again. The Captain then ran the airspeed unreliable checklist to see if it would be any help but nothing in the checklist seemed to be a fit for the problem we were encountering. The cause of the problems was not obvious and the checklist did not seem like it was of any help. Also; during the climbs; the FCP was not usable. All of the windows for airspeed; heading; and altitude only showed - - - -.With the problems we were having and not being able to identify the cause combined with the prospect of having to hand-fly and the encounter weather; we felt that continuing to ZZZ1 was not an option should the problems become worse. If we did in fact have an unreliable airspeed; we would be directed to disconnect both AFS and FD switches off which would mean having to leave RVSM; which would lead to higher fuel burns and hand-flying in heavy thunderstorms. [The problem was identified] when the amber IAS and ALT alerts were presented on our PFDs (Primary Flight Display). If I had to guess I would say that the aircraft had some sort of computer issue that was presenting the airspeed and altitude to the displays with differing data and a comparator caught the difference and alerted us. Our first reaction was to fly the plane. I started asking the Captain what his airspeed was as I figured that was more important than altitude. Once we were at a safe altitude and maintaining a safe airspeed; we began to troubleshoot and discuss the problems. There is nothing that we could have done as pilots to prevent this.


Second narrative: On the climb out the EIS detected intermittent ALT and IAS miss-comparisons between Captain and FO's (First Officer) PFD (Primary Flight Display). Stopped climb at 16;000 ft.; the issue went away. We then decided to continue to climb but had to stop at FL260. I had the FO put his STATIC AIR to ALT. Problem went away for about 1 minute and then came back. I then selected Captain STATIC AIR to ALT. This time; the issues went away for a few minutes. We decided to continue the climb but had to stop at FL320 because the problem came back. I ran the emergency non-alert AIRSPEED UNRELIABLE checklist; but the issue was not resolved. At 27100 ft.; Captain's IAS/MACH 336/.830; FO's IAS/MACH 330/.816. At every level change attempt FCP window indicated - - - -. The altitude difference between [the] Captain and FO was over 200 ft. Since AP (Autopilot) #2 was deferred and if we disconnected AP #1 we would not be able to remain in RVSM; we contacted company and returned to ZZZ. No emergency or special handling was required. [The problem was identified by the] amber IAS and ALT on Captain's and FO's upper left-hand corner of the PFD. [The cause was] failure annunciations… cross-side miss-comparisons generated when the EIS detected significant differences between the displayed data of the Captain's and First Officer's instruments AIRSPEED and ALTITUDE. [The response was to] climb to [a] safe altitude; maintain speed; and attempt to address the issue at hand. Maintenance issues cannot be forecasted.


Synopsis: MD-11 flight crew reported an airspeed and altimeter cross-side mismatch and elected to perform an air turn back and precautionary landing.