Several times a year your airline’s training department or flight safety department will release information concerning the upcoming weather season. These bulletins are intended as primers for your review, just to get you thinking about the ever-changing conditions in which you fly.
For instance, prior to winter’s start you can expect to see review information about ground de-icing procedures, operation of the aircraft de-icing and anti-icing equipment, and operating on snow- or ice-contaminated runways. Even a normally simple procedure like taxiing can become a nightmare in blustery winds on a frozen surface, when braking is almost nil and the nose wheel steering capability is little better. You’ll also receive reminders about potential fuel problems (water in the fuel can turn to ice at cold temperatures and clog a fuel system), how to operate the APU if the aircraft is cold-soaked, and proper preheating procedures.
IFR flights in the winter months may face a plethora of meteorological challenges such as snow squalls and blowing snow with associated obscurations, severe turbulence associated with a shifted jet stream, ice pellets, and freezing drizzle. Even in clear weather, snow on the ground at airports can create unexpected problems. As an example, brightness is one of the visual cues pilots use to judge distance at night. Crisp, glittering snow can increase airport luminosity and this can make the pilot believe the airplane is closer to the airport than it actually is, so the pilot initiates the final descent too early, or establishes too steep a glide angle. This can cause an undershoot—or the pilot might add too much power once he recognizes the error, resulting in touch down far beyond the threshold.
Likewise, summertime presents unique challenges to a flight crew, beginning with atmospheric turbulence. Turbulence can be generated in an unstable air mass as hot air rises (convective turbulence) or in a stable air mass due to strong wind shears, also known as mountain wave turbulence or clear air turbulence (CAT).
Convective turbulence typically occurs during spring and summer due to surface heating. The resulting turbulence is usually light, but if moisture and condensation are added to the heating, convection can lead to showers and thunderstorms with associated moderate to severe turbulence.
CAT and mountain wave turbulence begins with an unbalanced set of vertical temperature and wind conditions. Usually this involves a temperature inversion—a warm layer above a cold layer of air—and a wind speed shear through the layers. If a parcel of cold air is forced upward into the warm air, basic meteorology dictates that due to the air density, the cold air parcel will descend back to its original level. Now, if this air mass has a horizon wind component (air wave), the cold air will be displaced not only vertically, but also horizontally. The resulting air mass will become unstable and break, therefore forming clear air turbulence.
A mountain wave is a specialized case of the above, with mountains literally acting as a barrier to impart the initial upward vertical motion to the air. Wind speeds on the windward side of the mountain increase with height, while wind direction remains relatively constant. Mountain waves may develop when:
- Wind direction lies within 30 degrees of perpendicular to the mountain ridges;
- A stable air mass is present near mountain top elevations.
Under the above conditions, the air being lifted up and over the mountain ridge will descend to its original altitude once it passes the ridge, but the momentum of the descending air can make it overshoot its original altitude, giving rise to wave-like oscillations. Depending upon the wind/stability/terrain relationships, the actual wave patterns can range from laminar and smooth to chaotic and turbulent. Generally, from the surface to FL200 you can expect turbulence associated with the mountain wave. Above FL200, the mountain wave can be smooth or turbulent depending on the amount of interaction between upward wave energy and the tropopause. The most intense cruise level mountain-wave turbulence is usually found near the tropopause when:
- The tropopause height is between FL340 and FL450;
- The temperature at or near the tropopause level is colder than standard;
- The tropopause is falling rapidly over the mountain wave zone.
Be alert. Atmospheric waves can form anywhere. I have experienced some of my roughest rides over the Pacific. Atmospheric waves can form between any two layers with significant density and wind speed differences.
There are four basic ingredients that go into making for severe weather: water, temperature (thunderstorms require relatively warm temperatures in the lower layers of the air for the simple reason that warm air can hold more water), a lifting mechanism (fronts, mountains, sea breeze, etc.), and instability.
Thunderstorms pose a significant problem, as they are associated with high winds, shear, turbulence, lighting, downdrafts and hail. I am amazed that in spite of all the recent research and literature on thunderstorm turbulence, many pilots continue to operate on the assumption that they can deal with the clouds and the rain shafts and skirt around the heart of a thunderstorm cell.
There are two basic types of thunderstorms: frontal and air mass. The frontal, or pre-frontal, squall line typically consists of a narrow line of individual storms, while air mass thunderstorms occur randomly over much of the United States. In the summer season, air mass thunderstorms occurring west of the Rockies are most frequent over the higher mountains, deserts, and plateaus. Moisture to feed these thunderstorms can be brought in from as far away as the Gulf of California or even the Gulf of Mexico.
Another type of thunderstorm known as the Mesoscale Convective Complex (MCC), develops rapidly, moves slowly, persists for long periods of time, is circular or elliptical in shape, and can cover several thousand square miles—in other words, it’s the classic summer thunderstorm. Characteristics of MCC include light winds aloft, high moisture content at low levels, low-level warm-air advection, late afternoon development, production of widespread areas of rain, and IFR conditions over the thunderstorm area.
Hail can be one of the worst hazards of thunderstorm flying. Large amounts of hail are found in the taller storms and if hail is encountered it can damage windshields and the leading edges of wings, or cause engine flameouts. For these reasons, it is common practice to avoid flying beneath the anvil of a thunderstorm.
Lightning is associated with any thunderstorm. If an aircraft happens to be in the vicinity of lightning—whether cloud-to-cloud or cloud-to-ground—it may end up acting as a conductor for a lightning bolt. If the temperature band is within -5 to +5 degrees Celsius in a cloud, the possibility for lightning strikes exists. Having encountered a few lightning strikes in my career, the physical damage included holes burned in the metal skin of the aircraft and one shattered radome. One Electra aircraft blew up after a lightning strike caused a fuel tank explosion. New “computer-driven” aircraft are potentially more vulnerable to the indirect effects of lightning since the bolts have an electromagnetic field associated with them that could damage delicate circuitry.
Turbulence is not the only mountain-wave hazard, as smooth waves can generate updrafts as strong as 3,000 feet per minute. Depending on the aircraft design limits, penetrating such a wave could result in aircraft over speed and mach buffet. Therefore, when flight planning you need to consider the potential of encountering mountain waves, and when airborne be alert for visual cues from the clouds that may suggest the presence of mountain waves:
- Cap clouds are low-hanging clouds with bases near or below the mountaintop level, with a relatively smooth top only a few thousand feet above the mountain ridge. At times, a cap cloud will appear to roll over the ridge and down the lee slope like a waterfall.
- Lenticular clouds are stationary lens-shaped clouds, which form in bands parallel to the mountains and may be present as high as 40,000 feet.
- Rotor clouds form under the crests of the mountain waves. These clouds often appear as tubular lines of cumulus or fractocumulus clouds. The base usually is at or below the ridgeline, and the tops may merge with lenticular clouds. The cloud usually appears to rotate, with the upper portion moving forward, while the lower portion moves back, toward the mountain.
Generally speaking, microbursts associated with thunderstorms will occur within the heavy rain portion of the storm. A microburst is usually less than one mile in diameter as it descends from the cloud base, but its downdraft can be as strong as 6,000 feet per minute with horizontal winds of 45 knots, resulting in a 90-plus knot wind shear.
An additional consideration for pilots is that a microburst can intensify for up to five minutes after it strikes the ground, and can last up to 15 minutes before dissipating. However, once microburst activity starts, multiple microbursts in the same general area are not uncommon and should be expected.
Probably the most illustrative case of the fury a microburst can unleash took place August 2, 1985 at Dallas-Ft. Worth International Airport (DFW), when an arriving Delta Air Lines L-1011 was forced down short of the runway.
DFW was forecasting isolated thunderstorms with moderate rain that day. The flight crew reported lightning from a cloud directly in front of them, but decided to continue their approach. Between 550 and 850 feet above ground, the aircraft penetrated the main downdraft of a microburst which came from a Level 4 thunderstorm with a top over 40,000 feet. The rain was falling at a rate of 3.7 inches per hour; horizontal wind shear was 73 knots; the downdraft was 29 knots; and the aircraft experienced a change of wind from a 27-knot headwind to a 40-knot tailwind. The on-board flight data recorder showed within a period of one second the airspeed dropped from 140 knots to 120 knots and the angle of attack increased from six degrees to 23 degrees, with a rapid roll to the right requiring full aileron deflection to correct. At 280 feet above the ground the descent rate was 5,000 feet per minute with a nose down pitch attitude of 8.3 degrees. Then the aircraft entered an updraft while the crew was still trying to recover from the downdraft and had full power and control reversal applied. One hundred and thirty five of the 163 passengers and crewmembers on board died in the crash.
I can recall sitting on the taxiway at Denver several years ago, with my radar painting red in almost every direction. We had declined takeoff clearance and decided to taxi clear of the runway and wait out the weather. I shut the engines down and had the APU running, while we watched the operations of aircraft attempting to land. It started to rain heavily—to the point where we could not see 100 feet in front of us with our wipers on high speed. The tower was reporting wind-shear alerts and a microburst alert with winds of 35 knots—and still the aircraft continued their approaches! One actually landed after encountering a severe downdraft at 500 feet, from which he barely recovered after going well below the glide path.
During the annual proficiency check, most airlines include a review of wind shear and microburst awareness, so it shocks me to know some aircrews continue to attempt takeoffs and landings under such weather conditions. Don’t get trapped in such a condition! If the captain attempts such a maneuver, speak up and protest. If that doesn’t work, don’t be afraid to become assertive—even aggressive—with your objections. I guarantee the flight office will support your position.
Flying the B-737-100 out of Denver on a summer day at maximum gross weight was always a problem. As any pilot knows, as the airport altitude increases and the temperature rises, performance decreases. Couple this with a full load of passengers, luggage, and cargo and you have a recipe for trouble.
It is absolutely critical—especially on the earlier models of the DC-9, B-737, and B-727—to check your takeoff performance on hot summer days at high-altitude airports. You easily could be weight restricted and not have the takeoff performance or single-engine climb performance expected. Your flight dispatch office should be on top of this and will decide if passengers and/or freight has to be left behind to ensure your on-time departure. Still, you always need to remain aware of heat and weight values and how they interact with altitude and available runway lengths.
CAVU conditions, long daylight hours, and clean taxiways can lull you into a sense of complacency after you’ve endured a hard season of overcast, snowstorms, icy runways, and excessive delays. But stay alert, the summer months can be treacherous!
Bob Norris is a retired captain/flight manager for a major U.S. airline. Type rated in the B-737, B-757, B-767, DC-8, and DC-10, he has been an active flight instructor for more than 40 years.
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