Narrative:

During the enroute portion of our flight over the colorado rockies while in clear air; we observed the weather radar to be painting several thunderstorms normally; displaying the correct azimuth; range and intensity of the weather that we observed visually out the window and thus we believed the radar system to be operating normally. The flight operated uneventfully until arrival in the terminal area where there were operational delays due to widespread rain showers and imbedded thunderstorms. Upon descent into the terminal area below FL180; we entered an area of wide spread stratus clouds with occasional light rain showers and thus we were unable to visually observe the weather ahead. Over a three to five minute period during our descent; the weather radar display slowly deteriorated to a completely unusable display due to severe attenuation of the radar signal. Once this condition was observed I attempted to adjust the radar using manual gain (vs the previously selected auto gain) with no appreciable change in the display. A change from the left to the right radar unit resulted in no change to the display. With the radar gain control in the auto position; the radar displayed a band of yellow returns out to 6 to 8 miles and a green return for several miles beyond that. No radar returns were visible in the area beyond this area of attenuation. During the arrival we were rerouted by ATC and issued a clearance to hold at FL090. During vectors for the approach we were unable to navigate around the weather due to the attenuation of the radar and the aircraft was struck by lightning while at FL090. This lightning strike resulted in numerous skin burns on rivet heads on the right side of the fuselage. At approximately 1;000 AGL on final approach; we broke out of the stratus clouds and light rain showers into clear air and we visually identified the band of weather to the north of us that we had just come through and saw that the area to the south of us was clear. A check of the radar unit while in clear air showed that the attenuation was now gone and the unit once again was painting weather targets correctly. We continued the approach to an uneventful landing. Upon arrival at the gate; the ground crew observed an 18 inch diameter sheet of ice accumulation on the radome. Approximately 20 minutes after block-in I was able to take a picture of the radome and at that time the ice was approximately 8 inches in diameter. In the past nine years of operation on the B777; I have observed an occasional reoccurrence of this radar attenuation in similar weather conditions. That is; with the aircraft at low altitude and in very light to light rain showers; occasionally the radar display slowly attenuates to a point that all that is visible on the radar is a band of green or yellow returns from the radome out to approximately 8 to 10 miles and then no echo or ground returns are visible beyond that point; indicating severe attenuation of the radar energy. What makes this issue so problematic is that the visible moisture we were in was just stratus cloud or very light rain showers; which you would not expect to cause severe attenuation of the radar energy. Changes in the antenna tilt only served to increase or decrease the size of the yellow/green returns that extended out from the radome. I do not believe that this is a mechanical problem with the specific radar right/T unit on this specific B777. This appears to be an engineering problem with all the radar units installed on the 777 and this problem needs to be addressed by the manufacturer to correct this attenuation while operating in light rain showers. This is the second report that I have filed regarding this issue. The march 2004 report has virtually identical observations as in this report. It should be noted that during my nine years on this fleet I have made dozens of approaches in weather conditions very similar to that which we encountered here; with no attenuationof the radar signal being observed. If this problem is allowed to continue; it will only be a matter of time before we have major damage (or worse) to the airplane as a result of our inability to safely circumnavigate a thunderstorm at low altitude while in instrument meteorological conditions. This safety of flight issue requires the immediate attention by the radar manufacture and the active involvement of the FAA; boeing; the B777 fleet maintenance controllers and engineering departments of my air carrier. Since this anomaly can only be identified while in flight during specific conditions; a communications bulletin needs to be sent to all B777 pilots explaining this phenomenon and asking for pilot input and encouraging a detailed logbook write-up. The write up that is often seen needs to have more information included such as; altitude and whether the flight was operating in visible precipitation at the time of occurrence. The only thing I have seen since 2004 is a standardized explanation of the radar attenuation problem in the flight manuals. After years of operation with this inadequate equipment and knowledge that this problem exists; we need an engineering fix to the issue; not a 2 paragraph warning to pilots in a 1400+ page flight manual advising us in essence to 'be careful out there'.

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Original NASA ASRS Text

Title: B777 Captain reports radar degradation during descent and approach that resulted in damage to the aircraft from lightening after flying into a thunderstorm.

Narrative: During the enroute portion of our flight over the Colorado Rockies while in clear air; we observed the weather radar to be painting several thunderstorms normally; displaying the correct azimuth; range and intensity of the weather that we observed visually out the window and thus we believed the radar system to be operating normally. The flight operated uneventfully until arrival in the terminal area where there were operational delays due to widespread rain showers and imbedded thunderstorms. Upon descent into the terminal area below FL180; we entered an area of wide spread stratus clouds with occasional light rain showers and thus we were unable to visually observe the weather ahead. Over a three to five minute period during our descent; the weather radar display slowly deteriorated to a completely unusable display due to severe attenuation of the radar signal. Once this condition was observed I attempted to adjust the radar using manual gain (vs the previously selected auto gain) with no appreciable change in the display. A change from the left to the right radar unit resulted in no change to the display. With the radar gain control in the auto position; the radar displayed a band of yellow returns out to 6 to 8 miles and a green return for several miles beyond that. No radar returns were visible in the area beyond this area of attenuation. During the arrival we were rerouted by ATC and issued a clearance to hold at FL090. During vectors for the approach we were unable to navigate around the weather due to the attenuation of the radar and the aircraft was struck by lightning while at FL090. This lightning strike resulted in numerous skin burns on rivet heads on the right side of the fuselage. At approximately 1;000 AGL on final approach; we broke out of the stratus clouds and light rain showers into clear air and we visually identified the band of weather to the north of us that we had just come through and saw that the area to the south of us was clear. A check of the radar unit while in clear air showed that the attenuation was now gone and the unit once again was painting weather targets correctly. We continued the approach to an uneventful landing. Upon arrival at the gate; the ground crew observed an 18 inch diameter sheet of ice accumulation on the radome. Approximately 20 minutes after block-in I was able to take a picture of the radome and at that time the ice was approximately 8 inches in diameter. In the past nine years of operation on the B777; I have observed an occasional reoccurrence of this radar attenuation in similar weather conditions. That is; with the aircraft at low altitude and in very light to light rain showers; occasionally the radar display slowly attenuates to a point that all that is visible on the radar is a band of green or yellow returns from the radome out to approximately 8 to 10 miles and then no echo or ground returns are visible beyond that point; indicating severe attenuation of the radar energy. What makes this issue so problematic is that the visible moisture we were in was just stratus cloud or very light rain showers; which you would not expect to cause severe attenuation of the radar energy. Changes in the antenna tilt only served to increase or decrease the size of the yellow/green returns that extended out from the radome. I do not believe that this is a mechanical problem with the specific radar R/T unit on this specific B777. This appears to be an engineering problem with ALL the radar units installed on the 777 and this problem needs to be addressed by the manufacturer to correct this attenuation while operating in light rain showers. This is the second report that I have filed regarding this issue. The March 2004 report has virtually identical observations as in this report. It should be noted that during my nine years on this fleet I have made dozens of approaches in weather conditions very similar to that which we encountered here; with no attenuationof the radar signal being observed. If this problem is allowed to continue; it will only be a matter of time before we have major damage (or worse) to the airplane as a result of our inability to safely circumnavigate a thunderstorm at low altitude while in instrument meteorological conditions. This safety of flight issue requires the immediate attention by the radar manufacture and the active involvement of the FAA; Boeing; the B777 Fleet Maintenance Controllers and Engineering Departments of my air carrier. Since this anomaly can only be identified while in flight during specific conditions; a communications bulletin needs to be sent to all B777 pilots explaining this phenomenon and asking for pilot input and encouraging a detailed logbook write-up. The write up that is often seen needs to have more information included such as; altitude and whether the flight was operating in visible precipitation at the time of occurrence. The only thing I have seen since 2004 is a standardized explanation of the radar attenuation problem in the flight manuals. After years of operation with this inadequate equipment and knowledge that this problem exists; we need an engineering fix to the issue; not a 2 paragraph warning to pilots in a 1400+ page flight manual advising us in essence to 'be careful out there'.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.