Is there any legitimate reason for adding momentary power when descending at idle with full carburettor heat on? Scenario: While demonstrating a simulated engine failure emergency landing a flight instructor puts on carburettor heat and reduces the throttle to idle of a small single engine Cessna. After a couple of minutes descending to the simulated emergency landing site, the CFI adds throttle "to clear the plugs" for a few seconds, then brings it back down to idle to complete the demonstration.

Is this is a legitimate thing that needs to be done for safety?

I've seen several pilots do this, and to me it always seems like cheating, because they are coming in a little short of their landing target.


How much plug fouling can occur in 2 or 3 minutes of idle with carburettor heat at low altitude?
How much good can 3 to 4 seconds of full power do?
Is there something else about this that I'm missing? (I couldn't find anything on this after a brief Google search).
straight out of the Airplane Flying Handbook, pp 9-4


  Operating the engine at idle speed for any prolonged period 
  during the glide may result in excessive engine cooling, spark 
  plug fouling, or carburetor ice. To assist in avoiding these 
  issues, the throttle should be periodically advanced to normal 
  cruise power and sustained for a few seconds.


This procedure is common during emergency descents or steep spirals to prevent the engine from cooling off too much.  If the engine mixture is excessively rich, spark plug fouling can occur.  Fouling can be cleaned off the plugs by running the engine at Cruise power settings with a properly leaned mixture in about 10 or so seconds.