Pitot-Heater

[mderdem]

Hello,

I am trying to build a simulation software which simulates the airspeed indicator. The documents I have didn't help much about the pitot-heater though.

Could anybody tell me what the airspeed indicator is supposed to show when the pitot-heater is not on ?

I understand that the pitot-heater is only needed on cold environments. Is there a regulation or a rule of thumb which tells how cold it should be to use the pitot-heater ?

Thanks a lot in advance,

MDE

[NickLappos]

The pitot heat provides anti-ice capability by electrically heating the probe with a standard resistance heating element. Heat has no affect on the reading of the airspeed system. It provides about 10 watts per square inch to the tube (about like a 150 watt light bulb would produce, so it can remove the skin from your hand quite easily!)

The usual procedures are to turn it on when in visible moisture (cloud, rain, snow, etc.) when the outside temp is below +4 degrees C or so (the specifics are in the flight manual for the type.)

If not used, the pitot tube gradually blocks, and generally starts to lose indicated airspeed, and will eventually show zero knots.

Sometimes, the static ports on the fuselage (if provided) are heated by the same switch.

[robipilot]

...when the pitot tube blocks in flight, and your altitude does not change, the airspeed indicator will indicate the last airspeed it had before it blocked. Then, as you descend, the airspeed indicator will indicate HIGHER until it pegs (or you stop descending). If you climb, the reading goes down.

[NickLappos]

Just so we are clear, if the ice forms on the pitot, the indicated airspeed will go slowly and erratically to zero. Only a theoretical, sudden application of a perfect plug in a millisecond will do as robipilot's instructor must have told him, a very unlikely event.

And since the ice is probably not an airtight seal, the airspeed will likely just read zero no matter what you do to your altitude.

I guess we found another Myth.

[Arm out the window]

I think it's the other way round in that case, too - with a pitot blockage the airspeed indication would increase as you climbed, in the event it didn't just read zero as Nick said.

[212man]

I think people are getting confused between static ports and pitot inlets! A blocked STATIC port will give the indications robipilot suggests. Of course, a pitot heater also heats the static ports as well as the pitot tube.

Further to the original query, normally there will be a cauition or advisory caption somewhere in the cockpit to remind the pilot that he has selected the pitot on.

[MightyGem]

normally there will be a cauition or advisory caption somewhere in the cockpit to remind the pilot that he has selected the pitot on.
Unless, perhaps, you're flying a Eurocopter. Both the 355 and 135 only show a caption when the Pitot heat is off.

[SASless]

No matter how many times you brief a passenger to keep his mitts off the pitot tube as they walk by the front of the helicopter....they will insist upon ignoring your request.

However, the first time you forget (on purpose or not ) and leave the Pitot Heat on....they will never touch it again (especially if they leave their finger prints behind in the form of shriveled up burnt skin!).

Of course 212Man will never admit doing that before....right?

[skadi]

Unless, perhaps, you're flying a Eurocopter. Both the 355 and 135 only show a caption when the Pitot heat is off.

EC changed the system some time ago, In the newer 135s you have now a green advisory light, when you switch pitotheat on, if it is off, you still have yellow caution

Happy new year

skadi

[212man]

Skadi,
the 155 still uses the old logic. Personally I would prefer to see:

Off: Blank screen
On: Green Advisory
On but failed: Amber Caution

That way you can operate with a blank screen in tropical climates without having to turn the pitots on!

[Camp Freddie]

In the AW139,

Off in temp of +4 degrees C and above : No Caption
Off in temp of less than +4 degrees C : 1(2) Pitot Heat Off
On in any temperature : 1(2) Pitot Heat On
On but failed : 1(2) Pitot Heat Fail

Further to this what about arming floats I am use to seeing this in green but the AW139 shows as amber, I reckon green would be better, what do you think ?

Regards

CF

[skadi]

Skadi,
the 155 still uses the old logic. Personally I would prefer to see:

Off: Blank screen
On: Green Advisory
On but failed: Amber Caution

That way you can operate with a blank screen in tropical climates without having to turn the pitots on!

212man, that would be nice,
but the system of the AB139 ( described by "Camp Freddie" sounds even better.
I'm also nerved by yellow PITOT HTR Caption in nonfreezing enviroments.

Skadi

[SASless]

I think I would prefer to have a green advisory if the Pitot heat is on....having a very short attention span, it might save someone getting burned if the pitot tube is within reach.

[Hidden Agenda]

Can anyone shed any light as to why the Eurocopter MMEL for the EC 120, which is a VFR only helicopter, (and perhaps several other aircraft) requires there to be a temperature of more than 4.5 degrees C AND no visible moisture as conditions for flight under the MEL if the Pitot Heater is inoperative?

For those operating in warmer climates it is unlikely that they will get anywhere near 4.5 degrees C whilst operating below 10,000 feet but if they can see rain cloud etc. it is a no go item. I would have thought from the comments above that providing you are above 4,5 degrees C you could fly quite happily in rain without any adverse effect on an unheated pitot head.

Before I start the battle to try and have the MMEL changed I would appreciate an insight, if anyone has one, as to why / how this condition / remark in the MMEL has evolved or been arrived at.

[OEI and Still Flying]

It has always been my understanding that pitot heat is used in the conditions of visible moisture (non-icing) to help burn off moisture before it can travel through the pitot system.
Thus preventing a build-up of water in the lines.

[Greenie from Hell]

OEI..

On a Bell 212, most pilots will turn on the pitot heat (and static vent heaters, as they are usually controlled by the same switch), when it is raining.

There are sometimes water traps in the air pipes, but these don't always work very well, and water in the pitot head can cause lots of problems.
I've never seen water travel as far as the ASI, as there's quite a pipe run to get to it, but I'm sure it has probably happened somewhere.

Before I escaped from Nigeria, we often had problems when it rained, as 212 pitot/static heaters aren't the most reliable things around.

When I was called to a waiting aircraft with this snag, water in the pitot system could easily be removed by sucking very gently (after making sure that the heater wasn't working!!), and this often caused the pax to enquire why the greenie was giving their heli a BJ.

[robipilot]

Nick, I thought back through this and you are right. Initially, when the pitot plugs, the IAS will go to zero. After it is plugged, the IAS becomes an altimeter. As you descend, the airspeed will go up, and as you climb, it will go down. So if you plug the pitot and stay at the same altitude, it will stay at zero. If you change altitude, the indication will change.

[419]

After it is plugged, the IAS becomes an altimeter. As you descend, the airspeed will go up, and as you climb, it will go down.

Surely it must be the other way around, with the indicated airspeed decreasing when you descend.
As the pitot pressure inside the bellows is fixed due to the plug, the only thing that can change is the static pressure acting on the outside of the bellows.
As this would increase as you descend, the bellows would be compressed, and cause the IAS to drop.