As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APPROXIMATELY 45 MINUTES AFTER DEPARTING KBIL WHILE IN CRUISE FLIGHT, THE CREW OBSERVED THE PILOTS ALTIMETER CLIMBING RAPIDLY WITHOUT STOPPING. THE CREW RETURNED TO KBIL WITHOUT INCIDENT. MAINTENANCE REPLACED THE PILOTS ALTIMETER WITH AN OVERHAULED UNIT AND OPS CHECK GOOD.