Date: 201205
Local time of day: 1201-1800
Location: ZID.ARTCC, IN
Weather elements / visibility: Turbulence
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Cruise
Airspace: Class A ZID
ATC / advisory: Center ZID
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Other / Unknown
Second reporter function: Captain; Pilot Flying
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control; Person Flight Crew
Result: General None Reported / Taken
Contributing factors: Human Factors; Weather
Primary problem: Weather

Narrative: An E170 was on my frequency transitioning through my airspace at FL350. There were reports of constant light to occasional moderate chop being reported and most aircraft were descending to lower altitudes to get better rides. Another aircraft checked on my frequency and after telling the aircraft of the rides to expect they requested lower. The aircraft was not in my airspace so I called on the land line to coordinate lower. While I was off line coordinating I heard over the speaker somebody request something. I got off the line; transmitted in the blind that I was off the line coordinating and I would be right back with them. I then finished my coordination; and descended the aircraft that had requested lower. Then I observed the E170 descending out of FL343. I called the E170 to verify level at FL350. He told me that he had encountered constant moderate chop bordering on severe and the way I understood it the aircraft had to descend to keep his speed. He told me something about being near the safety envelope and needing to descend immediately. He told me he didn't require assistance now that he had descended. I worked the sector for another hour and never had a report of worse then occasional moderate chop. It's the first time in my experience that I've heard of a plane needing to descend so urgently; without an emergency. I felt I was taking care of the aircraft on first come; first serve basis. I'm pretty sure if I could have heard the E170's first request transmission instead of going over the speaker since I was on the line; I could have gotten him his clearance in time for him to not have to take matters into his own hands.


Second narrative: While flying at FL350; we encountered an area of severe turbulence. We were operating in VMC. The aircraft had large changes in pitch airspeed and altitude. I took the controls to exit the area of turbulence with a descent to FL330. We called ATC to inform them that we were unable to maintain FL350 and were descending to FL330. We received no response from ATC. Once we exited the severe turbulence and level at FL330 we called the cabin to ensure there were no injuries. ATC then asked us to verify our altitude as FL350. We explained that we called and told him we were descending and that we encountered severe turbulence at FL350. He asked if we were declaring an emergency. We said 'NO'. We were out of turbulence and there were no injuries on board. ATC said he was 'on a land line and missed the call'; and then he gave us a phone number to call for possible pilot deviation. We did what was needed to maintain control of the aircraft and the safety of the people on board. Upon landing Maintenance inspection found some damage.


Synopsis: ZID Controller described an unexpected descent by an Air Carrier at FL350 during a turbulence encounter. The Controller did not hear the initial descent request because of land line coordination.