Date: 202006
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: MBB-BK 117 All Series
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Tower ZZZ
Reporter organization: Air Taxi
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Commercial
Human factors: Workload; Other / Unknown; Troubleshooting
Anomaly: Airspace Violation All Types; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Aircraft; Weather
Primary problem: Ambiguous

Narrative: Enroute to pick up passenger noted some turbulence crossing the mountains and into the desert region east bound and a 10 knot head wind; I would categorize the turbulence as mild to moderate with some up drafts and down drafts.  Cruse altitude was 075 VFR. Upon subsequent passenger pick up followed the same track back to the receiving facility; having experienced the turbulence and winds I elected to come back westbound at 10;500 [feet] VFR to cross the highest peaks and then descend to 065 [feet] [feet].  At this altitude there was still some mild turbulence; and was experiencing a tail wind that was fluctuating in wind speed; but nothing abnormal given the prevailing weather; aircraft speed was normal for cruise flight.  After crossing the highest mountain peaks I began a descent; with the AFCS; altitude acquire upper mode engaged for a level off at 065 [feet].  The aircraft nose dipped down and began a normal descent; approximately 2 to 3 minutes into the descent the aircraft nose pitched down dramatically in what I interpreted as a severe down draft or severe gust from the tail wind; forcing the nose of the aircraft down; the speed rapidly increased.  I applied a slight aft cyclic; as I decoupled the AFCS; the aircraft then violently pitched up into at least a 45 to 50 degree nose up attitude; and began a left rolling moment.  Given the similarity to a (fixed wing) approach turn stall; I applied left pedal and followed the rolling moment with cyclic to the left; reduced collective; and rolled the aircraft back to the right; the aircraft recovered wings level in about a 45 degree nose down attitude.  I then gradually applied aft cyclic and increased collective until the aircraft was in level flight; I believe the aircraft went through about 90 degrees of turn.  The only CAD indication at this time was MM EXCEED.  I turned the aircraft back on course westbound; I reviewed the check list for the MM EXCEED Caution Procedures.  No indications of AFCS malfunctions or hydraulic system malfunctions; the controls 'felt' normal.  I and the crew were very rattled (upset); we discussed what happened and I calmed them and told them we were going to land direct to the airport.Once the situation calmed and I completed the EP review I became aware I was now in the Class B upper tier airspace; I immediately made a turn and continued a descent.  I then called ATC and requested a 'Bravo transition;' whereupon I was given a squawk and clearance.  At this time I continued on a slow descent and airspeed toward the ZZZ airport; my consideration was to not make any dramatic or abrupt maneuvers but continue on this flight profile to the airport given the Mast Moment Exceed.  I arranged ground transport for the passenger and crew.  All con notified and passenger with crew transferred to ground transportation.My summation of the causal factor: given the track of the aircraft and a review of the prevailing weather: the aircraft experienced retreating blade stall; set on by High altitude; High heat index; mountain topography wave activity and an abrupt tail wind or down draft forcing the nose down and a rapid increase in airspeed. I recall no rotor speed horn(s); no 'high' torque tone/gong. No VNE 'LIMIT' on the VEMD; gong or associated master caution; the system seemed to be indicating normally but airspeed was changing abruptly in cruise flight; and I was constantly adjusting power to maintain nominal cruise airspeed. Weather was not as forecasted for that region. Topography at the incident is a dramatic drop off into a valley; charted and known as a glider area.  Training department failure to properly address the known factor(s) with the BK117 and its susceptibility to Retreating Blade Stall.




Synopsis: BK117 Captain reported temporary loss of aircraft control during flight over mountainous terrain in turbulent weather.