This was my point Doug... the manual in the airplane recommends this. I hope everything works out for you on this, but the NTSB loves for you to tell them you didn't follow the manual. Be very careful if you have future conversations with them. Just my two cents!
I understand your point. The following discussion is not me trying to prove you wrong but to provide you with an understanding of where my head was at during the flight.
I just looked at my copy of the PoH and the section on Descent says carb heat as required, not on and doesn't discuss RPM's so I'm not sure I did anything outside of what the PoH recommends. In retrospect, any time I pull power, carb heat would be a good idea but that's not close to how I was trained and doesn't seem to be what the PoH says.
Addendum:
The expanded descent does call for adding carb heat during a low power descent. I did not think a 100 rpm drop to 2200 qualified as low power. If I'd dropped below 2000 rpm (green band), yes, carb heat would have been on.
I went back and looked and the actual cruise settings are pretty broad; the 2400 lower limit is only on a new engine. We routinely flew at 2350 which put us between 88 and 90 knots. Descent below pattern is down to 1500 rpm as is slow flight (carb heat on in both cases).
So again, I didn't think I was flying contrary to the PoH.
<edited to add extended descent information and added cruise section>
From PoH:
DESCENT
1. Power - AS DESIRED (2750 RPM maximum)
2. MIXTURE Control - ADJUST (if necessary to make engine run smoothly)
3. Altimeter - SET
4. Wing Flaps - AS DESIRED (UP - 10° below 100 KIAS)
(10° - 25° below 85 KIAS)
(25° - FULL below 70 KIAS)
5. CARB HEAT Control Knob - ON (as required)
DESCENT (expanded)
Normal descent from altitude may be made with flaps retracted or with
flaps extended to increase drag for a steepened descent angle.
Caution should be used to observe flap limit speeds. Slips may be
made in all flap configurations and are useful to increase descent rates
while allowing power to be kept above idle. Carburetor heat should be
used as needed for engine roughness and applied before reducing
power to prevent carburetor ice from forming during low power descent.
Since heated air causes a richer fuel mixture, readjust the mixture
setting when carburetor heat is to be used for extended descent. If a
low power descent is made, it is recommended that the throttle be
cycled occasionally to check for engine roughness at higher power.
CRUISE
Normal cruise is performed between 40% and 75% power. The engine
RPM and corresponding fuel consumption for various altitudes can be
determined by using the data in Section 5.
NOTE
Cruise flight should use 75% power as much as possible
until the engine has operated for a total of 50 hours or oil
consumption has stabilized.