The Clean Aircraft Concept
by Phyllis-Anne Duncan
A clean aircraft flies better, right? Is more aerodynamic, right? Pilots over the years have come to believe that regularly washing and waxing their aircraft somehow improves its performance. Every Spring you see lots of aircraft getting their post-winter baths. This may be a myth as far as performance is concerned, but that shiny, clean aircraft surely makes you feel good. We need to make our mythology reality when it comes to cleaning the aircraft of ice, snow, or frost adhering to its control surfaces.
The clean aircraft concept is actually supported by the FAR: FAR ¤ 91.527 (as well as similar sections in FAR Parts 121, 125, and 135) prohibit takeoff when frost, snow, or ice is adhering to "any propeller, windshield, or powerplant installation or to an airspeed, altimeter, rate of climb, or flight attitude instrument system...," when snow or ice is "adhering to the wings or stabilizing or control surfaces," or when any frost is "adhering to the wings or stabilizing or control surfaces, unless that frost has been polished to make it smooth." [FAR ¤ 91.527(a)]
There are many aspects of the FAR peripherally tied into the clean aircraft concept--PIC responsibility, careless and reckless operation, preflight requirements, etc., but perhaps the most important is the fact that the pilot in command has the ultimate responsibility to assure that the aircraft can be operated safely. Recognizing that even small amounts of ice, snow, or frost can degrade aircraft performance unpredictably means you're beginning to accept that responsibility. Accepting the consensus of the aviation community that the best way to verify the condition of your aircraft before takeoff is to perform a visual/hands on inspection indicates that you are living up to your responsibilities. Not taking off with snow, ice, or frost on the aircraft means you've fulfilled your clean aircraft concept responsibilities for that flight.
FAR ¤ 91.527 is contained in FAR Part 91, Subpart F, which applies only to large and turbine-powered, multiengine airplanes, but pilots of smaller aircraft under FAR Part 91 can use the definitions and prohibitions in FAR ¤ 91.527 to enhance the safety of their winter aircraft operations. In this article--Part One of a three-part series--we'll outline the clean aircraft concept and encourage pilots of small aircraft to avail themselves of de-icing and anti-icing procedures when practical.
Understanding the Need
Before you can fulfill your PIC responsibilities, you must understand the need for a clean aircraft. This involves acquiring knowledge of the adverse effects of ice, snow, or frost on aircraft performance and flight characteristics. Degradation of performance is wide-ranging, unpredictable, and dependent upon make and model, but some of the typical problems are decreased thrust, increased drag, decreased lift (alteration of airflow over the airfoil), increased stall speed, trim changes, and altered stall characteristics and handling. All in all, nothing you want to mess with.
Additionally, the pilot committed to the clean aircraft concept needs to have knowledge of:_ Various and appropriate ground de-icing and anti-icing procedures, including the use and effectiveness of freezing point depressant (FPD) fluids_ Capabilities and limitations of these procedures in various weather conditions_ What the critical areas of the aircraft are, i.e., wings, props, and control surfaces, as well as airspeed, altimeter, rate of climb, and flight attitude instrument systems Takeoff should not be attempted in any aircraft with contaminated surfaces, and the PIC is responsible for determining that the critical components listed in the FAR are free of frozen contamination. Operators under FAR Parts 121, 125, and 135 have to develop specific procedures for the PIC to follow to assure this. Those procedures could include qualified ground personnel and adequate equipment and supplies to remove frozen contamination. (Advisory Circular 135-17, "Pilot Guide: Small Aircraft Ground Deicing," contains information that will help pilots to understand aircraft ground deicing issues. It defines frozen contaminants and how they can affect performance and flight characteristics. What follows in this article is general information. For aircraft type specific procedures, consult aircraft flight manuals or other manufacturer documents for that particular type aircraft.)
Frozen Contaminants and their Causes
Ice, snow, and frost are frozen contaminants, and they can form and accumulate on exterior aircraft surfaces on the ground. Weather causes this accumulation as do ground operational conditions conducive to icing. In either case, atmospheric conditions vary the type of accumulation, the amount, etc. Generally, icing conditions (during flight or ground operations) occur and ice protection systems or procedures should be activated when the outside air temperature (OAT) is below 50_F (10_C) and visible moisture is present or when there is standing water, ice, or snow on runways or taxiways.
Aircraft in flight experience a variety of atmospheric conditions which alone or together can produce ice formations on the aircraft and its components. These conditions include:_ Supercooled clouds.
These are clouds containing water droplets that have remained in the liquid state even though the ambient temperature may be below 32F.
These droplets are very small (five to 100 microns), and they freeze on impact with another object. Water droplets have remained liquid even at temperatures as low as -40F.
Cloud liquid water content, ambient temperature, droplet size, and the aircraft's size, shape, and velocity all contribute to the rate of accretion and the shape of the ice formed. (One micrometer or micron is one millionth of one meter or .00003937 inches.) Ice Crystal Clouds.
These clouds exist at very cold temperatures where their moisture has frozen to the solid or crystal state. Mixed Conditions. These clouds have an ambient temperature below 32_F and contain a mixture of ice crystals and supercooled water droplets.
Freezing Rain and Drizzle. These are precipitation that exist within or below clouds at ambient temperatures below 32F. Rain droplets remain in a supercooled liquid state. Freezing rain is different from freezing drizzle only by virtue of droplet size. (Rain droplets range from 500 to 2,000 microns, and freezing drizzle droplets are less than 500 microns in size.)
Aircraft on the ground are susceptible to many of the same conditions as in flight even when they are parked or when they are operating on the ground. There are also conditions specific to ground operations. On the ground, the aircraft is exposed to:
Frozen precipitation--snow or sleet
Residual ice from a previous flight--usually on the leading edges of wings, the empennage, trailing edge flaps, etc.
Moisture, slush, or snow on ramps, taxiways, and runways--which can remain in place on the aircraft if the temperature is low enough; particularly susceptible to this kind of frozen contamination are wheel wells, landing gear components, flaps, undersurfaces of wings, horizontal stabilizers, etc.
Supercooled ground fog and ice fog--much like supercooled clouds and caused by advection or nighttime cooling
Snow blown by ambient winds, other aircraft, or ground support equipment; the source can be snow drifts, other aircraft, buildings, etc.
Recirculated snow--whipped up into the air by engine, propeller, or rotor wash
High relative humidity with temperatures below the dew or frost point can cause frost; this is common during overnight storage after descending from higher altitudes, especially on lower wing surfaces in the vicinity of fuel cells
Frost--a crystallized deposit formed from water vapor on surfaces at or below 32F
Polished frost--Takeoff is allowed by FAR Part 135 and other rules for small aircraft if the frost has been polished smooth; FAA recommends removing frost, but if you choose to polish it, use the aircraft manufacturer's procedures
Clear ice-usually around integral fuel tanks, difficult to see, and usually detectable only by touch or ice detector
Other Locations of Frozen Contamination
There are areas of the aircraft other than the ones we've mentioned where frozen contamination can accumulate and not be detected except by careful, visual inspection. Some of these areas are not found in smaller, general aviation aircraft, but pilots of those aircraft can get an idea of how extensive a visual examination has to be.
Anti-icing fluids may not reach areas under leading edge slats and portions of trailing edge flaps. Without a protective film of anti-icing fluid, these areas may be exposed to icing during precipitation or high relative humidity when taxiing, waiting for takeoff, or when in a takeoff configuration.
Residual ice, in particular, from previous flights can "hide" on the leading edges of wings, on the empennage, in slotted flaps, engine air inlets, etc., of arriving aircraft. If not discovered and removed, residual ice can then affect aircraft performance and handling characteristics on takeoff after turnaround.
During ground operations, propellers and other rotating components are exposed to icing-forming conditions similar to those in forward flight. Some aircraft require operation of inflight ice protection equipment when operating on the ground.
Effects of Contamination
Ice, snow, or frost, with the thickness and surface roughness of medium or coarse sandpaper, on the leading edge and upper surfaces of wings can reduce wing lift by as much as 30%! Drag can increase by 40%!
Such changes in lift and draft can greatly increase stall speed, reduce controllability, and can even alter flight handling characteristics. As the frozen contamination gets thicker and rougher, the adverse effects also increase, and, in addition to the stated effects on lift, drag, stall speed, and performance, the aircraft's inherent stability and control can be lost. Without warning, the aircraft can depart from the commanded flight path. Consequently, it is essential that the pilot not attempt takeoff unless the PIC has made certain these critical surfaces and components are free of frozen contaminants.
Snow, frost, slush, and other ice formations can cause undesirable air flow disturbances or can restrict air and fluid vents. Mechanical interference can also occur, resulting in restricted movement of flight controls, flap and slat operation, landing gear mechanisms, etc. Ice formation on turbine engine and carburetor air intakes can cause power loss. If the ice dislodges a turbine engine may ingest it, and engine damage or failure can occur. Ice on external instrumentation sensors (pitot/static ports, angle of attack sensors) can result in improper indications on cockpit instrumentation or improper operations of certain systems.
Summary - Cold Weather Preflight Inspection Procedures
No one looks forward to preflighting in the cold, and sometimes pilots will cut the preflight short for the sake of saving fingers and toes from frostbite. When it comes to icing, abbreviating your preflight could be deadly. Conducting the complete, normal preflight inspection offers the opportunity to detect ice, snow, frost, slush, etc., which has accumulated on your aircraft from any of the ways we've already discussed.
A thorough preflight is actually more critical in extremely low temperatures because of the degradation of performance that can occur from frozen contamination on critical surfaces. Even though the preflight may make you physically uncomfortable, wintertime is the time to do your most thorough preflight.
The areas requiring special attention during a cold-weather preflight depend on the aircraft's design. FAR Part 135 and 125 certificate holders will identify these areas in the training program they are required to develop. Borrowing from what's required for that training program, general aviation pilots should pay particular attention to:
Wing leading edges, upper and lower surfaces
Vertical and horizontal stabilizing devices, leading edges, upper surfaces, lower surfaces, and side panels
Lift/drag devices (e.g., flaps)
Spoilers and speed brakes
All control surfaces and control balance bays
Engine inlets, particle separators, and screens
Windshields and other windows necessary for visibility
Exposed instrumentation devices, e.g., angle-of-attack vanes, pitot-static pressure probes, static ports_ Fuel tanks and fuel cap vents
Cooling and APU air intakes and exhausts
Landing gear If you know or suspect that the aircraft has been subjected to blowing snow, check any openings where snow can enter and freeze. In addition to and including the above, check:
Pitot tubes and static system sensing ports
Wheel wells/wheel pants
Engine air intakes and carburetor intakes
Elevator and rudder controls
Now, what to do if you discover frozen contamination on your preflight? Remove it.
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