ERJ-175 flight crew reported encountering wake turbulence from a preceding B777 on descent into DEN.
Synopsis
ERJ-175 flight crew reported encountering wake turbulence from a preceding B777 on descent into DEN.
Narrative
Just prior to reaching our initial descent into DEN on the LAWGR 4 Arrival; the First Officer (FO) and I noticed Denver Center vectoring a 777 around us to the right side of the aircraft. We were assigned a speed to maintain (Mach .74) and noticed the 777 was passing off our right 2000 feet above us; with about 3 - 4 mile spacing between us. At that particular time; the FO and I both deemed that spacing was adequate. Seeing as the 777 was flying much faster than we were (estimated around Mach .79); spacing continued to increase. The 777 was then vectored in front of us; and according to our TCAS system; spacing between us was about 12 miles at the time of the event.A few minutes later; we were in straight and level flight at about 34 thousand feet. The FO and I began accomplishing our descent tasks; which included obtaining the Digital ATIS; calculating the landing performance data from ACARS; verifying the points on the arrival and approach; bugging" the Flight Mode Annunciator (FMA) / Primary Flight Display (PFD); and accomplishing the arrival briefing. During the FO's arrival briefing; I noticed the 777 ahead of us was now at our altitude; but spaced about 12 miles in front of us. ATC did not notify us of traffic ahead; and I did not inquire about the traffic; seeing as this is a typical "in trail" spacing technique used by ATC. I also noted the winds aloft were showing a direct headwind at 33 knots.Shortly thereafter; our aircraft began "buffeting" and a few seconds later entered into two separate; aggressive thirty-degree banks in either direction. Autopilot was automatically turned off; and the FO promptly returned to straight and level flight. Seeing as this was not an "upset" the Standard Operating Procedures recovery procedure was not required. The FO (pilot flying) maintained control of the aircraft; and we eventually turned the autopilot back on.After ensuring control of the aircraft; I immediately notified ATC that we needed to descend due to wake turbulence. ATC promptly cleared us to descend and maintain FL320; and we quickly exited the wake turbulence.It was determined that this event occurred due to lack of communication / separation by ATC. We were not notified of a wake turbulence caution; and the 777 was vectored in front of us on the arrival. No TCAS RA / TA was triggered.After ensuring aircraft control; descending to a lower altitude; and communicating with ATC; I made a PA to the flight attendants (FAs) informing them to be seated immediately. Shortly thereafter; I called them on the interphone to ensure nobody in the cabin was injured during the wake turbulence event. Both FAs informed me that no passenger was injured; and that they were seated on the floor in the aft galley. I informed them it was now safe to get up and resume their duties.Promptly after speaking with the FAs; I decided to make a PA to the passengers; explaining the nature of the wake turbulence. The fasten seat belt sign remained on for the rest of the flight. The flight continued to DEN and landed with no further incident.Lack of communication between ATC and pilots regarding spacing / wake turbulence separation. Aircraft was vectored in front of us and then descended from FL360 down to our altitude (FL340) with inadequate separation.Suggest modifying spacing techniques; taking into consideration wake turbulence categories. For example; an Airbus A350 would not require as much spacing as an E-175 when flying behind a 777; because it is a much larger aircraft.Suggest better communication between ATC and aircraft with regards to wake turbulence. I notice I get many wake turbulence warnings in the terminal environment; but not as many when I am flying at a higher altitude."
Second reporter narrative
My Captain and I were doing our normal duties of flight at FL340; beginning prep for our descent into Denver on the LAWGR4 Arrival. As we were flying; we requested a higher altitude because of bumps at 340; but were told we couldn't get higher because of a trailing heavy aircraft at 360 on the same arrival into DEN. Later; we did see that trailing traffic overtake us (we were going approximately .74 Mach). They overtook us about 2.5 - 5 miles to the north of us at 36000 feet. Once we saw them past us; we considered them not a threat as they were faster and offset from us; and as of that time; hadn't encountered any wake turbulence to suggest we were crossing their flight path.As we were getting close to our descent; we began our descent tasks; and I began briefing the arrival into DEN when we began to pick up some buffeting. Immediately; my Captain said he thought it was wake turbulence from the jet ahead of us; and as he said that; we began to experience significant buffeting and roll induced by the wake turbulence. I reacted by counteracting the roll by inducing opposite aileron and attempting to maintain a level attitude. The automation was automatically turned off as we were banked up to 30 degrees to the left and right. We regained control quickly and were able to get the plane back to its original position while my Captain requested lower and put the altitude bug at 320 while waiting for clearance. We got the automation back on and did not think we had an UPSET" as the bank was never more than 30 from what I recall. We were ready to descend to lower if buffeting continued. We then received clearance for lower and descended.ATC never gave us a warning of traffic ahead or that we could encounter wake turbulence; but they were also about 12 miles ahead; so spacing seemed proper. The winds; however; were a direct headwind above 30 knots; and that probably played a part in the dissipation of the wake turbulence.During the descent things returned to normal and we had no other indication that we were too close to the 777 and we were then given warnings by the next control of possible wake turbulence. We continued our approach to landing without any other issues and were able to land the plane safely.After the event the Captain made sure to address the FAs and passengers to make sure no one was injured. We received word that everyone was seated despite the seat belt sign being off and that there were no injuries reported. The Captain had the FAs resume securing the cabin and prepare for landing. The seat belt sign was turned back on and remained on for the remainder of the flight.We believe this event was caused because of lack of ATC separation; with no verbal warning of the 777 ahead of us and possible wake turbulence. While they did have 10 - 12 miles of separation; because they descended ahead of us and directly in our track with the arrival; it led to us getting hit by the wake turbulence. In the future; I will be more aware of how quickly wake turbulence can affect the aircraft and will be ready to turn off automation in the case of experiencing wake turbulence. If we are ever in trail of a heavy aircraft; a warning of wake turbulence even if the aircraft is 10 miles ahead would be a good warning as well. Especially if the winds are coming from the west as they were today."
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.