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I'm merely an amateur simulator (X-Plane) pilot. On a flight yesterday the airspeed indicator stopped working. After the initial panic I remembered reading something about pitot tubes freezing, found the pitot heating switch, and turned it on. The problem was resolved immediately.

This led me to think: Is there a good reason to not turn pitot heating on before take-off, and leave it on for the entire flight?

EDIT: I'm flying a Sud Aviation SE 210 Caravelle

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a defect in flight which forces you to turn it off –  ratchet freak May 12 at 12:58
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to add to @ratchetfreak's comment, you don't want your pitot heat on with a failed generator/alternator for example (unless you really have to), as it'll drain your battery rather quickly. –  falstro May 12 at 14:58
    
@falstro Unless you are in icing conditions in which case you have bigger problems if you turn it off. A better example would be an electrical short in the pitot heat system. –  Lnafziger May 13 at 17:03
    
@Lnafziger that's the "(unless you really have to)" part. An electrical short in the pitot heat system would probably trip a separate breaker, so turning it off doesn't actually do anything in that case (if the breaker would have covered multiple systems you could turn it off and reset the breaker though). –  falstro May 13 at 17:11

5 Answers 5

up vote 4 down vote accepted

The aircraft checklist will determine when the pitot heat should be on. Strictly speaking, if the checklist says it should be on, the only reason it should not be on is if the system is inoperative or is causing some issue (if the generator system fails it will drain the battery, as falstro commented). The checklist linked by Lnafziger has the pitot heat ON before takeoff and OFF after landing. Other times it may be "as required" by conditions.

The pitot heat makes sure the pitot system remains free of ice. Failed airspeed, especially in IFR conditions, can be serious. The failure may not be obvious, leaving you to stall/overspeed. Leaving the heat on can cause the system to overheat. While some systems automatically protect against this, not all will (especially with smaller aircraft).

Checklists for larger jets also seem to have probe heat on before taxi and off right before shutdown. The reason for this is probably safety. Modern aircraft rely heavily on pitot static systems. AF447 crashed partially due to icing on the pitot tubes. Why leave it off and run the risk of icing? The cost of leaving it on is fairly low.

On smaller aircraft where the pilot may even be able to see the pitot tubes, the situation may be different. These aircraft have less automation, and the cost of repairing a pitot heat system is higher relative to the rest of the aircraft's maintenance. However, the issue of safety remains.

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Actually, very few light GA aircraft should be operated with pitot heat on all the time. Unless it's in the checklist, it should be turned on before (or, at the least, upon) encountering visible moisture. I've not seen any checklists for Cessna, Piper, or other light singles that suggest the pitot heat should be on during normal VMC operations. –  egid May 13 at 22:18

I'm not sure what kind of airplane that you are flying in the simulator, but the checklists for jets typically do have you turn the pitot heat on just before takeoff and leave it on until after landing, just so that this is less likely to happen.


Update: I found a French checklist for the Caravelle and it says to turn on the pitot heat before takeoff:

Rechauffage Pitot.....MARCHE

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OK, that makes sense. I'm flying a Caravelle, will look into its checklist. –  Vidar S. Ramdal May 12 at 14:59
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Piston aircraft checklists usually have "pitot heat on" before takeoff and "pitot heat off" after takeoff,. But that doesn't mean it is expected to leave it on all the time. Other checklists (such as "flight into icing conditions") have "pitot heat on". Is pitot heat expected to be on all the time with jets? –  DJClayworth May 12 at 15:53
    
@DJClayworth Yes, and that's why they are written that way, even in the piston airplanes. They put it in the icing checklist just in case it was forgotten previously. –  Lnafziger May 12 at 16:20
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Jets that fly up into the flight levels are always flying in conditions well below freezing. It makes sense for them to have Pitot Heat on for the duration. –  Skip Miller May 12 at 16:36
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@SkipMiller Usually, but they are also usually above the weather and not in icing conditions for the vast majority of their flying time. It's simply a precaution and most manufacturers would rather have the pitots on and not need it than the other way around. –  Lnafziger May 12 at 17:18

At our airclub, the checklist for C172 shows:

  • ... stuff ...
  • Pitot heat on, check tube warm, turn off.
  • ... complete the walk-around... start... taxi to engine run-up... do run-up...
  • Do final pre-take-off checks, one of the last of which is "pitot heat: ON".

I've seen some C172 checklists that say "as required" instead of "on" near the end.

There are no further pitot heat items on the checklist except for the "after landing checklist". So we're supposed to just leave it on.

During my initial training my instructor mentioned that we turn it off until just before take-off because it can overheat and fail if operated while stationary. I don't remember seeing this in the C172 operating handbook, so maybe they're just being overly cautious or something.

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Although pitot heat is quite reliable, it can burn out much like an incandescent light bulb. Why waste its useful life without immediate reason?

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Because the price of needing it and not having it on can be quite high. The same could be said for reserve fuel. Why do we carry it around all of the time if we don't need it? It's there for those times that we do (and you might not even know that the pitot heat saved the day if it were already on). Personally, I just follow the manufacturers recommendation. If they say that it should be on during flight, I turn it on for the entire flight and don't second guess them in order to possibly save a few dollars. –  Lnafziger May 12 at 17:23
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If the pitot tube is giving a partial reading that is 95% accurate... You may need it on and not even really know it. –  Jay Carr May 12 at 17:24

On the ground, without airflow over the pitot tube to cool it, it will burn out very quickly. By operating something like this when it's not needed you risk not having it available when you need it.

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While this may be true for some aircraft, others automatically prevent it from overheating. If the checklist tells you it should be on, then it's probably fine to turn it on. –  fooot May 13 at 16:31
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To clarify, the aircraft I'm familiar with, large part 121 transports, do not have any means to prevent overheating of the pitot tube. Newer aircraft do have means to prevent the heaters from coming on while on the ground if at least one engine isn't running and also lower power while on the ground than in the air. Old birds (e.g. 737-200) just put the heat on when the switch is on. –  Sports Racer May 13 at 22:26

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