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.
Synopsis
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 be 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 different frequency 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.
Second reporter narrative
Nearing 140 west; our aircraft encountered unforecasted/unreported severe turbulence. As we were coordinating with ATC; the airspeed and altitude fluctuations grew to an unacceptable level with respect to the safe operation of the aircraft. At this time; the Captain (pilot not flying) made the decision to descend out of the turbulence in the interest of safety. Shortly thereafter; clearance was received from ATC for our descent. As our preflight winds and turbulence plots had indicated the possibility of turbulence in this area; we had made the decision to sit the passengers and flight attendants down prior to this encounter; which undoubtedly warded off any injuries. Our crew had requested PIREP's from other aircraft operating on the same route (both company and other) at the same altitude; but no other aircraft had experienced any turbulence in excess of occasional moderate turbulence. Making matters worse; the event happened coincident with a compulsory reporting point which was also a course change. Handling the turbulence; communications; and navigation was a chore but other than the altitude deviation; no other errors were noted. Although we had made many requests to Dispatch through ACARS prior to the event regarding weather on our route and fuel calculations should we need to descend; no replies were received. For the majority of our flight; ACARS showed 'No Comm' although there were no system faults with the HF radios. Although this event would have been difficult to predict by Dispatch; a more reliable system for transmission of data would have helped forewarn company aircraft following in our path. Instead; warning the company aircraft following behind us by approximately 30 minutes became our task; only adding to the workload on the flight deck.[We had] a rapid onset of severe turbulence at our flight level; which was not experienced by aircraft passing through the same region ahead of us and avoided by trailing aircraft due to our PIREP's was the cause. Weather predictions obtained prior to flight and PIREP's from preceding aircraft also failed to predict this episode.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.