Date: 201811
Local time of day: 1801-2400
Location: GGW, MT
Flight conditions: IMC
Weather elements / visibility: Icing
Light: Night
Aircraft: Cessna Aircraft Undifferentiated or Other Model
Operator: Air Taxi
Operating under: Part 135
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class E GGW
ATC / advisory: Center ZLC
Reporter organization: Air Taxi
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Commercial
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Altitude Overshoot; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter Unstabilized Approach
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: Human Factors; Weather; Procedure
Primary problem: Weather

Narrative: Cleared by Salt Lake Center for the GPS 30 into GGW via JITAG. Cleared to cross JITAG at 5;800 [feet]. I descended early and crossed the fix at 5;800 [feet]because earlier in the trip departing Billings; we only picked up trace icing at that altitude. We flew the 11 miles to HUKAL picking up light to moderate ice; with the intensity growing. My plan was to blow the boots once we had collected enough ice to get a good break and hand fly the approach. There was no trim movement and our indicated airspeed was around 140 KIAS; plenty of speed in my opinion. We turned inbound at HUKAL and as the glideslope came alive; I went in range and lowered the gear preparing to disengage the autopilot and blow the boots. The Pilot Monitoring called airspeed. I noticed the airspeed decaying to about 110 [knots] followed by the yoke moving forward and aft in my hands as if the tail plane was starting to stall. I disconnected the autopilot and added power and leveled off; and with the added power; the aircraft started a shallow climb. We deviated high and to the left of course; but upon exiting the tail plane buffet; navigated back towards the localizer and glideslope while blowing the boots multiple times. The course was regained easily enough to continue the approach because I certainly did not want to stay in the icing environment. We broke out at about 1;000 AGL and came over the numbers at about 110 knots. I experienced another tail plane buffet through the yoke again during the flare; and landed without incident. I rolled out to the end and exited to the left on Taxiway Charlie. Starting our descent early to cross JITAG at 5;800 [feet] put us in the clouds for 11 miles. We could have crossed HUKAL at 5;800 [feet] and still would have been okay for the approach. The airspeed decay could be attributed to the ice buildup since normal procedures for power settings were observed during the flight.Also; the delay in blowing the boots may have attributed; but I make it a practice to try not to blow the boots prematurely. When the boots were blown; a sizable amount of ice was removed from the wing and tail. I would try to stay in VMC longer; even though we experienced trace icing earlier.


Second narrative: Starting the approach level 5;800 [feet] at JITAG intersection for the RNAV GPS 30 into GGW; we were picking up moderate ice. We continued on to HUKAL inbound. As we were wings level and the glideslope was coming alive; we went in range followed immediately by the before landing flow. Our airspeed started to decrease and the Pilot Monitoring called to watch airspeed. Within a couple seconds; a buffet and vibration of the controls occurred. No stall horn went off because it was completely iced over. The Pilot Flying took corrective action and disengaged the autopilot and added power. After we recovered and the airplane was flying again and we regained positive control; we navigated back to become established again on the glideslope and localizer. We were waiting to blow the boots in the descent on final to get more ice off of the leading edge. We blew the boots multiple times coming down after we turned onto the final approach course after the stall had occurred. We did not go missed because we did not want to go back into the icing at this point. We flew the approach fast and in the flare stalled over the runway and rolled down to the end to exit the runway environment.We descended early when we could have stayed high and crossed JITAG above 5;800 [feet]. Waiting to blow the boots in the descent to try and get more ice off the leading edges. Loss of situational awareness at night in IFR conditions. [Suggest] blowing the boots earlier and staying higher until absolutely necessary to descend.


Synopsis: Air Taxi flight crew reported an early descent led to ice accumulation and possible tail stall.