Some met conundrums

[WX Man]

Working my way through some feedback and I have come across the following "issues":

1. Temperatures condusive to the formation of rain ice:
-7 to -25, or -30 to -40?

My theory is that at the higher of those two temp ranges, SCWD would be larger and therefore pose a greater icing risk. However answer given is -30 to -40. Obviously lower airframe temperature means more of the SCWD freezes on impact (thirty eightieths as opposed to seven eightieths). Is this the right tree to be barking up?

2. Triggers for a thunderstorm:
"convergence in a trough or a low", or "a steep ELR"?

I say steep ELR. Someone else says convergence. Both seem to be correct, but I would say the steep ELR is more correct than "convergence in a trough or a low", because convergence can still be associated with stability.

3. Setting QFE on final. Altimeter gives:
(A) height of the wheels above the RWY
(B) height of the static port above the AD datum
(C) height above AD elevation

I say (B). However answer given as correct is (A). ((D) was a crock of sh*te). Do I take it that all altimeters are corrected for the height difference between the static port and the bottom of the landing gear?

Thanks! :uhoh:

[pugzi]

WX Man

In your first question.
Rain ice is a unique airframe icing. Rain must be produced from a layer of warmer air. This rain must then fall into an airmass where the temperature is below zero. As a result these rain size drops will eventually become supercooled. Now, if that airmass is very very very cold then the drops will become super cooled very quickly indeed. AND your aircraft will be flippin cold too.
If that airmass is not so cold, the drops may take a while to freeze and become supercooled.
Your theory is right but for Rime and Clear ice, not for Rain Ice or Freezing rain. Closer to 0°C the larger the droplet.

Question 2.
The question is asking for a TRIGGER.
These are the conditions needed for a thunderstorm.
1) Unstable atmosphere (steep/high ELR)
2) Moisture
3) Trigger action, these are
a) Convergence
b) Convection
c) Orography
d) Frontal
Hope this helps. Obviously, CONVERGENCE IS THE ANSWER
By the way convergene and stability have no relation to each other what so ever.

Your third question
I agree with you. But, instrumentation was never my favourite. This isn't really a "met" question, so I'll bow out.

Hope this helps

Steve Francis

[Send Clowns]

The rest have been answered, but 3 is (a) - the altimeter is calibrated with respect to the wheels. Thus QFE will give a zero reading when the wheels are on threshold/airfield datum. This is, of course, relevant for a larger jet, as the difference could be more than a millibar!

[Stoney X]

Send Clowns, moving away from the original question a bit, would the position of the static port make much of a difference? For example, if the static port was positioned near the nose of an aircraft then would the altimeter be calibrated for touchdown with a flared attitude or with all the wheels on the ground?

Regards
Stoney X

[Send Clowns]

For a well-designed static port it should not matter, and in fact once you're in the flare the barometric altitude should no longer be important so this should not give significant concern. There is certainly nothing in the ATPL syllabus concerning position errors, except to say that they can exist!

With a name like that, I take it you are local? :)

[High Wing Drifter]

I thought the risk of icing reduces rapidly below -25 and is negligible below -30 and where it does exist it is in the form of rime ice. The worst icing (clear), so I was told, is between 0 and -10. However, the question doesn't make much sense to me. Rain ice from a warm front above (and above freezing) in to a cold air mass which is suddenly -30? That don't sound right :confused:

[BillieBob]

With the exception of airflow effects, the position of the static port in relation to a barometric altimeter is irrelevant - it is the pressure affecting the capsule in the altimeter that determines the reading.

Consider, for example, a barometric altimeter positioned at the top of the CN Tower, connected to a 1000ft lenth of tubing hung over the edge of the balcony . What would be indicated on the altimeter?

The answer, of course, is the vertical position of the altimeter, not the open end of the tubing 1000ft below. The pressure of the air inside the tubing will decrease with altitude in exactly the same way as the air surrounding it and removing the tubing from the back of the altimeter case will make no difference at all to the reading.

[Stoney X]

Good point BillieBob. But I suppose it all becomes a bit academic cause an aircraft that is large enough to have a few millibars difference in the cockpit flared to un-flared would be using a Radio Altimeter anyway.

SC, good guess, we may even had bumped into each other at BFC/BCFT ;)

Regards
Stoney X

[Jinkster]

Which feedback are you using? Use the Oxford stuff (well done Pugzi) it got me through that subject.

All the best,

Jinkster