Airborne Weather Avoidance<\/p>\n By Captain Shem Malmquist \u00a0Please also see my book, written with Roger Rapoport, available via the following link: \u201cAngle of Attack<\/a>\u201c.<\/p>\n You are on approach with convective weather in the area.\u00a0 Lightning is present, and you can see the storm in front of you has some lightning in it.\u00a0 You are in the \u201cconga line\u201d, and aircraft are landing in front of you with no problems.\u00a0 Your radar is showing just light to moderate rain between you and the airport.\u00a0 Will you continue the approach?<\/p>\n On August 2, 1985, Delta 191 approached the DFW area[i]<\/a>.\u00a0 The ATIS described the weather as benign, scattered clouds at 6,000\u2019, 10 miles visibility and calm wind.\u00a0 There were scattered thunderstorms in the area, and the flight made a few deviations accordingly.\u00a0 At 1756 CDT, ATC transmitted that \u201c\u2026there\u2019s a little rainshower just north of the airport\u2026\u201d.\u00a0\u00a0 The Delta 191 crew told ATC they were at 5,000 feet at 1800. At 1802 they were 6 miles from the outer marker.\u00a0 At 1804:18 the first officer stated that there was lightning coming out of the cloud in front of them.\u00a0 They reached 1,000 AGL at 1805:05, and the aircraft crashed at 1805:58.\u00a0\u00a0 During this same time period, NWS radar showed a level 3 cell off the end of the runway at 1756, which had intensified to a level 4 cell by 1804.\u00a0 But what did the crew see on their radar?<\/p>\n Where do you set your radar tilt control on approach?\u00a0 If you\u2019re like most pilots, it is likely that your radar is set to less than 5 degrees nose up. A tilt setting of 5 degrees nose up will place the radar beam at 5,000 feet above your altitude at a distance of 10 miles (the beam moves 1,000 ft\/degree at a distance of 10 miles).\u00a0 On final, that means that most aircraft will be scanning the weather between themselves and the airport that is below 10,000 feet. If you are flying an aircraft with the \u201cauto\u201d setting, odds are that you are leaving it in \u201cauto\u201d.\u00a0 What altitudes is it scanning?\u00a0 If you do not know the answer, a review of the system manual might be worthwhile.<\/p>\n Why does this matter?<\/p>\n Doctor Fujita, who created the tornado scales, also did a significant amount of research on microbursts.\u00a0 What he found was that during the building stages of a thunderstorm extreme updrafts can literally hold the rain at relatively high altitudes (15-20,000 feet).\u00a0 Eventually the water gets so heavy that it overcomes the updraft and it essentially dumps.\u00a0 It is not unlike one of those big buckets at a water park that eventually reaches the point where it is full enough to tip over and dump its contents.<\/p>\n The column of water is now falling downwards rapidly, and as it encounters warmer air, starts to evaporate.\u00a0 The evaporation causes cooling, which further accelerates the vertical column of water and air.\u00a0 It eventually hits the ground and fans out \u2013 a classic microburst.<\/p>\n Modern Terminal Doppler Weather Radar (TDWR) and Predictive Windshear systems will detect the event when it starts to fan out.\u00a0 Both of those systems are designed to measure horizontal (x-axis) wind shifts.\u00a0 However, they will not \u201csee\u201d the column of water when it is still suspended in the updrafts or when it first starts downwards and has not started to fan out.<\/p>\n NASA contracted with MIT\u2019s Lincoln Laboratory to conduct a study of aircraft thunderstorm penetrations in the terminal area. [ii]<\/a> The study found that flight crews were regularly penetrating severe storms, and the only reason we do not have more accidents Is chance.\u00a0 The probability of a microburst occurring over the close in final approach course of an airport while an aircraft is there is very low.\u00a0\u00a0 Pilot training as well as TDWR and PWS have led to crews avoiding those microbursts that were in the mature stage.\u00a0 However, there is little or no protection against crews flying under the rapidly falling column of water that has yet to start fanning out.\u00a0 Our radar training and the wind shear systems do not address this type of scenario.<\/p>\n To combat this, consider varying the tilt control.\u00a0 If you tilt the beam upwards to the maximum limit and get a red return, but find that the return is showing much less precipitation at lower altitudes, that might be an indication that something is, literally, \u201cup\u201d.\u00a0 There are only a handful of possibilities that would lead to this.\u00a0 One, as has been described, is that the water is (or was) suspended up high due to updrafts \u2013 meaning that at some point gravity will win that tug of war. The second might be VIRGA.\u00a0 That will show rain above, but below it is going to dry out.\u00a0 The last might be hail, where wet rain up higher is freezing, so not showing on your radar when you are scanning the lower altitudes.\u00a0\u00a0 Any of these can be a bad scenario to fly into, so if you see that, be very aware and consider all the indications you are seeing.\u00a0 Proceed with caution.\u00a0 Consider asking ATC what they are depicting and pay close attention to any secondary indications of hazards.<\/p>\n Delta 191 encountered storm that was developing significant weather at higher altitudes, which, once reaching critical mass, would essentially dump the rain downward, along with the significant wind, which would doom the flight.\u00a0 At 1800 CDT, and 5,000\u2019, the flight was about 20 miles from the airport.\u00a0 If, like many crews, they had their radar tilt less than 5\u02da nose up, they would be viewing weather at around the 15,000 foot range.\u00a0 It is probable that this was not high enough to detect the severity of the storm at that time.\u00a0 Tilting the radar higher would have yielded the storm\u2019s deadly secret of having a lot of moisture up high, and little down below.\u00a0 The recipe for a microburst.<\/p>\n What if you\u2019re already in the rain and your radar indications are limited?\u00a0 Unlike airborne units, ATC radar can power through the strongest precipitation without attenuation can be of value if you\u2019re not sure what might be on the other side of the radar echo you\u2019re seeing.\u00a0 The aircraft radar runs about ten times the frequency of ATC radar, and it is much<\/em> less powerful (peak outputs 150 watts vs. the 25KW range).\u00a0 This means that our airborne radar attenuates easily.\u00a0 On the plus side, though, our higher frequency allows the airborne system to \u201csee\u201d smaller water droplets and provide more definition than what ATC can get.\u00a0 We also can focus it to get an idea of the vertical development of the weather.<\/p>\n \u00a0<\/span>Enroute in North America<\/span><\/strong><\/p>\n We want to look for weather that is extending vertically into the flight levels.\u00a0 Use the tilt formula, and find the altitude where the bottom of the beam is first hitting the weather.\u00a0 If you are at FL200 and the bottom of the beam is hitting weather aligned with your altitude, you are probably looking at convective weather.\u00a0 If you are at 10,000\u2019, you will want to rotate it upwards.\u00a0 Let\u2019s assume the weather is 50 miles in front of you.\u00a0 If you are hitting weather with the bottom of the beam 2\u02da above level, you are seeing something that is extending up to 20,000\u2019.\u00a0 If you are 30,000\u2019, you will want to tilt it down<\/em> to ensure that you are capturing the weather.\u00a0 The reason is that the precipitation at FL300 will likely be frozen, and may not be picked up by your radar.\u00a0 Momentarily turning your gain off theCAL position might help with this, but frozen precip just does not reflect very well. It is, therefore, very important to remember that just because you are not showing precipitation at your altitude, that does not mean that you do not have severe weather at your altitude.\u00a0 The rain shaft might stop at FL 250, but the severe weather may extend thousands of feet above that, and just not be reflecting due to it being frozen.<\/p>\n Another scenario that can occur is when the radar is filled with returns, such as the image below.\u00a0 In those cases we might need to reduce\u00a0<\/em>the gain in order to highlight the most significant areas of the storm. In the U.S., Center weather depiction on the individual controllers scope utilizes a Weather and Radar Processor (WARP), which integrates weather data from one or more NEXRAD sites (unfortunately not available outside the U.S.).\u00a0 It does not display anything less than moderate precipitation.\u00a0 Controllers can set their scopes to display weather in four altitude blocks, starting at the surface to FL600, starting at the surface to FL240,\u00a0 FL240 to FL 330, and starting FL 330 to FL600.\u00a0\u00a0 You can ask them to toggle through their altitude blocks to get a better idea about the extent of a storm.\u00a0 Remember they only display weather that is stronger than 30dbz!\u00a0 Below figures outline that more:<\/p>\n Our airborne radar has an advantage where we can get a lot better definition and see gaps that are appearing at our altitude \u2013 just be cautious to ensure that the gap is not just a dry pocket within<\/em> a storm.\u00a0\u00a0 We lose that advantage, however, if we are flying along in the teens in an area of general heavy rain.\u00a0 In this scenario our radar is likely to just depict precipitation in a solid arc of weather across the display.\u00a0 It can be just a little better than useless.<\/p>\n If you find yourself in this situation, flight planned to fly at 10-15,000 feet, stuck in the rain, another strategy might be to continue up to the lower flight levels, to a point that you are out of the rain and able to utilize the airborne weather radar.\u00a0\u00a0 Another choice, and an equally valid one, would be to request ATC\u2019s assistance in this scenario.\u00a0 ATC will not be faced with the attenuation issues, and should be able to identify areas of more intense weather. \u00a0As previously stated, you might consider asking Center to toggle their display to only depict the weather FL 240 to FL330, as anything that was depicting at that altitude is very probably significant convective weather.<\/p>\n ATC weather depiction ability in the U.S. has improved greatly since Delta 191, but one thing that has not changed is the ATC primary function of separating known traffic.\u00a0\u00a0 Telling us about the weather is not ATC\u2019s primary responsibility, but controllers do want to help where they can.\u00a0 Many controllers are not aware of the limitations of our radar displays, and they are also limited by their equipment and FAA legal has placed additional limitations on what they are, and are not, allowed to tell us.\u00a0 Through understanding of what they can depict, we can directly request the information that we need.<\/p>\n Enroute Maritime Tropical<\/strong><\/span><\/p>\n Storms in regions outside the CONUS are different. Contrary to the majority of textbooks and training material provided to pilots, the profile for a storm varies regionally based on a combination of factors that include topography, ocean influences and temperatures, seasonally and the time of day or night. As the profile changes, so must our methods of detecting storms. As an example, storms in areas such as the mid- Pacific and Atlantic, and other regions similar in nature to the Bay of Bengal tend to \u201crain out\u201d at quite low altitudes\u00a0 (generally below FL200) while the storms themselves extend up into the high flight levels. This means that if we are using the radar guidance as is typically outlined in the manuals during cruise flight we are overscanning the area of reflectivity. The areas at the nominal cruise altitudes for most jet transport aircraft is comprised of ice crystals, which have little or no reflectivity. This can also be somewhat true of storms in other regions, including Northern Africa.<\/p>\n Storms over the higher latitudes, both north and south, tend to have a lot of energy but top out at much lower altitudes, thus the \u201ctraditional\u201d guidance of seeking returns around FL250 may not work. On top of that, they may not present as the solid red that most pilots have been trained to expect.<\/p>\n Below is an image of a storm with the radar in auto mode in an oceanic tropical region (warm water). Notice that it does not present as particular threatening as the water is mostly frozen at altitude. When we choose to display a lower altitude with the gain turned up we start to get a better idea of the storm, but still not as threatening as a similar storm over other regions.<\/p>\n And, of course, out the window we get a clear picture!<\/p>\n On some systems pilots will have to switch to manual and work around the software to get the best idea of what to do.<\/p>\n Pilots need to ensure that they have a solid understanding of both how their radar system operates, as well as the way weather manifests in different regions of the world.<\/p>\n [i]<\/a> http:\/\/www.airdisaster.com\/reports\/ntsb\/AAR86-05.pdf<\/a><\/p>\n![\"IMG-20120801-00119\"](\"https:\/\/airlinesafety.blog\/wp-content\/uploads\/2012\/05\/img-20120801-001192.jpg\")
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