Freezing rain / drizzle
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Freezing rain / drizzle
I've never really had any specific guidance on operating in freezing rain or drizzle, but it is something I would prefer to stay well away from. How do your operators treat this issue and could anybody point to me to any official sources or research that could tell me more about the implications of encountering it?
My last employer forbade departures in freezing rain although landing was allowed subject to braking action.
The problems are that freezing rain adheres to all surfaces around zero and below.
It is extremely difficult to deice except with heated fluid.
On untreated surfaces the braking action is a gnats f*rt above zero - a neighbour landed a DC8 and had to use differential reverse thrust to steer the beast.
On approach I haven't found it a problem but due to anti icing only available on leading edges I wouldn't consider departing.
It is difficult to see the deposit on the wings from inside the aircraft - as dead heading crew I was sent forward to the cockpit as even the ground crew who had inspected the wings hadn't noticed the glazing. (my skipper was seated adjacent to the over wing exits).
The problems are that freezing rain adheres to all surfaces around zero and below.
It is extremely difficult to deice except with heated fluid.
On untreated surfaces the braking action is a gnats f*rt above zero - a neighbour landed a DC8 and had to use differential reverse thrust to steer the beast.
On approach I haven't found it a problem but due to anti icing only available on leading edges I wouldn't consider departing.
It is difficult to see the deposit on the wings from inside the aircraft - as dead heading crew I was sent forward to the cockpit as even the ground crew who had inspected the wings hadn't noticed the glazing. (my skipper was seated adjacent to the over wing exits).
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Ice will destort and destroy the lift performance of your wing. Lots of CRM case studies with regards to accidents/incidents resulting from departures with ice adhering to the wing. 
Freezing rain with signs of ice accretion before departure? Then de-ice and apply anti-ice measures if applicable to your aircraft type.
During flight? Anticipate (visible moisture below 10c) and deploy anti-ice and de-ice measures as are available to you. Not possible? Then climb or descend or return the way you came (if its an option) until clear of the condition. NASA did some research and found that ice appears in layers of 2000-3000'....apparantly. Not sure how they worked that out...
Or better still, listen to the man from the CAA:
See ice? Dont think twice! De-ice!
Freezing rain with signs of ice accretion before departure? Then de-ice and apply anti-ice measures if applicable to your aircraft type.
During flight? Anticipate (visible moisture below 10c) and deploy anti-ice and de-ice measures as are available to you. Not possible? Then climb or descend or return the way you came (if its an option) until clear of the condition. NASA did some research and found that ice appears in layers of 2000-3000'....apparantly. Not sure how they worked that out...
Or better still, listen to the man from the CAA:
See ice? Dont think twice! De-ice!
My take on freezing rain - having nearly once crashed as a result of it!
1. It can be forecast - there's a specific set of circumstances where it can occur, under the 'ledge' of warm air under a warm front while flying in the cold sector. This is where you may see it forecast. However, This may not always be the case and it can be present in circumstances where it is very difficult to predict.
2. When you get it, at first it's innocuous but can build up very quickly - very quickly indeed.
3. Use of anti ice/de-ice can make things worse in clear ice. The ice melts off the leading edge, runs back and re-freezes a bit further downstream on the aerofoil section. This is worse than an even layer of ice over the whole aerofoil.
In my case, it wasn't forecast. We were being vectored into an airport in a temperate zone on a winter's day in high pressure system with the gound temperatire only a couple of degrees above freezing. We were level at 3000' above a layer of stratus and had asked for a late descent because air aircraft had engine intake anti-icing, but no airframe anti-icing and we had just completed a long high level transit and knew our airframes would be super cooled. We were told to descend to accommodate traffic going into an adjescant airport so complied. But there was drizzle in the stratus and we started to accumulate ice rapidly. We declared a Pan and aked for a climb out of the straus. This was granted, but now we found we didn't have enough power to climb, so we slowed down and immediately entrered buffet, at a speed some 60% more than the usual buffet onset. Now we were in trouble as we couldn't remain level at full power as the intake de-icing wasn't coping under there conditions.
A mayday was declared and ATC realising we wouln't make our destination offered us vectors to join the ILS at a third airport which we gladly accepted. We flew the ILS at full throttle accepting the increse in speed and made a safe landing, but hardly being able to see forward due to the ice. We were a formation of two and the wingman had only our strobes to formate on. I could see his aircraft and he was covered in a glassy sheet of ice. Luckily, he had a bit of power reserve on me.
The problems didn't end on landing as this third airfield was in another country - one to which we had no clearance to fly. And when I got out of the cockpit, I failed to see the sheet of ice coating the wing and immediately slid off - landing on the ground on my bum looking very undignified. It hurt too!
Subsequent investigation showed the conditions we were in were a very unlucky set of circumstances and had only been encountered rarely. The met office were aware of the danger and had forecast the icing risk as moderate, but not warned of rain ice as it didn't fit the classic pattern. Also, our aircraft was a brand new type and we were still learning about it's performance in icing conditions.
Rain ice - it's very very dangerous.
1. It can be forecast - there's a specific set of circumstances where it can occur, under the 'ledge' of warm air under a warm front while flying in the cold sector. This is where you may see it forecast. However, This may not always be the case and it can be present in circumstances where it is very difficult to predict.
2. When you get it, at first it's innocuous but can build up very quickly - very quickly indeed.
3. Use of anti ice/de-ice can make things worse in clear ice. The ice melts off the leading edge, runs back and re-freezes a bit further downstream on the aerofoil section. This is worse than an even layer of ice over the whole aerofoil.
In my case, it wasn't forecast. We were being vectored into an airport in a temperate zone on a winter's day in high pressure system with the gound temperatire only a couple of degrees above freezing. We were level at 3000' above a layer of stratus and had asked for a late descent because air aircraft had engine intake anti-icing, but no airframe anti-icing and we had just completed a long high level transit and knew our airframes would be super cooled. We were told to descend to accommodate traffic going into an adjescant airport so complied. But there was drizzle in the stratus and we started to accumulate ice rapidly. We declared a Pan and aked for a climb out of the straus. This was granted, but now we found we didn't have enough power to climb, so we slowed down and immediately entrered buffet, at a speed some 60% more than the usual buffet onset. Now we were in trouble as we couldn't remain level at full power as the intake de-icing wasn't coping under there conditions.
A mayday was declared and ATC realising we wouln't make our destination offered us vectors to join the ILS at a third airport which we gladly accepted. We flew the ILS at full throttle accepting the increse in speed and made a safe landing, but hardly being able to see forward due to the ice. We were a formation of two and the wingman had only our strobes to formate on. I could see his aircraft and he was covered in a glassy sheet of ice. Luckily, he had a bit of power reserve on me.
The problems didn't end on landing as this third airfield was in another country - one to which we had no clearance to fly. And when I got out of the cockpit, I failed to see the sheet of ice coating the wing and immediately slid off - landing on the ground on my bum looking very undignified. It hurt too!
Subsequent investigation showed the conditions we were in were a very unlucky set of circumstances and had only been encountered rarely. The met office were aware of the danger and had forecast the icing risk as moderate, but not warned of rain ice as it didn't fit the classic pattern. Also, our aircraft was a brand new type and we were still learning about it's performance in icing conditions.
Rain ice - it's very very dangerous.
Recently was faced with FZ DZ and the company has standard policy of no departures in FZ RA. De-Iced using Kilfrost type 4 with 100% mix and this allows a departure in FZ DZ with a HOT of 40 minutes. Another question, can you land in FZ RA, most operators stipulate TO only as a No-Go? I'm not trying to be smart I honestly don't know and personally avoid freezing rain at all cost. I saw it at Anchorage once and it stuck to the wipers like super glue so you can imagine what it's doing to the wings. Great care needs to be taken in deciding the difference between DZ and RA, if you need the wipers don't go. 'Blind P' you say your operator allowed landing in freezing rain subject to braking conditions, I assume 'Good' was required?
Originally Posted by By George
...allows a departure in FZ DZ with a HOT of 40 minutes.
Jeez, that sounds like an awfully long holdover time for freezing drizzle - if I remember correctly, the HOTs published by Transport Canada only allow a HOT of something like 2 to 5 minutes for this condition. I don't have access to those documents right now, I'm just going from memory, so I might be wrong... but I kind of suspect that 40 minutes might be a bit optomistic.
Michael
I never argue with Canadians when it comes to cold weather but the HOT times vary greatly depending on the mix and type of fluid.
I used 'Kilfrost ABC-4 substain Type IV with a ratio of 100/00
The tables say: Freezing Fog 1 -1.50 hrs
Snow 30 - 55 mins
Freezing DZ 40 -1.05 min/hr
Temp range -3c and above. I took the conservative low end of 40 min
We could never go with 2 to 5 mins, it takes us around 8 minutes to push back and start. (four eng and no auto-start 747). Normally can get airborne around the twenty minute mark. Also the HOT times start from the begining of application so your tables are not the same as we use. Anyhow you can have your 'white scenery' I have a few trips in the sun now.
I used 'Kilfrost ABC-4 substain Type IV with a ratio of 100/00
The tables say: Freezing Fog 1 -1.50 hrs
Snow 30 - 55 mins
Freezing DZ 40 -1.05 min/hr
Temp range -3c and above. I took the conservative low end of 40 min
We could never go with 2 to 5 mins, it takes us around 8 minutes to push back and start. (four eng and no auto-start 747). Normally can get airborne around the twenty minute mark. Also the HOT times start from the begining of application so your tables are not the same as we use. Anyhow you can have your 'white scenery' I have a few trips in the sun now.
asked for a late descent because air aircraft had engine intake anti-icing, but no airframe anti-icing ... The met office were aware of the danger and had forecast the icing risk as moderate, ... now we found we didn't have enough power to climb, so we slowed down and immediately entrered buffet, at a speed some 60% more than the usual buffet onset. Now we were in trouble as we couldn't remain level at full power as the intake de-icing wasn't coping under there conditions. ... A mayday was declared
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Probably the most important aspect of the ZL/ZR issue is to recognize that these conditions lie outside of the engineering standard envelopes used for icing certification. These conditions fall into the category of SLD (Supercooled Large Droplets) .They are conditions that were not considered during the design, testing and certification of the aircraft ice protection systems, and the aircraft handling characteristics were not evaluated with ice shapes derived from ZL or ZR.
The engineering standard envelopes use a single-mode, bell-shaped droplet size distribution which is centered on a median droplet size of 40 to 50 microns. Under the far right hand tail of the curve, some ZL-sized droplets may be found, but they are extremely sparse. The vast bulk of the droplets in this standard envelope fall well under the size of ZL. This engineering standard is believed to cover 99% of the icing environment, although that number was originally based on the entire scope of icing in the continental US. It does not accurately describe such places as the Great Lakes region, the Canadian maritimes, or the Alps of northern Italy.
Like most design choices, the extent of the protected area on the wing is an engineering optimization. One common method for determining the aft extent of the protected surface is to determine how far back a 50 micron droplet will impinge. There is no requirement for protection further aft because the larger droplets that will impinge there are not part of the envelope and are considered quite rare. Some manufacturers will opt for more conservative choices; the 50 micron drop estimation is part of what got ATR and Embraer in trouble with their turboprops. ATR resolved this in the weeks following Roselawn with a retrofitted set of wing leading edges which extended the protected surface further aft by around 1% MAC.
The industry is in the process of developing certification standards for a new icing envelope which is centered on large droplets. No airplane has yet been certificated to this new standard; it remains to be seen just how it will work.
Although ZL and ZR are thought to be rare, they actually make up 1.8% of the reported surface precipitation in the continental US. In the icing accident database that I maintain, ZL or ZR are associated with approximately 30% of the accidents and incidents combined. This makes a pretty strong case that SLD is a very serious threat to safe operations. As with all icing, aircraft scale plays a significant role, with larger transports not experiencing the same degree of difficulty. However, the 30% number remains pretty consistent up through the large turboprop scale.
In terms of vertical extent, an extensive study of atmospheric measurements done by Dick Jeck at the FAA (now retired) identifies ZR environments as deep as 7000 feet and ZL environments as deep as 12,000 feet. I have a couple of reports of crews making altitude changes in excess of 4000 feet and not escaping the condition. That said, most of these environments are shallower and an altitude change is definitely in order when they are encountered.
While ZR generally exists under an inversion, ZL can exist without an inversion through collision coalescence. One fairly good method for anticipating ZL or ZR aloft is the presence of ice pellets at the surface.
The elephant in the living room for the past sixteen years has been the existence of holdover tables for light ZR and light to moderate ZL. As far as the FAA's Aircraft Certification service is concerned, the aircraft is not certificated for these conditions and should not be operated in them. As far as the FAA's Flight Standards Service is concerned, they are an acceptable norm of all-weather flying. This makes it almost impossible to generate a consistent, factually based message. When taking off in moderate ZL, for example, the fluid work quite well within the holdover times. Once the airplane rotates, the fluids shear off, and one is suddenly airborne in an environment that the airplane's ice protection system was not designed to cope with. For years, this was tolerated on the premise that the SLD layer was very shallow; this is not at all true in a reasonable number of cases.
There are still no real good, black-and-white answers, which is what makes the icing issue such a squirrel. Scale is important, but you can expect some performance degradations even in larger scale aircraft. Proactive operation of the ice protection system is essential.
Here are a couple of links that may provide some good references:
http://www.tc.faa.gov/its/worldpac/techrpt/ar0945.pdf (Dick Jeck's paper)
http://www.skybrary.aero/bookshelf/books/531.pdf (AIAA paper on the icing accident database)
http://www.skybrary.aero/bookshelf/books/532.pdf (AIAA paper on the history and nature of the airframe icing threat)
http://flightsafety.org/fsd/fsd_jan96.pdf (John Dow's excellent article on roll upset)
The engineering standard envelopes use a single-mode, bell-shaped droplet size distribution which is centered on a median droplet size of 40 to 50 microns. Under the far right hand tail of the curve, some ZL-sized droplets may be found, but they are extremely sparse. The vast bulk of the droplets in this standard envelope fall well under the size of ZL. This engineering standard is believed to cover 99% of the icing environment, although that number was originally based on the entire scope of icing in the continental US. It does not accurately describe such places as the Great Lakes region, the Canadian maritimes, or the Alps of northern Italy.
Like most design choices, the extent of the protected area on the wing is an engineering optimization. One common method for determining the aft extent of the protected surface is to determine how far back a 50 micron droplet will impinge. There is no requirement for protection further aft because the larger droplets that will impinge there are not part of the envelope and are considered quite rare. Some manufacturers will opt for more conservative choices; the 50 micron drop estimation is part of what got ATR and Embraer in trouble with their turboprops. ATR resolved this in the weeks following Roselawn with a retrofitted set of wing leading edges which extended the protected surface further aft by around 1% MAC.
The industry is in the process of developing certification standards for a new icing envelope which is centered on large droplets. No airplane has yet been certificated to this new standard; it remains to be seen just how it will work.
Although ZL and ZR are thought to be rare, they actually make up 1.8% of the reported surface precipitation in the continental US. In the icing accident database that I maintain, ZL or ZR are associated with approximately 30% of the accidents and incidents combined. This makes a pretty strong case that SLD is a very serious threat to safe operations. As with all icing, aircraft scale plays a significant role, with larger transports not experiencing the same degree of difficulty. However, the 30% number remains pretty consistent up through the large turboprop scale.
In terms of vertical extent, an extensive study of atmospheric measurements done by Dick Jeck at the FAA (now retired) identifies ZR environments as deep as 7000 feet and ZL environments as deep as 12,000 feet. I have a couple of reports of crews making altitude changes in excess of 4000 feet and not escaping the condition. That said, most of these environments are shallower and an altitude change is definitely in order when they are encountered.
While ZR generally exists under an inversion, ZL can exist without an inversion through collision coalescence. One fairly good method for anticipating ZL or ZR aloft is the presence of ice pellets at the surface.
The elephant in the living room for the past sixteen years has been the existence of holdover tables for light ZR and light to moderate ZL. As far as the FAA's Aircraft Certification service is concerned, the aircraft is not certificated for these conditions and should not be operated in them. As far as the FAA's Flight Standards Service is concerned, they are an acceptable norm of all-weather flying. This makes it almost impossible to generate a consistent, factually based message. When taking off in moderate ZL, for example, the fluid work quite well within the holdover times. Once the airplane rotates, the fluids shear off, and one is suddenly airborne in an environment that the airplane's ice protection system was not designed to cope with. For years, this was tolerated on the premise that the SLD layer was very shallow; this is not at all true in a reasonable number of cases.
There are still no real good, black-and-white answers, which is what makes the icing issue such a squirrel. Scale is important, but you can expect some performance degradations even in larger scale aircraft. Proactive operation of the ice protection system is essential.
Here are a couple of links that may provide some good references:
http://www.tc.faa.gov/its/worldpac/techrpt/ar0945.pdf (Dick Jeck's paper)
http://www.skybrary.aero/bookshelf/books/531.pdf (AIAA paper on the icing accident database)
http://www.skybrary.aero/bookshelf/books/532.pdf (AIAA paper on the history and nature of the airframe icing threat)
http://flightsafety.org/fsd/fsd_jan96.pdf (John Dow's excellent article on roll upset)
In the early eighties Northern Europe had a particularly cold spell.
Zurich was virtually closed and Swissair's fleet was grounded for a couple of days.
Zurich Kloten is situated in a mountainous bowl which often retains the katabatic air. When a front passes over in winter there is often a sudden but often temporary period of freezing rain.
I think on this occasion two fronts passed over before the freezing air was displaced.
It was impossible to walk or drive - although in the windless conditions it was possible to maneuver an aircraft but not to push it back.
The main operator that flew into ZRH during this time was BA who did not have the braking coefficient restrictions of Swissair.
It is interesting to note that they were also allowed to operate for 11hrs across the Ogin after an engine failure and fly 777s in colder air than other operators.- but that it is straying off the subject.
Some of you might remember the 747 that broke up in anchorage - although the runway had reasonable braking action the taxiway didn't.
The crew gave up trying to move the aircraft in strong cross winds and shut the engines down.
The wind then took over, weather cocked the jumbo and blew it down the embankment.
Think everyone had a lucky escape.
Zurich was virtually closed and Swissair's fleet was grounded for a couple of days.
Zurich Kloten is situated in a mountainous bowl which often retains the katabatic air. When a front passes over in winter there is often a sudden but often temporary period of freezing rain.
I think on this occasion two fronts passed over before the freezing air was displaced.
It was impossible to walk or drive - although in the windless conditions it was possible to maneuver an aircraft but not to push it back.
The main operator that flew into ZRH during this time was BA who did not have the braking coefficient restrictions of Swissair.
It is interesting to note that they were also allowed to operate for 11hrs across the Ogin after an engine failure and fly 777s in colder air than other operators.- but that it is straying off the subject.
Some of you might remember the 747 that broke up in anchorage - although the runway had reasonable braking action the taxiway didn't.
The crew gave up trying to move the aircraft in strong cross winds and shut the engines down.
The wind then took over, weather cocked the jumbo and blew it down the embankment.
Think everyone had a lucky escape.
Originally Posted by ByGeorge
...Anyhow you can have your 'white scenery' I have a few trips in the sun now.
Thanks for double-checking the data for the Type 4 fluid. That type of fluid cannot be used on the small turboprops (only Type 1 and Type 3 is permitted), so, that's probably why I was a bit out in left field with my question.
As for the "white scenery" - well, here's a picture of what I was doing between Christmas and New Year's when I had to ferry the plane from Canada to the Seychelles. The picture was taken far north of the Arctic Circle, de-icing with Type 1 in -35 degree weather. That's me on the top of the plane.
I gotta admit I don't miss the white scenery at all.
Michael
A rather classic example of what happens when you violate an aircraft operating limitation that prohibits flight into "known icing conditions".
Except this was a military aircraft which had no such limitation.
Except this was a military aircraft which had no such limitation.
Not irresponsible as the aircraft was cleared into icing conditions. It was a military aircraft and had no restrictions on flying in icing as a result of no airframe anti-icing - not many do. I can't remember the exact wording in the manuals (close to 20 years ago) but it mentioned exposure to moderate icing should be minimised. It had engine and canopy de-icing. Both were subsequently upgraded, partly as a result of this and other incidents. It was a brand new aircraft and we were still learning about it.
My point - relevant to the thread subject - is that rain ice can form extremely quickly and is very dangerous. And airframe de-icing may not protect you.
Mansfield - thanks for the info. I shows that we are still learning.
My point - relevant to the thread subject - is that rain ice can form extremely quickly and is very dangerous. And airframe de-icing may not protect you.
Mansfield - thanks for the info. I shows that we are still learning.
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Holdover times for freezing rain and drizzle are taken from the tables as : " Light freezing rain", "Freezing Drizzle" "Rain on cold soaked wing"
We have no restrictions operationally after the correct proceedure is applied, and just follow Boeing guidlines for thrust, engine run ups etc. I agree it is not to be taken lightly, but it is not an issue in most cases for our operations.
We have no restrictions operationally after the correct proceedure is applied, and just follow Boeing guidlines for thrust, engine run ups etc. I agree it is not to be taken lightly, but it is not an issue in most cases for our operations.
Dan
Don't let the banter get you down
I personally scrubbed many many sorties when faced with light to moderate icing sausage side, just glad I didn't have your irresponsible pressonitis attitude
Oscar said (possibly!) " Sir, a mind is like a parachute......... only useful when open!"
Don't let the banter get you down
I personally scrubbed many many sorties when faced with light to moderate icing sausage side, just glad I didn't have your irresponsible pressonitis attitude
Oscar said (possibly!) " Sir, a mind is like a parachute......... only useful when open!"
Ha ha. Thanks for the support Angel but i can take the 'banter'. To be fair, the forecast on departure only had stratus and drizzle (the icing forecast was issued after we took off), the aircraft didn't have a tendancy to collect ice (this was rain ice and very different) - and it wasn't my pressonitis!
We were taking a four ship to show off our new toys to a foregin nation, but due to the weather, the boss elected to spilt us into two pairs for the arrival with me leading the second formation. Boss and his wing man went ahead, didn't get icing and landed at the intended destination. We ended up elsewhere.
So now we had two aircraft instead of four for the presentation and two crews arriving late at the event by ferry from another country! He wasn't happy and atttempted to 'ream me out' blaming me for maiking him look a pr!ck. (He didn't need me to do that!). We had a blazing row, with the d!ckhead completely failing to realise how close he had been to losing a couple of aircraft with four of his squadron bobbing around in the sea in winter in single seat liferafts!
He gave me a dreadful ACR that year and I redressed him! Wasn't great for my career.
We were taking a four ship to show off our new toys to a foregin nation, but due to the weather, the boss elected to spilt us into two pairs for the arrival with me leading the second formation. Boss and his wing man went ahead, didn't get icing and landed at the intended destination. We ended up elsewhere.
So now we had two aircraft instead of four for the presentation and two crews arriving late at the event by ferry from another country! He wasn't happy and atttempted to 'ream me out' blaming me for maiking him look a pr!ck. (He didn't need me to do that!). We had a blazing row, with the d!ckhead completely failing to realise how close he had been to losing a couple of aircraft with four of his squadron bobbing around in the sea in winter in single seat liferafts!
He gave me a dreadful ACR that year and I redressed him! Wasn't great for my career.
