Pitot heat

[FlyingForFun]

I was randomly thinking about flying the other day (not unusual, I know) when I got around to thinking about pitot heat... and it occurred to me that it seems like a bit of a waste of space in, say, a PA28, C172 or similar.

The reason I say this is because the aircraft is not allowed to fly in icing conditions anyway. In which case, the pitot tube isn't going to encounter any icing.

Or is it? Does pitot icing occur at higher temperatures than airframe icing, or in different conditions? Or is simply a safety item, on the basis that if you accidentally encounter icing in IMC then you're likely to be able to fly out of it safely, but only if your instruments (including ASI) all work correctly?

Personally, I would always use pitot heat in any visible moisture below 10 degrees C, just because it's what I've been taught to do. But now that I stop and think about it, I'm not convinced on how useful it is.

Thoughts, anyone?

FFF
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[tmmorris]

In basic training I was not taught to use the pitot heat, but since training for my IMC rating I always turn it on as I enter the runway for departure (along with the transponder and strobes), and off when I exit after landing (ditto), for the simple reason that the compass is swung with it switched on and it can make a significant difference in some planes.

As for its usefulness, I think the point is that a very thin layer of ice can completely block the pitot tube, before you've had a chance to notice it building up on the wing and hence make the decision to descend/climb away from the ice.

Tim

[High Wing Drifter]

I suppose there is a slight risk that any condensed water in the tube may freeze soon after departure with a low freezing level, especially if the wind is not strong enough for radiation fog.

[Brooklands]

I've always been under the impression that you don't just use it to prevent ice formation, it also helps stop water getting into the pitot system when flying in cloud or rain - consequently I always turn it on when flying through cloud or rain.

[Deano777]

surely if the OAT is 0 deg then any moisture forming on the pitot could potentially freeze? I was taught to check the OAT using the thermometer in the cockpit, I flew last week when it was cold and the OAT was -1 at 3,000ft, what I do is if im flying and its cloudy the pitot heater goes on, if Im flying and its marginally cold the pitot goes on.
Better to put it on than to wish you had put it on imho

D.

[Lowtimer]

Well, put it this way. Obviously none of us would deliberately fly in known icing conditions, in an aircraft not approved for such flight. We are far too wise and sensible to want layers of ice building up on our flying surfaces and prop. However, some people may have occasionally been in the following position.There may be a thin layer of cloud well below the freezing level. You may be cruising at 10,000 feet and have your airframe thoroughly cold-soaked. When you descend through that thin (but warm) cloud layer, the residual coldness of the airframe may lead to very short-term icing. If your wing ices up you will see it happening, it will start as light icing and get heavier at a certain rate, then it will melt and recede. The effect is proportionate and you can probably see what's going on. Pitot icing may exhibit itself in a sudden and apparently counter-intuitive way, as instrument readings go from sensible to haywire in seconds. Pitot heat will prevent this happening.

[IO540]

You can get rapid pitot blockage (or partial blockage) due to ice without any visible ice on the wings. I've had this a number of times, in hazy cold VMC. The plane isn't making the expected IAS for the power setting (and the fuel flow rate); switch on the pitot and hey there's another 5-10kt

Re icing, while the plane won't be cert for known ice, people still pop into clouds here and there, and a few mm won't (generally!!) get noticed especially once the speed is up. But if the pitot gets blocked, you have lost a very important instrument for landing.

I switch it on anytime the OAT is below +5C.

[FlyingForFun]

Well, seems like the majority view is that it's there because of the rapid effect of a tiny amount of pitot ice, which makes sense.

More thoughts welcome, but thanks to everyone who responded already.

FFF
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[Flyin'Dutch']

FFF,

IO540 has said it all so no need to post a lot more other than to say that his advice to use it from +5 degrees C and below is sound advice.

FD

[A and C]

I,m not sure where these numbers come from for preventing ice , so far we have had OAT of +5 to +10 banded about.

Most jet airliners will require the engine anti-ice to be swiched on at about +10C ( B737 NG) when visible moisture is presant but the Pitot heat is swiched on after start and off at the end of the flight what ever the weather.

I think that the numbers being banded about in the flying clubs have more to do with what some wannabe airline pilot instructors have picked up on the ATPL theory course than practical aplication.

The water in the pitot tube is not an issue as pitot heads have a small drain hole to clear this.

As far as I can see the two big factors for the use of pitot heat are accidental flight into icing conditions and the fact that the compass will have been swung with the pitot heat on.

Using the pitot heat has no penaltys except if you run it for to long on the ground , so as far as I can see if you are in any doubt as to if it should be used switch it on during the before take off checks and switch it off when you clean up after landing.

[IO540]

A and C

Could it be that the airliner procedures are based on the much higher airflow speeds and having to allow for consequent errors in the OAT measurement? I'd imagine the penalty for frozen up pitot(s) would be nasty when in auto throttle mode?

There cannot be anything wrong with having the pitot on all the time, except the CFI will moan because he doesn't want to pay for burnt out pitot heaters (most trainers I flew in had the heater burnt out years ago)

[FJJP]

All commercial and military aircraft fly with the pitot heat on - it's SOP. In any case, most commercial and military aircraft are inevitably going to fly above the zero deg isotherm.

You don't have an electricity bill to pay, so why not go belt and braces and switch it on before you get airborne. You never know what atmospheric conditions you may encounter, and, as Deano777 says, better to put it on than to wish you had put it on.

[bookworm]

Could it be that the airliner procedures are based on the much higher airflow speeds and having to allow for consequent errors in the OAT measurement?

It's along those lines. The prospect of airframe ice when the indicated air temperature is above zero is mostly due to the pressure-, and therefore temperature-variation over the wing. Where the pressure is low, the temperature is also lower than in the freestream. In aeroplanes with high wing-loading, the pressure coefficients have to be higher, and therefore some parts of the wing surface will be at a much lower temperature than the rest.

A typical heavy jet has a wing loading around 100 pounds per sq ft, while a typical light aircraft has a wing loading around 10 to 20 pounds per sq ft. Hence the effect is much more prominent in heavy jets, and the indicated temperatures at which airframe icing precuations are taken are that much higher. I've never seen airframe icing in a light aircraft with an indicated OAT greater than 1 degC.

While that applies to airframe icing, I can't speak for engine icing, though similar principles may apply. Because of the more serious consequences and rapidly developing timeframe of pitot icing, it seems to make sense to have pitot heat on when in flight.

[Cypres]

Additional to the above comments about the compass being swung with the pitot heat on etc.. just consider this:

When during an otherwise uneventful trip your flight visibility starts to diminish, your navigation workload starts increasing exponentially and your spare mental capacity is rapidly disappearing to zero.
Will you have the presence of mind to check the OAT, then correctly evaluate the prevailing conditions and respond by turning the Pitot heat on?

Will you notice the insidious errors creeping into your pitot/static instruments in sufficient time?

Does the reduced electrical load and extended life of the pitot heaters in any way justify not using pitot heat?

Better to turn the sucker on whilst lining up and you still have two spare brain cells to rub together.

[Cusco]

I first encountered icing when training ten years ago:

I was pretty low hours and the instructor drew my attention to the little knob of ice building on the fuel cap handles of the PA38 *into * the airstream long before any ice appeared on the wings.

The litttle knobs of into- the- airflow ice built up in a matter of minutes to about 3cms long.

As this is precisely where the orifice in the pitot tube is it provided me with a never to be forgotten demonstration of the need for pitot heat.

Needless to say we descended and landed PDQ.

Safe flying

Cusco.

[Chuck Ellsworth]

Some interesting comments here.

I especially like the one that advises you to turn on the pitot heat while you still have two spare brain cells working.

Are there really pilots out there flying airplanes IFR that are that short on airmanship skills that flying IFR taxes their ability???

scary, scary...

[Bodie]

I'm not so sure about the statement of not having the presence of mind to include pitot heat when under pressure. That could apply to so many aspects of flying, hence the reason we have FREDA checks and the like.

During my IMC training I was taught to include Pitot Heat in my FREDA checks. I'd include Ice at the end of the check, checking the wings for ice, then checking the temperature and if necessary putting the pitot heat on.

[Flyin'Dutch']

On these shores there are very few times that while flying in the clag you are that comfortably high above freezing that freezing of pitot etc is not an issue at all.

So I just switch it on when I go into the clag and off when I get out.

Not burned a pitot out yet or have had it frozen up, so must be doing something right.

FD

[mad_jock]

I must admit I usually stick the thing on with the power checks and turn it off on the afterlanding or where ever its mentioned in the checklist. Doesn't matter if i am VFR or IFR summer or winter.

Asked the engineers if it is going to cost anything. Reply was no its a bloody great thick wired heater only time they had changed one was if some plonker had used the pitot tube as a hand hold.

I am sure some people will say its a high current device and your putting more load on the Alternators. But who cares, if it stops the failure of a primary instrument once in your flying career its been worth the small expense.

MJ

[IO540]

Asked the engineers if it is going to cost anything. Reply was no its a bloody great thick wired heater only time they had changed one was if some plonker had used the pitot tube as a hand hold

That could be because many schools don't change them when they go, to save money