They may be inconsequentially small in comparison to many components on a Twin Commander, but heated fuel vents carry an outsize responsibility. Their job is to collect ambient air and channel it to the fuel bladders in the wing to replace the space vacated by fuel as it is drains from the bladders. Without replacement air, a vacuum would develop in the bladders, leading to an undesirable outcome – either the excruciating quiet of a fuel-starved engine, or the deformation of the fuel bladders when the snaps, lacing or Velcro that hold the bladders in place disconnect.
(Heated fuel vents are found on 690 and 690A and B-model Twin Commanders. The 690C and D and 695s have unheated NACA-scoop fuel vents.)
To ensure the small-diameter aluminum-tube fuel vent lines remain unobstructed in icing conditions, they incorporate a wire-wound heating element. A thermistor reads the temperature of the vent tube and when it senses 85 degrees F it opens a control circuit to remove electrical power to the vent tube heating wire.
Proper operation of the fuel vent heating element is one of the anti-ice/deice functions checked in the 150-hour aircraft inspection. However, that seemingly simple inspection can be done incorrectly, possibly leading to unnecessary replacement of the vent line and heating element.
If the inspection is conducted when the ambient temperature is near or above 85 degrees and the vent is not artificially cooled, the thermistor will not close the circuitry and the vent will not heat. Instead of grasping the vent line by hand to try and feel it warming, spray compressed air or some chilling agent on the vent. It will quickly cool the vent, even frosting it over, triggering the thermistor to close the electrical circuit and apply heat to the vent. If the frost quickly melts, the vent line heating system is working properly.
Patience is a virtue when it comes to testing heated fuel vents. The thermistor sensor should detect when the vent line temperature rises to about 85 degrees, and open the circuitry to remove the heat from the vent line. Give the system time to work. Wait a few minutes after the frost melts to grasping the vent line. If it is warm, but not hot, it is confirmation that the thermistor sensor is working and has opened the heating element circuitry. If it is not working correctly, the vent line will be hot to the touch, and the system vent will need replacing.