Brake Smoke and Falling Air Pressure on a Mountain Grade
You are driving a 30-passenger bus with air brakes down a 7-mile 7% grade. Near the bottom, you smell burning rubber and see smoke from the right rear wheel. Your air pressure gauge reads 75 psi and is falling slowly; the low-air warning has not sounded. You are on a narrow shoulder with a guardrail on the right and a rock wall on the left. There are 24 passengers, including two with limited mobility. The rear service door is jammed, but the front door and roof hatch open. The engine is still running. Traffic is passing at 60 mph. You must decide whether to stop, evacuate, or continue to the next exit one mile away.
1. Do you stop, continue, or evacuate? Explain the air-brake and fire reasoning.
Read the worked answer
Stop. Smoke and burning rubber suggest a wheel-end fire or dragging brake, and 75 psi is already low and falling. If pressure continues down, the low-air warning at about 60 psi will activate and the spring brakes will apply at about 45 psi, possibly abruptly. A fire can also damage air lines and make brake control worse. Continuing one mile to the exit risks losing braking ability or having a fire spread while the bus is moving. Pull as far onto the shoulder as the guardrail and rock wall allow, stop, set the parking brake, shut down the engine, and chock wheels if possible. Do not open the engine compartment if a fire is suspected. Get passengers off and away from the bus and traffic.
2. How do you evacuate 24 passengers, including two with limited mobility, when the rear service door is jammed?
Read the worked answer
Use the front door as the primary exit and the roof hatch as a secondary route. Assign two or three capable adults to help the passengers with limited mobility; do not let them block the front door. Move passengers away from the bus, uphill and upwind if possible, and well away from the traffic lane and guardrail. Keep the group together on the safe side of the guardrail or as far from passing traffic as the terrain allows. Have someone call 911 and report the fire, location, and passenger count. Account for all 24 by name or seat count. Do not send people back for belongings. If the roof hatch is needed, use it only with assistance and a safe landing area. Keep the rear door closed and marked as unusable.
3. After evacuation, what do you tell emergency responders and dispatch about the air-brake system?
Read the worked answer
Report that the bus has air brakes, that pressure was 75 psi and falling before shutdown, that the low-air warning had not yet sounded, and that smoke came from the right rear wheel. Explain that spring brakes may apply as pressure drops and that the rear service door is jammed. Tell them the front door and roof hatch were used and give the passenger count and any injuries. Do not restart the engine or attempt to move the bus; fire damage to air lines or wheel-end components could cause an unexpected brake release or loss of control. Ask dispatch to send a replacement bus and a mechanic. Keep passengers away until firefighters or the mechanic declares the area safe.