Lycoming Carb Heat

Subject is Lycoming O-360 engines. I have flown behind a number of them in
Pipers and the POH instruction on carb heat is always "as required."

Carb ice happens when it happens. You need carb heat when you need it.

If the OAT is 30F and the dewpoint -5F, no airplane needs carb heat -
there's simply not enough moisture in the air to build any carb ice.
When flying an approach with an OAT of 65F and the dew point the same,
through thick, fluffy cloud every carbureted airplane needs carb heat -
as soon as you throttle back for the descent, you will be building up
carb ice even if you're running a Lycoming-powered Piper.

Most situations fall somewhere between the extremes and call for a
pilot to monitor the engine and decide when carb heat should be used.
Some airplanes call for carb heat at specific times because most pilots
are not terribly good at determining when they need carb heat. Pilots
manage to ignore all the warning signs of carb ice right to the point
where the engine quits with disturbing regularity.

The induction system on Cessnas is not really the same as it is on
Pipers. The installation makes a big difference. Before Piper started
making Lycoming O-320 powered Cherokees, Piper made Lycoming O-320
powered TriPacers. I used to own one, and it iced every chance it got.
The current owner installed an STC'd modifications to the oil cooler,
and it doesn't ice up the way it used to. So what I'm trying to tell
you is this - there's more to carb icing potential than the engine, or
even the airframe it's on. The particular modifications installed can
even make a significant difference.

I would suggest that the procedural use of carb heat on approach to
landing (VFR ir IFR) is wise in any case, and ignoring the POH
requirement for it would be unwise at best unless you're confident you
can reliably detect the formation of carb ice in the descent based on
engine sound and instrument indications. My experience is that most
pilots can't. If you forget to turn off the carb heat on go-around,
performance will be somewhat anemic - but it takes only a second to
realize it is anemic and turn the carb heat off. But if you develop
carb ice on the descent to landing, the go-around can get REALLY
exciting.

Michael