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Date: Mon, 23 May 2011 10:49:44 -0400
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From: Paul Miller <paul@tbm700.com>
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Subject: Re: [LML] Re: Fuel restriction or air, Fuel Vent Check Valves
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Having a check valve or float valve in the vent line is common in 
certified aircraft so I disagree that vent lines must be free flowing in 
both directions.  The twin cessnas use a float system to block the vent 
when fuel is present. This has led to problems with unanticipated 
blockage of the vent line leading to ADs like this one:

> Compliance: Required as indicated, unless already accomplished.
>  To provide an alternate source of fuel tank venting in case of fuel 
> tank vent obstruction by
> foreign material and/or sticking of the fuel vent valve, within the 
> next 100 hours time-in-service after
> the effective date of this AD, accomplish the following:
>  (A) Install applicable vented fuel cap(s) with related adapters and 
> fuel servicing placards
The fix has been a "sniffle valve" on fuel tanks or a sniffle valve on 
the fuel cap itself to prevent vacuum in flight.  If the Lancair fuel 
caps had a vacuum vent that would be the simplest solution for a backup 
in addition to switching to the other tank.

Paul Miller
Legacy