Narrative:

The severe wake turbulence took place on a flight which was operating under part 91; at the time; to bfl. We had 5 passengers on board. The encounter took place between the hec and pmd vors. Sometime after our initial descent we were given ATC instructions to turn left 10 degrees for our descent. Shortly after that we were cleared to descend to FL300. We began a 1;000 FPM descent to FL300 and it was at that time I noticed a TCAS target at our altitude; but descending; at our 3 o'clock. I acquired the target visually and noted its size and assumed it was a B747. Its position was gradually moving from our 3 o'clock toward our 12 o'clock as it descended. I also noted that it was descending at about the same rate as we were. It stayed at 800-1;000 ft below us and 8-10 miles from us throughout our descent. I assumed that this was the traffic we were being vectored for and I anticipated a turn back on course (toward this aircraft's flight path) and further descent. I increased our descent rate to 1;500 FPM thinking that the sooner we reached FL300 the sooner we would be turned back on course. My thinking was that this would get us on the other side of the plane's wake sooner; allowing for a larger buffer between our plane and where I perceived the wake to be. It was at this time I noted that the winds aloft were from the northwest at around 40 KTS; moving the wake toward us.I commented to the co-pilot that this situation was making me a bit uneasy; but after a short discussion we both thought that the 8-10 miles between us and the large jet was adequate. It wasn't. Seconds later we encountered the wake. The plane instantly rolled 45-50 degrees to the right. I reacted by applying full power and pitching up to approximately 20 degrees to escape. Once things settled down we advised ATC of our altitude deviation and the cause. Strangely; the controller seemed not to believe us at first. He advised us of the distance from us to the large jet; which turned out to be an A340; and stated that the plane was always at least 10 miles from us and 2;000-3;000 ft below us. According to our TCAS this was not true. After checking on the passengers I reported to ATC the scenario as we saw it and gave him a winds aloft reading. I think that one of the large contributing factors to us encountering the wake was the wind aloft. I don't think the controller that directed us onto a heading downwind of the A340's wake had taken into consideration the winds aloft. On my part; I should have gone with my intuition and requested a vector and/or a descent or climb to avoid the wake. However; all of those requests would have been made on the assumption that we were being vectored in reference to the A340. ATC never mentioned the A340 until after we had encountered the wake.

Related reports:

Google
 

Original NASA ASRS Text

Title: A C560 Captain reported encountering very strong wake from an A340 at FL300 that resulted in a 45-50 deg roll.

Narrative: The severe wake turbulence took place on a flight which was operating under Part 91; at the time; to BFL. We had 5 passengers on board. The encounter took place between the HEC and PMD VORs. Sometime after our initial descent we were given ATC instructions to turn left 10 degrees for our descent. Shortly after that we were cleared to descend to FL300. We began a 1;000 FPM descent to FL300 and it was at that time I noticed a TCAS target at our altitude; but descending; at our 3 o'clock. I acquired the target visually and noted its size and assumed it was a B747. Its position was gradually moving from our 3 o'clock toward our 12 o'clock as it descended. I also noted that it was descending at about the same rate as we were. It stayed at 800-1;000 FT below us and 8-10 miles from us throughout our descent. I assumed that this was the traffic we were being vectored for and I anticipated a turn back on course (toward this aircraft's flight path) and further descent. I increased our descent rate to 1;500 FPM thinking that the sooner we reached FL300 the sooner we would be turned back on course. My thinking was that this would get us on the other side of the plane's wake sooner; allowing for a larger buffer between our plane and where I perceived the wake to be. It was at this time I noted that the winds aloft were from the northwest at around 40 KTS; moving the wake toward us.I commented to the co-pilot that this situation was making me a bit uneasy; but after a short discussion we both thought that the 8-10 miles between us and the large jet was adequate. It wasn't. Seconds later we encountered the wake. The plane instantly rolled 45-50 degrees to the right. I reacted by applying full power and pitching up to approximately 20 degrees to escape. Once things settled down we advised ATC of our altitude deviation and the cause. Strangely; the Controller seemed not to believe us at first. He advised us of the distance from us to the large jet; which turned out to be an A340; and stated that the plane was always at least 10 miles from us and 2;000-3;000 FT below us. According to our TCAS this was not true. After checking on the passengers I reported to ATC the scenario as we saw it and gave him a winds aloft reading. I think that one of the large contributing factors to us encountering the wake was the wind aloft. I don't think the Controller that directed us onto a heading downwind of the A340's wake had taken into consideration the winds aloft. On my part; I should have gone with my intuition and requested a vector and/or a descent or climb to avoid the wake. However; all of those requests would have been made on the assumption that we were being vectored in reference to the A340. ATC never mentioned the A340 until after we had encountered the wake.

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.