This business aviation blog post continues from our article last week, entitled, “Snow – Its Effects on Aircraft & Runways: Part 1 – Types of Snow.”

For business aircraft operators, there are a number of particular hazards associated with snow and snow accumulation. Best practice is to take advantage of all available forecasting tools to “know before you go.”

The following is an overview of what you need to know about snow:

1. Snow forecasting

Many forecasting techniques and models are used to predict snow. While overall accuracy in predicting snow conditions has improved over the years, it can still be a challenge to accurately forecast snow amounts.

2. SNOWTAMs

The International Civil Aviation Organization defines a “SNOWTAM” as “a special series Notices to Airmen (NOTAMs) notifying the presence, or removal of, hazardous conditions due to snow, ice, slush or associated standing water on aerodrome movement areas.” SNOWTAMs are issued by airport authorities, when warranted, to notify users of the presence or removal of hazardous conditions. Maximum validity period for a SNOWTAM is 24 hours, and new SNOWTAMs are issued whenever there’s a significant change in conditions.The link below provides further information on how SNOWTAMS are issued (courtesy of Skybrary and Eurocontrol):

http://www.skybrary.aero/bookshelf/books/2074.pdf

On METARs and TAFs, snow is indicated by the descriptor “SN.”

Note that even though “SNOWTAM” may be an ICAO listing, it isn’t listed under FAA JO 7930.2P or AC 150/5200-28D. This means that operators shouldn’t expect to find SNOWTAMs in the United States. It’s recommended that operators review the FAA NOTAM information (FAA JO 7930.2P and AC 150/5200-28D) to understand how U.S. NOTAMs handle snow events.

02/05/2015: Updated by reader

3. Hazards to flight

Snow and blowing snow can greatly affect aircraft on the ground, as well as during ascent and descent. When visibility is significantly reduced, as a result of snow activity, aircraft acceptance rates are often reduced. This causes operational delays, and at times aircraft may need to divert to alternate airfields. Significant snow accumulation can impact airport operations, and runways may close for plowing. If the rate of snowfall is such that plowing and treatment are not able to keep runways clear, a particular airfield may be forced to close. For operators landing at airports experiencing snow events there is a concern of snow and slush freezing on the aircraft brakes, so it’s recommended to avoid taxiing through loose snow, slush, or puddles that may adhere to the brakes and freeze.

02/05/2015: Updated by reader

4. Snow accumulation

Snow is designated by intensity, based on rate of accumulation and visibility restrictions. Heavy snow is indicated by “+,” moderate snow has no designation, and light snow is indicated by “-.” As snow is often accompanied by fog, using visibility alone as an intensity descriptor will not necessarily be accurate. Snowfall rates can be calculated, but if snow is melting as it falls or while on the ground, projected snow depth may not be accurate. Pilots need to be aware of these differences.

5. Visibility impact

Snow can greatly affect visibility and has potential to reduce visibility rapidly. This is particularly true in situations in which the intensity of snowfall is increasing. When snow already on the ground is blown about by strong winds, this often leads to rapid reduction in visibility. The following definitions use visibility in determining snow fall intensity. Use these with caution as other factors – such as liquid water content, temperature, and daylight/darkness – also impact visibility.
Below you will see the visibility based on the snow intensity:

• Light Snow: Snow conditions in which visibility is greater than 0.5 mile
• Snow: Snow conditions in which visibility is in a range from 0.25 to 0.5 mile
• Heavy Snow: Snow conditions in which visibility is less than .25 mile

6. Flat-light and white-out conditions

The FAA Safety Library provides a good reference how to avoid white-out situations.

Flat light

An optical illusion also known as “sector or partial white out.” It’s not as severe as a “white out,” but this condition causes pilots to lose depth-of-field and contrast in vision. Flat-light conditions are usually accompanied by overcast skies, inhibiting any good visual clues. These conditions can occur anywhere in the world but primarily occur in snow-covered areas. Flat light can completely obscure features of terrain, creating inability to distinguish distances and closure rates. As a result of reflected light, this condition can give pilots the illusion of ascending or descending when they’re actually flying level. With good judgment and proper training/planning, however, it’s possible to safely operate aircraft in flat-light conditions.

White out

As defined in meteorological terms, is a condition where a person becomes engulfed in a uniformly white glow. This glow may be a result of being surrounded by blowing snow, dust, sand, mud, or water. In a white-out condition, there are no shadows, no horizon, and no clouds, and all depth-of-field/orientation are lost. White-out situations can be severe in that there are not any visual references to depend on. Flying is not recommended during any white-out situation. Be aware that flat-light conditions can lead to a white-out environment quite rapidly. Both atmospheric conditions are insidious. They sneak up on you as your visual references slowly begin to disappear. White outs, in snow-covered areas, have been the cause of several aviation accidents over recent years.

Self-induced white out

Typically occurs when a helicopter takes off/lands on a snow-covered area. Rotor down-wash picks up particles and re-circulates them through the rotor system. The self-induced white-out effect can vary in intensity, depending upon the amount of light on the surface. This phenomenon can occur even on the sunniest, brightest day with good light contrast. When it happens, there can be a complete loss of visual clues. If the pilot has not prepared for this immediate loss of visibility, the results can be disastrous.

7. Dealing with snow events

Operators should play close attention during trip planning stages to the potential for significant snow events. With the advent of today’s more accurate long-range global forecast models, snow events are often accurately predicted seven-10 days prior to operation.

When choosing airport alternates, bear in mind how extensive and impactful a local or regional snow event might be. During winter, Northeastern U.S. airfields – from Washington, D.C. to Maine – may be impacted by significant snow conditions. Keep in mind that in some parts of the world, suitable alternates may be hundreds of miles from your planned destination. After significant snow events, certain airfields may close for several hours. In the case of a major blizzard, an aerodrome may be closed for several days. At many airports, snow can’t be cleared from all surfaces at the same time. This will likely result in having the snow cleared in order of priority (i.e. runways, taxiways, roads, and aprons).

02/05/2015: Updated by reader

Conclusion

Significant snow events, although usually predictable, have potential to develop rapidly. It’s important for flight crews to be aware of how this atmospheric phenomena may impact visibility. In some cases, the best course of action is to divert to an alternate airfield or make plans to operate on different days to avoid the snow event.

Questions?

If you have any questions about this article or would like weather planning assistance for your next trip, contact me at stevearbogast@univ-wea.com.