Date: 202101
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 12
Light: Daylight
Aircraft: Bonanza 36
Operator: Personal
Operating under: Part 91
Flight plan: SVFR
Mission: Training
Flight phase: Parked
Aircraft component: Reciprocating Engine Assembly
Component problem: Failed
Reporter organization: Personal
Reporter function: Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Private
Second reporter function: Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew
When detected: Pre-flight
Result: Air Traffic Control Provided Assistance; Aircraft Aircraft Damaged; Flight Crew Requested ATC Assistance / Clarification; General Evacuated; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We had completed the pre-flight check of the aircraft and pulled it from the hangar at ZZZ Airport to the alley way between hangars.We had completed the pre-start checklist items; picked up the ATIS and our clearance to transit out of the SFRA (Special Flight Rules Area).The aircraft started normally. There was a single backfire after or as the engine started but not too uncommon. The throttle was retarded following normal procedures.Shortly after engine start the cockpit began to fill with an acrid smelling smoke from under the left side of the panel. The engine was immediately shutdown and we exited the plane. All electrical (master/alternator/ignition) was shut off.Co-pilot grabbed the fire extinguisher as we exited. After exiting the Tower was called. The Tower called the on-field fire company. Response time was about <5 min as the on-field company was sent to the wrong side of the field.We opened the engine cowling to see if we could identify a source and possible extinguish. No fire was evident in the engine compartment. The fire departments had not yet arrived.The prop began a very slow rotation as if the engine was trying to crank.We went to check to verify that the Master Switch was off. It was.Smoke from the panel was much heavier. Appeared to mostly originate from the pilot side.Close evaluation showed flames from top of the left side-wall upholstery.We used the fire extinguisher to extinguish the flames along the sidewall. The amount of extinguishing agent or time for a full discharge seemed less than expected (perhaps 30 seconds).Fuel valve was moved to Off.A person called indicating that the Emergency Locator Transmitter had activated. We began a search for the beacon to deactivate. Determined that it is behind the panel in the back of the passenger area behind the stretcher. Requires panel removal.Fire department from ZZZ arrives.They verify that there is no excess heat in area where the flames were extinguished or other locations.The mechanic we used at [Company] arrived and began assisting with assessment and possible causes and removal of the ELT.Fire departments determined that no action on their part was needed and departed. On the scene perhaps 30 minutes.No immediate cause was noted the A&P from [Company].We returned the plane to the hangar. We called [Company1] to see if they could determine any immediate cause. We were trying to decide if we needed to use [Company1] vs [Company2].The technician from [Company1] saw no immediate issues.We disconnected the ground side of the battery; wrapped and secured to make certain no contact could occur that might provide a power source.We left the airplane doors open or ajar to allow for ventilation and secured the hangar.The plan at this point was to have the aircraft moved to [Company2].


Second narrative: We pulled the aircraft out of the hangar; shut the hangar doors and climbed into the cockpit for the start. We completed the normal procedures checklist setting the parking brake; received our SFRA (Special Flight Rules Area) clearance and shut down the avionics for start and noted the time as XA:X0 pm. The engine was primed with the auxiliary fuel pump; mixture was set to full rich and throttle to full open. The ignition was turned to start and as the engine began to turn over it ignited; and the throttle was retarded to slow the RPM to approximately 1000 RPM. As the RPM was beginning to stabilize; there was a backfire explosion and almost simultaneously smoke began pouring out from the defroster duct? at the left side of the wind screen and from underneath the dash panel by the PIC (Pilot-in-Command)'s rudder pedals. The smoke intensified filling the cockpit and the door was opened to relieve the smoke. Since the smoke was continuing; the mixture was pulled; the ignition was turned off removing the key and the battery and alternator switches turned off. Co-pilot exited the aircraft to determine a cause of the smoke and as he was investigating; I pulled the fire extinguisher from behind the co-pilots seat. I then exited the aircraft while he continued to walk around the nose of the aircraft looking for any source of fire. None was noted and the smoke was ventilating and dissipating. I called the tower on my cell phone to notify them of our situation and asked for fire assistance. I told Tower we were [at] the hangars on the east side just south of the terminal building at hangar. Tower took immediate action to alert the airport firetruck and ZZZ City Fire Dept. We did not see any evidence of a fuel fire or flames exiting the engine compartment although we did notice that the propeller began to rotate slowly for a few terms before stopping completely. The engine compartment was opened and there was no evidence of any fuel or oil leaks or any fire or smoke damage to the engine compartment. We went back behind the right wing and looking through the cockpit door; co-pilot noticed flames between the sidewall and left rudder pedal. He activated the fire extinguisher and sprayed the area of the flame; extinguishing it.While waiting for the fire equipment to arrive; I called Maintenance to ask them to have a mechanic come over and evaluate the situation. Fortunately; their lead was on the way to the east side and he arrived to look at the aircraft. Shortly thereafter the Airport Operations Director was first to arrive on scene and supervised the arrival of the fire equipment. The airport team was first ARFF (Airport Rescue and Fire Fighting) unit on the scene and were very professional in evaluating the situation so that no unnecessary action was taken. The smoke gradually dissipated and the aircraft was checked for residual hot spots; none of which were found. The fire equipment returned to their stations and the aircraft was hangared until it could be moved to the local maintenance facility.


Synopsis: BE-36 pilots reported a fire of unknown origin at engine start requiring evacuation and the airport ARFF to respond.