Narrative:

Feet wet; several minutes south of diw; clear air; smooth air; sudden extreme mountain wave activity caused the aircraft to be unable to maintain desired altitude; airspeed; and N1. Some choppiness and minor airspeed deviations had occurred when feet dry over north carolina - suspected mountain wave activity. Conditions smoothed when feet wet. Wind from the west at 190+/- KTS; then sudden windshear +/- 30 KTS. Had to throttle right back (<50 percent N1) to keep from over speed; needed to descend. Declared emergency to descend to FL380 because I could not maintain altitude and airspeed. During the descent the wind speed fluctuated wildly between 120 and 190 KTS. This caused 'coffin corner ' to be observed about 3 times during the extreme wind speed fluctuations; the min spread was about 5 KTS and the wide spread was about 50 KTS on the airspeed tape. I was busy locked onto the 'ball and needle' while coaxing the plane down to FL380 and while waiting for the engines to spool back up to the normal N1 range for cruise. After 3 to 4 minutes; regained complete control of altitude; airspeed; and N1. Continued toward ozena fix. Then contacted dispatch/maintenance after clearing the emergency condition with ATC. Concurrence between PIC; mc; and dispatch that extreme turbulence was not encountered which would have required an inspection of the aircraft by mc. Upon later debrief with the lead F/a while enroute after the discussion with dispatch and mc; learned that the passengers experienced nothing more than the choppiness of the occurrence; and didn't seem concerned; but that the flight attendants had recognized something unusual had occurred. Then continued to destination without further incident.

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Original NASA ASRS Text

Title: B737-700 Captain reports encountering mountain wave like conditions at FL400 over the western Atlantic. Large airspeed excursions are reported and altitude cannot be maintained. An emergency is declared until clear of the conditions then canceled.

Narrative: Feet wet; several minutes south of DIW; clear air; smooth air; sudden extreme mountain wave activity caused the aircraft to be unable to maintain desired altitude; airspeed; and N1. Some choppiness and minor airspeed deviations had occurred when feet dry over North Carolina - suspected mountain wave activity. Conditions smoothed when feet wet. Wind from the west at 190+/- KTS; then sudden windshear +/- 30 KTS. Had to throttle right back (<50 percent N1) to keep from over speed; needed to descend. Declared emergency to descend to FL380 because I could not maintain altitude and airspeed. During the descent the wind speed fluctuated wildly between 120 and 190 KTS. This caused 'coffin corner ' to be observed about 3 times during the extreme wind speed fluctuations; the min spread was about 5 KTS and the wide spread was about 50 KTS on the airspeed tape. I was busy locked onto the 'ball and needle' while coaxing the plane down to FL380 and while waiting for the engines to spool back up to the normal N1 range for cruise. After 3 to 4 minutes; regained complete control of altitude; airspeed; and N1. Continued toward OZENA fix. Then contacted Dispatch/Maintenance after clearing the emergency condition with ATC. Concurrence between PIC; MC; and Dispatch that extreme turbulence was not encountered which would have required an inspection of the aircraft by MC. Upon later debrief with the lead F/A while enroute after the discussion with Dispatch and MC; learned that the passengers experienced nothing more than the choppiness of the occurrence; and didn't seem concerned; but that the flight attendants had recognized something unusual had occurred. Then continued to destination without further incident.

Data retrieved from NASA's ASRS site as of July 2013 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.