The Carburettor Temperature Probe (CRB) is used to detect temperature drop in the carburettor.
For successful ignition to occur, the air-fuel mixture has to be perfect and this can only happen at the right temperature. Now when fuel and air is mixed, the temperature inside the carb will drop dramatically and this is compounded due to external cold air and cold temperatures. Next thing you know – the carburettor is frozen solid – and this is not something you want to happen when flying; not by a long shot.
So, the carburettor is kept warm and steady at its ideal temperature through a rather ingenious way by installing an anti-icing system and using air heated by the aircraft engine. This marvel of modern aviation engineering keeps the aircraft carburettor as warm as a kitten sleeping next to a fire. Not too hot – just right temperature keeps the kitten and carburettor happy.
But an aircraft being what it is i.e. an amalgamation of thousand different parts; moving and non-moving, anything can happen so the pilot needs to know the moment the carburettor is not too comfy because it’s impact on the engine and aircraft can be far reaching and catastrophic. Enter the CRB probe.
The Carburettor Temperature Probe (CRB) is connected either to an independent analog or Digital EGT Gauge (the type of probe used will depend on type of display you have in the cockpit). The moment the pilot detects a drop in the carburettor temperature, he can start the pre-heater and it will heat the air flowing over the carb thereby preventing the carburettor from freezing over.
To ensure accurate temperature reading, the Carburettor Temperature Probe (a sensor), is mounted directly on the carburettor wall. This, along with other indicators (e.g. drop in RPM, rough sound etc.) serve as a warning to the pilot(s) that all is not well.
Modern CRB probe like those manufactured by J.P. Instruments, are meant to be immune to factors such as oil, humidity and gasoline. They are also engineered to withstand wide temperature fluctuation.
Each CRB has a sensing coil within it. This coil is fully coated in an epoxy resin which in turn sits within a metal tube thick enough to withstand repeated backfires, but thin enough to be highly sensitivity to temperature change. This fine balance is perfectly achieved by only a few CRB manufacturers and J.P. Instruments is one of them.
The CRB Probes being delicate, should be handled with kid gloves i.e. fit it gently but firmly avoiding the use of harsh tools such as hammer or pliers. The maximum tightening torque should be no more than 4 foot-pounds. The single-spacing washer that is provided should contact the carburettor casting, and the lock washer should be in contact with the shoulder on the probe.
Use counterbore to ensure the CDT Probes reaches all the way into the carburettor barrel. The counterbore will reduce the thickness of the casting slightly at the outside of the hole. Recommended torque is no more than 3 to 4 foot-pounds.
For more information on aircraft CRB probes, please visit https://www.jpinstruments.com/shop/crb-probe/
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