Dash 8-100
Flight Crew Air Transport Pilot (ATP)
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Diverted; Flight Crew Regained Aircraft Control
Weather
As we were beginning the approach for Runway 9R into PHL; just outside the final approach fix (KELEE) we encountered severe turbulence causing a momentary loss of aircraft control. We initially lost about 400 feet of altitude then an immediate reversal of the descent to about 2000 FT per minute climb to about 3;500 FT MSL before regaining control over the altitude. We also received roll deviations which were corrected. Upon the initial encounter of the turbulence; I called for a go-around and proceeded to advise ATC of the severe turbulence and advised we were not able to maintain our assigned altitude of 3;000 ft. The First Officer maintained wings and pitch level as best he could and added power as I brought the condition levers up. We received a vector to exit the approach corridor and proceeded south where we continued to receive moderate turbulence. We reconfigured the aircraft for normal flight and advised ATC we would need to divert.  Heavy rain showers in the area created moderate turbulence all around PHL. ATC advised the approach corridor looked acceptable on the radar so we accepted the approach clearance. We also did not detect any signs that severe turbulence may occur on the aircraft radar. We proceeded with a go-around and exited the area of severe turbulence and diverted. A maintenance write up was made because of the severe turbulence encounter. I believe there is a stigma about calling turbulence severe. If the aircraft in front of us had advised ATC that the turbulence was in fact severe; we would have never proceeded with the approach. After landing; the pilots of that aircraft in front of us had also diverted and admitted they should have reported the encounter as severe. Pilots should be reminded that it not only affects them when they label turbulence severe; but also keeps other aircraft from going through that same area.
Dash 8 Captain reported encountering severe turbulence on approach to PHL; which resulted in momentary loss of control. Flight crew chose to divert to a suitable alternate.