Date: 202102
Local time of day: 1201-1800
Location: ZSE.ARTCC, WA
Flight conditions: VMC
Weather elements / visibility: Turbulence
Aircraft: Caravan Undifferentiated
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Airspace: Class E ZSE
ATC / advisory: Center ZSE
Reporter organization: Air Taxi
Reporter function: Captain
Reporter qualification: Flight Crew Commercial; Flight Crew Instrument
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; 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 Regained Aircraft Control; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Weather
Primary problem: Weather

Narrative: While approaching Mt Jefferson area on course direct ELKES at 14;000 aircraft encountered a mountain wave. First indications were not unusual; a loss of altitude followed by the autopilot pitch correction to maintain and resulting indicated airspeed loss. It is not uncommon for a momentary loss of a couple hundred feet and airspeed to drop to 90 KIAS; however; at the point where it would 'normal' the situation quickly escalated into an uncontrolled altitude loss as the autopilot rapidly pitched above 15* and I noted a 90+ headwind on the G600 wind vector indicator. I immediately contacted ATC that I was unable to maintain altitude; which they issued a lower clearance (13;000) and vector away from terrain. With the autopilot turned off; I maintained about 10* nose up at 75 KIAS; flaps 10; full throttle and noted a 1200FPM altitude loss. As I approached 13;000 I informed ATC I was unable and inquired about the MVA. I was fully prepared to activate emergency power; declare; and turn back; however ATC stated that the heading I was on gave me about 9000 for an MVA. I elected to ride the wave as the uncontrolled altitude loss was trending up and was able to stabilize as I approached 12;000. While altitude was stable; I remained below the MEA and published minimum off route altitude unable to climb or increase indicated airspeed above 80KIAS for several minutes. As the aircraft exited the mountain wave to the south; I was able to recover the airspeed slowly back to normal cruise as the aircraft moved over the ridge lines.This is the second time a mountain wave event has resulted in altitude loss of more than a few hundred feet. During the 'normal' events I have always notified ATC that I was encountering a wave and only one time previous did the incident result in the necessity for a lower clearance at 13;000'. However; in this event; I would characterize the resulting altitude loss as uncontrolled at minimum control airspeed and max continuous power for the altitude. I would consider this to be an escalation of what might be a normal inconvenience; and causes me to inquire about alternate routing. In all cases of my experience; these waves seem to be encountered over the Mt Jefferson area and a southerly re-routing of to the south is the normal avoidance procedure when the waves are encountered. But why wait until one is encountered? Might it be more prudent to take a different airway further south to begin with? Might be worth exploring.




Synopsis: C208 Captain reported encountering mountain wave action which resulted in momentary loss of control and altitude excursion.