"Marc Lattoni" wrote in message news:c3%hc.1386$2q2.192@edtnps84...


Do modern airliners not have an alternate static source as on my 172?


They have alternate static, but it's not as simple as the one on
your 172. If yours is like most spamcan alternate static sources, it
opens the static system to the cabin air. Simple and almost
foolproof, but that sort of system wouldn't work on a pressurized
airplane. The artificially high cabin pressure would generate
erroneous readings. A pressurized airplane would have to have its
alternate static source open to the outside air (where it can be taped
over).

John Galban=====N4BQ (PA28-180)