Aviation Maintenance Technician, Second Class (AMT2) SWE Practice Exam

The sight glass in an ECS refrigerant loop is used to verify that the refrigerant is circulating properly and is in the liquid phase as it passes the metering device. When the system is properly charged and operating, you should see a steady stream of liquid Freon flowing through the sight glass. This indicates the refrigerant is circulating at the correct rate and not flashing into vapor or starving the evaporator.

Frost on lines would suggest the evaporator or lines are excessively cold or that moisture may be present, which isn’t normal during steady operation. Bubbles in the sight glass imply that gas is present or the refrigerant is flashing, typically pointing to undercharge, air, or other flow irregularities. No flow means the system isn’t circulating at all, which is not acceptable during normal operation.