Flight Crew Commercial; Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Flight Crew Last 90 Days 120; Flight Crew Total 4700; Flight Crew Type 380
Other / Unknown
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Weather / Turbulence
Flight Crew Returned To Clearance
Aircraft; Human Factors; Weather
On a typical; hot; Texas day I was flying a relatively light payload of cargo with building thunderstorms near Dallas and Houston. Our original flight plan was scheduled to make a fuel stop in Houston Hobby; then continue to destination. Upon looking at the current radar summary and satellite; the Captain and I decided to change our flight plan to Shreveport as the cells appeared to be moving in separate directions. Houston appeared to be in the path of the largest part of the storm. I had an XM weather display as well as the on-board WX radar to monitor the storms as we approached. The movement of the storms changed as they were growing rapidly; and the gap between them was getting smaller. About 50 miles to the south of the storms Houston Center advised that the gap was closed and aircraft had not penetrated the line for almost an hour. After discussing with the Captain; the on-board radar still showed almost a 15 mile gap in the cells with heavy precipitation. The tops in all the adjacent areas were reported 24;000 FT and lower. We were cruising at 17;000 FT and requested a block altitude of 16;000 to 18;000 FT in case of turbulence. Houston Center handed us off to Ft. Worth Center; who subsequently climbed us to FL190; deleting our block altitude clearance. We were told that the area was clear and we were given 'Deviations as necessary for Weather'. I slowed the plane to our turbulence penetration speed before entering the area between the cells 'painted' on the radar. The flight conditions included some light and moderate precipitation; trace rime ice; and light to moderate chop. The Captain issued a PIREP to the Controller as light chop; however; for just a moment or two we were caught in a moderate down draft and lost about 400 feet of altitude. We were close to the company limit on our Turbine Temps; and I elected that it was not necessary to add power. This resulted in slightly more time spent in our altitude deviation. Fort Worth acknowledged our deviation and advised us to return to FL190 when able. I returned to FL190 as soon as the aircraft was able; and within 15 more minutes we were out of IMC and descending for Shreveport. We continued the rest of the way to destination without further incident and delivered out freight.
SD3 First Officer reports loosing 400 feet from FL190 while passing between thunderstorms in IMC with with some moderate down drafts. The engines were operating near their limits; delaying the return to assigned altitude.