Embraer Jet Undifferentiated or Other Model
Training / Qualification; Situational Awareness; Confusion
Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter CFTT / CFIT
Flight Crew Took Evasive Action
Weather; Human Factors
During descent into LBB radar was indicating several cells in vicinity of the field and approach control reported deteriorating weather conditions on the field. Due to reported weather I had briefed and planned on doing the RNAV 35 L approach into LBB. Approach reported that the visibility and ceilings were good to the south of the field and we accepted vectors so that we could make a visual approach to 35L.  When we broke out of the clouds conditions were as advertised by approach control and we accepted a visual approach to 35L. We were already on a downwind for the runway so I decided to continue on the downwind and turn base when passing the FAF for the RNAV 35 L approach. When we passed the FAF I began to make a descending left turn towards the runway to intercept final and line up with the VASI. About half way through the base turn we got a 'Caution Obstacle' aural alert around 4500 FT MSL. The captain immediately told me to climb which I did until the alert message cleared. He was able to visually identify the obstacle which was a tower a few hundred feet below us and about a mile away. During this time I had climbed up to around 5200 FT MSL and maintained that altitude until intercepting the final approach course and descending on the final approach course using the VPI guidance.  The major threat in this approach was expecting to do a full instrument approach and not expecting that it would turn into a visual approach. Because I assumed I would be on charted approach courses and vectored by ATC I didn't brief or even look for any possible obstacles or terrain that might need to be avoided to make a visual approach to the runway.I did not know that the GPWS even had an obstacle alert feature.  It would have been nice to know about this feature and to know immediately how to respond to this alert. Most importantly though; I will now always pay close attention to terrain and obstacles charted on the approach plate so that in case an instrument approach turns into a visual I will know the location of terrain or towers between the aircraft and the runway.
An air carrier flight crew planning to shoot an IAP into LBB with convective weather in the area instead became visual and were cleared for a visual approach to 35L.  While maneuvering from a left downwind to base they received an EGPWS obstacle warning and flew the appropriate escape maneuver.