A B767-300 EICAS alerted TAT PROBE failure with the loss of VNAV; LNAV; Autothrottles and wind read out. Because icing was expected; an emergency was declared and the flight returned to the departure airport.
Synopsis
A B767-300 EICAS alerted TAT PROBE failure with the loss of VNAV; LNAV; Autothrottles and wind read out. Because icing was expected; an emergency was declared and the flight returned to the departure airport.
Narrative
After takeoff; we received a TAT PROBE EICAS indication along with failure of the Autothrottles; LNAV; VNAV; Thrust Reference Indications; and wind readout. We had seen this exact item written up twice in the last day in the logbook. I remained the pilot flying and talked with ATC while the Captain got out the QRH procedure as we had briefed. The Captain also looked up in the MEL what the restrictions may be for this problem and found that the aircraft could not be dispatched into known or forecast icing. We both agreed that we would likely encounter icing conditions throughout this flight and decided it best to return to our departure airport. While the Captain notified the company and spoke with our maintenance; I worked with ATC to avoid weather on our departure. The Company agreed that we should return and as we advised ATC of our intentions; we encountered icing conditions and subsequently were required to turn on our engine anti-ice due to temperatures below 10 degrees. At this point; we could not be completely sure of the reliability of our instruments and discussed the need to pay particularly close attention as we descended back toward our departure airport in IMC conditions. We decided to declare an emergency at this point; which afforded us the time needed to prepare for and brief the approach. This also allowed us relief on the crossing restrictions on the arrival since we still had some altitude to lose and needed more time to prepare for landing. While I flew and coordinated with Approach; the Captain briefed the Flight Attendants and passengers and then joined me in setting up for an approach to the longest runway. We landed uneventfully 30;000 LBS below maximum landing weight. The Captain put the discrepancy into the logbook for the third time in the last few days.
Second reporter narrative
First Officer's leg. Aircraft was below maximum landing weight at takeoff. Flight was to be the last flight into and out of our destination before a tropical storm. Forecast was for flight to descend through the leading edge of the tropical storm probably in icing conditions from cruise altitude. Weather legalities (winds and ceiling forecasts) were met at time of departure into and out of the departure airport. Log book entry: After lift off; TAT PROBE EICAS and light illuminated with associated failure of LNAV/VNAV/thrust reference indications/wind speed readout and auto-throttles. Returned to land in our departure airport. After landing all of the above appeared to return to normal. Landing was not overweight. The identical item was written up twice today. After takeoff; First Officer climbed to an altitude clear of the terminal area for pilot not flying (Captain) to evaluate QRH (and possible MEL requirements). Note on QRH for TAT PROBE states: Flight in icing conditions may result in some erroneous flight instrument indications. Climbing out of 10;000 we entered IMC and did; in fact; have to turn on engine anti-ice due to temps below 10 degrees. However; pitch attitude and aircraft instrument indications appeared to remain normal. Icing conditions required increased pilot vigilance for the possibility of errors. Some Mode Control Panel and FMC functions did not work normally; for example; VNAV and flight level change would come on simultaneously causing us to doubt which systems were operating normally. However; white (command) ADI display seemed to work normally. Additionally; the lower hook on the airspeed tape indicator did appear as described in the Special Message causing an additional distraction. I contacted Dispatch and got a link to maintenance. After discussing our situation with a technician and considering the probability of a descent through icing near our destination; I instructed ATC that we needed to return to our departure airport. I contacted the Purser and told her we were returning to land; then made a PA to the passengers informing them of our situation. We wanted relief from the STAR altitude constrains and priority handling for the longest runway with a precision approach; therefore I declared an emergency. Although we were below max landing weight; we were still heavy at 280;000 LBS. We also needed more time to descend; prepare for the approach and run checklists (not having a mechanical checklist was a further distraction). The First Officer landed smoothly without incident.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.