Well, there's no escaping it now. Minnesota has had several days of snow, although none of it lasted more than a few hours; it's currently 12 degrees in Minneapolis. When I got done with a trip yesterday morning, my short walk from the bus stop to our apartment froze my fingers and ears so thoroughly that I didn't venture outside again all day. My six month hibernation begins now.
If I could spend the entire winter curled up inside with a warm blanket and hot chocolate I very possibly would, but alas, the need to earn a living saves me from being a total recluse during winter. Being a pilot is a good job for someone who hates long cold winters but lives in Minnesota, as it gets me above the clouds into sunshine rather frequently and also involves the occasional layover in Phoenix or San Antonio. Unfortunately two of RedCo's three hubs are in northern states, so I do spend a lot of time operating in wintry conditions. This rather frequently involves deicing. I deiced for the first time of the season about a month ago. Given that I hadn't done it since last winter, I took that as my cue to open our Deicing/Anti-icing manual and study up.
As I mentioned in my last post, anti-icing systems on jets are beautifully effective in flight but provide no protection on the ground. For that, we use propylene glycol-based deicing fluids somewhat similar to the antifreeze fluid used in your car. Similar fluids have been used in aviation for quite a while, but for a long time they were poorly understood and their use was nonstandard throughout the industry. After some notable accidents involving airliners attempting takeoff with iced-up wings, the FAA got serious about ground deicing. Now every Part 121 carrier is required to maintain and distribute an FAA-approved Ground Deicing/Anti-icing Manual. It spells out in great detail the various ground and flight crew roles and responsibilities, what parts of the aircraft must be "clean" for takeoff, approved methods of removing contamination, the limitations of various anti-ice fluids, and the checks that must be accomplished before an airplane may take off.
As the name of the manual suggests, there are two stages to ground deicing/anti-icing. Deicing is intended to remove any previously accumulated contaminants that are adhering to the airframe. This may include frost, freezing rain, snow, or airframe ice from the last flight. Anti-icing prevents further accumulation of freezing precipitation between deicing and takeoff. In very mild conditions, both of these steps may be accomplished in a single application of deicing fluid, but more often the steps are accomplished with separate coats of different fluids.
The deice fluid used to remove previous contaminants is called Type I fluid. It is fairly thin, slippery, and is dyed red to help deicing personnel see which parts of the aircraft have already been sprayed. Type I fluid is normally heated between 130 and 180 degrees F, and is sprayed out at considerable pressure to aid in knocking contaminants off the airframe. As Type I fluid isn't very viscous, it doesn't adhere to the airframe well. It also has a somewhat limited ability to absorb moisture. For both of these reasons, Type I fluid is used for anti-icing purposes only for short periods or in very light ground icing conditions.
The more commonly used anti-ice fluid is called Type IV. It has thickening agents added and is dyed green. Type IV is always applied cold after a previous application of Type I fluid. It is sometimes diluted with water, not only to save costs but also to (rather counter-intuitively) lower its freezing point. A solution of 75% fluid and 25% water has a freezing point of around -55C; undiluted fluid has around the same -30C freezing point as a 50-50 mix. Type IV fluid's greater viscosity helps it to better adhere to the airframe. It is designed to shear off at around 100 knots airspeed on the takeoff roll, leaving a clean wing. It also has greater moisture absorption capabilities than Type I fluid. All this means that it protects the airframe against contamination for much longer periods than Type I fluid.
The most common apparatus used to spray deicing fluid is a boom truck. The truck drives around the aircraft while the deicer sprays the fluid from a basket on top of the boom. Both Type I and Type IV fluid can be and often are sprayed from the same truck in subsequent applications. A fairly recent development is infrared deicing. Infrared deicers melt existing snow and ice off the airframe as the plane taxies through a hangar-like structure. This arrangement still requires a truck to spray Type IV fluid for anti-ice protection. There are only a few infrared deice installations in the US.
Occasionally ground crews will deice aircraft before the flight crew arrives in the morning if there was frost or snow accumulation overnight but ground icing no longer exists. Usually, though, crews must request deicing. At small stations, the ground crew needs advance warning to make sure the truck has enough fluid and to heat it up. If there are any contaminants adhering to the airframe, or if there's steady snowfall, the decision to deice is an easy one. It's less clear when the temperatures are just warm enough that the snow is melting on contact, or if flurries aren't quite sticking to the airframe. You don't want to deice unnecessarily due to the time and cost involved, but you also don't want changing conditions to make you come back for deicing at the last minute either.
At outstations, I'll inform operations or a member of the ground crew that we need to deice at least 30 minutes before departure. At most of these airports, we get deiced right after pushback from our gate, before we've started our engines. At the smallest stations, the same rampers that just loaded the bags and pushed us back will then hop in the deice truck. At large northern outstations and hub airports, there are dedicated deice pads located near the end of the runways and manned by a virtual army of trucks and staff during winter weather. At Minneapolis, there are four deice pads that can each handle half a dozen aircraft simultaneously! The usual drill is to call "The Iceman," or central coordinator, as soon as you know you'll need deicing. They'll either assign you a pad or dispatch a truck to deice you near the gate when you push back. If assigned a pad, you confirm it with the Iceman when you begin taxiing and then contact that pad on a discrete frequency to be assigned a lane. You usually deice with engines running at deice pads.
Before the crew begins deicing, the deice coordinator will plug into the ground intercom or transmit via radio to confirm what type of fluid we want and that we are configured for deice. This involves ensuring the flaps are retracted, the stabilizer trim is full nose-down (to prevent fluid from entering internal mechanisms), and turning off all the bleed and pack switches. This last step is especially important to prevent passengers from breathing atomized glycol. Unlike ethylene glycol (antifreeze), propylene glycol is supposed to be fairly non-toxic - but it's not exactly pleasant to inhale, either. After deicing is complete, we wait a full minute before selecting bleed sources on, and another minute after that before turning the packs back on.
Once the process is complete, the deice coordinator will tell us the type and amount of fluid used, the fluid/water mixture and freezing point, and the time that fluid application began. The last bit of information is especially important, for it is the time that our holdover time is calculated from. Holdover time is the length of time that the fluid can be expected to provide anti-ice protection in active ground icing conditions. It varies by type and mix of fluid, type and intensity of precipitation, outside air temperature, and a few other factors. We use a series of tables to calculate our holdover time for the given conditions. The tables give us a range of times; for example, with moderate snowfall at a temperature of -5C, Type I fluid has a holdover time of five to eight minutes and undiluted Type IV has a holdover time of 20 to 40 minutes. The Captain chooses a holdover time within this range that most closely corresponds to prevailing conditions, and may adjust the time upward or downward in response to changing conditions. The holdover time begins when the application of the last coat of anti-ice fluid begins; takeoff must be commenced before the holdover time expires.
Of course, in any weather that makes holdover time a factor, there are likely air traffic delays that may hinder a timely takeoff. ATC is usually pretty good about asking pilots their holdover expiration and resequencing aircraft if necessary. Sometimes best efforts aren't enough and the holdover time expires. Different airlines handle this in different ways. Some require the aircraft to return for secondary deicing. Others, including NewCo, permit the use of a Pre-Takeoff Contamination Check. This is a tactile exterior inspection conducted by trained deicing personnel to confirm that the aircraft is still clean and the fluid has not failed. If the aircraft passes this check, it is still allowed to take off so long as the takeoff happens within five minutes of the check. Close coordination between the crew and operations is necessary to ensure there are personnel available at the end of the runway in case holdover time expires.
In any case, shortly before takeoff the flight crew must perform their own Pre-Takeoff Check to ensure the fluid has not become saturated with moisture. Basically you just look at the few airframe parts visible from the cockpit and observe the sheen of the fluid. If it is dull and milky, the fluid has failed and the aircraft must be deiced again. If it's still glossy, then you're set to go. Hopefully you're headed South where you can thaw a bit. Unless they're having a cold snap, that is; the only thing worse than winter weather in the North is winter weather in the South! While Minneapolis and Detroit are true pros at fast, effective deicing, the same is decidedly not true of places that only see winter weather once or twice a year. They simply aren't equipped or staffed for it, and there's always a learning curve.
Portland is one notable exception. They're blessed with fairly mild winter weather but when they do get it, it's in the form of truly nasty ice storms. Accordingly, Horizon puts heavy emphasis on training and equipping their PDX deicing crews for those days. In the end, it's usually all for naught; they soldier on for a few hours before freezing rain closes the airport entirely and grounds a large portion of Horizon's fleet. A few years ago a friend of mine was the FO on the last flight that landed before a particularly nasty ice storm closed PDX for several days. In the last few minutes of the flight, they encountered heavy freezing rain; by the time they landed, every inch of the airframe was encrusted in several inches of ice! The deicing team's last act before conceding to Mother Nature was to deice my friend's CRJ so they could get the main cabin door open to deplane the passengers.