Cessna Single Piston Undifferentiated or Other Model
Flight Crew Commercial; Flight Crew Instrument
Situational Awareness; Communication Breakdown
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter Loss Of Aircraft Control
Air Traffic Control Issued New Clearance; Flight Crew Regained Aircraft Control
Environment - Non Weather Related; Weather
Flight to COS on an IFR flight plan. After crossing LUFSE at 9000 ft we turned south on V389 after calling COS approach on 124.0; we were told to expect 'vectors for the Visual 17R' into COS. On V389; approximately 15 NM north east of COS; we received the first vector of heading 230 for the Visual 17R. About 8 NM ahead we could see light rain shafts; that we associated would have some downdrafts. I requested a left deviation of 20 degrees to avoid the rain shaft. During the deviation of 20 degrees we started to experience downdrafts. The aircraft began pitching up to around 8 degrees nose up and began decreasing in airspeed. Full power was already applied and aircraft slowed to around 66 KIAS and was unable to hold altitude; at the same time the autopilot was disconnected and straight and level flight was maintained. The aircraft continued to pitch up to what was around 12 degrees. I made COS Approach on 124.0 aware that we were descending in what occurred to be a downdraft at the altitude of 8800 ft; during which our VSI indicated a descent at 1400 FPM. The aircraft stall warning briefly sounded. I was told by approach to maintain 9000; and I continued to tell approach we were unable to maintain 9000 with the current situation at hand. They understood and told us that the minimum enroute altitude was 9000 and to turn to heading 150 for an emergency vector. We complied with Approach's instructions and began the turn; which was about the time the downdraft ceased; we were able to maintain level flight at 8500 and continue the climb back to 9000. At 8500 feet we cancelled our IFR flight plan because we could maintain our own terrain and obstacle clearance and were in VFR conditions. The rest of the flight was continued without incident under VMC with flight following with approach to COS. Corrective action was taken on the event during the loss of altitude due to weather. Cancelling IFR earlier during the loss of altitude would have relieved the controller of having to issue emergency vector due to altitude below MEA. If on an IFR flight plan; consider flying higher than the MEA assigned for the routing to account for unknown variables; such as weather.
Cessna pilot reported being unable to maintain altitude and airspeed due to a downdraft on approach to COS.