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Date: Tue, 10 Nov 1998 17:05:13 -0600
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Subject: Re: Water in the tanks???
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Andrew McCoy wrote:

> After taking off from the "shorter" runway (6,000') my instructor told
> me to go to Vx to clear a ridge and trees about a mile off the 
> departure end.  We had plenty of room, but better safe then sorry.

> As I approached 65KIAS (in a c-172) there was a strong buffeting.
> Realizing we had the ridge cleared, I lowered the nose and it stopped.
> My instructor asked "what was that?".  When I told him what I thought
> and why I did what I did, he said no, that was the engine going rough

> Any thoughts?  Carb Ice seems unlikely as the problem was gone as soon
> as the nose was lowered, we didn't have time to apply heat.

Just a comment on carb ice, if I've been flying low power for a while
(as in the pattern, or on an approach) I've had it happen that the
engine ran very rough for a second or two some time (30 seconds?) after
I applied full power.  My mechanic thought this was carb ice, a small
amount not enough to reduce the plane's performance at a given power
setting, which was melted by the increased heat produced by full-
throttle ops.  

This made sense to me (fit the lack of subsequent symptoms, the
atmospheric conditions, and the circumstances).  So I think it's
possible to pick up a little carb ice then melt it without the carb 
heat but just by going to full power.  And it made me rethink my
plane's POH instructions not to apply carb heat routinely on 
approach or in the pattern.

I wouldn't have described the 'roughness' as buffeting though; the
engine ran very rough, but I couldn't feel it in the controls.

Not saying that this is what happened to you, just that I think
carb ice can form at low power and be melted at full throttle
without carb heat in some conditions.

I'm afraid I can't quite picture how it would work for water to
get into the engine as you pitch up, then stop immediately when
you lower the nose, unless lowering the nose/stopping the 
roughness was just a coincidence.  It's my understanding the fuel
in the lines of most GA airplanes will last more than a moment.
It's been known to happen that a low-wing pilot switches to a dry
tank just before takeoff, and the fuel lasts just long enough to
get the plane into the air.

Snowbird


