Deicing demystified

Understanding the value of pretakeoff spraying.

By Douglas Wilson
Private/Helo. Dir Line Ops, Galvin Flying Services

Deicers such at this Premier MT35P18 deicing a Cessna Citation X, contain 2 storage tanks for both deice and anti-icing fluids. Typically, FBOs operating vehicles of this nature will offer both Type I and Type IV fluids.

In all likelihood, the word "deicing" conjures up the image of a clumsy vehicle in low visibility conditions maneuvering ridiculously close to expensive aircraft in order to spray them with a fluid that, in terms of price by volume, is on par with a small latte.

In initial training, most pilots receive a perfunctory overview of FAA's "clean aircraft concept," which is woefully lacking in, among other things, useful information about the practice of deicing itself.

Furthermore, depending on the FARs under which those pilots eventually operate aircraft, inadequate and conflicting information continues to permeate the regulations, creating an opportunity for winter weather accidents.

Notwithstanding the Dec 2007 crash of a King Air 200 in Salmon ID, which remains under investigation, one of the more prominent deicing-related accidents involving a business aircraft remains the tragic crash of a Challenger 604 that unsuccessfully attempted to depart BHX (Birmingham, England) on Jan 2, 2002.

According to the Air Accidents Investigation Branch (AAIB)-the UK's equivalent of NTSB-immediately after takeoff, the Challenger began to bank rapidly to the left, rolling through 50 degrees some 2 sec after liftoff.

Despite the immediate application of full right aileron and full right rudder, the roll continued and the aircraft struck the ground seconds later at some 111 degrees of left bank and a 13 degrees nosedown.

While the findings of the accident were chilling-a stall of the left wing at an abnormally low angle of attack (AOA) as a result of asymmetrical frost adhesion-most compelling was the failure of the crew to have the aircraft deiced. This decision-made even though an estimated 1-2 mm of frost was adhering to the Challenger's wings-was fatal.

Conflicting regulations

For the pilot community, FARs 91.527, 135.227 and 121.629 are effectively the regulations pertaining to deicing. Truly, it is only the last of these that outlines the requirements of an effective deicing training program.

Most alarming is the first of the 3 regulations, which still to this day articulates the idea of "polished frost" as an acceptable practice. Specifically, 91.527(a)(3) states that no person may take off an airplane which has "any frost adhering to the wings or stabilizing or control surfaces, unless that frost has been polished to make it smooth."

In the wake of the 2002 BHX crash, AAIB recommended that "the US Federal Aviation Administration, and all authorities who follow FAA practice, delete all reference to 'polished frost' within their regulations and ensure that the term is expunged from operations manuals."

Although FAR 135.227 makes a similar reference to the acceptability of polished frost, FAR 121.629 (b) is abundantly clear in its contradiction, stating, "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..."

To add to the confusion, the pilot's operating handbook (POH) or operating manual of an aircraft often provides guidance to a flightcrew that is different, yet understandably stricter, than the FAA regulations regarding frozen contamination.

Aircraft with aerodynamically supercritical wings-such as the Bombardier Challenger 604 or Cessna Citation X-are particularly susceptible to the adverse effects of frozen contamination.

For instance, "Supplementary Procedures" (Cold Weather Operations)" for the Challenger 604 states, "Takeoff must not be attempted if snow, ice or frost are present in any amount on the wings and tail surfaces of the aircraft." Prudence suggests that pilots take a conservative approach and observe the more stringent guidelines provided by the OEM.

That syrupy smell

Regulations notwithstanding, once the choice has been made to deice, the following questions need to be asked. What fluids does the FBO have available? Is the deicing taking place during a continuing event with additional frozen precipitation falling? If so, how long until the aircraft has to be deiced again? These questions are asked each winter, but the nuances can catch a crew off guard.

Bombardier's Challenger 600 series, such as the FlexJet Challenger 604 shown here being deiced by Vail Valley Jet Center at EGE (Eagle CO), is the subject of a recent airworthiness directive requiring a revision to the airplane flight manuals to "include a requirement for flightcrew training regarding enhanced takeoff procedures and winter operations."

Like many things involving aviation, the devil is in the details. In this case, the devil has a name-holdover time (HOT). According to FAR 121.629(3), holdover time is the estimated time deicing or anti-icing fluid will prevent the formation of frost or ice and the accumulation of snow on the protected surfaces of an aircraft.

This time is obviously of critical importance. For Part 121 operators, understanding HOT is a requirement of their approved ground deicing and anti-icing program. However, for a Part 91 operator, without proper planning, the first reminder to review and interpret these tables may come in the middle of a raging northeastern US snowstorm.

In a manner reminiscent of performance tables for aircraft, the "HOT tables" published by and available through FAA can be absurdly complex with their diverse scenarios, but they provide details on every possible combination of factors.

The tables consider outside air temperature (OAT), the prevailing weather conditions encountered, each type of deicing or anti-icing fluid available, the manufacturer's brand of deicing fluid, and the mixture ratios, if applicable, of that fluid.

Since the weather outside the cockpit window is an observable phenomenon, it is important to ask the FBO what types of deicing fluid are available in order to select the appropriate HOT tables.

Typically, most US FBOs that offer aircraft deicing services carry Type I deicing fluid. Some also carry Type IV anti-icing fluid. In its most basic description, Type I is a propylene glycol-based fluid, orange in color, which is mixed with water and heated prior to application to an aircraft.

Ideally, Type I fluid should be heated to a minimum temperature of 140ยบ F at the nozzle prior to application. This heating helps break the bond of the frozen precipitation to the aircraft's surface and of course, lengthens the fluid's holdover time.

In circumstances such as a continuing winter weather event, refilling deicing trucks is a frequent occurrence-as such, crews should be cautious, since it often takes many minutes for a newly refilled deicing truck to heat its fluid to the proper temperature for application.


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