Narrative:

While transiting an area of weather; we were forced to utilize the weather deviation without a clearance procedure as outlined in the fom and quick reference guide. We were not the only ones that night; either. Air carrier behind us; also required to utilize the procedure.at XA15Z we received nfff/nadi SIGMET number 1 showing stationary weather across our route extending as far east as s13e166 and as far west as s12w175 with tops to FL480. This effectively bisected our route for hundreds of miles both left and right of our course. As we approached the line of weather at FL350; it painted from behind and to the left (7:00-8:00 [o'clock]) across our route to our 1-2:00 position (both to the edges of effective radar coverage). At XB20Z; we received nfff SIGMET number 4 altering slightly the geographic points of SIGMET number 1. Our radar display clearly favored a right deviation; so at XB24Z we initially asked for and received clearance for up to right 70 NM. As we continued north eastbound; it quickly became necessary for larger deviations of 120 and finally 128 (XB32Z and XB45Z respectively); the maximum that nfff could approve. Radar returns were growing and appearing further and further ahead of us as the storms reached the edge of our radar range (200 NM or so).while this was taking place (and with a dynamic and rapidly changing weather situation/picture; seating our flight attendants; etc.) we realized that for deviations greater than 99 NM; we needed to go to heading select mode; since the FMC (and therefore LNAV) will not accept greater than 99 NM offsets. This greatly increased our workload. Worse yet; beyond this distance (99 NM) we were not able to precisely determine 128 NM; our cleared limit; as offset distance is no longer displayed in the FMC. As we passed 99 NM; right off course; the prog. 2 page showed no usable data. To get around this limitation; we tried deleting the (99 NM FMC) offset completely in an effort to show a distance from the original route rather than from the 99 NM (FMC) offset; to no avail. Once beyond 99 NM off original course; the FMC does not show offset distance (at least on the prog. Page 2)! We therefore had to utilize the map range marks on the nd (at low ranges) as an approximation (while selecting at times more appropriate radar ranges); creating significant task saturation. As we approached what I estimated to be 128 NM right; I complied with the weather deviation without a clearance procedure including climb of 300 feet; lights on; etc.; as we still could not safely come back left. Nfff subsequently cleared us block 350-360; perhaps seeing that we had climbed 300 feet. Nfff was also asking if we had new routing in mind since he could not approve greater than 128 NM. We felt that with the radar display limited in distance and our task saturation; that we did not have time for a lengthy discussion with dispatch regarding routing options; nor did we have time then to figure it out on our own. Dispatch was sending messages at roughly the same time indicating where he saw lightning; but had indicated earlier that his radar picture of the area was not that great.I finally found an area that; while not ideal; was at least our first and perhaps only hole. I believe this to have been somewhere near our clearance limit of R128 NM; but was unable to exactly determine the distance from course; for the already mentioned reasons. I took the left turn and we were able to transit the remaining affected area with a reasonable ride. Around this time we received nfff SIGMET number 5 (issued at XC28Z) changing the boundaries of the affected airspace to slightly further both east and west. However; by this time we were well in the battle and had begun our correction back to our course.obviously; the weather around us was changing rapidly. Perhaps this was why we had little warning of it prior to leaving [departure airport]. Several company aircraft transited the area in the hours before wedid; with no pireps; so this further illustrates the dynamic nature of the weather that evening.I have two concerns with this event: first; the FMC limitations that I illustrated dramatically increased our workload. When combined with the rapidly changing weather picture; we had limited bandwidth to coordinate with dispatch for a revised routing beyond the clearance limit of 128 NM right of course. Our job at that point was to keep us on a heading away from harm's way.secondly; the B787 multi-scan radar is completely different from what we have operated in the past. I found this out during my first few months on the line with the 787; but did not have any significant training on it. It tends to paint weather as more intensely in auto mode than previous radar models I have used (727; 737; 757/767; and 777). I knew this on our flight; however; being in the equatorial convergence zone; my desire was to err on the side of caution; in case of dry-top storms. Using various manual mode settings to see something more comparable to my past experience definitely added to an already task saturated event. It was only through using manual mode in various configurations that we were able to find our 'hole' to fly through. I encourage company to develop a comprehensive training course on the differences of this radar and how to most efficiently utilize its capabilities.

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Original NASA ASRS Text

Title: B787 flight crew reported concerns with the Flight Dynamics; Navigation; and Safety Systems.

Narrative: While transiting an area of weather; we were forced to utilize the Weather Deviation without a clearance procedure as outlined in the FOM and Quick Reference Guide. We were not the only ones that night; either. Air Carrier behind us; also required to utilize the procedure.At XA15Z we received NFFF/NADI SIGMET Number 1 showing stationary weather across our route extending as far east as S13E166 and as far west as S12W175 with tops to FL480. This effectively bisected our route for hundreds of miles both left and right of our course. As we approached the line of weather at FL350; It painted from behind and to the left (7:00-8:00 [o'clock]) across our route to our 1-2:00 position (both to the edges of effective radar coverage). At XB20Z; we received NFFF SIGMET Number 4 altering slightly the geographic points of SIGMET Number 1. Our radar display clearly favored a right deviation; so at XB24Z we initially asked for and received clearance for up to Right 70 NM. As we continued north eastbound; it quickly became necessary for larger deviations of 120 and finally 128 (XB32Z and XB45Z respectively); the maximum that NFFF could approve. Radar returns were growing and appearing further and further ahead of us as the storms reached the edge of our radar range (200 NM or so).While this was taking place (and with a dynamic and rapidly changing weather situation/picture; seating our Flight Attendants; etc.) we realized that for deviations greater than 99 NM; we needed to go to heading select mode; since the FMC (and therefore LNAV) will not accept greater than 99 NM offsets. This greatly increased our workload. Worse yet; beyond this distance (99 NM) we were not able to precisely determine 128 NM; our cleared limit; as OFFSET DISTANCE IS NO LONGER DISPLAYED IN THE FMC. As we passed 99 NM; right off course; the Prog. 2 page showed no usable data. To get around this limitation; we tried deleting the (99 NM FMC) offset completely in an effort to show a distance from the original route rather than from the 99 NM (FMC) offset; to no avail. Once beyond 99 NM off original course; the FMC DOES NOT SHOW OFFSET DISTANCE (at least on the Prog. page 2)! We therefore had to utilize the map range marks on the ND (at low ranges) as an approximation (while selecting at times more appropriate radar ranges); creating significant task saturation. As we approached what I estimated to be 128 NM right; I complied with the weather deviation without a clearance procedure including climb of 300 feet; lights on; etc.; as we still could not safely come back left. NFFF subsequently cleared us block 350-360; perhaps seeing that we had climbed 300 feet. NFFF was also asking if we had new routing in mind since he could not approve greater than 128 NM. We felt that with the radar display limited in distance and our task saturation; that we did not have time for a lengthy discussion with dispatch regarding routing options; nor did we have time then to figure it out on our own. Dispatch was sending messages at roughly the same time indicating where he saw lightning; but had indicated earlier that his radar picture of the area was not that great.I finally found an area that; while not ideal; was at least our first and perhaps only hole. I believe this to have been somewhere near our clearance limit of R128 NM; but was unable to exactly determine the distance from course; for the already mentioned reasons. I took the left turn and we were able to transit the remaining affected area with a reasonable ride. Around this time we received NFFF SIGMET Number 5 (issued at XC28Z) changing the boundaries of the affected airspace to slightly further both east and west. However; by this time we were well in the battle and had begun our correction back to our course.Obviously; the weather around us was changing rapidly. Perhaps this was why we had little warning of it prior to leaving [departure airport]. Several company aircraft transited the area in the hours before wedid; with no PIREPs; so this further illustrates the dynamic nature of the weather that evening.I have two concerns with this event: First; the FMC limitations that I illustrated dramatically increased our workload. When combined with the rapidly changing weather picture; we had limited bandwidth to coordinate with dispatch for a revised routing beyond the clearance limit of 128 NM right of course. Our job at that point was to keep us on a heading away from harm's way.Secondly; the B787 Multi-Scan radar is completely different from what we have operated in the past. I found this out during my first few months on the line with the 787; BUT DID NOT HAVE ANY SIGNIFICANT TRAINING ON IT. It tends to paint weather as more intensely in auto mode than previous radar models I have used (727; 737; 757/767; and 777). I knew this on our flight; however; being in the Equatorial Convergence Zone; my desire was to err on the side of caution; in case of dry-top storms. Using various manual mode settings to see something more comparable to my past experience definitely added to an already task saturated event. It was only through using manual mode in various configurations that we were able to find our 'hole' to fly through. I ENCOURAGE COMPANY TO DEVELOP A COMPREHENSIVE TRAINING COURSE on the differences of this radar and how to most efficiently utilize its capabilities.

Data retrieved from NASA's ASRS site 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.