By Walt Hunter

PHILADELPHIA (CBS) — The crew of a Philadelphia medic unit, returning from a run, was startled to find two dual rear wheels on the left side of their apparatus were falling off Tuesday afternoon.

The medics, who did not have a patient aboard, were able to pull the unit safely to a stop on Bustleton Avenue near Benton Street in Northeast Philadelphia.

Exclusive CBS 3 video showing all 8 stems and lug nuts holding the tires onto the axle had broken off. At least one was found along the roadway behind the now disabled unit.

City officials tell CBS 3 preliminarily it appears metal fatigue may be involved. The unit involved, Medic 258, is a reserve unit which has been in service for 12 years, with more than 185,000 on its odometer.

“The fleet is aging,” Firefighters’ Local 22 Union President Joe Schulle told CBS 3’s Walt Hunter. “The city does not have a plan to replace these vehicles.”

Medic 258 was covering for a frontline unit, Medic 6, which has been placed out of service due to mechanical problems.

This breakdown is just the most recent documented in a series of incidents that began surfacing after a medic unit, bursting into flames, heavily damaged the firehouse at 4th and Arch Streets on September 27,2013 (See Related Story).

“A medic unit was driving along Germantown Avenue and the wheels just fell off,” Schulle added. “Medic 11 with a critical care patient aboard just shut off.”

City officials responded that the city has put 74 new medic units into service in fiscal years 2002 through 2012.

“Fleet Management and the Administration take issues like this very seriously,” stated Mayor Michael Nutter’s Press Secretary Mark McDonald.

“To argue the city skimps on maintenance is not supported by facts.”

More than 2 hours after the incident, the Medic unit was placed onto a flatbed as officials begin a full investigation into what caused the wheel problem. Meanwhile, Union President Schulle says, following this latest incident, the union is seriously considering taking legal action against the city to ensure it keeps fire apparatus safe for firefighters, medics and patients.