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Most (all?) aircraft have a Pitot Heat Switch which must be turned on in certain conditions.

Seems Pitot Heat is one of those things you don't want to forget to switch on when it's needed, or worse, not recognize you need it on and take action based on false readings.

The pre-flight checklists I've read always say something like:

Pitot Heat - As Required

Why not always have it on? Or not have a switch and it just comes on with the aircraft's other power systems?

Is there a downside to it being always on?

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    $\begingroup$ related: Why turn off pitot tube heating? $\endgroup$ – Dan Pichelman Jan 10 '17 at 18:51
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    $\begingroup$ @DanPichelman That question talks about somewhat the same thing, but never really gives a definitive answer. On an airliner, we have the system on from the time we push back until shortly before approaching the jetbridge (don't want to risk fabric from the curtain on the jetbridge melting onto a hot pitot tube), so WE essentially always have it on. For aircraft that operate otherwise, I'd be interested to know definitive reasons why. $\endgroup$ – Ralph J Jan 10 '17 at 19:16
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    $\begingroup$ I agree, the linked question is the same, but apparently none of the answers exactly address it. $\endgroup$ – kevin Jan 10 '17 at 19:25
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    $\begingroup$ Not to mention, a simple 30 cent temperature diode could measure temperature of the element, allowing the system to cycle heat on/off automatically, ensuring it never overheats. $\endgroup$ – SnakeDoc Jan 10 '17 at 23:58
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    $\begingroup$ @Ralph J - I think one thing we often forget about with questions like this are human factors. Even if we assume there is some sort of perfectly ideal pitot probe that won't oxidize, won't fail after so many hours, etc... There is still the issue that people will tend to just leave the switch "ON" all the time. After all, the question is asking, "Why not always have it on?" This could cause serious injury to personnel who inadvertently touch the pitot probe on the ground, and it could also cause damage to the probe if you attempt to put pitot covers on it. That's at least one reason why. $\endgroup$ – Frank Jan 13 '17 at 21:55
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I can generally only speak for commercial aviation. The reasoning is probably the same or similar for General Aviation, but seeing as that is such a wide and varied field, with a tremendous number of different aircraft, probe types, etc., someone somewhere could probably find some niche corner-case to dispute the reasoning. My guess would be that, generally speaking, in General Aviation, certain probes are most likely only meant to run for a certain period of time. After all, it's not like a Piper Cub is going on an 18-hour non-stop flight to China like a Boeing 777 might. I'm not saying that a probe will fail after 18 hours of being on, just that repetitive use in that way might necessitate a different type of probe with a more robust design.

In commercial aviation, many modern airliners use certain logic to turn pitot heat on or off. With the exception of a manual override switch equipped on some aircraft, to force it off, the pitot heat is generally on during most flight phases and works as such:

On the ground, with engines off, the pitot heat is turned off. On the ground, with engines running, pitot heat is on low. Finally, once the plane takes off, the pitot probes are put into a "high heat" condition and remain that way for the remainder of the flight (unless manually turned off).

The pitot heat is generally monitored with current sensing relays such that if the pitot heat draws excessive current or not enough current, the crew will be alerted to a pitot heat failure.

Why not keep them full hot all the time? For one, that would most likely cause premature failure of the heating element. Another reason is that, on the ground, pitot covers are normally applied to prevent bugs from building homes inside the tubes as well as preventing other debris from entering. The pitot covers will melt and destroy the pitot probes if the covers are left on while heat is applied. This can also be problematic and dangerous if the pitot cover partially melted, wasn't noticed by ground crews, and the drain holes or entry holes were blocked by the melted cover. Finally, and probably most importantly, the pitot probes get extraordinarily hot. Speaking from experience, and from having touched one that had failed in the "full hot" phase, it takes only a brief second to receive excessive burns on one's hand. Most of us that have been around them in our lifetimes have been burned by them, and a savvy mechanic will always approach them with caution!

So, it makes sense to have the probes off, on the ground, with engines not running. Otherwise, commercial aviation has more or less decided that "full hot" in the air, and low heat on the ground with engines running is a design characteristic.

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Depends on the airframe and where it is operating. For light aircraft operating a low altitudes (<10,000 ft) pitot heat use is going to be dependent upon whether the pilot anticipates encountering ice or freezing conditions. This should keep the tube unclogged be free of ice should it be encountered - though I have my doubts given how anemic the pitot heat systems on light Cessnas are.

Business jets and airliners are another story altogether. Designed specifically to operate in the flight levels and hard icing conditions, the anti icing systems on these aircraft are much more capable compared to smaller GA aircraft. They can safely operate in all but severe icing conditions and are VERY effective compared with their bugsmasher counterparts. The pitot probes, static ports, air data sensors and AoA vanes get so hot during use it causes oxidation on their surfaces. Pitot/AoA heat is usually engaged just prior to takeoff and disengaged just after landing to prevent ramp hands from being burned by the probes.

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    $\begingroup$ Okay, but what drives the non-continuous operation on the light aircraft? What is gained by leaving the pitot heat off when it's clear & VMC? Saving electricity? Prolong the life of the heating element? Risk of burning it out if you forget to turn it off after landing? Something else? $\endgroup$ – Ralph J Jan 10 '17 at 21:20
  • $\begingroup$ As I interpret such a checklist item, pitot heat is to be used AS REQUIRED for the safe completion of the flight, usually if the pilot anticipates entering clouds, or other areas with moisture where freezing conditions exist and icing might be encountered. Could you switch on pitot heat for the full duration of the flight? Sure, but it's unnecessary. $\endgroup$ – Carlo Felicione Jan 10 '17 at 22:16
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    $\begingroup$ Unnecessary for clear skies, but for airliners we leave it on full-time, regardless. (We only turn ENGINE anti-ice on "as required" when we're entering potential icing conditions -- IMC + low temperatures -- because there is a drawback to running it continuously: loss of power / fuel efficiency.) In many GA aircraft, pitot heat is not run continuously, and the question is, why not? What's lost by leaving it on from takeoff through landing that offsets the (presumed) advantage of having it already on when you need it, no "remembering to turn it on" required? SOMETHING is different; what it is? $\endgroup$ – Ralph J Jan 10 '17 at 22:45
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If you heat the pitot tube while in flight, it will keep a reasonable temperature. If you heat it when you are on the ground at a normal temperature and without any forced convection, it will reach a temperature of some hundreds degree. This is very bad for three reason.

1) Wasted energy: why to waste energy? It has no reason to waste precious (and expensive) electric energy. It is a reason also to keep it switched off during the flight in all those conditions that do not present ice risk.

2) Corrosion and wear: the higher the temperature, the faster the corrosion process and the wear of the component. The more you use something, the sooner you will have to inspect and replace it.

3) Safety: if you are a technician and you touch the pitot tube on the ground you lose your hand (and I can tell you for sure that it happens sometimes, when you have to perform tests and you turn it on with weight on wheel).

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