By Parvez Dara
MD, FACP, ATP, CFII, and MEI
(Previously published on-line by Aviation Safety and SAFE)
I was barely a pilot, quite private in my actions and decisions on that February afternoon. I was armed with the littlest of experience that comes with the fewest of hours and the largesse of ego. Flying was a new adventure and, even though the tiniest of butterflies fluttered in the middle of my being, they did not overwhelm the logic between go and no- go. I was naïve. I was green. I was inexperienced. Oh, I was Instrument rated all right and my Mooney was fashionably instrument-equipped; yet the bulk of that promise was definitely overrated and under-deserved.
Flying through the ragged clouds on that cold winter’s day for a 200-mile journey, I had the “Weeping Wings” weeping Glycol; yet at 8000 feet, the view of the grey beyond seemed to get “fixed” with the mosaic of a frosty windshield. And then there was panic!
Why did this happen? What went wrong? This was not supposed to happen! It did. I’ll get to what happened later but let me answer the question, why, first. What makes those clouds cover a perfectly fine beautiful airplane with ice? Okay, before we go there, a better question would be what makes the clouds? Stay with me on this one, as it is essential to the storyline down the line.
Clouds form from the moisture they suck up from the earth. This lifting action from heating the earth surface creates forces of evaporation and condensation (the gaseous liquid gets suspended as moisture when it meets the star-dust in the atmosphere which forms the nucleus of every rain drop and snow flake), as the lifted mass of moisture meets undersurface of another mass of air with a different temperature dew point spread. There, the clouds stop building and a soft bubbly flattened stratus layer spreads from here to where the eye can see. The action with the low pressure is somewhat more dramatic; it is concentrated, localized and the lift from the counter-clockwise rotation carries the moisture higher and higher, building the cumulus type clouds that seem suspended in the air as Grecian columns.
What is interesting about the clouds is that they represent a suspension of very tiny droplets of moisture. These droplets are 10-40 microns or less than half the thickness of human hair (Human hair 100 microns and 1 micron = 0.001mm). In the cumulus clouds they can range to 200 microns. Tiny by all standards, they are roiled in the commotion of the agitated state rubbing, deflecting, colliding, and coalescing with their counterparts and in so doing getting larger. The largest ones are present at the top-most layer of the clouds. Here due to gravitational pull and lack of any further development based on the limits of the atmospheric dynamics these micro-droplets float carrying their precious cargo of water. Imagine a snowball gathering more snowy mass as it rolls down the mountain, only here it’s in reverse the droplet gets larger climbing the wave of the lift in the clouds. This is not running down the mountain and getting fat like the snowball, but taking the elevator and eating yourself to obesity before you reach the top floor. So from a practical point of view the most rime icing one can get would be in the top crust of a stratus layer. Above is bright sunshine and below warmer temps and smaller droplets.
As the inexorable climb reaches a colder and colder temperature, the droplet becomes super-cooled when it reaches -20 degrees Celsius. Above -20 degrees Celsius the droplets freeze and as they fall from height once again into the warming temps they turn into super-cooled droplets (SLD).
Before that frozen state however and having no further climb left, the micro-droplet reaches a critical mass of 200-500 microns and with gravity’s help falls down towards the earth as a freezing drizzle. The limits of the size are by virtue of the elevation of the cloud dynamics. In a cumulus cloud these droplets can range as large as 2 micrometers and even up to 6 micrometers but beyond that size it is mechanically impossible to sustain togetherness due to physics of surface tension, as they fall in the form of rain.
Having deciphered that little painful truth let me venture into what makes ice form on the surface of the aircrafts, but first an interesting anecdote to prove the preceding statement about agitation and collision of micro-droplets: Imagine flying through a cold cloud with all its accoutrements of droplets. Now, this massive (relative term) aluminum bodied aircraft flying through, agitating those micro-droplets as it moves in space, causing the droplets to collide and coalesce will by this sheering force cause the once stratus cloud to spit out a brief spill of snow to the ground! (Popular Science October 2010).
But what happens to the aircraft?
Certain conditions are necessary for structural icing in flight: (1) the aircraft must be flying through visiblemoisture such as rain or cloud droplets, and (2) temperature at the point where the moisture strikes theaircraft must be 0° C or colder. Aerodynamic cooling can lower temperature of an airfoil to 0° C eventhough the ambient temperature is a few degrees warmer. So now imagine flying through this thick layer of clouds and the temperatures are between +5 to -20 degrees Celsius, as the super-cooled micro-droplets hit the surface of the cold aluminum or even a cold composite, the friction from this contact results in latent heat generation which raises the temperature of the micro-droplet above 0 degrees Celsius, hence a very small portion of the droplet sticks to the leading edge of the aircraft. If the droplet is large enough, the remainder wanders over the surface; if the droplet is small, the remainder dissipates back into the atmosphere. It is the constancy of the cloud moisture content and the size of the droplets that dictate the form and intensity of icing on the aircraft skin.
Raindrops versus Micro-droplets: Size and Volume
Your next question would be what is with these micro-droplets? I see big splatters of raindrops on my head on rainy days. Those aren’t micro by any consideration. And you are right. The suspended moisture in the clouds cannot sustain the droplet size past 100-200 microns. Once achieving that size, the droplet starts to fall as in drizzle due to the tug of gravity. If the clouds are cumulus type, then in its journey towards the earth as it falls, it merges forces with other’s like itself and grows like a snowball. This raindrop can reach sizes of 2 micrometer and sometimes 6 micro-meters. Now that is your conventional raindrop. And you are thinking well that is still not large enough to what I have felt.
A little painful physics has the answer – size and volume are different. The volume of water content is based on the cube of the radius; therefore, a 2-micrometer droplet will contain 1,000,000 of water content within it compared to a 20-micron size cloud droplet. A 100-fold increase in size increases the volume by 1,000,000 fold. So those pesky raindrops are loaded with water content and they splatter on contact.
Aircraft Ice Accretion
I still haven’t answered the question of ice accretion now have I? So here we are flying though the clouds and those droplets are hitting the surface constantly causing friction, raising the temperature of the colliding molecules of aluminum and water. Slowly, and in incremental steps, the super-cooled droplets hit and lay themselves down as rime ice. Most of the rime ice forms on the leading edges. A steady-state icing research done by NASA on a Twin Otter shows a constant increase in the angle of attack at the rate of 1.2 degrees per 300 seconds in cruise mode and 1.6 degrees in a holding pattern. It is therefore a matter of time before the angle of attack is exceeded due to increasing inefficiency of the elevator and the aircraft stalls if active mitigation is not accomplished.
Rime, Clear, and Mixed Ice
You ask, what about the mixed and clear or glassy ice? Mixed or clear ice is nothing more than abundant moisture and larger droplets. Imagine the droplets getting turgid from coalescing with each other getting heavier. Finally reaching that critical mass they get a call from gravity and start falling. Reaching a boundary layer of colder temperature as they fall especially in cases of a warm front over-riding a cold front the droplets get super-cooled turning into “Freezing Rain” and upon striking the cold surface of the aircraft, instantly freeze and get glued to the surface. Since these are larger droplets, they can and do hit the airfoil (wing) at a point past the leading edges thus disrupting and detaching airflow over the upper surface of the wing and destroying lift – the carrier/floater of all things heavy in the air. Being larger droplets, they form clear ice or with a mixture of small and large droplets a mixed version of the same.
Interestingly, as the super-cooled droplets and the frozen (snow) fall from colder atmosphere to relatively warmer temperatures, the electric charges of the droplets reverse on the precipitate surfaces. The friction between the “colder and warmer temperature precipitates” become negative and positive electrical charged particles respectively, which can create thunder snow with lightening in the winter and of course our trusty thunderstorms in the summer months, replete with microburst, rain shafts and anvils. The concept remains the same; just the variation in temperatures and build-up of clouds makes the difference in outcome. You can get icing in the clouds in the dead of summer at altitude in the clouds as you can near the surface in the winter. It is dependent on the adiabatic rate.
GA and the Carriers
If you are thinking that us mere mortals flying in small General Aviation aircraft are exposed to the physics of the atmosphere suffer the wrath of nature’s indulgence, think again. Icing has been cited as the cause of the American Eagle ATR turboprop crash during a holding pattern over Illinois Another ice-related accident involved an Air Florida Boeing 737-200 at Washington, DC in 1982. And as recently as 2008, a British Airways Boeing 777-236ER was brought down short of the runway in London by contaminated fuel icing in its huge engines with resultant power failure. The meek and the mighty are both vulnerable to nature’s fury.
My Story continued…
You must be wondering by now what happened to me. Well I am here to write this tale, so it went well. The time it took for you to read the rest of the story is the time it took me to fly out of the clouds. The only remnant of that encounter was a 1 inch ice-horn from the leading edge light covers, a cool chill down the back of my spine, and an untimely tremor in my mental peace with words emblazoned in my mind in neon like “Never Again.”
So what do we do about aircraft icing?
Here are all the total effects of aircraft icing:
- A loss of aerodynamic efficiency due to reducing aircraft efficiency by increasing weight, reducing lift, decreasing thrust, and increasing drag.
- A loss of engine power; “Ice frequently forms in the air intake of an engine robbing the engine of air to support combustion. Thistype of icing occurs with both piston and jet engines, and almost everyone in the aviation community isfamiliar with carburetor icing. The downward moving piston in a piston engine or the compressor in ajet engine forms a partial vacuum in the intake. Adiabatic expansion in the partial vacuum cools the air. Ice forms when the temperature drops below freezing and sufficient moisture is present for sublimation. In piston engines, fuel evaporation produces additional cooling. Induction icing always lowers engineperformance and can even reduce intake flow below that necessary for the engine to operate.”
- A loss of proper operation of control surfaces, brakes, and landing gear.
- A loss of pilot’s outside vision.
- A false flight instrument indications; “Icing of the pitot tube reduces ram air pressure on the airspeed indicator and renders the instrument unreliable. Most modern aircraft also have outside static pressure port as part of the pitot-static system. Icing of the static pressure port reduces reliability of all instruments on the system – the airspeed, rate-of-climb, and altimeter.”
- A loss of radio communication: “Ice forming on the radio antenna distorts its shape, increases drag, and imposes vibrations that mayresult in failure in the communications system of the aircraft.”
Knowing this will alert you to the remedies needed and required if you encounter icing and what necessary mitigation strategies you need to have to arrive safely on terra firma. Also the Icing Forecasts are to be taken with a grain of understanding. A Boeing encountering mild to moderate icing would be a “Blizzard” for a GA pilot. So Pireps from equivalent GA aircrafts have equivalency in meaning. Also the position where icing is reported does not stay static, it moves as weather moves therefore the playing field becomes wide open from a single pilot encounter.
And how to get out of the icing encounter:
- Never knowingly fly into known icing conditions even if equipped with anti-icing equipment. The freezing drizzle and or rain can quickly overwhelm all anti-icing functions.
- If you encounter rime icing which is mild with less than 1 inch of accumulation and no deviation in speed or control input keep flying but have an exit strategy.
- If accumulation exceeds your comfort zone of a thin layer to less than 1-inch accumulation, ask for a deviation in altitude of up or down 2000 feet from your current altitude.
- If accumulation is moderate to heavy, change altitude, declare emergency and ask ATC for help in locating VFR conditions.
- If the horizontal stabilizer is iced, fly the aircraft manually to determine the change in control surface function and the required input to determine the degree of lift destruction. The concept is to detect the loss of elevator effectiveness.
- Upon landing with structural icing, do not extend flaps for change in the camber and loss of lift generation. Ice accumulation on the horizontal stabilizer is a potentially hazardous condition particularly during approach and landing. Extension of the flaps can increase the “downwash” that can seriously reduce tail-plane stall margin.
- Anticipate before drastic action has to be taken. In mixed or clear icing conditions even the hint would require you to take immediate action of deviation from altitude upwards to reach warmer temperatures aloft (warm front over-riding a cold front). 90% of pilots gather information through visual cues, which is not reliable for aircraft mechanics. Here anticipation and mitigation of potential risks is a reliable means to flight safety. Accuracy of information regarding the atmospheric flight conditions and the Eigenstate of the aircraft determine the pilot’s ability to predict a potential stall. 89% pilots in simulator training wrongly predicted the stall state of the aircraft in icing conditions.
- Always fly manually in icing conditions since aircraft stall and elevator saturation can occur when the autopilot is engaged in altitude hold state. This altitude hold state with a roll command can saturate the vertical component of lift and lead to a stall in a turn.
- Once the ambient temperatures are below -20 degrees Celsius and the moisture falls as snow, flying is at best bumpy. The snowflakes deflect of the airframe and do not accumulate.
- It is better to see snow and especially freezing rain from the comforts of an armchair.
Always have an exit strategy, a Plan B, an alternate option, another choice. Be Safe.
- Pilot Outlook—Icing
- Cloud Dynamic Structure by Professor Steven Rutledge, University of Colorado, June 3, 2009.
- Detection of the loss of elevator effectiveness due to aircraft icing. Robert H. Miller * William B. Ribbers, Department of Aerospace Engineering, The University of Michigan, Ann Arbor, MI 48109 AIAA-99-0637
- European Aviation Safety Agency: British Airways, G-YMMM Boeing 777-236ER Accident.
- Smart Icing Systems for Aircraft Icing Safety, Michael Bragg et al, Univ of Illinois.
- The Ohio State University American Institute of Aeronautics & Astronautics 27/01/2009
- TP 185 – Aviation Safety Letter
- Aircraft Icing handbook, Version 1, Civil Aviation Authority New Zealand.