B757-200
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Physiological - Other; Distraction
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Aircraft Equipment Problem Dissipated; Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control; Flight Crew Took Evasive Action
Weather; Aircraft
We departed and climbed uneventfully and in accordance with the flight plan to FL380. Due to light and moderate turbulence over the south at FL380 and below; we climbed to FL400 in order for a better ride. However; FL400 became unbearable with continuous light to moderate turbulence with lots of jolts. No precipitation was noted along our flight path visually or on the radar; however; we were above the clouds which topped out around 33;000 FT without any noticeable vertical development. Multiple PIREPS were sent to the Dispatcher and acknowledged. FL280 and below were the only altitudes reported as smooth by center. So; we requested lower. After being stepped down several times from FL400; we were cleared to FL280 and experienced smooth air. I had descended at 290 KTS and remained so with the intention to speed back up if it remained smooth. We were in IMC conditions from around FL330 to FL280. The weather was thick and hazy with very little or no precipitation. Engine anti-ice was used in the descent and while in level flight at FL280 due to the temperature. As we approached ZZZ City area we were cleared direct. This course took us well clear of an area of thunderstorms in the region. There was no weather indicating along our flight path except for a few spurious green pixels on the radar display. Soon; we experienced continuous light to moderate turbulence at FL280 with moderate jolts just as we had experienced up high above the clouds earlier. The radar display still indicated no precipitation along our route. Visually; it appeared that the weather was thicker; but no heavy precipitation or icing on the windscreen was observed by me. We did have a few flashes of St Elmo's fire on windshield and the radios were unreadable and scratchy with static. Next; we had a progression of unusual things occur. First; we had a IAS disagree message followed by a Rudder Ratio and Mach speed trim EICAS messages. The Captain informed me that his airspeed indicator was indicating around 140 KTS. I looked over and observed his airspeed fluctuating in the 140s. His bug was at 290 as commanded and his Mach speed was blank. The aircraft was stable at FL280 and 290 KTS. Since I was using the Center Autopilot; I disengaged the autopilot and selected the right autopilot to mitigate any issues with his airspeed indicator. Pitch and thrust were normal for the phase of flight. The aircraft remained on speed and altitude at FL280 and my airspeed indicator indicated 290 KTS as commanded. Continuous light and moderate turbulence persisted. The Captain then referred to the QRH for the EICAS messages and the airspeed indicator. The Captain informed me he was selecting an Alternate Air Data Source on his side; however; his airspeed remained at 140 to 145 KTS. I attempted to call ATC to give them a PIREP and inform them of our problem plus request a different altitude; but there was no answer. The static was too great to hear anything on the radio. The radios were scratchy and unresponsive. I took a quick scan across the panel to build situational awareness regarding our instruments and to back up the Captain. The Captain was in alternate on air data; his airspeed indicator was bouncing around in the 140s; his attitude indicator was just as mine (straight and level with a normal pitch of a few degrees); and our flight path was clear of any weather. While scanning the engine instruments on the EICAS; I notice our EPRs gauges were blank and I confirmed our TAI (thermal anti-ice) was on for both engines. I informed the Captain about the EPR gauges being blank. As my scan progressed to my side; suddenly without warning my airspeed indicator went to around 145 KTS; the aircraft started an uncommanded climb with about 2000 FPM rate and with a slight pitch up and then the throttles began to retard. I grabbed the yoke and the throttles to control the airplane. As I did this; the autopilot disengaged with the warning alarm and the throttles went to 'Throttle Hold.'I silenced the autopilot alarm and clicked off the autothrottles. I initially pushed the throttles up; but the Captain directed me to set 3500 PPH on each engine and I did. The unanticipated and uncommanded climb plus the autopilot warning; and the moderate and light turbulence in the weather was disorientating and confusing. It took all my focus on controlling the airplane and trying to get it back to FL280 and stable. I didn't see more than a 300 or 400 foot altitude deviation; but I was not sure how accurate my systems were and the Captain had informed me to be cautious regarding our altitude since fluctuations had been observed by him. I attempted to call ATC twice to inform them; but still no answer. We had static only. As I leveled off at FL280 on my altimeter; I saw a clear and bright area off to the right. Then blue sky appeared and I headed for the clear air. As I was doing so; we were finally able to talk to ATC and tell them what happened and that we were deviating right to get out of the clouds. My best recollection was that ATC said our altitude was 300 FT high. ATC acknowledged the issues with our instruments and offered us any assistance if needed. I hand flew the aircraft at FL280 for approximately 5 minutes while the Captain called the Dispatcher and Maintenance on the radio. All systems returned to normal except for the engine EPR gauges which remained blank. The pitch attitude and thrust appeared normal for our phase of flight so we turned the Flight Directors on and I continued to hand fly the aircraft at FL280. A few minutes later; I was confident the systems were normal so I engaged the right autopilot; stayed above the weather until being vectored for the approach; and uneventfully landed at our filed destination from the ILS. Maintenance checked the pitot-static system and found no moisture or abnormalities. Multiple codes were present in their download of information from the computers in the E&E bay. The computers were reset and BITE checks accomplished. All systems returned to normal and Maintenance deemed our aircraft to be good to go. The aircraft was then flown by us back to the previous airport with no problems.
The reporter stated that the air data computers had to be reset by Maintenance after landing. But there was no single system anomaly could be found for this event. He noted that the OAT at the time was -15C when he would normally have expected somewhere near -30C and he suspects that somehow the pilot static system was disrupted by the warmer than normal OAT along with the precipitation.
A B757 experienced airspeed; autothrottle and autopilot anomalies while flying in icing conditions with turbulence and St Elmo's Fire.