Scenario 89: An emergency application of the trailer brakes happens automatically when:

Question: Scenario 89: An emergency application of the trailer brakes happens automatically when:

Answer options:

• Engine RPM is too high ✅ The emergency (supply) line loses pressure (e.g., line break or disconnected glad-hand)
• The driver shifts gears
• Headlights are turned on

Correct answer: The emergency (supply) line loses pressure (e.g., line break or disconnected glad-hand)

Explanation: Loss of supply-line pressure triggers an automatic emergency application of trailer brakes via the relay/emergency valve. The correct answer is "The emergency (supply) line loses pressure (e.g., line break or disconnected glad-hand)". This capacity-fill scenario 89 reinforces the same competency for the airbrake bank and follows the certified explanation standard.