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By Jonzarno
#754162
Peter

Thanks for your thoughts on this and sorry not to have replied earlier but I wanted to check the temperature readings on the plane before doing so.

You are at least partly right: the readings on the ground are 2 deg C apart and I will get the sensors checked at the forthcoming annual.

Yesterday at FL120 and 160 KTAS they were up to 7 deg C apart and, later on the way back, at FL90 and 155 KTAS they were 6 deg C apart.

The sensor on the wing seems to be right because I started to see some icing when entering cloud with a reading of -3 and the ice cleared as the indicated temperature rose above 0 Deg as I descended to get out of it. The TKS system did a good job of keeping the leading edges of both wings and elevators clear whilst I was getting clearance to descend.

I don't know if this sheds any light but would be interested to hear your further thoughts.

Many thanks

John
By bookworm
#754788
peterh337 wrote:Bookworm is the man for this (he has a PhD in the subject) but would somebody mount one OAT probe under the wing and another one sticking up on top of the wing ;)


With respect to Jonzarno, I don't believe a true 5 degC difference between probes.

I have a PhD in something different, but I did once run the numbers to work out the temperature fall over the wing. It depends on wing loading (as well as the peak to mean Cp ratio) and rarely exceeds 1 degC for typical GA (10 kg/m^2). For a 747 (70 kg/m^2) it can get up to 10 degC, hence the admonitions to airline crews to use anti-icing in the 0 to 10 degC range.
By peterh337
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#755280
In this case the OP needs to borrow an accurate thermometer and find out (on the ground) the static temperature errors. These should be within 1 C which is probably all one can achieve given that the display resolution is 1 C anyway.

I have a PT100 thermometer accurate to 0.1C and would be happy to meet someone at Shoreham with it. It's only a little instrument but was fairly pricey.

I would expect two probes mounted perpendicular to the airflow, anywhere on the airframe, to be fairly close to each other.

It's important to get this right because there is a big difference between flying in +2C and flying in -2C, in terms of possible icing if IMC is entered. Also, temperature drops in IMC, generally by 2-3C, so if flying in sunshine at +2C towards IMC, one can expect icing conditions in the cloud.

What kind of instruments are used to display these readings, and where are the probes for each mounted?
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By Jonzarno
#756050
I can quite understand why this should be puzzling as I do not believe the difference should be this big myself.

To provide one answer to the question as to where the values are displayed: the wing mounted sensor displays on the primary flight display and fuselage displays on the multifunction display.

As stated in my earlier post, the difference on the ground is 2°C and the difference in the air can be up to 7°C as it was again yesterday.

I quite take your point about the importance of the difference between -2°C and +2°C

The aircraft is booked in for an annual in a couple of weeks time and I will get the probes calibrated and the problem looked at and will post the results of that when I have them.

John Zarno

PS by the way did anyone look at the video whose link I posted? That was the main reason I did the post in the first place as I for one certainly found it to be a real eye-opener.
By peterh337
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#756570
It seems unreal to see a 4C diff airborne but only a 2C diff on the ground.

But temperature measurement needs to be done very carefully. Got to make sure both probes really are at the same temp on the ground - this is not going to be the case outdoors, due to solar radiation, airflow, etc.

Without accurate instrumentation this is all a bit meaningless.
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By Jonzarno
#756586
Yes, I take your point. I'll get it checked at the annual and post the results.
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By Jonzarno
#773972
peterh337 wrote:It seems unreal to see a 4C diff airborne but only a 2C diff on the ground.

But temperature measurement needs to be done very carefully. Got to make sure both probes really are at the same temp on the ground - this is not going to be the case outdoors, due to solar radiation, airflow, etc.

Without accurate instrumentation this is all a bit meaningless.


Well I promised that I would post the results of the calibration check I asked for when my annual inspection was done and I must confess to a certain embarrassment.

It turns out that both probes are accurate and the problem results from where they are mounted. One probe is mounted on the right wing and provides an accurate indication of potential icing problems. The other probe is mounted on the right side of the fuselage on the outside of the engine cover and about 3 feet above the exhausts. Guess which one reads high...........

I understand that Cirrus have now removed the second probe from new aircraft. Quite why they fitted it where they did in the first place is beyond me.

Many thanks to everyone who contributed thoughts on this: I am sorry I have wasted your time. :oops: :oops: :oops:
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By debiassi
#787547


Probably worth mentioning that ice can accrete form +2 degrees to minus 20 degrees. Whilst not impossible, it is highly unlikely outside of this temperature range. The most dangerous type being clear ice as this rolls back and freezes in a clear and smooth formation and is very heavy and almost impossible to shift. The most common cause is SLD. super cooled liquid droplets. Have a look at the clip above, the pilot of the 206 is more than lucky to be alive. It makes very interesting listening. Ice is a big killer and those conditions deserve the utmost respect. The most effective combatant in the general aviation market for anti icing has been the invention of tks systems or the weeping wing as they are more commonly known. Of course there are the old boots which need more care in when to utilize as if not used with caution, then ice can build up behind the boots coverage area. TKS weeps anti icing fluid from the leading edges on wings, elevator and tail fin which is verry effective.
By peterh337
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#788774
Ice should not be a big killer if people flying non-deiced aircraft scrapped any flight in which IMC below 0C will be found and the 0C level is anywhere near (or below) the MSA.

Most certified aeroplanes can carry a bit of ice - even a Cirrus can ;) - if flying at a reasonable speed. One problem is the one I give above; another one is collecting ice when flying near one's operating ceiling i.e. close to Vs (a possible stall/spin scenario).
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By debiassi
#788818
Also may be worth mentioning Pete regarding operating near ceiling that if ice is encountered, in most cases, as climb or descent of 3000ft will take you out of icing conditions. The likelihood is if your operating near the aircraft ceiling, then the only option is a descent. If your near your ceiling and flying near mount everest you may be unlucky but in the majority of cases, a descent would be possible unless poor planning is at fault.