INFLT ENCOUNTER WITH ENRTE WX NEAR TSTM ACTIVITY CREATES AN ALTDEV ALT EXCURSION WHEN LOSS OF ACFT CTL OCCURS.

Narrative

DEPARTED HYS ARPT ENRTE TO SLN ARPT. LEVEL FLT AT 7000 FT MSL. NIGHTTIME (PM LCL). VISIBILITY WAS 25 MI OR BETTER; SCATTERED LAYER OF CLOUDS BELOW; ESTIMATE TOPS 5000 FT AND BASES 4000 FT MSL. AREAS OF WIDELY SCATTERED TSTMS IN ALL DIRECTIONS (N/S/E/W); NO STORMS ALONG RTE OF FLT; ESTIMATE CLOSEST TSTM WAS 50+ MI AWAY. BTWN HYS AND SLN ENCOUNTERED AREA OF MODERATE TURB (OCCASIONAL LIGHT TURB PRIOR TO MODERATE) AND NOTICED SMALL RAINSHOWER DIRECTLY ALONG RTE OF FLT. I WAS GOING TO REQUEST A LOWER ALT OR DEVIATE AROUND WHEN I ENCOUNTERED VERY HVY RAIN AND SEVERE TURB AND STRONG DOWNDRAFTS. ACFT WAS UNABLE TO MAINT ALT DUE TO THE STRENGTH OF THE DOWNDRAFT AND LOST APPROX 800 FT (7000 FT MSL TO 6200 FT). AT 6200 FT DOWNDRAFT STOPPED AND AIRPLANE CLBED NORMALLY BACK TO 7000 FT. NO FURTHER DOWNDRAFTS AND TURB MODERATE BAL OF FLT. THERE WAS NO WAY TO PREVENT THE ALT LOSS. VELOCITY OF THE DOWNDRAFT GREATLY EXCEEDED THE ABILITY OF THE ACFT TO CLB. ONLY ACTION POSSIBLE WAS TO CLB AT VY AND MAX CONTINUOUS PWR; AND RIDE THE DOWNDRAFT OUT UNTIL AIRPLANE FLEW OUT THE BOTTOM OR SIDE OF THE AREA OF HIGH SINK. AT NIGHT THERE WAS NO WAY TO SEE THE RAINSHOWER; SINCE THE STORM DID NOT HAVE ANY LIGHTNING IN IT (IT WAS NOT A TSTM). AIRPLANE WAS NOT EQUIPPED WITH WX RADAR. PRIOR TO ENTERING SHOWER; FORWARD VISIBILITY WAS GOOD; ALTHOUGH RAIN WAS HVY WHEN DOWNDRAFT WAS ENCOUNTERED; IT DID NOT BLOCK VISIBILITY UNTIL ACTUALLY FLOWN INTO.