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Should Pitot Heat ON (in dry weather conditions) decrease airspeed indication because warmer (so less dense) air flows through the tube?

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No, there is no decrease or change with the use of pitot heat.

The pitot system measures pressure. There is no airflow through the system in normal operation. Any airflow through the system would be due to a system failure which led to leakage.

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  • $\begingroup$ Pressure is proportional to temperature... $\endgroup$ – Notts90 supports Monica Jan 9 '17 at 19:06
  • $\begingroup$ @Notts90, no, it isn't. There are complex relations between pressure, temperature and density that depend on which variables are fixed by external conditions and they are totally irrelevant in this case, because the variable fixed here is directly pressure, specifically the stagnation pressure of the oncoming air, and that air is not heated (yet), so its pressure is not affected (if it was heated, its pressure would be lower, as the question correctly states, but it is not yet). $\endgroup$ – Jan Hudec Jan 9 '17 at 20:01
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    $\begingroup$ @Notts90 You are correct insofar as concerns a fixed volume system, which the pitot system is not. $\endgroup$ – J Walters Jan 9 '17 at 20:17

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