B747-400 flight crew reported an IAS disagree EICAS message on climb. Crew referenced checklist and safely returned to departure airport.
Synopsis
B747-400 flight crew reported an IAS disagree EICAS message on climb. Crew referenced checklist and safely returned to departure airport.
Narrative
We were in the initial climb approximately V2 and received an IAS DISAGREE EICAS momentarily. Then within a minute the EICAS message came back; and I called for the Airspeed unreliable checklist. At nearly the same time; ATC gave us direct to a point can a climb to 16000 ft. I shut the autopilot off and began to hand fly. Initially we thought that the FOs data was correct; so I cross referenced the standby; right and left data. I chose the slowest indicated airspeed (left) at a safe speed; and flew that while the observer and pm began to run checklists. We were VFR; and since I had an instructor in the right seat performing my line check I continued to fly. While they were running the checklist for IAS unreliable; we got altitude disagreement around 7000 ft. I was flying; the two observers were running checklists; and the Line Check Airman was on the phone with maintenance and dispatch; so we asked for delayed vectors and to level at 10000 ft. After talking to dispatch and discussing it; we notified ATC; and decided to return to ZZZ. The plane had been in maintenance for a few days; there was forecast for convection over the area; and we felt this was the safest course of action. We completed the checklists; dumped 50;000 kilos of fuel; and returned to ZZZ. I selected the center ADC; and right ADC; and initially thought the right was the closest to being correct; but were unable to determine that was true. I bugged the airspeed fast; and used the performance pitch and power from the QRH once the checklist reached that point. When the fuel dump was complete; we flew the ILS for runway XX in ZZZ because it was the longest runway. The crew worked together flawlessly; and we landed safely; using the entire runway to prevent the brakes from overheating. There were no status messages throughout the flight or upon blocking in.Cause: Airspeed and altitude disagree; maintenance is still inspecting the aircraftSuggestions: Maintenance let us know what malfunction they find
Second reporter narrative
We were a 4 man crew taking off of XXR out of ZZZ. A line check was taking place with an Line Check Pilot in the FO seat. I was in the 1st observer seat and another FO was in the second observers seat. Almost immediately after take off; we received an IAS DISAGREE eicas message.The captain asked the Line Check Pilot to run the checklist for IAS disagree. We were in a critical phase of flight so the Line Check Pilot in the right seat handed me (observer seat) the QRH to run for the condition. I began to run the checklist as the two captains were trying to determine which instruments were reliable.I ran the checklist and once the Line Check Pilot made the decision to call dispatch; I took over the radios while in observer seat so the flying captain could focus on flying the aircraft. At this time the auto pilot was off as instructed by the checklist for IAS disagree. At roughly 7;000' we also got an eicas message for ALT disagree. The 4th seat observer began to pull that checklist up on his iPad. I retrieved immediate pitch and power numbers from the performance inflight section of the QRH for 10;000 feet as it was decided to stop the climb there to deal with the situation. They stabilized the aircraft at that altitude with those numbers. I continued with the checklist while watching safety of flight as the flying captain was flying by hand with no automation and the Line Check Pilot in the right seat was talking to dispatch. I had a lot that I was handling at this time. The decision was made to make an air return as this aircraft had just come out of a major maintenance delay and we had just taken off for an almost 8 hour flight with a lot of weather to deal with and a situation that was likely going to get worse with altitude. And due to the altitude disagree that we had (up to 250' at one point) we knew couldn't go RVSM airspace. The ALT disagree disagree eicas message was intermittent but we saw the issue.We made the decision to notify ATCWe were heavy coming out of ZZZ and determined that we needed to jettison roughly 50 tons of fuel to land under our max landing weight.At this point I was extremely task saturated in the 3rd seat running multiple checklists from several sections of the qrh and backing up the crew in a fuel jettison calculation as well as watching the captain fly by hand while the Line Check Pilot was on a satcom call with dispatch. I was also still answering radios at this time.While running all of these tasks the Line Check Airman and the flying captain had tried switching the ADCs and thought they determined one to be reliable and were flying on that. Due to the fatiguing nature of hand flying for as long as the flying captain did; and it appearing that we were stable at that time with some reliability; I mentioned that if they were comfortable they may be able to try to re-automate to reduce workload. They did and the aircraft appeared to be operating normally and reliably at that time. I was backing them up looking at ground speed vs the current indicated airspeed. They were within a normal range of each other for our altitude. We were then setting up for landing in ZZZ for runway XY. I was running performance for 2 configurations (flaps 30 thrust reverser credit; and flaps 25 thrust reverser credit).Because the tables in the performance inflight showed 25; that's what we decided on for landing.I was double verifying in the performance inflight section of the QRH with Flight Data Performance and running hand calculations just as a back up for performance numbers since my work load through reading several checklists was very high. With how task saturated I was bouncing between checklists; I wanted to be sure for our weight and speed; landing configuration would be suitable. The autopilot was on and the flight profile was normal again and everything seemed to be working as it should again so I was comfortable to be heads down verifying performance numbers while in the observers seat. By thetime I had finished; I told them that the performance works for current landing plan.Ultimately we made a normal approach and landing to ZZZ and that in the end; after switching between all ADCs we couldn't definitively determine which side had the actual reliable airspeed to continue. With this condition being worsened as altitude increased we feel we made the best decision by turning back to the field. As we descended lower the deviations from side to side were less (only a couple kts) This is indicative of a true pitot static issue.On the ground after block in several maintenance personnel came on board and one of them said to another it appears there's ice in a port". Which this couldn't be true because the temps were in the 20s Celsius. But this proved to us that there was something obstructing the pitot tube(s). We explained to maintenance on the ground that even after switching through all of the ADCs we never could fully and truly determine which ADC provided reliable data to continue the flight. We had moments after making the decision to return to ZZZ where we thought one side was good and reliable but was later proved to be false. We all agreed this was the best course of action due to the multiple factors that played into this flight.Cause: Some likely potential pitot static issue or adc failure.Suggestions: I felt like from the 3rd seat I was very overloaded. I was being pulled into multiple sections of the QRH at one time so it was difficult to get through them and watch safety of flight. I felt very rushed but looking back we all worked together to get the aircraft on the ground with no damage and all agreed that it was the best course of action. I feel like we could have slowed down a good bit in the actions taken and had a bit more discussion as to what was truly happening. I feel like we made some decisions quickly even though we had time on our side."
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.