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Date: Fri, 31 Aug 2012 09:58:25 -0400
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From: jeffrey liegner <liegner@ptd.net>
Subject: Unexpected MAP Reduction with Light Icing (LIVP)
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I have noticed, since getting the engine overhauled and the annual done, th=
at the engine loses some manifold pressure in icing conditions.  First, be =
reassured this is not significant icing =E2=80=A6 a light frosting to trace=
 mixed ice at flight levels in IMC.  

Instead of cruising at 34"/2500 (19gph) as before, I'm now cruising at 31.5=
"/2350 (16.4gph), which perhaps makes the turbos spin less fast and more vu=
lnerable to restrictions on intake.  

As expected, the first sign of icing is a reduction in airspeed.  But I'm s=
eeing a quicker reduction in MAP from 31.5" to lower numbers (maybe to 29")=
, requiring adding back throttle to restore MAP.  At the 31.5/2350 in the f=
light levels, there is still plenty of throttle available.

In the past, I have seen MAP reduce with light to moderate icing, but not s=
o soon with trace, and not with minimal icing conditions without observed i=
cing evidence.  Also, I have not heard the alternate air door open, as has =
occurred once or twice in the past.

Even with presumed icing (and reduced airspeed) but no visible icing, this =
will happen.

I wonder about obstructing intake icing tendency, alternate door function, =
and size or status of air filter as my possible issues.  Any other insights=
 or suggestions by those with more experience?

Jeff L
LIVP
TSIO550E