Defining Freezing Rain FZRA

ZapBrannigan

ZapBrannigan

If it ain’t a Boeing, I’m not going. No choice.
Ok weather whizkids. Need your help here.

Let's say you're sitting at a Podunk airport with no deicing equipment and no weather observer.

AWOS says the temp is 0 degrees with light rain.

Is it fair to assume that any liquid precipitation falling when the temperature is 0 or less is FZRA or FZDZ?

Therefore, if my company SOP says thou shalt not take off in FZRA, it is fair to put it in the hangar if its raining with a temp of 0 or less?

Technically FZRA is liquid until impact with a below freezing surface -- but since we can not measure the skin temperature of the aircraft all we can go on is OAT. Even an airplane that has been sitting in a warm hangar overnight has a "holdover time" that can not be measured because we don't know how rapidly the skin temperature will fall below freezing.

And if i'm way off base here, then how would you determine FZRA while sitting in Podunk under those conditions?
 
I would say this is one of those grey areas of the FARs and SOPs that we have to deal with day to day in aviation. When those come up, I take the most conservative approach, which in this case would be sit it out.

Can't really go wrong there.

I know that really doesn't help you in the definition of what is freezing rain. But who cares what the definition is, I would much rather save the hassle of potentially being called to the carpet.
 
ZapBrannigan said:
Ok weather whizkids. Need your help here.

Let's say you're sitting at a Podunk airport with no deicing equipment and no weather observer.

AWOS says the temp is 0 degrees with light rain.

Is it fair to assume that any liquid precipitation falling when the temperature is 0 or less is FZRA or FZDZ?

Therefore, if my company SOP says thou shalt not take off in FZRA, it is fair to put it in the hangar if its raining with a temp of 0 or less?

Technically FZRA is liquid until impact with a below freezing surface -- but since we can not measure the skin temperature of the aircraft all we can go on is OAT. Even an airplane that has been sitting in a warm hangar overnight has a "holdover time" that can not be measured because we don't know how rapidly the skin temperature will fall below freezing.

And if i'm way off base here, then how would you determine FZRA while sitting in Podunk under those conditions?
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1. Our airline procedures state that the type of precipitation and intensity is determined by the most current weather report, ATIS or pilot observation...whicher the captain deems to be most appropriate.

2. Freezing rain is rain falling as supercooled drops. Not to be confused with "rain on a cold soaked wing". Both of which can form ice on contact. If the rain is falling from colder air into warmer air...I'm not sure the precip would be "supercooled" and not necessarily FZRA. If the rain was falling from an inversion into 0C surface...it possibly could be.

So my answer to the situation would be...the pilot has to make the determination and it is not automatically FZRA.

Although like Seg says...a conservative decision would probably not be a bad idea.
 
If liquid precipitation is falling, and the temperature is below 0 degree c... what else could it be?

Essentially, i'm trying to justify my opinion to the other pilot that IF there's liquid precip...and IF the temp is below zero... we have to deice or, if deicing is unavailable, we don't go. (and of course if FZRA is reported on ATIS or other weather observation we dont go anyway).

Why? Because we can not guarantee that once the surface temperature of any part of the aircraft falls below freezing, supercooled drops won't instantly freeze upon impact.

http://www.influks.com/post1121.html

It was easy at the airlines. Deicing fluid was readily available. Out here in part 91 land, there is still the pressure to get the job done, but a lot of times we lack the resources to do so safely. I will not compromise safety but without a satisfactory definition for FZRA, it is difficult for me to make my case when the other pilot says, "It's just rain!"

So...if it's rain with a surface temp below 0 degrees C.... then?
 
I think there could be lots of different things in affect here...and that's why the FAA gives the pilots some lattitude for decision making.

For one, the temperature could be indicating zero...but maybe it's slightly above in actuality.

Also, and my understanding anyways, is that there is still some question on how water droplets become supercooled. Not every water droplet becomes supercooled at 0 degrees C. It depends on the size of the droplet, the latent heat that is given off during the change of state, etc.

How many times have you been flying at the freezing level and you get no ice? The temperature drops one degree...and there it is. It drops 5 degrees and the icing is gone. Theres lots of factors that go into determining when the water droplet will become supercooled or freeze.

The big test, IMO, is does the rain freeze at impact? If the answer is yes...you've got freezing rain.

However, if a captain made a decision not to fly with liquid precip falling at 0 C...I would back up that decision.
 
is it freezing rain or not? Is the ramp getting slippery? Are the cars in the parking lot getting covered in ice? If those conditins exist, you probably should think twice. If you pull the airplane out, you aren't picking up ice, you do your pre-takeoff contamination check and the wings are clear, you're good to go. The good thing about this situation is either the airplane has ice or it doesn't.... you've got a way to determine if you can go or not.

This situation has actually come up with me twice. Both times I had no ice, so both times I went. Why wasn't it freezing? I don't know. But it wasn't. One thing that came to mind is that the temp could've been 32.1 degrees, or 32.4 degrees or something like that... above freezing but the machine still calls it 0.
 
The lack of any de/anti ice would worry me. Whose to say you make the GO decsion, you then load up people, take fuel, taxi out...and in that time there has been accumlation of ice on your airplane that you have no protection on and have no way of telling that its now there.
 
That's my concern as well. The wings have warm fuel in them fresh out of the ground, so the pre takeoff contamination check may be good, but who knows what is going on up there on the t-tail?

How quickly does a hollow metal surface cool to the freezing point? (This is why I think the "cars in the parking lot" argument is a little suspect)

The bottom line is that there are a lot of variables at play. So the only choice we have is to look at the hard information we have: Surface temperature and precipitation.

Now I'll be the first to admit that as a former 121 pilot, i'm probably the most conservative part 91 pilot out there. If I would have deiced under 121 and deicing fluid is unavailable, I won't even depart under 91. I just need some justification for my decision... which is what I was looking for here.
 
A little clarification of my thinking is in order, so go with me on this for a second.

So what would you do?

You're sitting on the ramp ready to depart. Temp is -1 and rain is falling. There is no deicing fluid available and you need to make your go/no-go decision.

The wings are wet, but then again...there's warm fuel in there. You have no idea as to the status of the horizontal stab.

By the time you load up the pax, close the door, start up, taxi out...probably 10 minutes or so.

Basically i'm looking for justification to call it quits and wait for the rain to stop. Since we can not tell if a droplet is supercooled or not, the only safe decision is to assume that it is and that, once the surface it is falling on reaches the freezing point, the droplet will freeze on impact or shortly thereafter.

What do the NetJets guys do when operating into an airfield with no deice capability? I'm interested in how other operators deal with some of the situations where it's not really black and white.

i/e light snowfall (not accumulating), liquid precip below freezing, freezing fog, freezing mist, etc.
 
Is there ice on anything outside? Other airplanes that have been sitting? Airport signs? The ground? It's not freezing rain unless it freezes to something, right? And if it's not freezing to buildings, flagpoles, or airplanes that have been sitting overnight, I don't see why it would freeze to yours.

If I were in your shoes, I'd consider loading the cargo, pax and fuel. Do one last inspection of the aircraft surfaces to make sure there's no ice (and none on anything else outside), then get going. Doesn't sound like you're going to be sitting on the taxiway, number 11 for takeoff behind a fleet of United jets. Taxi out and go. Just be prepared to deal with icing once in flight. And if you do pick up ice on the taxi, you can always change your mind. I would think that ice accumulation will occur on the aircraft nose, somewhere on the wing, or the wipers (somewhere you'd notice from the pilot's seat) just as easily as the tail.

Then again, it's easy to say that when I'm sitting on a couch drinking coffee.....
 
ZapBrannigan said:
Basically i'm looking for justification to call it quits and wait for the rain to stop. Since we can not tell if a droplet is supercooled or not, the only safe decision is to assume that it is and that, once the surface it is falling on reaches the freezing point, the droplet will freeze on impact or shortly thereafter.
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I think that's your justification right there. If you are not 100% comfortable with the operation, the show stops.
 
The simple answer is that if you yourself are not comfortable, don't go. It's that easy.

As for your situation you listed, there is no for sure way to tell that the horizontal stab isn't picking up ice unless you can look at it. But... and the big but... if you make that decision in this situation than any pre takeoff contamination check you ever do is nullified- with or without being deiced. You'll never be able to see the tail in most airplanes. Once you get accustomed to your airplane you'll start learning where to look for ice. I see ice forming on the windshild wipers in my metro prior to the wings. That's my personal indicator. If I taxi out and see ice on the wipers I'm not going to go.
 
ZapBrannigan said:
Since we can not tell if a droplet is supercooled or not, the only safe decision is to assume that it is and that, once the surface it is falling on reaches the freezing point, the droplet will freeze on impact or shortly thereafter.

What do the NetJets guys do when operating into an airfield with no deice capability? I'm interested in how other operators deal with some of the situations where it's not really black and white.
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Zap-

From the sound of it so far, it seems that you have standard rain possibly falling from some warmer air above. If you had freezing rain above you, then you would either be seeing ice pellets, or the actual freezing rain itself falling at ground level. It doesn't sound like supercooled water droplets (yet) because by definition, they would then freeze on contact to a surface (ground, airfoil, etc).

As far as the Netjets approach, it seems that the owner generally dictates where we go when going into an airport. If it is obvious that there is going to be too much contamination (snow, ice) for it to melt off upon arrival, then Netjets might try and negotiate with the owner to use a different field and then set up ground transportation for them from there. I have had a few instances when we were sent into an airport with no deice capabilities with owners onboard. After landing, the aircraft still had residual ice attached, so we sat and waited for it to melt off before leaving again. In one case, the temps were going to be below freezing for a while, so we left the aircraft there!

That being said, when an owner requests to come OUT of an airport with no deice capability, it is a different game. In my experience, the company either tells them that: if they insist on using the airport and icing conditions exist there is a possibility that the flight may not operate; or they offer the option for the owner to depart from a nearby field that has deice capability (this seems to happen most often).

In the end, it obviously comes down to the crews decision. Here at Netjets, when we get to those gray areas, it has to be a team effort. We have our dispatchers, weather personnel, etc. to contact as resources. If one member of the crew doesn't think it is safe, the whole operation stops. We've called it quits a few times and have NEVER caught any flak for it. Ever.

ZapBrannigan said:
Basically i'm looking for justification to call it quits and wait for the rain to stop.
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Like other guys above me have said, sounds like your concern is all the justification you need. I'd rather end up in the CP's office explaining a decision I made to err on the side of safety, than to get airborne and realize I made a bad call.

Good luck!
 
ZapBrannigan said:
If liquid precipitation is falling, and the temperature is below 0 degree c... what else could it be?
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freezing rain has nothing to do with the air temp at the surface and everything to do with the air temp aloft. Freezing rain droplets are supercooled and if you were to measure their temperature you would note that the water temp is below freezing. When the rain impacts a surface it causes the water to leave the supercooled state and spontaneously freeze to whatever it just hit.

At an airport with no observer or weather reporting equipment you would not freezing precipitation by the fact that when the water droplets hit your airplane they are freezing on contact and ice is building all over your airplane. The taxiways will probably be getting pretty slippery as well.

You can have surface temps above freezing and still have FZRA because the rain is falling from cooler air aloft. It will still freeze on contact but will begin the melting/sublimating process as soon as that happens.

Normal rain into subzero temperatures would be remain liquid and freeze over time, not instantly like FZRA would. Even in above freezing temperatures you can run into issues if you spend a lot of time in high cruise as the residual fuel tank temperatures may be cold enough to cause precip/condensation to freeze to the wing.

I'd say your best bet to determine FZRA vs RA is what kind of ice you are getting. If its mostly clear ice then you have RA slowly freezing to the airplane. If you have rough ice formations then it is probably FZRA because that water is already below freezing and will crystallize as ice wherever and however it hits your airplane.
 
Ok. So let's simplify. If you had an SOP that said, "Thou shalt not depart in FZRA" and you noticed it was raining with a temperature of -1C...

What would you do?

I get that most of you are suggesting a pre-takeoff contamination check and that is the only true determiner of airframe icing prior to departure. I guess I was just trying to figure out if there were any environmental conditions that could be used to make that determination earlier in the planning process and therefore allow for less impact to the passengers. (i/e allow them to stay at the office and keep working if we're going to delay, etc.)
 
ZapBrannigan said:
Ok. So let's simplify. If you had an SOP that said, "Thou shalt not depart in FZRA" and you noticed it was raining with a temperature of -1C...

What would you do?

I get that most of you are suggesting a pre-takeoff contamination check and that is the only true determiner of airframe icing prior to departure. I guess I was just trying to figure out if there were any environmental conditions that could be used to make that determination earlier in the planning process and therefore allow for less impact to the passengers. (i/e allow them to stay at the office and keep working if we're going to delay, etc.)
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Zap,

No need to tell you, as with your experience I'm sure you know, but all-weather flying is a dynamic situation and that requires tactical decisions.

If you are a CFI looking to make a go/ no-go decision for a student pilot...certain scenarios will prohibit the flight.

As a professional...it would be tough to cancel ahead of time based upon expectations. At minus 1 centigrade and rain...mostly likely you are going to delay or cancel because of the icing conditions. However, those are decisions that more than likely will be made at the time of takeoff.

I'm not sure there is a black and white template for the situation you are describing. Bottom line, the captain must ensure the airplane is free of contaminants.



As a side note, a couple of years ago the FAA stopped endorsing the procedure of "polishing" frost on airplanes. Frost must be removed prior to flight. I know there are a lot of pilots out there unaware of this.
 
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