The Invisible Killer

There is a weather phenomenon known to pilots which has caused the loss of many lives. This weather phenomenon is called a microburst and it has been compared to tornadoes and hurricanes. Microbursts can significantly affect people or property on the ground, but it is considered to be more dangerous to pilots. The reason microbursts are said to be “invisible,” is because they are difficult to detect.

Microbursts are a form of wind shear. Wind shear is a sudden and drastic change in wind direction, speed, or both within a small area. Wind shear can be divided into two components: vertical wind shear, wind shear measured in the vertical direction and horizontal wind shear, wind shear measured in the horizontal direction. The effects of a microburst are more dangerous when encountered near the surface. When wind shear occurs at lower levels in the atmosphere near the surface it is known as Low-Level Wind Shear (LLWS).

LLWS can form in several ways, but most of the time thunderstorms are the cause of the most severe type of wind shear, if under the right conditions. Certainly changes in vertical motions constantly happen through the life cycle of a thunderstorm. During the dissipating stage, the final stage of the thunderstorm cycle, the thunderstorm dissipates due to the overcoming downdrafts (downward moving–negatively buoyant air) that can reach approximately 6000 feet per minute.

Thunderstorms create the anvil–like shapes during the dissipating stage. The anvil–like shape is due to the weakening of vertical motions near the top of the storm and downdrafts overcoming the updrafts and dissipating the cloud from the bottom up. Downdrafts are formed by hydrometeor drag and evaporative cooling.

Precipitation (water and hail) drags air downward contributing to the generating of the downdraft is known as hydrometeor drag.  However, the biggest contributor to the downdraft formation within a thunderstorm is evaporative cooling.

Evaporative cooling is evaporation of rain and cloud drops or sublimation of ice crystals, making the air parcels colder than their environment and thus negatively buoyant. As cloud droplets becomes heavy enough to overcome the updraft and fall, they travels into a warmer environment, evaporating (changing state from liquid to gas) or sublimating (changing state from solid to gas) by taking heat away from its surrounding and cooling the air around. Entrainment, mixing in of drier air occurring along cloud edges, is also known to enhance the downdraft strength because it leads to evaporative cooling.

A lot of the mixing of drier air that occurs aloft is due in part by the jet stream at higher altitudes carrying dry air and mixing it with the thunderstorm. This increases the evaporative cooling and strengthens the downdraft and acting as a rear inflow jet by carrying the cold, dense and negatively–buoyant air to the ground.

When the downdraft occurs at the bottom of the thunderstorms, reaches the ground and spreads out, it is called a downburst. Downbursts can be subdivided into two: macrobursts and microbursts. A macroburst is a downburst in which a region of more than 4 kilometers (km) is affected; consequently, a microburst is a downburst affecting a region of 4 km or less.

Macrobursts can last up to 30 minutes with wind speeds of up to 116 knots. Microbursts however, are difficult to detect because they occur over a very small area. Some microbursts have been known to occur in a space of less than one mile horizontally and within 1000 feet vertically. The lifespan of a typical microburst is of approximately 15 minutes and although they are smaller than macrobusts, their wind speeds can be much higher, as high as 145 knots, the same as category 5 hurricane winds.

Dry microbursts also exist. They are harder to identify and are more commonly found in western U.S. in areas where there are dry conditions at lower altitudes and an increase in moisture with altitude. Dry microbursts often occur with less than 0.01 inch of precipitation as opposed to wet microbursts of 0.01 inch or more of precipitation. Virga, rain that evaporates before reaching the ground, is associated with dry microbursts and is present most of the time. Dry microbursts are as dangerous as wet microbursts.

But is this “killer” so “invisible”?—to an extent. Even though there are common systems already being used such as the LLWS Alert System and the Doppler Radar, there are other much simpler ways for pilots to detect and warn themselves of possible LLWS and/or microbursts as a last resort.

Taking a closer look at microbursts, there are some features that can be used to identify them. These features include some of the types of clouds generated by a thunderstorm as well as the use of a sounding.

A shelf cloud or arcus cloud is a cloud that forms as a result of the warm air lifted by the gust front of a thunderstorm and can be an indication of a microburst in the region. The downdrafts within a thunderstorm that reaches the surface as downbursts (microburst) transports the cold, dense, negatively–buoyant air and high wind speeds from higher altitudes to the surface from the rear inflow jet. Areas, sometimes called “pools” of cold air at the surface are known as the thunderstorm’s outflow. The storm’s outflow can be identified as small scale cold front, known as a gust front, decreasing temperatures at the surface and lifting warmer air.

Behaving as small scale cold fronts, under the right conditions, gust fronts can start the formation of other thunderstorms. Pilots inadvertently flying in thunderstorms can avoid entering a downburst situation if they spot a shelf cloud indicating the existence of a downburst.

Another indication of possible downbursts is the peculiarly–shaped Mammatus clouds. Mammatus are basically "blobs" of cooled air that sink under the anvil as a result of strong evaporational cooling by the interaction with dry air below the anvil. If flying into a thunderstorm inadvertently, pilots can predict the possibility of microburst situation through observing Mammatus clouds as an indication of strong evaporational cooling. However, this is still highly improbable since they form in a very turbulent environment. 

Soundings can also be used not only to predict the presence and strength of thunderstorms and thus the possibility microbursts, but also the type of microburst. “Inverted–V” soundings indicate dry microbursts are possible. This type of sounding indicates dry air at lower levels and moist air at higher levels, the opposite of the conditions expected with wet microbursts.

Quote:

“Success is not measured by what you accomplish, but by the opposition you have encountered, and the courage with which you have maintained the struggle against overwhelming odds.” Orison Swett Marden

3 2 1 Descent

I’ve recently been taught that using the VNAV (Vertical Navigation) function to meet crossing altitude can get pilots into trouble. This for example, in terms of meeting a crossing restriction while descending on an Instrument Approach Procedure (IAP) or a Standard Terminal Arrival Route (STAR). This isn’t new. Pilots should know never to fully trust technology. There have been many incidents and accidents that are as a result of complacency.

Whether using VNAV or not, stabilized descent planning is important. A 3:1 descent is a good stabilized descent to use. Here’s how to plan one:

In order to establish a 3:1 descent and meet a crossing restriction, proper planning should begin various miles before, depending on the altitude to lose. The altitude to lose should be first determined by finding the difference between the crossing restriction altitude and the aircraft’s cruising altitude. Since 3:1 descent is going to be used, the difference in thousands of feet is multiplied by 3 to determine the distance to start the descent before the crossing restriction.

Speed reduction is an additional factor that needs to be taken into account when calculating the distance to begin the descent. Since transport category aircraft can’t reduce their speed and descend at the same time, pilots must level off to reduce speed. A rule of thumb to use to determine speed reduction distance is 1nm/10kts of speed reduction. This is then added to the distance determined to be used for a 3:1 descent.

In addition to finding the distance to start the descent before the crossing restriction, the Rate of Descent (ROD) that will maintain a 3:1 descent must also be calculated. For this we use True Airspeed (TAS), but since wind is always present, Ground Speed (GS) is used instead. The formula (GS/2) * 10 is the rule of thumb used to calculate the ROD. As the aircraft moves through different altitudes and descends to thicker air, GS will be changing through the descent and so the ROD must be periodically recalculated in order to continue to maintain a 3:1 descent. Below are some examples.

Example 1:

An airplane is being flown at FL200 on an arrival procedure and is 40 nautical miles (nm) from its next fix, FIRST. FIRST is at FL120 and the instructions are to cross it at 280 knots. The airplane is currently flying at 300 knots. There is a 40 knots headwind at FL200.

Distance to descend: 20000ft – 12000ft = 8000ft/1000 = 8 * 3 = 24 nm

Speed reduction distance: 300 knots – 280 knots = 20 knots/10 = 2 nm

Begin speed reduction: 26 nm from FIRST

Start descent: 24 nm from FIRST

3:1 ROD: 280 knots – 40 knots = GS = 240 knots/2 = 120 * 10 = 1200 ft. /min (initially)

Example 2:

Pilots flying an airplane at FL380 are instructed to descend to FL310 before passing its next fix, SECON. The pilots are also instructed to cross SECON at 330 knots. The airplane is currently flying at 360 knots. There is a 75 knots headwind at FL380.

Distance to descend: 38000ft – 31000ft = 7000ft/1000 = 7 * 3 = 21 nm

Speed reduction distance: 360 knots – 330 knots = 30 knots/10 = 3 nm

Begin speed reduction: 24 nm from SECON

Start descent: 21 nm from SECON

3:1 ROD: 360 knots – 75 knots = GS = 285 knots/2 = 142.5 * 10 = 1425 ft. /min (initially)


Quote:

“You are not only responsible for what you say, but also for what you do not say.” Martin Luther