Narrative:

We descended from our cruise altitude without a clearance due to unforecasted turbulence on our route. As we were approaching 140 west; the ride was getting worse. We had already requested; and flew; in a block altitude from FL350 to FL370 in order to accommodate a better ride for our customers. We had been flying at FL360; but returned to FL350 because the ride seemed to better lower. We noticed on the TCAS ahead and below us at FL310 an aircraft over 140 west. We queried the aircraft about their ride. They told us their ride was pretty decent at FL310. We had sent an ACARS message to dispatch to run the fuel numbers for a flight at FL310 to see if that was an option; and gave our dispatcher notice of what we were contemplating. By 140 west; our current flight plan dictated a step climb to FL370; but based on the current conditions this was not a viable option. The fasten seat belt sign was on. I requested a discreet HF frequency and a patch to dispatch because I had not received an answer for a flight plan burn at FL310. We were told to contact them on 6640 and to standby for the patch. About 20 to 15 nautical miles from 140 west the ride deteriorated further; and we experienced a large variance in indicated airspeed; and large changes in attitudes. Realizing that the ride kept getting worse; I elected to use my captain's emergency authority and told the first officer to descend before we did some serious damage to the aircraft. We remained on assigned frequency and advised that we had encountered severe turbulence and were descending to FL320. To add to the workload that was before us; we had to make sure our course and distance for our next leg was correct; and had to get ready to mark our position as we crossed 140 west and write the parameters on the flight plan. I made sure the TCAS switch was selected to 'below' to make sure no conflict traffic laid ahead. The workload jumped exponentially from one second to another. The first officer called the flight attendants to make sure no one had been hurt and everybody remained belted. We encountered this polar jet stream and area of light turbulence that was shown on the prognostic chart; prior to the location on the chart. According to the chart; the light turbulence area and jet stream were still further to the east of us. Company traffic ahead of us had provided a pilot report saying the ride was decent at FL370. Our optimal altitude from the FMC still did not provide FL370 as an option; therefore; we were left with the options of staying at FL350 or going lower. We were well aware of the weather system ahead of us and had discussed the options. What the prognostic chart did not show; was the turbulence was greater than light! With the prognostic chart and company aircraft to back up the ride reports we were mislead. No one could have predicted this based on the information at hand. Based on how fast this event developed; and how fast we got our new clearance; I believe we did due diligence on our part. At no time was the safety of our passengers compromised by a potential collision with another aircraft. I believe that if satcom would have been available; the decision to go to a lower altitude would have been made earlier; and this situation could have been avoided. 'No comm' on the ACARS delayed messages sent from dispatch and us. Having a direct communication dispatch link would have been a great help. Also; the ability to talk to commercial radio that evening was degraded due to the heavy volume of communication taking place on HF.

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Original NASA ASRS Text

Title: During a Pacific crossing a flight crew reports encountering unforecasted severe turbulence and descends to a lower altitude using Captain's emergency authority prior to receiving ATC clearance via HF radio.

Narrative: We descended from our cruise altitude without a clearance due to unforecasted turbulence on our route. As we were approaching 140 west; the ride was getting worse. We had already requested; and flew; in a block altitude from FL350 to FL370 in order to accommodate a better ride for our customers. We had been flying at FL360; but returned to FL350 because the ride seemed to better lower. We noticed on the TCAS ahead and below us at FL310 an aircraft over 140 west. We queried the aircraft about their ride. They told us their ride was pretty decent at FL310. We had sent an ACARS message to Dispatch to run the fuel numbers for a flight at FL310 to see if that was an option; and gave our Dispatcher notice of what we were contemplating. By 140 west; our current flight plan dictated a step climb to FL370; but based on the current conditions this was not a viable option. The fasten seat belt sign was on. I requested a discreet HF frequency and a patch to Dispatch because I had not received an answer for a flight plan burn at FL310. We were told to contact them on 6640 and to standby for the patch. About 20 to 15 nautical miles from 140 west the ride deteriorated further; and we experienced a large variance in indicated airspeed; and large changes in attitudes. Realizing that the ride kept getting worse; I elected to use my Captain's emergency authority and told the First Officer to descend before we did some serious damage to the aircraft. We remained on assigned frequency and advised that we had encountered severe turbulence and were descending to FL320. To add to the workload that was before us; we had to make sure our course and distance for our next leg was correct; and had to get ready to mark our position as we crossed 140 west and write the parameters on the flight plan. I made sure the TCAS switch was selected to 'BELOW' to make sure no conflict traffic laid ahead. The workload jumped exponentially from one second to another. The First Officer called the flight attendants to make sure no one had been hurt and everybody remained belted. We encountered this polar jet stream and area of light turbulence that was shown on the prognostic chart; prior to the location on the chart. According to the chart; the light turbulence area and jet stream were still further to the east of us. Company traffic ahead of us had provided a pilot report saying the ride was decent at FL370. Our optimal altitude from the FMC still did not provide FL370 as an option; therefore; we were left with the options of staying at FL350 or going lower. We were well aware of the weather system ahead of us and had discussed the options. What the prognostic chart did not show; was the turbulence was greater than light! With the prognostic chart and company aircraft to back up the ride reports we were mislead. No one could have predicted this based on the information at hand. Based on how fast this event developed; and how fast we got our new clearance; I believe we did due diligence on our part. At no time was the safety of our passengers compromised by a potential collision with another aircraft. I believe that if Satcom would have been available; the decision to go to a lower altitude would have been made earlier; and this situation could have been avoided. 'NO COMM' on the ACARS delayed messages sent from dispatch and us. Having a direct communication dispatch link would have been a great help. Also; the ability to talk to Commercial Radio that evening was degraded due to the heavy volume of communication taking place on HF.

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.