THUNDERSTORM AND WEATHER RADAR

 

 

THUNDERSTORM AND WEATHER RADAR

            My first aircraft, the De-Havilland Canada DHC-3 Otter (1940s design which first flew in late 1951), was the simplest aircraft with few basic instruments. I flew it extensively, single pilot mostly, all across the Western and Northern sector of the country. Weather posed the greatest challenge where we had only our eyes and luck to rely upon. So I was very excited, on my conversion to AN-32 (1993), to have weather radar which would tell me, through color codes, the areas to be avoided. My joy, though, remained short-lived as I frequently got tossed around in weather which my radar showed as benign.  During those early days when I was still at sea with weather radar, I cannot forget a sortie from Agra to Jamnagar. The initial picture painted on the weather radar displayed a continuous 10 mile thick green arch followed by black (no weather) beyond 10 miles. I pressed on hoping to clear the rain patch soon. In the process I flew for about 15 minutes in most turbulent and stormy weather. The ordeal is narrated below as INTO THE EYE OF THE STORM.  Another brush happened while flying in the eastern sector. We were in clouds and the weather radar was not painting anything. I had some premonitions because I heard two commercial aircrafts asking for wide deviations, in the same area, due weather. Frantic manipulation of radar revealed nothing except very light green in front. Fortunately we broke clouds and were face to face with a massive and monstrous built up towering till above 40-45 thousand feet. I immediately deviated like other two aircrafts. The incident troubled me. Why were we not able to pick it up? And if there was no break in clouds, probably we would have entered it. Fast forward 10 years by which I had become little more learned about weather and weather radar. While undertaking Courier flights to Nicobar Islands we could see a massive built up with well formed tentacles and hooks. One glance was enough to tell that it was a most violent area except that the weather radar displayed almost nil returns. There were few green spots on the fringes but mostly the radar showed black. I continued straight ahead after emphasizing to the young pilot that the radar was showing all clear. As I closed, it turned into light green showing mild rain contents. I continued. We were still around 20 miles (visually) from the dark portion when it got wildly turbulent shaking the aircraft violently. Soon there were tiny pebble like strikes on the wind screen increasing in intensity and frequency. I did a sharp turn away from the cloud. Now we were 90 degree to the cloud and the weather radar displayed solid red to our right. “This is one of the many limitations of weather radar called ATTENUATION or SHADOWING”, I pointed out to the young pilot. Radar shadow is a phenomenon when radar energy is unable to penetrate the storm and is attenuated (weakened) so badly that it cannot get back to the receiver to advise the pilot of the real danger.  In explaining this phenomenon I always compared it with Black-holes which are defined by Astrophysicists as “Points in space that are so dense they create deep gravity sinks. Not even light can escape the powerful tug of a black hole’s gravity. And anything that ventures too close –be it star, planet or spacecraft-gets stretched and compressed like putty in a theoretical process aptly known as spaghettification”. Similarly the holes (apparent) in thunderstorms are powerful energy concentrations not easily visible to radar and capable of pounding up any intruding aircraft. Like the gravity pull of black holes these too deceptively lure and trap aircrafts into them.

TRAPPED INTO THE (BLACK) HOLE

            A DC-9 of Southern Airways flying as Flight 242 crashed on 04 Apr 1977 in New Hope Georgia. The aircraft lost both engine thrust when it flew into a severe thunder storm. The aircraft disintegrated while attempting an emergency off field landing killing 72 people including both pilots. The crew mistook the dark portion (hole) between two cells as clear area and entered it. Post accident analysis of radar plot showed that the aircraft went into the largest thunderstorm cell having extreme turbulence and hail.

            A Cessna-421, on 30 Aug 1996, crashed in an open field. Miraculously all occupants survived. In pilot’s own words, “I turned on the on board radar scope and could see rain to the left and right of the path. The centre area of around 15 miles wide was clear on the scope. This was the area that I planned to go through the clouds. Before we reached these clouds about 10 miles before we encountered hail. It first appeared as rain and then within one minute of first encounter a large piece of ice broke through the left windshield.”

            On 07 May 1998 a DC-9 of Airtran Airlines crew during the initial climb after takeoff noticed that they were approaching a line of thunderstorms. The Captain noticed a 10 mile gap in the line that was depicted on the radar and decided to fly through the gap. The aircraft encountered extreme turbulence and hail. An emergency landing was carried out on a nearby airfield. The airplanes radome had separated and portions of it had been ingested into the right engine. All three outer panes of the cockpit front shield were shattered. The wing leading edge devices, horizontal stabilizer leading edge, vertical stabilizer leading edge, and both left and right engine inlet cowls were dented and damaged. Both engine sustained foreign object damage. Overlaying the ground weather surveillance radar pictures on the ac track from ATC radar data showed that the ac penetrated a level 6 echo intensity which is considered as the maximum. These radar echo intensity levels are on a scale of one to six VIP (Video Integrator Processor) level 6 is ‘extreme’ with severe turbulence, lightening, large hail, extensive surface wind gusts and turbulence.

            On Jan 16 2002 a Garuda Indonesia Flight 421 a Boeing 737-300 engines had a dual engine flame out. Engine restarts failed and the aircraft was ditched into a river. The pilots reported that they were attempting to fly through a gap in between two red cells that were displayed on their color weather radar. About 90 seconds after entering the TS as the ac descended through FL180 at flight idle power setting, both engines flamed out.

                        The gap between two convective cells which these aircrafts observed could have been the most intense weather where all radiations were absorbed. Alternatively these could also have been areas of no returns due to absence of water contents. Remember that the weather radar picks up only liquid water. It does not pick up turbulence and hail. But some of the most turbulent conditions are found in area between two convective cells. Hence venturing into such holes is to be avoided like plague. The airborne weather radar is not for penetrating but avoiding thunderstorms.

            The investigation conducted for Flight 242 revealed that the loss of thrust was not due to ingestion of hail and water. The thrust changes resorted by crew led to compressor stall which got further aggravated when high power setting was maintained thereafter. This was happening when the engines were ingesting massive amount of water. Although the exact mechanism of these water induced engine stalls could not be determined, it was felt that thrust changes may have an adverse effect on engine stall margins in the presence of massive water ingestion. Hence when flying in such conditions the crew must avoid thrust changes. While studying engine performance inside Thunder storms it was found that all engine flame out events occurred as the aircrafts were descending with throttles set at a low power setting. A low power setting in combination with high airspeeds, can adversely affect engine operation in intense rain and hail because, typically , there is less centrifugal slinging of rain and hail away from the engine core at slower fan rotation speeds and higher airspeeds. The subsequent evaporation of the water from rain or hail inside the engine will have detrimental effects on the combustion process and drive the engine toward a flame out or loss of power. Engines are most vulnerable when they are at low power and high airspeed. In this combination lot of water and hail make way into the core of the engine disrupting the normal working. Increased power and reduced speed are beneficial.

UNDERSTANDING AIRBORNE WEATHER RADAR

            Weather radar is no panacea when it comes to weather detection. In-fact it is a very crude instrument for weather detection which uses indirect logic to represent weather. The three most common threats to aircraft are turbulence, wind shear and hail. Interestingly none of these three elements are detected by weather radar. What it detects is ONLY the presence of water. The weather radar works on the principle of reflectivity of water droplets.  It sends out a signal which bounces off the water droplets and the bounced energy, apportions of it, is received by the antenna. This returned energy is displayed on the CRT as different color coded bands. These bands actually represent graded levels of reflection of water contents which are (deceptively) referred to as representing various levels of danger in the radar manuals. It is simplistically assumed that turbulence is directly proportional to the quantity of water in a cell and thus the areas

The reflectivity of water is governed by number of drops and size of the drops and not the actual water contents. Also drop size determines echo intensity to a much greater extent than does drop number. A one fourth inch drop (diameter) reflects the same amount of energy as 64 one eighth inch drops even though there is 729 times more liquid in the one eighth inch drops. To make matter further complicated water exists in atmosphere in various forms representing different level of dangers as well as different reflectivity. Water in liquid form has the maximum reflectivity. But not all precipitation is in liquid state. Hail and snow are two forms of dry precipitation. But there can be wet hail and wet snow which has better reflectivity than dry hail and dry snow. The reflectivity of ice particles is drastically reduced depending if it is dry or wet ice. Reflectivity of ice with water on its surface (wet ice) is one fifth of that of water. If on the other hand the beam strikes ice which is totally dry then the radio waves will be absorbed by the ice surface and will not reflect anything back.

            The top of a thunderstorm is mostly composed of ice crystals which have low reflectivity whereas the lower part would have liquid water. The weather radar could show the upper portion as green and lower portion as red whereas the upper portion would be more dangerous due presence of hail. At times the upper portion could have dry ice which might not send any returns. Thus reflectivity of particles, the basis of weather detection, is not directly proportional to the hazard that may be encountered in a cell.          Radar shadows, as brought out earlier, are caused by weather system so dense that radar energy cannot penetrate it akin to a shadow from a torch caused by an object so dense that light is not able to penetrate it. Radio waves act rather like light waves where they are unable to penetrate solid objects. Failure to recognize a shadow and entering it has been the cause of 90 percent of weather accidents as shadowed areas contain extreme weather consisting of microburst, downbursts, large hail and extreme turbulence.

            In a survey, conducted by Boeing to test the awareness of aircrew on weather radar, it was found that five in every eight pilots incorrectly thought green radar targets shown near to cruise levels above FL310 need not be avoided. At these altitudes targets are less reflective as it mainly consists of hail which is most dangerous for aircraft. There were many other questions regarding tilt, beam width and the optimum setting in terms of range for their specific type of weather radars. The knowledge reflected was less than optimum. Another study by FAA amongst General Aviation pilots observed: - “The study highlighted deficiencies in pilot education, training and skills when confronting various weather conditions.” I spoke to some of the ex-IAF pilots active in civil flying about this aspect. It was a mixed response as some agreed to the finding but some denied saying that knowledge of majority of pilots was adequate.

            Interestingly the manufacturers of the weather radar and the operating aircraft companies, when it comes to weather avoidance, give primacy to the general meteorological awareness and knowledge of the crew. Down below are some paragraphs taken from bulletin of Airbus regarding use of weather radar emphasizing this aspect (https://safetyfirst.airbus.com/optimum-use-of-weather-radar/)

- To obtain the maximum benefit from the weather radar system requires the crew to carefully optimize its use. This relies primarily on a good meteorological knowledge of weather phenomena, along with a good understanding of the available radar functions.

-A key element of the adverse weather avoidance strategies is the active monitoring of the overall met situation by the crew in addition to the optimum use of weather radar and correct understanding of the information displayed. WE MUST NOT FORGET THAT WEATHER RADAR IS OF HELP, BUT THE CREW OVERALL ASSESMENT OF THE WEATHER SITUATION PLAYS THE CENTRAL ROLE.

-In fact the management of adverse weather still relies primarily on the crew to actively monitor the met situation throughout the flight.

INSIDE THE STORM

            After flying just about 30 minutes I saw rain bearing grey and black clouds. Weather radar showed a green band-10 mile in depth at about 15 miles. I interpreted it as rain at 15 miles (which matched with visual observation). The assumption, though, that it was only 10 miles deep and penetrable was fatalistically flawed. The area we were to enter was a well marked low which had turned into depression overnight. The multiple thunderstorms there would not let the radar wave’s travel beyond 10 miles and the returned signals were badly attenuated. It was obvious that I had not taken met briefing, a criminal negligence on my part. Expecting some rough ride but oblivious to the dangers in front I got to the turbulence speed, approximate power setting and the attitude on AH to maintain that. This was inbuilt into my cognition from Otter days when these were my life line for surviving inadvertent ingress into weather. Severe turbulence can cause structural damage due to airframe exceeding g limitation due to gusts and turbulence. It can also stall the aircraft due to increase in angle of attack as the relative air changes wildly in direction and intensity. The rough air speed protects us from both these conditions as it is fast enough to preclude stall and optimally low not to impose limiting forces on aircraft structure. Flying at this speed would guarantee that the aircraft would not structurally fail which would be catastrophic if it does. The speed ensures that before the aircraft reaches these limiting g loads it will stall thus relieving the airframe loads. Under these circumstances a stall is a better option as it is recoverable and most aircrafts have easy stall and recovery characteristics. Unless the crew mismanages, a stall even in storm can be recovered. We held on to the rough air speed even before entering the turbulence. To cater for the icing, power was increased by about 10 degrees so that we maintain rough airspeed.

            The type of auto pilot and its limitation would dictate its use in storm but almost always all locks like height, speed, auto-trim and auto-throttle should be disconnected as the pressure in the storm wildly fluctuates leading to erratic speed and height indications that would put autopilot to do wild corrections leading to upset. In mild turbulence Auto Pilot use can be made but a sharp eye is to be kept as the control forces can abruptly cut off in unusual attitude. Hand flying, though, is a better option as it ensures that the pilot is always dynamically aware of the attitude at any instant and is not caught out of phase with the corrections. In such situations it is a good practice to let only one person handle the controls as two people cannot simultaneously put synchronized inputs. We disconnected the AP at the first jolt and the Copilot hand flew the aircraft.

             The pressure instruments are the most affected in storm due to partial/complete blocking of pitot system. They are to be ignored. Reacting to fluctuating speeds has been cause of many upsets leading to accidents including Air France 447. In the storm it-self there is wide fluctuation of ambient pressure which shows up in pressure instruments. We also noticed wide variation in height and airspeed. The temptation to react to them was inhibited by me verbalizing repeatedly to ignore all pressure instruments and concentrate on maintain wing level and correct attitude on AH. The attitude plus power, which we had set for rough air speed, would ensure that the speeds are correct. A more reliable instrument is the angle of attack indicator which was always showing that we were nowhere near the stalling regime. If the Air Bus 320 which was being flown as Air France 447 had had an angle of indicator in all probability the crew would have realized that they were stalling and need to lower the attitude.

            Flying inside a storm is not easy. It gets thrown around in all directions and even the instruments shake so much that it is not easy to read them. The young pilot hand flew the aircraft concentrating on flying the attitude and keeping wings level, on AH, and avoid any abrupt control input in case of aircraft deviating from the desired attitude. The corrections were to be prompt (both in pitch and bank) but gentle. Stresses are least if the aircraft is held in a constant attitude and allowed to ‘ride the waves’. The chances of structural failure due to turbulence are remote however investigation into such cases shows that aircraft break up in severe turbulence results from over controlling and exceeding design limits while recovering from upsets. If I was to listen to the CVR it would have me parroting away, nonstop, to Co-pilot ‘Wings level- attitude correct’. He was to concentrate on these two aspects only and was to remain head down. All cockpit lights were put on and fully bright. We never encountered lightning strike but in anticipation had put on the cockpit dome light (white light) on to avoid getting blinded. I tried to look out into the storm. It was black grey and scary.  The World War 2 era pilot when engulfed by storm would meander towards the darkest portion (eye of the storm) as that area would be the calmest and the shortest route out of the storm. The temptation to turn around, though, should be avoided as many aircrafts have got into unusual attitude while maneuvering.

             The aircraft construction is all about balancing contrasting requirements. The aircraft manufacturers give extra strength to any forward facing part like wind shield, leading edges, nose cone, engine cowling, empennage etc. as they are vulnerable to bird or hail strike. The control surfaces, normally flushed behind the leading and stronger parts of the aircraft, are structurally not that strong. Exposing them to hail while turning can damage them. Moreover maneuvering can increase stress on the airframe. That is yet another reason for avoiding turning in hail.  Also encounters with hail are generally of short duration and studies indicate that shortest route through hail is generally straight ahead.

            A paradoxically interesting feature of air borne weather radar is that it’s capability to show weather deteriorates markedly once inside the weather due to the radome getting sheathed in rain or ice. This protective layer on the radome interferes with the pulses both when sending and receiving. That is the reason there are bold capital captions WEATHER RADAR IS FOR WEATHER AVOIDANCE AND NOT FOR PENETRATING IT. For max effectiveness interpretation of weather radar displays should be accomplished when aircraft is in areas free of water vapor or precipitation.

            In 15 minutes, which looked like eternity, we encountered severe turbulence, torrential rain and saw wide fluctuations in airspeed and altitude. The aircraft was being tossed around like hay and it was a task to even make semblance of instruments. Our bank had registered more than 45 degrees and attitude too varied considerably.  L The aircraft though came back to level and responded to all inputs. Inspite of ingesting insane quantity of water the engine worked perfectly. There was not even a flutter.

SUMMING UP

     The Hurricane hunters, a team within the National Oceanic and Atmospheric Administration (NOAA) in USA, regularly fly in storms to gather data and predicting the path of the storm. Similarly 53rd Weather Reconnaissance Squadron a flying unit of USAF flies through storms for research purposes. They use aircrafts like C-130, P-3 Orion and Gulfstream IV in their basic version (without any structural strengthening or modification). Aircraft are designed and built to withstand environmental forces far greater than any turbulence can inflict on them. The wings are designed to sustain 1.5 times the designed ‘g’ limits.  Hence if due to bad decision or luck one gets inside the storm remember aircrafts are quite rugged and if flown properly can sail through any rough weather. Most of the aircraft accidents in weather are due to mishandling of controls and getting into unusual attitude and exceeding design limits. Flying the turbulence speed and not getting the aircraft to get into unusual attitude is the key to get out of stormy encounter.

     A study was carried by NASA to study the pilot’s propensity to penetrate thunderstorm instead of circumventing it as mandated by SOPs. It was found that a vast majority of encounters near the airport resulted in penetrations. The study concluded that aircrafts were more likely to penetrate convective weather when they were:-

-Near the destination airport rather than further away.

-Following another aircraft.

-When running late i.e. behind schedule.

-Flying after dark.

            I would like to add one more bullet- Foolishness. It was foolishness on my part not to get met briefing and using a system (weather radar) with little understanding.

Also the thunderstorm are inherently unpredictable grows at a very fast rate. Thunder storms are a result of unstable atmosphere and are inherently unpredictable. The clear area between two cells is filled up in no time with the rapid growth of convective cells.  It is constantly changing, every moment, while one flight could pass this area with light turbulence the other might encounter sever wind shear.