2011-05 · NASA ASRS report 950798
B767 flight crew experiences pitot static system anomalies at FL370 approaching the North Atlantic Tracks east bound and elects to divert for maintenance.
While in cruise flight at FL370; with both the autopilot and the autothrottles engaged; the throttles suddenly retarded to idle and the overspeed warning sounded. I immediately disengaged the autopilot and autothrottles and started to move the throttle levers back to a higher setting. I looked at my airspeed indicator and noticed that it was at or slightly above the barber pole; but within moments; it suddenly and immediately returned to our previous cruise speed of about M.80. My First Officer mentioned that he had glanced at my altimeter at the same time and noticed that it had jumped to about 200-300 FT high. That also quickly returned to its previous reading of 37;000 FT; when the airspeed needle jumped back to M.80. His instruments stayed constant at M.80 and 37;000 FT. I stabilized the thrust setting and after a brief wait; re-engaged the autopilot and autothrottles. The aircraft flew fine for a few more minutes; but then the same things suddenly happened again. After re-stabilizing the aircraft and thrust setting again; the First Officer and I discussed the possibility of the problem being the left ADC. I switched my ADC to the alternate source and again; the aircraft flew fine for a few more minutes. However; it all happened again; but this time it was the First Officer's airspeed that jumped to the barber pole and his altimeter read high. During all of these events; the standby airspeed indicator and standby altimeter were reading fine. At this point; and while hand-flying; I attempted to re-engage the center autopilot but was unable to do so. We considered the possibility of the problem being with the center autopilot; so we attempted to engage both the left and right autopilots; but were unable to do so. We noticed that the VNAV indicator on our ADIs were indicating a degraded pitch mode. Neither Flight Display would display their pitch bars; so we turned those off; and I continued to hand-fly the aircraft and manually set the thrust. The First Officer and I discussed the situation and decided that continuing flight into ETOPS airspace and on to destination would no longer be feasible; so we contacted ATC (Moncton Center) and requested a clearance to divert. While we were doing all of this; I had the First Officer call and get the Relief Pilot back to the cockpit to review the Flight Manual to look for any help in diagnosing the problem; to interact with the cabin crew; review the diversion guide; and to keep the passengers informed about our situation. The First Officer discussed our situation with Dispatch and Maintenance while I flew; communicated with ATC and worked with the Relief Pilot on troubleshooting the problems. We had very few EICAS messages; no popped circuit breakers; and no status messages. The EICAS messages we had were RUDDER RATIO; AILERON LOCKOUT; and then when I extended the speed brakes during descent; we got AUTO SPEEDBRAKES. We had warning lights for these also. I am not sure; but I think I recall a quick EEC EICAS message; but if it was displayed; it was only for a few moments. At one point during all that was happening; the fast/slow indicators on our ADIs were reading slow when the all three airspeed indicators were reading on-speed; then they suddenly jumped to a too fast indication with no change to our airspeed. Also; there was a point during my initial descent from FL370; where my VSI was indicating a 400-500 FPM descent; and I glanced over to the First Officer's VSI and it was reading a 400-500 FPM climb. The standby VSI was also indicating a descent and we could see visually that the aircraft was actually descending. The entire diversion was in VMC conditions. Later during the descent; I was able to engage the left autopilot. The others may have worked; but I didn't try them at this point. I flew the aircraft in V/S Mode and maintained the airspeed by manually adjusting the throttles. We were unsuccessful in re-engaging the autothrottles. I disconnected the autopilot at about8;000 FT and hand-flew the aircraft to the landing. We landed at 319;000 LBS; autobrakes set to level 1; and I manually deployed the speedbrakes because of the AUTO SPEEDBRAKES EICAS message. The landing was uneventful; the brake temperatures did not get close to or exceed any limits and we taxied to the gate. The crew worked well together; there were no human factors. We still do not know exactly what things failed or degraded; but the outcome was a successful diversion for a significant mechanical issue.
I was on first rest break in the passenger cabin. While seated I felt various throttle and pitch changes; a little unusual for that phase of flight. Flight Attendant then gave me the word the Captain wanted me back on the flight deck. When I arrived I saw the Captain hand flying with the autopilot and autothrottle cautions displayed. Other First Officer was on the radio with Dispatch and Maintenance. During this period it was decided to divert before the tracks. It was conveyed to me that the auto flight/air data system may have malfunctioned in some way. During the return; we also received the rudder ratio; aileron lockout; and auto speed brakes cautions. Ran the checklists for each and continued. Dispatch wanted us to return to departure airport; but upon conferring with each other we were not sure what was wrong; and had thought there may be other issues as well. Maintenance agreed and since a closer airport was clear we landed there. During the return the Captain was able to engage the left autopilot but not the autothrottles. We coordinated with Dispatch for new landing data and also the station for passenger service. The flight attendants secured the cabin for arrival and we conducted a visual approach and we were under maximum landing weight.
The reporter is not aware of what maintenance was required to return the aircraft to service. He was told that the air data computers checked OK and did not seem to be the cause of the anomalies.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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