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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
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