Severe Weather Flying

For although we'd all like to say that we would never fly in the worst of conditions, the term "severe" itself is subjective, and the ability to navigate around it depends in part on the equipment on board the aircraft and the experience of the pilot. In the real world there are often cases where we must choose whether and where to fly when conditions might become severe, or when severe conditions are nearby but safe passage may still be made with knowledge of how adverse conditions form, evolve and move.

In the preface to the third edition of Severe Weather Flying (Aviation Supplies & Academics, Inc., 2002), author Dennis Newton lays the groundwork for the book. "As any prudent person would probably guess," he writes, "it is mostly a book about how not to fly in severe weather." Newton's goal is to provide readers with the knowledge needed to anticipate and avoid severe weather. A degreed meteorologist as well as a weather research pilot and flight instructor, Newton goes to great lengths to "characterize weather elements!which are so complex that they defy characterization, without resorting to mathematics or without so many except for's and whereas's that the thing degenerates into technical obscurities." His focus is to "worry less about true in tedious detail" and to hone in on what is "useful." As he puts it, Severe Weather Flying is "primarily a book written for pilots by a pilot."

Earlier editions of Newton's work, in continuous print since 1983, could not cover the dramatic changes in weather information gathering, forecasting and dissemination that have come about with the advent of the Internet. A number of landmark aircraft accidents involving icing, turbulence and thunderstorms have promoted changes in pilot education and scientific research regarding weather avoidance. In-cockpit technologies bring increasing weather sophistication to the cockpit. The most significant change in the third edition, however, is that earlier editions focused primarily on the Visual Flight Rules (VFR) pilot and the less-experienced Instrument Flight Rules (IFR) aviator. Newton comments that "there is still a good bit of that [material] here. However, [a] change that has come about in the 1990s is the rapid increase in the rate at which pilots have advanced from piston engine equipment into turboprops and often into jets." The author has "therefore tried to balance the book!so that it will be of benefit to pilots flying this more advanced equipment." The result is a very readable and eminently informative pilot's guide to the worst of flying weather.

The Basics
All understanding comes from knowledge of the basics. Newton begins his book by detailing the elements that make up severe weather: water, temperature, lifting action and stability. These "Four Fundamentals" act together and in unison to create all weather phenomena---and the effects are often cumulative. In particular, from a pilot's standpoint, "water is the enemy". So knowing where the water is (in the atmosphere), a pilot can begin to predict where severe weather may form. "When you look at any sort of weather chart," Newton teaches, "ask yourself where the water is. What are the dew points? Are the winds coming from dry land, or from a source of moisture?"

The final form that moisture takes in the air depends on three additional factors:

In combination, the Four Fundamentals can produce truly dramatic weather events.

Theory Into Application
How do we take this elementary knowledge and use it to be safer pilots? How can we avoid canceling flights just because of the threat of storms or ice, and instead intelligently plan a flight to avoid severe weather hazards? Newton steps readers through discussions of various types of severe flying weather, from the standpoint of development, growth and movement of each.

Air-mass thunderstorms. Air-mass thunderstorms are those that pop up away from fronts, on warm days when the air mass is moist and unstable. The lives of individual air-mass storms are brief, sometimes lasting as little as half an hour from beginning to end, although if conditions are widespread and enough moisture is present the general area may be visited by storms for some time. Specific air-mass storms may be difficult to detect in aviation weather reports. "The distance between primary weather stations is about 100 miles, on the average" says Newton, "and the distance between upper-air [observation] stations is more like 200 miles. Surface observations are made hourly, but usually only every 12 hours at the upper-air stations. This essentially limits [air-mass thunderstorm] forecasting to a statement that storms are probable in an area much larger than the storms themselves." In other words, air-mass storms can slip through the weather-reporting cracks. "If you don't have radar or a lightning detector" on board your aircraft, "you must use your eyes, and what you know about moisture, stability, temperature, and lifting, to fill in the gaps."

Newton details the causal factors in air-mass thunderstorm development, the life cycle of air-mass storms, and the hazards present in each phase of that life cycle. He proposes techniques for avoidance based on an understanding of the factors that create air-mass thunderstorms.

Steady-state thunderstorms. Newton calls steady-state thunderstorms the "Mama Bear" to the air-mass thunderstorm's "Baby". As he puts it, "the steady-state storm is a genuine killer." This is not to suggest it's safe to fly through lesser storms---"no airplane can be said to be certified for flight in thunderstorms"---but Newton warns that the steady-state storm will typically be much stronger than air-mass storms.

"To get a steady-state thunderstorm," Newton writes, "we have to take the brakes off. We must get the water out of the updraft. The simply way for this to occur is to have the storm develop in an environment in which the wind changes with height!. The more the growing thunderstorm leans, the more water will go elsewhere than right back down through the updraft [and thus reducing the lifting action's force], and the more the storm can grow." Newton discusses ways to evaluate the severity of thunderstorms by correlating hail, wind shear and severe turbulence to a sloping radar profile of the thunderstorm cloud. He gives five rules for avoiding steady-state thunderstorms, punctuated by the first rule: "When in doubt, always treat [a thunderstorm] as a steady-state storm."

Severe thunderstorms. If there's a Baby Bear and a Mama Bear, there must be a Papa Bear---and that's what Newton calls the "full-blown severe thunderstorm." Severe thunderstorms, identified by high winds, torrential downpours and massive hail, and possible tornadoes "require large quantities of moisture and deep unstable layers to form."

"Once the moisture and instability are there," he continues, "and the wind field is favorable for steady-state storm development, all we need is lifting. This lifting is often provided, not by a cold front, as you might expect, but by lifting of the low-level jet stream air over a warm front." Newton goes on to more closely detail the elements that converge to create severe thunderstorms and tornadoes. He then provides very specific things to look for, in radar imagery and other weather products, that indicate a severe storm is forming, all the while strongly encouraging a healthy respect and a wide berth for these most powerful thunderstorms.

More Severe Weather
Other by-products of thunderstorms receive Newton's treatment:

Low-level wind shear. "The downburst," as Newton describes, is defined as a "localized, intense downdraft with vertical currents exceeding a downward speed of 720 feet per minute at 300 feet above the surface." Caught in a downburst unaware, the pilot of even the most capable aircraft might not react in time to recover before impact---borne out by severe air carrier accidents that helped scientists discover the phenomenon, and the smaller yet even more intense microburst. When a downburst hits the ground it veers outward in a circular "outburst", creating the classic Low-Level Wind Shear pattern and the Low-Level Wind Shear Alerting System (LLWAS). Newton discusses research into LLWAS and the FAA's three programs for improving the system at major airports. More importantly, he provides a table of Microburst Wind Shear Probability Guidelines, correlating observed weather phenomena (for example, virga or temperature/dew point spreads) and the probability of dangerous wind shear. And as always, Newton gives specific avoidance techniques, and how to escape if those attempts at avoidance fail.

Lightning. The most obvious indicator of a thunderstorm, albeit not the most dangerous, lightning nonetheless poses a threat. "There have been many, many lightning strikes on airplanes that have done no more damage than a small hole somewhere in the skin," Newton writes. "Once in a while, though, something!happens." In one case, illustrated in the book, "about the last six feet of wing came wide open at the first line of rivets aft of the leading edge. Once in a while also [a lightning-struck airplane] goes down." Newton tells us "lightning protection provisions in modern aircraft are excellent" and research "has certainly proved that a properly protected and maintained airplane can take an essentially unlimited number of strikes. Nonetheless, the power of lightning should be treated with great respect."

Newton details the formative process of lightning and relates that to research conducted in lightning-test aircraft. "More has probably been learned about lightning since about 1980 than in the whole of human history" as a result of testing for aviation applications. He pinpoints the altitudes and areas around storms where lightning most commonly formed, relating it particularly to the freezing level, and describes the types of damage most commonly resulting from lightning strikes. This leads to a list of four recommendations for avoiding adverse effects of lightning when flying in proximity to storms, and a chapter on in-flight thunderstorm detection equipment (radar and lightning detectors).

The Thunderstorm Briefing
Newton provides guidance for obtaining a preflight weather briefing to better understand the severe weather threat. His recommendations for a thunderstorm threat include detailed explanations of the use of the:

Lastly, Newton suggests pilots "look at what the forecast said the weather would be right now," using forecasts from your earlier weather checks as a guide, "and see how the forecast is panning out so far."

Ice
Airframe icing, according to Newton, is "probably the weather hazard second only to thunderstorms in the opinion of those pilots who spend a lot of time at relatively low altitudes." Interestingly, "the average experience of pilots involved in icing accidents was nearly 3000 hours, and over 70 percent of them were instrument rated"---confirming ice is a hazard even to "high-time pilots and sophisticated aircraft."

Newton covers the important aspects of flying under the threat of airframe ice:

Ice formation. "The accretion of ice on the components of an airframe is caused by!factors includ[ing] cloud liquid-water content, temperature, cloud-droplet size, and the size and speed of the ice-collecting object." The shape of accumulated ice depends on airplane speed and water-droplet size. The type (rime, clear or mixed) is also dependent on all factors including aircraft velocity through the air. The location of ice on an airframe is determined by the size of the airframe component ("small parts will collect ice first"; "it is possible to ice the!empennage with little or no icing" on the wings) and the size of water droplets (freezing drizzle or freezing rain, the result of large water droplets, can cause ice to form aft of protected leading edges of ice-protected airframes).

Icing clouds. "If you don't want ice, the first and most important thing to do is stay out of clouds with high liquid-water content. The primary cause of high liquid-water content is lifting of moist air." Newton ticks off a list of cloud types---cumulus, stratus, stratocumulus and wave clouds---and details the specific icing hazard associated with each.

Ice forecasts. Several ice-related commuter airliner crashes in the 1990s led the FAA to turn its focus to improving ice-related flight forecasts. Freezing drizzle and freezing rain, in particular, were labeled "severe icing conditions", and nearly every type of deice-equipped airplane was subject to an Airworthiness Directive that required its Pilots Operating Handbook be amended to prohibit flight in these conditions. Research and development of new and more usable ice reporting and forecasting models resulted in products like the Current Icing Potential and Forecast Icing Potential on the Aviation Digital Data Service Website. Progress, says Newton, "has been much better" than before, while in his opinion it is "still short of the ultimate goal of improving the quality and dissemination of icing weather information!."

Recognizing and avoiding ice in flight. "Avoiding ice, or getting out of it if you have been unsuccessful at avoiding it, really boils down to just two things. Either get to an area where the temperature is warm enough to melt the ice, or get out of the liquid parts of the clouds." In addition to some caveats about temperatures aloft reports and the effect aircraft type and speed has on ice accumulations (and any resulting PIREP), Newton lists several rules for detecting and avoiding the liquid-water portion of clouds.

Effects of airframe ice. "A lot of data has been taken in icing [wind] tunnels and in flight testing," reports Newton. "It is possible to give some ballpark numbers to point out the relative magnitude" of ice-related degradation. For instance, "the weight of ice does not, in itself, present a serious problem." However, "even small buildups of ice on airfoil leading edges can decrease maximum lift coefficient by about 30 percent. Most of the damage to lift is done by the first accumulation!even a small buildup will significantly reduce the angle of attack at which an airfoil will stall." Settling an argument about whether it's better to keep the propeller(s) clean (for thrust) or the wings, "the propeller advocates lose the contest handily, according to the data."

The legality of flight in ice. "Since about 1972, the FAA has been altogether prohibiting newly manufactured airplanes from flight into known icing conditions unless and until they have been tested and shown capable of safely operating in the icing conditions specified for transport category airplanes!." Earlier airplanes "were in many cases neither certified nor restricted" from flight in "known" ice. This topic is hotly debated (forgive the pun) and the whole concept of what constitutes "known ice" is under review by the FAA.

The Icing Weather Briefing
As he did with thunderstorm flying, Newton provides guidance for a preflight weather briefing when icing is a threat. He recommends referencing:

In other words, Newton says, it's "the Four Fundamentals again." He asserts that the state of icing forecasts is "not very good." It takes "judicious preflight planning" and knowing "what to do to stay out of [ice], or to get out of it" if encountered in flight. Newton completes his discussion on airframe ice with a chapter each on the operation of ice protection equipment and the process of certifying an aircraft for flight in icing conditions.

Summary
At 187 pages with copious black-and-white photographs and illustrations (a few reproduced in color in an Appendix) and a good index, a well-read and earmarked copy of Dennis Newton's Severe Weather Flying third edition belongs on the bookshelf of any serious cross-country pilot.

Postscript
It is sadly ironic that the forward to this book was written by test pilot legend Scott Crossfield, heralded for over a decade as "the fastest man alive" for his pioneering work with X-series aircraft including the X-15 rocket plane. Crossfield writes: "The degree of confidence to operating in areas where severe weather may occur and the confidence to now one's limits is a measure of true proficiency." The legendary test pilot perished when he flew his Cessna 210 into a Level Six thunderstorm over northern Georgia April 19, 2006 while attempting to navigate through an area of intense storms. Crossfield's last flight teaches that even the best among us must constantly re-evaluate our severe-weather decision making. Crossfield's experience and Dennis Newton's Severe Weather Flying provide valuable lessons for all pilots.