Friday, March 06, 2015

Ice Ice Baby!

This past week has seen a large part of the country dealing with wintry weather so it seemed like a good time to address icing. Airplanes and ice have always had an adversarial relationship. Ice can prevent airplanes from getting airborne and should they be airborne, ice will do its best to facilitate an airplane's hasty return to Earth, willingly or not.

From the earliest days of aviation, airframe icing was recognized as a threat to flight. Icing will cause problems for aircraft in two ways. The first is the simple weight that icing can add to an aircraft. Many thousands of pounds of added weight from icing on an airframe will increase stall speeds and can prevent an airplane from climbing out of icing conditions.

The second pernicious effect of airframe icing is the addition of drag and the destruction of a wing's ability to create lift. As you'll recall, lift is generated due to the Bernoulli effect with regards to the flow of air over the wing. Faster moving airflow over the wing has lower dynamic pressure than the air passing beneath. This pressure differential generates the lift that keeps airplanes in the sky.

One requirement though is that this airflow must be laminar, or smooth, to work its magic. A coating of ice will destroy the smooth flow of air and result in what is known as boundary layer separation. When this happens, the wing stops producing lift and the airplane drops. As ice progressively coats a wing in icing conditions, the wing's lifting ability decreases and its drag increases to the point where flight is no longer possible.

Even a layer of frost over the top of a wing can have devastating effects on lift. Roughness approximating a piece of #40 grit sandpaper will reportedly reduce lift by 30 to 40%. This loss of lift can produce disastrous results especially during takeoff, which is why icing must be taken seriously.

Ice Can Kill On the Ground

There have been many accidents and incidents attributed to airframe icing over the years. One of the most famous ones was Air Florida 90, which crashed into the Potomac River moments after takeoff in a snowstorm in 1982. While the ultimate cause was determined to be pilot error, the series of errors which led to the crash were caused by the pilots' lack of understanding of the effects of ice on their aircraft.

Specifically, the crew inexplicably failed to use engine anti-icing and also allowed a dangerous buildup of snow to accumulate on the aircraft prior to takeoff. The failure to use engine anti-icing, which heats sensors that determine thrust settings, allowed a false reading from clogged sensors to show that the engines were at full thrust while they were actually set much lower. 

The lower thrust coupled with the added weight and increased drag from accumulated snow prevented the aircraft from being able to remain airborne. It hit the 14th St bridge 30 seconds after takeoff killing 69 of the 74 passengers and crew.

And is Also Deadly in the Air

Ice accumulation while airborne has been a well documented hazard to aviation over the years and also a staple of aviation film drama. Should an airplane fly into what is known as "icing conditions", supercooled rain droplets will freeze on the surface of an aircraft leaving a coating of ice. This coating starts at the leading edge of the wing and slowly travels back over the wing destroying the wing's ability to create lift as it progresses.

A simpler word for "icing conditions" would be cloud. Any time visible moisture is present and the temperature is below freezing, icing conditions are present and airframe icing is possible. Airframe icing is categorized as either "rime"or "clear". Rime icing is opaque in color and easily visible on the aircraft while clear ice is much harder to see and therefore more difficult to detect.

One of the more recent casualties of airborne ice accumulation was American Eagle 4184 which crashed due to icing induced loss of control in 1994. The aircraft, an ATR 72 enroute from Indianapolis to Chicago, had held in freezing rain conditions while awaiting further clearance to O'Hare. While descending to enter a second holding pattern, the pilots retracted the flaps which had been extended for the first holding pattern.

Upon flap retraction the aircraft became uncontrollable, rolling completely at least twice before crashing in a field near Roselawn, Indiana, killing all 64 passengers and 4 crew. The cause of the accident was attributed to a buildup of ice on the wing which only became critical after the flaps were retracted.

Many aircraft now have restrictions against holding in icing conditions with flaps extended as a result.

Clean Aircraft Concept

The mitigation of dangers posed by icing before takeoff and while airborne are two very different problems requiring different solutions, but the end objective is the same: to keep ice off the aircraft. And short of keeping an airplane safely in a warm hangar, solutions to icing have become ever more exotic as the dangers of icing have become better understood.

After many years of trying to come up with a regulatory framework which could be universally and simply applied, the FAA came up with the Clean Aircraft Concept. This formulation left no wiggle room as to how much freezing precipitation could be adhering to an aircraft readying for takeoff:

 The “clean-airplane” concept is derived from U.S. Federal Aviation Administration (FAA) Federal Aviation Regulation (FAR) 121.629, which states, “No person may take off an aircraft when frost, ice or snow is adhering to the wings, control surfaces, propellers, engine inlets, or other critical surfaces of the aircraft or when the takeoff would not be in compliance with paragraph (c) of this section. Takeoffs with frost under the wing in the area of the fuel tanks may be authorized by the Administrator.” 
The FAR also prohibits dispatch or takeoff any time conditions are such that frost, ice, or snow may reasonably be expected to adhere to the airplane, unless the certificate holder has an approved ground deicing/anti-icing program in its operations specifications that includes holdover time (HOT) tables.

The aim of this simple regulation was to put an end to the guessing game of how much snow and ice can safely be on the aircraft for a takeoff. The short answer is none (with occasional frost but only on the underside of the wing). No one would be able to say "oh, it'll blow off during takeoff", or " the exhaust from the plane taxiing ahead of us will melt the snow". The airplane had to be clean. Period.

Don't Drink the Deicing Fluid

Dating to the 1950s and earlier, deicing fluid for use on aircraft was based on ethylene glycol, commonly used as automotive antifreeze solution, or sometimes even ethyl alcohol (the drinking kind). Due to its toxicity to animals, ethylene glycol was mostly replaced by propylene glycol in the 1980s. Ethyl alcohol fell out of favor as a deicer after WWII due to it's popularity as a jaw lubricant with ground crews in Russia and other places. New fluids have been introduced over the years that not only remove ice, but also inhibit further accumulation.

It is important to make the distinction between the terms "deice" and "anti-ice" because they mean different things and the fluids used in each application are also different. The term deice refers to removing existing snow and ice from an aircraft while anti-icing means to apply fluid which inhibits continuing frozen precipitation from adhering to aircraft surfaces.

Specialty fluids have been developed over the years for these two separate functions. For most applications, fluids used to deice aircraft are known as "Type I" fluids while anti-ice fluids are "Types II, III and IV". They function differently.

While Type I fluids are used mainly for deicing, Types II, III, and IV have thickeners included and are designed to adhere to the wing and absorb moisture from additional snowfall or ice accumulations and to then shear off the wing during takeoff. This gives extra time between application and taking off.

This extra time is known as "holdover time" and differs depending on the type of fluid used, its concentration, the type and intensity of the snow or ice coming down, and the outside temperature. We have lots of very complicated charts to figure it all out. If holdover time is exceeded, we go back to the gate and get sprayed again.

A typical Type I fluid will be based on propylene glycol (PG) and will include other ingredients such as corrosion inhibitors, surfactants, or wetting agents and dye. It will usually be diluted with water and heated in the truck to be sprayed on the aircraft.

So as you sit in your window seat you might see the trucks make two passes during deicing. The first pass will be with Type I fluid to deice, while the second pass will be to spray Type IV fluid as an anti-icer. Type IV fluid is green in color and sticks to the wing but is designed to shear off.

Deicing Ain't Cheap

With a quick web search I found a vendor selling DOW UCAR PG Type 1 fluid in a handy 230 gallon pack for $4250. This will typically be diluted 70/30 with water making the solution about $13 per gallon. Keep in mind that it may take up to 500 gallons to properly deice a 737 or A320, two common airliners, so you can see that the process is expensive.

Another facet to consider is what happens to all that deice fluid after it hits the ground. Many environmental jurisdictions are starting to require capture and recycle systems for used fluid which further drives up the costs. Given the thin profit margins of most airlines, it's likely that flights that have been deiced are marginally profitable or unprofitable.

This begs the question of why airlines even fly in snow. Well for one, the airline has no sure way to tell when snow will fall, but the more likely answer is that cancelling flights prematurely is expensive and kills customer loyalty if the competition is still flying. Plus aircraft and crews may also be needed elsewhere.

The new tarmac delay law with it's heavy penalties for long delays certainly contributes to the cancellation equation, but that will have to be the subject of a future post.

Clean or "Cell Phone Clean"

After many years of ambiguity regarding the question of when and how to deice, everyone from the FAA, the airlines, unions, safety administrators and aircraft manufacturers are really on the same page concerning pre-takeoff deicing. The airplane has to be clean to take off. On this everyone agrees.

But in tearing a page from medicine, a new phenomenon of "defensive deicing" is making itself slowly apparent. Airlines managements, while fully onboard with the need to properly deice an aircraft, also don't want pilots to be spraying thousands of dollars worth of fluids unnecessarily. Thus pilots are routinely bombarded with memos to this effect.

Here is where a pilot's and the airlines' incentives may be somewhat misaligned. There are plenty of instances say where flurries may be coming down in windy conditions where no snow may be sticking to the aircraft. In this case it is perfectly appropriate, safe, and legal to depart without deicing.

Pilots also know however that in the back of the airplane are several hundred cell phone cameras with some owners only too eager to snap a picture of a snow flurry for forwarding to the FAA (believe me, I've seen it happen). And the FAA, being the ever loyal guardians of aviation safety, will dutifully send a letter of investigation to a pilot who thought he was doing the right thing advising him to retain a lawyer and to explain his actions.

Having one's livelihood potentially threatened does wonders to concentrate the mind and has resulted in a type of bunker mentality. If one airplane is getting sprayed, they all seem to end up getting sprayed if there's even a flurry still in the air.

And should the hourly weather observation list frozen precipitation at an airport, deicing seems to always continue regardless of whether snow is actually still coming down 45 minutes later or not. And so it goes.

But there's no doubt that a certain measure of over-caution, while an inconvenience, never ended with an airplane in the Potomac.

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Capt Rob