A Mechanic reports their bottle-type jacks used to raise the nose and main landing gears during tire and brake changes have sunk about three inches at times into the blacktop surface that was applied over the deteriorating concrete at their hangar ramp. Their CRJ aircraft have rolled off jacks and skidded forward; even with nose chocks installed.

Narrative

While performing a Service Check and other maintenance on a CRJ-200 aircraft out on the hangar ramp; there was a worn tire that needed to be replaced; I was working on the APU but noticed the problem. Mechanic Y was changing the left main inboard tire. The aircraft had chocks; and was jacked using a bottle-type jack. Right after Mechanic Y had finished the final torque of the [axle] nut for the [main] tire; the bottle jack sunk about three inches into the blacktop; then [the aircraft] kicked-off the jack and rolled the plane 8 inches forward; even with the [nose wheel] chocks in place. I have had similar occurrences of this and after talking to other mechanics; some have had the aircraft roll off the jack on them! This is a timebomb waiting to happen! I've seen mechanics change tires with their legs straddling both sides of tires; along with changing brakes; someone will get killed or injured soon! There was no damage to aircraft or personnel [in this instance]. My suggestion is cement the ramp to hold the weight [of the aircraft]; or at least the areas where the wheels sit.