H125 FLT CREW EXPERIENCES FAILURE OF THE AUTOPLT TO MAINTAIN ALT ABOVE FL370.

Narrative

CRUISE AT FL370. DSNDED (ATC REQUEST) TO FL350; THEN SUBSEQUENT CTLR CHANGE. REQUESTED RETURN TO FL370; CLRED TO FL380 FOR TFC. UPON REACHING FL380; THE AUTOPLT HAD A DIFFICULT TIME OF LEVELING OFF AT FL380. DIVERGING PITCH OSCILLATIONS ENSUED; TO THE POINT WHERE THE PF HAD TO DISENGAGE THE AUTOPLT AND MANUALLY TRIM FOR LEVEL FLT. AUTOPLT WAS REENGAGED AND OSCILLATIONS SLOWLY RESUMED. NEITHER ALT HOLD OR ALT CAPTURE FUNCTIONS WERE EFFECTIVE IN MAINTAINING LEVEL FLT. ALTDEVS WERE AS MUCH AS +/-300 FT. REQUESTED DSCNT TO FL370 WHEN TFC HAD PASSED UNDERNEATH US; BUT WERE DENIED UNTIL THE NEXT CTLR. DURING PITCH OSCILLATIONS AND SUBSEQUENT ALT EXCURSIONS; OBSERVED DIVERGENT AND OPPOSITE INST INDICATIONS FROM PF AND PNF'S INSTS. WHEN PF'S INSTS INDICATED A CLB ON THE ALTIMETER AND VSI; THE PNF'S INSTS OFTEN INDICATED OPPOSITE. STANDBY ALTIMETER INDICATED AS MUCH AS 500 FT DIFFERENCE FROM PRIMARY INSTRUMENTATION. WHEN HANDED OVER TO ZNY; REQUESTED AND RECEIVED DSCNT TO FL370. ACFT RESPONDED NORMALLY TO LEVEL OFF AT FL370 WITH AUTOPLT ENGAGED. WE WERE INFORMED BY NY THAT ZOB WANTED TO TALK TO US WHEN WE REACHED DEST AND WERE GIVEN A TELEPHONE CONTACT NUMBER.