There are many parts and components in an aircraft that do not generate their own heat and instead, rely on heat generated from the engine. Of all these non-heat-generating components or parts, the aircraft carburettor is perhaps the most vulnerable to humidity and low temperatures. The freezing of the aircraft carburettor would not just impede the smooth functioning of the engine, it will ultimately stop the engine from functioning!
To prevent the above from happening, an anti-icing system that preheats the air before it reaches the carburettor is put in place. This is intended to keep the fuel-air mixture warm enough so that freezing does not occur and the carburettor is kept free of ice. Despite this, higher humidity levels coupled with abnormal temperature variations when flying in certain locations and height, can cause the carburettor temperature to drop and ultimately freeze.
As the ultimate fail-safe, the pilot(s) must be able to detect temperature drop of the carburettor and this is achieved via the Carburettor Temperature Probe (CRB) connected to the Aircraft Engine Data Management (EDM). When temperature drops, pre-heat can be switched on to prevent the freezing of the carburettor.
Carburettor Temperature Probe is a sensor mounted directly on the carburettor wall and detects temperature drop. This, along with other indicators (e.g. rough sound, drop in rpm etc) serve as a warning to the pilot(s) of impending disaster.
Modern CRB probes especially those manufactured by J.P. Instruments, are meant to withstand effects of humidity, oil, gasoline, as well as wide temperature variations. The sensing coil within the CRB is fully coated in an epoxy resin which itself is within a metal tube thick enough to withstand repeated backfires, but thin enough to be highly sensitivity to temperature change.
When the CRB probe is fitted, care should be taken to fit it gently but firmly and without using harsh tools such as pliers. The tightening torque should be no more than 4 foot-pounds. Only one spacing washer can be used, and should contact the carburettor casting, with the lock washer in contact with the shoulder on the probe. Allow sufficient slack for the wires of the probe.
If the probe does not reach all the way into the carburettor barrel, the counterbore can be used again to reduce the thickness of the casting slightly at the outside of the hole. Recommended torque is 3 to 4 foot-pounds as anything more than this will likely damage to the threads in the carburettor or the probe. For more information on aircraft CRB Probes, please visit https://www.jpinstruments.com/shop/crb-probe/