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Water line heater temperature sensing

Aviation Mechanic Airframe (AMA) practice question 2 of 2 on Water and Waste Systems. ACS element AM.II.O.K2 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA mechanic is troubleshooting a water and waste line heater system whose heaters do not come on in freezing conditions. Which components supply the temperature data used by the control unit to switch the heaters on and off?

  1. AIce detectors mounted on the fuselage skin
  2. BA pressure switch in the waste tank drain line
  3. CThermostats located in the water lines
Show the answer and explanation

Correct answer: C. Thermostats located in the water lines

  • A — incorrect Ice detectors are part of the airframe ice detection system; they are not the sensing device described for the water and waste line heaters.
  • B — incorrect Heater control is based on temperature rather than pressure, and no pressure switch is described as controlling these heaters.
  • C — correct The handbook states that thermostats in the water lines supply temperature data to the control unit that turns the electrical heaters on and off.

BACKGROUND

Heater blankets, inline heaters, and heater boots are used to keep the water supply lines, water tank drain hoses, waste drain lines, waste tank rinse fittings, and drain masts from freezing. Thermostats placed in the water lines feed temperature information to a control unit. When the temperature falls below freezing, the control unit turns the electrical heaters on and keeps them on until the temperature has returned to a safe value. Knowing which sensing element drives the heaters tells a mechanic where to look first when the heat does not come on.

STUDY TIP

When a heated water or waste line stays cold, trace the circuit from the thermostat in the line to the control unit before condemning the heater element itself.

KEY POINTS

  • Thermostats in the water lines send temperature data to the heater control unit.
  • The control unit turns the electrical heaters on when the temperature falls below freezing and off once a safe temperature is reached.
  • Heater blankets, inline heaters, and heater boots protect supply lines, drain hoses, waste drain lines, rinse fittings, and drain masts.

SOURCES

  • FAA-H-8083-31B ch15 "Water & Waste Tank Ice Prevention" (sources/ama-airframe/ch15.txt)
Practice the Aviation Mechanic Airframe (AMA) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON WATER AND WASTE SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.