Medium Transport; Low Wing; 2 Turbojet Eng
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Conflict Ground Conflict; Critical; Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Took Evasive Action
Airport; Human Factors
We landed Runway 19 at SGU because of the winds and multiple other aircraft in the pattern for Runway 19 on Unicom. We heard [another carrier flight] taxi out to Runway 1 and pick up his clearance from LA Center (whom we were still monitoring; along with Unicom). We landed and exited at A4 to the ramp.Clearing the runway and established on Taxiway A; we heard a mumbled call from someone taking Runway 1 for departure VFR to the north AND a small aircraft (Seneca) taking [Runway] 19 for takeoff. We could see the small plane as we were taxiing north to the FBO. We immediately stopped the aircraft and we both said; 'I think they are taking off into each other'. We immediately called on Unicom to tell the aircraft to stop and because you are taking off into each other. We had to repeat the call again and the Seneca said I'm departing [Runway] 19 and [the carrier] said they were departing Runway 1. Both planes stopped on the runway; and at that moment they realized how close they were to taking off into each other. The Seneca elected to taxi clear and [the carrier flight] departed that runway.Had we not made the radio call; they would have taken off into each other and at that field altitude we feel it would have happened as [the carrier flight] rotated.  Nothing more was said over radio.One hour later we taxied out (Taxiway A to Runway 19) for our departure. There was a Baron and Mooney in closed traffic for Runway 19 at that time. [A carrier flight] was taxiing and getting outbound IFR clearance from LA Center over 133.3. They taxied to Runway 1 instead of Runway 19. Several Unicom calls were made indicating traffic flow was using [Runway] 19. [The air carrier flight crew] broadcasted that they were departing Runway 1; the Mooney and Baron in the pattern elected to modify their patterns to allow the [carrier] to depart and fly out opposite traffic flow. The communication from the [carrier] crew did not seem as vigilant as it should be at an uncontrolled airport when operating opposite and conflicting with traffic.Caution note for Runway 1/19 at SGU. Due to a slight hump and runway length; it is hard to determine and see aircraft that may be in position on the opposite end of the runway.Had we not communicated on Unicom to tell aircraft at SGU on runway that they were taking off into each other; there would have been an accident on the runway. The [carrier flight] did not rotate until well past A4 taxiway to the north. It seemed that the [air carrier] pilots were using Runway 1 because it was closer to the terminal ramp. If no one else was operating there it would have been fine. But in our 2 hours on the ground; we saw one potential accident and one other instance of [aircraft] going against traffic flow and not communicating their intentions effectively over Unicom.
Air carrier Captain reported observing several dangerous operations at a non-towered airport.