A B737-800 pilot reported a pitch over climbing to FL190 resulting in a 4;000' altitude loss. Maintenance reported an ADIRU fault with an altitude fault.
Synopsis
A B737-800 pilot reported a pitch over climbing to FL190 resulting in a 4;000' altitude loss. Maintenance reported an ADIRU fault with an altitude fault.
Narrative
I; copilot; was flying aircraft. Thunderstorms and icing were active in the area. Radar was on and aircraft was on a clear flight path in smooth air. Flight was running late so a modified climb speed of 310 was being used and a final altitude of 24;000 FT was requested from ATC due to a short leg length. Climbing through 18;000 FT both altimeters were switched to STD; autopilot was engaged at this time and lights were secured. At approximately 19;000 FT in the climb the pitch bar on the PFD disappeared and the aircraft pitched over hard from approximately seven degrees nose up to five degrees nose down. I disengaged the autopilot and auto throttles and slowly brought the deck angle back into a climb attitude. No column entanglement was apparent. No warning lights or messages were present. Windshield; pitot heat and engine anti-ice were ON and icing was present on the wipers. Aircraft increased in speed very rapidly and mach clacker sounded at 325 KTS during pull and recovery. Aircraft was returned to flight path and a slower airspeed used. Flight Attendants were not injured; felt like a hard level off. Maintenance was informed of problem and met the aircraft on jet bridge. Long and descriptive log book entry was completed by Captain. Mechanic was debriefed and after an hour; called the Captain with preliminary information. Two faults were found. An altitude discrepancy was found as a hard fault and a right ADIRU fault was indicated. His opinion was icing of the right pitot. Again heat was ON and no fault was indicated. Issues: Deviation from ATC clearance during pitch over and subsequent recovery. Aircraft exceeded VMMO during recovery by about 5 KTS.
NASA callback
The reporter described very clearly the sequence of events before and during the pitch over. He has a set pattern of actions at FL180 that include stating '29.92'; setting the altimeter; engaging the number two autopilot; turning the landing lights off and checking the pressurization. The reporter typically hand flies the aircraft to FL180 in order to maintain flying proficiency. During this flight the aircraft was in VNAV SPD at 300 KTS. At FL190 the aircraft began a firm but not violent pitch over to five degrees nose down. The total altitude loss was 4;000'. Upon arrival at the next airport the crew talked with Maintenance about the event and put the detailed report in the maintenance log. The Mechanic found two faults listed on the E and E Compartment boxes but neither explained the aircraft's action. Both the crew and aircraft overnighted. The aircraft was released for an on time departure the next morning. Days later the crew was told that it was determined that the crew did not set their altimeters within five seconds of each other and therefore the autopilot was attempting to resolve the discrepancy between them. The Reporter questioned this statement because he has never heard nor seen anything in writing which would verify that assertion. The flight conditions at the time were smooth although earlier in the climb the aircraft transited some rough air with icing. The reporter questioned if possibly ice had accumulated on the tail causing loss of tail airflow. The other ice related question he had was if the aircraft's pitot system was iced over by the earlier icing and just momentarily responded as the ice melted. The air data computer could also be called into question.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.