A320 Weather radar
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A320 Weather radar
Hi there!
I have a question about weather radar beam:
Considering a tilt of zero° just for example, how many degrees up and down is the beam wide?
Thanks much
Moto
I have a question about weather radar beam:
Considering a tilt of zero° just for example, how many degrees up and down is the beam wide?
Thanks much
Moto
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See Page 5-15 here. Honeywell radar but others will be very similar on beam width.
http://www.ce560xl.com/files/Honeywell_Radar_I.pdf
http://www.ce560xl.com/files/Honeywell_Radar_I.pdf
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Beamwidth is how far off boresight to the half power point, above plus below.
Beamwidth on the Airbus, Boeing, McDouglas airliner with 29 inch diameter flatplate antenna is 3 degrees. Sidelobes, as seen in the prior generation radars with dish antennas, are almost nonexistent. The return is from where you point the beam. Just be sure to point it down into the liquid part of the storm, below freezing level. Ice is a poor reflector.
GB
Beamwidth on the Airbus, Boeing, McDouglas airliner with 29 inch diameter flatplate antenna is 3 degrees. Sidelobes, as seen in the prior generation radars with dish antennas, are almost nonexistent. The return is from where you point the beam. Just be sure to point it down into the liquid part of the storm, below freezing level. Ice is a poor reflector.
GB
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Here it is, chapter an verse! The scientific way, if you have the patience to work through a very long post. Apologies for the imperfect scan-to-text too.
Antenna Tilt:
The Key Radar Control
Tilt management skills are critical to using
airborne radar properly, but first you
must understand the academics.
by Archie Trammell
We in aviation have known since first flight that a poor attitudean incorrect mind set can kill. Not quite so obvious, however, is that someone else's faulty attitude may cause your accident. We now have a crystal clear example that the latter can be true.
In its investigation of the Delta Lockheed L1011 crash in a thunderstorm at Dallas/Fort Worth Airport in August 1985, the NTSB interviewed Delta's systems manager for trainmg, who said, .... .with any airborne radar device, written instructions and classroom academics are highly inadequate.... It's largely something that has to be learned by experience."
The NTSB let the statement go unchallenged in its report on the accident, but we must not.
To let it be said that classroom instruction in the fundamentals of radar need not precede knob twiddling is to leave a dangerous attitude uncorrected. Such a belief is no different than thinking that the only way to teach a captain how to use a flight director is to load the airplane with 200 unsuspecting souls and dispatch him into an area of 200andaquarter weather. Moreover, the statement is totally contrary to historical experience. It says that each succeeding generation of pilots must start from zero and learn from his or her own mistakes, never benefiting from radar operating techniques and methods worked out by others in thousands of hours. The truth is that without a background in the basics, no amount of trial and error will teach you how to use an airborne radar.
It's also revealing of the abilities of the Delta systems manager himself, who has not learned the first fundamental about airborne radar operation. He doesn't understand tilt management, the prerequisite to all other radar skills. If he did, he would have never said in a followon statement to the NTSB interviewer, "The primary use of this type of radar, or any airborne radar with which I have any experience, is enroute weather avoidance. When you get into the approach environment.. to get any useful work out of the radar, you have to do an awful lot of playing with the antenna tilt, and [since] you are also very close to the ground...you get a lot of ground return. So, it's least useful in the approach phase of the flight."
About 20 minutes of radar ground schoolof "classroom academics"will prove him totally wrong. Setting up a radar so that ground retums are eliminated from the display is nothing more than applied sixthgrade geometry and fourthgrade mathematics.
Let's examine just two of the "academic" facts and see how they might have prevented the Delta 191 accidentif they had been passed on to the crew in a ground school.
Incidentally, these facts are not new. They were discovered by Captain Robert N. Buck of TWA during World War II and proven by the original Project Rough Rider pilot, Jim Cook of Dallas, in hundreds of thunderstorm penetrations between 1953 and the present.
Fact one is that a radar, which is an acronym for radio detection and ranging, detects and displays to the pilot on his indicator those objectsand those objects onlythat are swept by the radar beam.
That beam is a narrow cone ranging from three to 10 degrees in diameter. It sweeps in a plane relative to the earth, selected by the pilot with an antenna tilt control.
The pilot, therefore, has total control of whether ground returns are detected and displayed. All a pilot has to do is select a tilt setting that results in the swept area being above all ground objects. Ideally, the tilt should be set so that the bottom edge of the conical beam sweeps on a plane parallel to the surface of the earth, as in Figure 1.
With the bottom of the beam sweeping on a plane parallel to the surface of the earth, only those objects that extend upward through the altitude of the aircraft are detected and displayed. Thus, at terminalarea altitudes (any altitude below 15,000 feet), only tall threatening weather is displayed; ground returns are eliminated from the display.
Fortunately, setting the tilt to the proper position is very simple to do. First, adjust the tilt control until the bottom edge of the beam is sweeping along the ground on the 20nm arc
You can easily identify where the bottom of the beam intersects the earth. From the bottom of the beam outward the radar indicator will be filled solid with a band of returns all across the swept sector; from the bottom of the beam inward the radar indicator will be blank (see Figure 2).
Next, divide your altitude in thousands of feet agl by two. Finally, note the tilt setting with the bottom of the beam sweeping on the 20nm arc, then increase tilt by a number of degrees equal to the calculation. (Example: You are at 12,000 feet agl; 12 divided by two equals six. With the bottom of your beam sweeping on the 20nm arc, the tilt index is at minus four degrees, let's say. If so, raise tilt to a setting of plus two degreesminus four plus six equals a positive tw~and the bottom of your beam is now sweeping an area from 12,000 feet agl upward. Anything depicted on your radar indicator is an object with a height greater than 12,000 feet agl.)
Had the Delta 191 crew been aware of this procedure (we call it TIP, for threat identification procedure), they would have known that significant weather was on or near the approach to DFW Runway 17L at least 14 minutes before impact.
At 1751:19 hours (impact was at 1805:55), the second officer saw rain ahead. At 1752, just 41 seconds later, the aircraft was descending through about 12,000 feet agi, headed almost directly toward the airport, which was about 40 nm away. At 1752:00 TIP would have revealed a contouring cell at the 35nm range dectly between them and the airport.
At 1756:00, when the aircraft was at 7,000 feet, 25 nm from the airport and headed direcdy toward it, TIP would have revealed a contouring cell at the 23nm range. The crew would have known a storm was in progress on or very near the final approach course at least six minutes before flying into it.
"The Magic Radar Formula"
Had the Delta crew run a height evaluation procedure (HEP) on the storm at 1756:00, they would have known it was exceedingly hazardous for a terminal operation.
To understand how HEP works, one must first become familiar with what we refer to as "the magic radar formula," which is the basis for all radar tiltmanagement procedures, tricks and shortcuts.
Since the radar beam is angular (a cone with a width measured in degrees) and tilt management is strictly an angular manipulation of that beam, one must have a method for converting degrees into feet; namely, distance times 100 equals feet per degree at that distance.
The dimension across a degree changes constantly as the distance from the origin increasesyour radar antenna being the origin in this case. To determine how much the beam is shifted up or down per degree at a certain distance, put two zeros behind the distance in nautical miles and you have the answer in feet. At 10 nm, for example, one degree is 1,000 feet (10 followed by two zeros). At 20 nm, one degree is 2,000 feet across, at 37 nm, one degree is 3,700 feet across and so forth.
The formula isn't as accurate as trigonometry, but a pilot doesn't have time for trig during an approach. The formula is more accurate than your tilt control, however, which has a degree or more of variance in it under the best of conditions.
Incidentally, you can use that formula to prove why TIP works. You can also use it to modify TIP to better suit the situation. At high altitudes (above 15,000 feet agl) put the bottom of your beam on the 40nm arc and divide your altitude agl by four rather than two.
At 32,000 feet agi, for instance, 32 divided by four equals eight. Put the bottom of your beam on the ground at 40 nm, raise tilt eight degrees and the bottom of your beam is level to the earth at an altitude of 32,000 feet agl. (We can ignore the fact that the earth is round as long as the distance of interest is 60 nm or less.)
Why does raising the bottom of your
beam eight degrees place it parallel to the surface of the earth? Because at 40 nm, each degree raises the bottom of the beam 4,000 feet and 4,000 feet times eight equals 32,000 feet. If the bottom of your beam is on the ground 40 nm ahead and you raise it eight degrees, the bottom of your beam at a point 40 nm ahead will now be at the same altitude as your antenna; therefore, the bottom of your beam will be parallel to the earth's surface at an altitude of 32,000 feet agi.
If you prefer to divide by three, five or eight, place the bottom of your beam on the ground at 30, 50 or 80 nm, respectively. TIP still works. We'll let you calculate why.
Antenna Stabilization
Before proceeding, we must clear up a prevalent misconception. Many pilots think that tilt directs the radar beam up or down relative to the pitch attitude of the aircraft. In fact, the pilot's manual for the radar that was installed in Delta 191 states that, "Selected (tilt) angle is in relation to the longitudinal axis of the aircraft."
Totally wrong. Selected tilt angles are relative to old Mother Earth. In aircraft that do not have ~ antenna stabilization, or when stab is turned off, tilt angles are relative to the longitudinal axis of the aircraft. However, all airliners, and virtually all corporate turbine aircraft, have stabilized antennas.
With stabilization, a reference from the aircraft vertical gyro is biased into the tilt knob logic. Therefore, tilt settings command the angle at which the center of your beam sweeps relative to the plane of the earth directly below the aircraft. Change pitch or roll attitude and the angle of your beam and beam sweep remain unchanged with respect to the horizon.
It's very important to understand that result. It's also very important that pilots of aircraft that do not have stabilized antennasor when stab is offalways correct tilt selections for deck angle excursions from level flight. Otherwise confusion will reign.
Notice what antenna stabilization does for you. Once a TIP is conducted, or once the beam is set to any desired angle relative to the earth, you do not have to mess with the tilt control again. With stabilization, the beam inclination stays where you left it relative to Earth, as your attitude and altitude change, which means you can perform a TIP once and the bottom of your beam will remain level with the earth as you climb or descend. TIP once and what you see on your radar intrudes through your current altitude even as that altitude is changing in a climb or de scent. (This discussion assumes that the aircraft is in reasonably stable flight. Maneuvers and speed changes introduce a number of errors into the antenna stabilization system due to gyro excursions. But those errors also affect the flight control system and other flight management procedures and so must be the subject of a broader, dedicated discussion at a later time. Suffice it to state here that when gyro errors may be a factor, TIP and other tiltmanagement procedures must be conducted more frequentlyat no more than twominute intervals in order to zero out the system.)
The HEP
Now, with "the magic radar formula" and stabilization well understood, how do you conduct a height evaluation procedure?
First perform TIP, which places the bottom of the beam level at your current altitude. Next, note the distance to any echo of interest and your tilt setting. Finally, raise tilt until the echo becomes so weak that you can just barely see it on the indicator. (The echo becomes weaker because the beam begins to overscan it.) Now, the distance to the echo multiplied by 100 multiplied by the tilt difference equals the height of the storm above your aircraft's current altitude.
Let's use Delta 191 at 1756:00 as the example. The flight was about 7,000 feet agl, 23 nm from that cell over the approach course.
At least seven degrees of uptilt from TIP would have been required to reduce the echo to just detectable (calculating 23 times 100 times seven: 16,100 feet). Adding the current altitude, 7,000 feet, yields a radar top of about 23,000 feet. A 23,000foothigh contouring storm located directly on the localizer is bad news to anybody.
A Shortcut HEP
But a reasonable question is, does a crew have time for that much knob tweaking and mental arithmetic during a busy approach?
Normally, yes. With practice, the entire TIP and HEP tilt manipulations require only 15 or 20 seconds. But even 15 seconds can be too long at times.
In that event, use the tiltup technique, TUT for short. That is, adjust tilt so that the center of your beam is angled up 10 degrees.
To accomplish that you must know where true zero degrees tilt istrue zero degrees tilt being the setting that results in the center of your beam being parallel to the earth. It's probably not at the zerodegree mark on the tilt control because of antenna installation errors and/or stabilization gyro excursions.
To find it, execute a TIP, then lower tilt onehalf your beam width (see Figure 3 to find your beam width). With a 12inch antenna (assuming Xband radar) true zero degrees tilt is at TIP 4.0 degrees; with an 18inch antenna, it's at ~P 2.5 degrees; with a 30inch antenna it's at TIP 1.5 degrees.
When using the + 10degree procedure, ignore the fact that your beam is a cone, that it has a width. Assume that radar tops are at the center of your beam, which will build in a safety cushion.
Now, with + 10 degrees on the tilt, echoes displayed at a distance of 30 nm or more have radar tops at least 30,000 feet above your current altitude. Those displayed at distances of 20 nm or more have radar tops of more than 20,000 above you. (Note: HEP and TUT give you an estimate of radar topsnot the top of the storm. For overflight planning, hazards may exist several thousand feet above the radar top of a weather system.)
Research has proven that the hazards associated with a weather system are directly proportional to its radar height. In the terminal area, any storm with a radar top in excess of 20,000 feet agl is a potential killer. In addition, any echo that contours at any distance with + 10 degrees of tilt is a nogo when operating below 15,000 feet.
From that comes three absolute, lifeanddeath, terminal area rules.
· With + 10 degrees of tilt, any echo that first appears on the display at 20 nm or greaterwhether contouring or notmust be avoided.
· Any echo that contours at any distance with + 10 degrees of tilt must be avoided regardless of how tall it is.
· With + 10 degrees on the tilt and after the gear is out, if a contouring echo is detected inside the fivenauticalmile arc and cannot be avoided, initiate a missed approach immediately. On departure, don't go.
Looking back over the string of 23 thunderstormrelated air carrier accidents that have occurred in the last quarter century, the TUT rules alone would have prevented 16. And the false alarm rate of the rules is very low. The best data available indicate that abiding by those rules will result in an average of only 16 delays per year per major airporta small price to pay, when you consider the cost of one fatal accident.
Surface Analysis
As important and useful as the + 10 degrees tilt procedure is, it must be frequently alternated with the Surface Analysis Procedure (SAP), especially in the terminal area.
The SAP requires that the tilt be adjusted so that the bottom of your beam slopes downward four degrees (TIP minus four degrees).
That tilt setting and the 50nm range setting (or as close to 50 nm as possible with your particular range selector) should be considered your normal terminal area radar configuration. Adjust tilt to TIP and TUT for several sweeps at frequent intervals, but always return it to the SAP position. Shorter or longer range selections should be used only intermittently, the exception being when within 10 nm of the runway on approach. As the airport draws near, a 25nm or even 10nm range selection is appropriate, but do not fail to go back out to 50 nm on occasion with tilt set at TUT to survey your missed approach pathjust in case.
Whenever the bottom of your radar beam is slanted down four degrees your radar display will provide you with three vital types of information:
(1) A terminal area radar mapWith the bottom of your beam down four degrees, the radar will paint a band of ground from 25 nm outward when the aircraft is at 10,000 feet agl. On arrivals, the 25 nm works inward 2.5 nm per 1,000 feet of descent so that when crossing the marker inbound, your radar will be displaying ground from about four nautical miles outward to the edge of your indicator.
Ground painting terminal areas frequently on VMC days is critical. A pilot who flies into a given terminal six or more times per year should know the radar signature of the area so well that he can fly to the center of the airport with no guidance other than radar. He should be able to point out, on the indicator, every lake, ridge, mountain peak, shopping center, tank farm and railroad yard in a 25nm radius of the middle marker. Such ability is called radar professionalism. It is often useful and sometimes lifesaving.
By knowing where the major radar landmarks are, a pilot can quickly identify storms in the airport vicinity just by glancing at the radar indicator and without touching any radar control:.
They are the strong echoes that weren't in a particular location last trip. Mountain peaks, suburban developments and tank farms don't grow overnight.
After identifying a weather echo either with TIP or by ground mapping, the wise pilot watches what happens with his tilt set at the SAP position. If the echo gets stronger or larger over a period of four or five minutes, it's a rapidly growing storm. If it gets smaller and weaker minute to minute, it's likely to be a dissipating storm.
Interestingly, during the eight minutes from 1751:19 to 1759:37 that Delta 191 was vectored straight toward DFW, a cell that was six nautical miles northeast of the airport over mostly farmland dissipated from Level 3 to nothing. At the same time, a second cell at four nautical miles southwest of the first, grew from nothing to Level 3. It was the second cell that spewed out the downburst as Flight 191 flew into it.
(2) Terrain threat warningWith a fourdegree down slant to the bottom of the beam, over flat terrain, ground should be painted from the 25nm arc outward as the aircraft passes through 10,000 feet agl. At 5,000 feet it should be on the 12nm arc. Crossing the marker it should be on about the fournauticalmile arc.
If your radar indicator doesn't agree during a climb or descent, something is not right, including the possibility that you may not have as much terrain clearance as you think.
Over mountainous terrain, a ground echo that intrudes inside the 10nm arc with tilt adjusted for SAP should command your undivided attention. You are less than 4,000 feet above a peak. Perhaps you are below it. One that intrudes itself inside the fivenauticalmile arc is cause for alarm, unless you have it visually, because you cannot be more than 2,000 feet above its peak. Within about two minutes you'll discover if you are below the peak.
(3) Radar shadow enhancementThis skill is the most important of all. Failure to identify a radar shadow is the greatest cause of convective weather accidents.
The tragedy is that they just about jump off the scope at you when you're aloft if you would just select the 50nm range and adjust your tilt so the bottom of your beam is angled down four degrees.
The radar shadow in the accompanying photo (Figure 4) is that black wedge trailing off into the distance at the 11 o'clock position. The black area on the right is a large lake.
Shadows always point directly away from the antenna. They have a feathered edge, as in the photo, and they continue outward as far as ground returns can be painted. (If the sides of a shadow come together in the distance, you are above the object casting it.)
Radar shadows are caused by a weather system so dense that radar energy cannot penetrate it just as a shadow from a flashlight is caused by an object so dense that light is not able to penetrate it).
Notice in the photo that energy is penetrating the left side of the storm; it cannot penetrate the right side.
The rule is, never, never, ever continue toward a radar shadow. Failure to recognize a shadow and abide by the rule is the cause of 90 percent of convective weather accidents. Eastern Airlines Flight 66 crashed in a shadow at JFK in 1975; Southern's Flight 242 crashed in a shadow at New Hope, Georgia in 1977; Air Wisconsin crashed in a shadow northeast of Omaha in 1980; Pan American crashed in a shadow on takeoff at New Orleans in 1982; USAir crashed in a shadow at Detroit in 1984; Delta Airlines Flight 191 crashed in a shadow at DEW in 1985; a corporate Westwind crashed near several shadows in 1986.
Evidence is abundant that airplanes cannot fly through weather that casts a shadow. Rainfall rates in a shadow producing storm frequently exceed the certification limits of the aircraft and engines. In addition, shadowing storms will contain microbursts, downbursts, large hail, extreme turbulence and very possibly, tornadoes. A storm casting a shadow can be identified with tilt up, but the clues are subtle and easily misread. With the SAP technique they cannot be missed. Those who know how to identify and read such indications are safe pilots; those who don't may as well turn off their radar. It's useless to them. (By the way, don't rely on the gimmicks of some newer radars that purport to identify shadows but often do not indicate a shadow where there is one. Put your tilt down and watch for the real shadows.)
In these few pages we have barely exposed the tip of an iceberg. We've touched on only two of hundreds of techniques, shortcuts and radar facts that can never be discovered in years of selfeducation. We hope that the NTSB will have second thoughts on their tadt agreement with the statement, .... .with any airborne radar device, written instructions and classroom academics are highly inadequate."
Meanwhile, don't let that attitude be the cause of your accident. Practice the TIP, REP, TUT and SAP techniques until they become second nature with you. Then steer far, far away from all radar shadows.
Those Confusing
Side Lobes
The fundamental truth in understanding tilt management is that radar detects and displays only those objects that fall within the limited area of the three degree to 1 degree conical beam. Objects not "Illuminated" by the finite dimensions of the beam are not detected.
But for every rule there is an exception. If the antenna installation is poorly engineered or if the radome is imperfect, ghost images appear on the indicator. Several weather echoes can be seen in the accompanying photograph. The one at 12 o'clock and two nautical miles, however, is not weather. It's a false echo.
False echoes are commonly referred to as "side lobe returns," or simply as side lobes, although in reality they are caused by refracted energy from the main beam. Metal objects in the radome cavity, water trapped In the honeycomb of the radome, a metallic paint strip. around the nose or one of those horrible plastic nose caps can generate side lobe returns.
The cause should be corrected by taking the aircraft to a firstclass radome repair shop, but almost no one bothers, so pilots must learn to live with false echoes.
In some instances a spear of refracted energy points downward, bounces off the ground and shows up as an echo at 12 o'clock. No matter where you place the tilt, there is the echo at 12 o'clock. In other instances the spear points to the side so that you see a sidelobe return, also at 12 o'clock, only when flying abeam a storm.
In still other instances, you see no side lobe return when your radome is dry; you see one only when your radome is wet or has Ice on it. Whenever you're in weather, you always have a false, contouring storm at 12 o'clock, never more than five or six miles distant.
A necessary radar skill is learning to recognize and ignore side lobe returns. There are several ways to identify them. First, just observe your radar on clear days. Execute TIP and if you see an echo, it's a side lobe. Mark it in your mind as something to ignore.
If you see side lobe returns only when in weather, however, identifying them gets a little trickier. One clue is movement. An echo that you cannot catch up to is a side lobe return because storms never move as fast as an airplane flies. An exception (there's that exception again) is when you descend. A side lobe caused by energy spilling off the antenna and pointed straight down will appear at a range roughly equal to aircraft altitude agl. Therefore, when you're descending or flying toward rising terrain, it will appear that you're overtaking a side lobewhich is something else you must observe on several flights and learn to ignore.
If all else fails, select the 50nm range (or as close to 50 nm as possible), run your tilt down until you have a solid ground paint from 10 or 15 nm outward, then back way off counterclockwise on your gain control. If a shadow appears behind the suspect echo, it's real; if not, it's a side lobe because side lobes do not cast shadows.
The Key Radar Control
Tilt management skills are critical to using
airborne radar properly, but first you
must understand the academics.
by Archie Trammell
We in aviation have known since first flight that a poor attitudean incorrect mind set can kill. Not quite so obvious, however, is that someone else's faulty attitude may cause your accident. We now have a crystal clear example that the latter can be true.
In its investigation of the Delta Lockheed L1011 crash in a thunderstorm at Dallas/Fort Worth Airport in August 1985, the NTSB interviewed Delta's systems manager for trainmg, who said, .... .with any airborne radar device, written instructions and classroom academics are highly inadequate.... It's largely something that has to be learned by experience."
The NTSB let the statement go unchallenged in its report on the accident, but we must not.
To let it be said that classroom instruction in the fundamentals of radar need not precede knob twiddling is to leave a dangerous attitude uncorrected. Such a belief is no different than thinking that the only way to teach a captain how to use a flight director is to load the airplane with 200 unsuspecting souls and dispatch him into an area of 200andaquarter weather. Moreover, the statement is totally contrary to historical experience. It says that each succeeding generation of pilots must start from zero and learn from his or her own mistakes, never benefiting from radar operating techniques and methods worked out by others in thousands of hours. The truth is that without a background in the basics, no amount of trial and error will teach you how to use an airborne radar.
It's also revealing of the abilities of the Delta systems manager himself, who has not learned the first fundamental about airborne radar operation. He doesn't understand tilt management, the prerequisite to all other radar skills. If he did, he would have never said in a followon statement to the NTSB interviewer, "The primary use of this type of radar, or any airborne radar with which I have any experience, is enroute weather avoidance. When you get into the approach environment.. to get any useful work out of the radar, you have to do an awful lot of playing with the antenna tilt, and [since] you are also very close to the ground...you get a lot of ground return. So, it's least useful in the approach phase of the flight."
About 20 minutes of radar ground schoolof "classroom academics"will prove him totally wrong. Setting up a radar so that ground retums are eliminated from the display is nothing more than applied sixthgrade geometry and fourthgrade mathematics.
Let's examine just two of the "academic" facts and see how they might have prevented the Delta 191 accidentif they had been passed on to the crew in a ground school.
Incidentally, these facts are not new. They were discovered by Captain Robert N. Buck of TWA during World War II and proven by the original Project Rough Rider pilot, Jim Cook of Dallas, in hundreds of thunderstorm penetrations between 1953 and the present.
Fact one is that a radar, which is an acronym for radio detection and ranging, detects and displays to the pilot on his indicator those objectsand those objects onlythat are swept by the radar beam.
That beam is a narrow cone ranging from three to 10 degrees in diameter. It sweeps in a plane relative to the earth, selected by the pilot with an antenna tilt control.
The pilot, therefore, has total control of whether ground returns are detected and displayed. All a pilot has to do is select a tilt setting that results in the swept area being above all ground objects. Ideally, the tilt should be set so that the bottom edge of the conical beam sweeps on a plane parallel to the surface of the earth, as in Figure 1.
With the bottom of the beam sweeping on a plane parallel to the surface of the earth, only those objects that extend upward through the altitude of the aircraft are detected and displayed. Thus, at terminalarea altitudes (any altitude below 15,000 feet), only tall threatening weather is displayed; ground returns are eliminated from the display.
Fortunately, setting the tilt to the proper position is very simple to do. First, adjust the tilt control until the bottom edge of the beam is sweeping along the ground on the 20nm arc
You can easily identify where the bottom of the beam intersects the earth. From the bottom of the beam outward the radar indicator will be filled solid with a band of returns all across the swept sector; from the bottom of the beam inward the radar indicator will be blank (see Figure 2).
Next, divide your altitude in thousands of feet agl by two. Finally, note the tilt setting with the bottom of the beam sweeping on the 20nm arc, then increase tilt by a number of degrees equal to the calculation. (Example: You are at 12,000 feet agl; 12 divided by two equals six. With the bottom of your beam sweeping on the 20nm arc, the tilt index is at minus four degrees, let's say. If so, raise tilt to a setting of plus two degreesminus four plus six equals a positive tw~and the bottom of your beam is now sweeping an area from 12,000 feet agl upward. Anything depicted on your radar indicator is an object with a height greater than 12,000 feet agl.)
Had the Delta 191 crew been aware of this procedure (we call it TIP, for threat identification procedure), they would have known that significant weather was on or near the approach to DFW Runway 17L at least 14 minutes before impact.
At 1751:19 hours (impact was at 1805:55), the second officer saw rain ahead. At 1752, just 41 seconds later, the aircraft was descending through about 12,000 feet agi, headed almost directly toward the airport, which was about 40 nm away. At 1752:00 TIP would have revealed a contouring cell at the 35nm range dectly between them and the airport.
At 1756:00, when the aircraft was at 7,000 feet, 25 nm from the airport and headed direcdy toward it, TIP would have revealed a contouring cell at the 23nm range. The crew would have known a storm was in progress on or very near the final approach course at least six minutes before flying into it.
"The Magic Radar Formula"
Had the Delta crew run a height evaluation procedure (HEP) on the storm at 1756:00, they would have known it was exceedingly hazardous for a terminal operation.
To understand how HEP works, one must first become familiar with what we refer to as "the magic radar formula," which is the basis for all radar tiltmanagement procedures, tricks and shortcuts.
Since the radar beam is angular (a cone with a width measured in degrees) and tilt management is strictly an angular manipulation of that beam, one must have a method for converting degrees into feet; namely, distance times 100 equals feet per degree at that distance.
The dimension across a degree changes constantly as the distance from the origin increasesyour radar antenna being the origin in this case. To determine how much the beam is shifted up or down per degree at a certain distance, put two zeros behind the distance in nautical miles and you have the answer in feet. At 10 nm, for example, one degree is 1,000 feet (10 followed by two zeros). At 20 nm, one degree is 2,000 feet across, at 37 nm, one degree is 3,700 feet across and so forth.
The formula isn't as accurate as trigonometry, but a pilot doesn't have time for trig during an approach. The formula is more accurate than your tilt control, however, which has a degree or more of variance in it under the best of conditions.
Incidentally, you can use that formula to prove why TIP works. You can also use it to modify TIP to better suit the situation. At high altitudes (above 15,000 feet agl) put the bottom of your beam on the 40nm arc and divide your altitude agl by four rather than two.
At 32,000 feet agi, for instance, 32 divided by four equals eight. Put the bottom of your beam on the ground at 40 nm, raise tilt eight degrees and the bottom of your beam is level to the earth at an altitude of 32,000 feet agl. (We can ignore the fact that the earth is round as long as the distance of interest is 60 nm or less.)
Why does raising the bottom of your
beam eight degrees place it parallel to the surface of the earth? Because at 40 nm, each degree raises the bottom of the beam 4,000 feet and 4,000 feet times eight equals 32,000 feet. If the bottom of your beam is on the ground 40 nm ahead and you raise it eight degrees, the bottom of your beam at a point 40 nm ahead will now be at the same altitude as your antenna; therefore, the bottom of your beam will be parallel to the earth's surface at an altitude of 32,000 feet agi.
If you prefer to divide by three, five or eight, place the bottom of your beam on the ground at 30, 50 or 80 nm, respectively. TIP still works. We'll let you calculate why.
Antenna Stabilization
Before proceeding, we must clear up a prevalent misconception. Many pilots think that tilt directs the radar beam up or down relative to the pitch attitude of the aircraft. In fact, the pilot's manual for the radar that was installed in Delta 191 states that, "Selected (tilt) angle is in relation to the longitudinal axis of the aircraft."
Totally wrong. Selected tilt angles are relative to old Mother Earth. In aircraft that do not have ~ antenna stabilization, or when stab is turned off, tilt angles are relative to the longitudinal axis of the aircraft. However, all airliners, and virtually all corporate turbine aircraft, have stabilized antennas.
With stabilization, a reference from the aircraft vertical gyro is biased into the tilt knob logic. Therefore, tilt settings command the angle at which the center of your beam sweeps relative to the plane of the earth directly below the aircraft. Change pitch or roll attitude and the angle of your beam and beam sweep remain unchanged with respect to the horizon.
It's very important to understand that result. It's also very important that pilots of aircraft that do not have stabilized antennasor when stab is offalways correct tilt selections for deck angle excursions from level flight. Otherwise confusion will reign.
Notice what antenna stabilization does for you. Once a TIP is conducted, or once the beam is set to any desired angle relative to the earth, you do not have to mess with the tilt control again. With stabilization, the beam inclination stays where you left it relative to Earth, as your attitude and altitude change, which means you can perform a TIP once and the bottom of your beam will remain level with the earth as you climb or descend. TIP once and what you see on your radar intrudes through your current altitude even as that altitude is changing in a climb or de scent. (This discussion assumes that the aircraft is in reasonably stable flight. Maneuvers and speed changes introduce a number of errors into the antenna stabilization system due to gyro excursions. But those errors also affect the flight control system and other flight management procedures and so must be the subject of a broader, dedicated discussion at a later time. Suffice it to state here that when gyro errors may be a factor, TIP and other tiltmanagement procedures must be conducted more frequentlyat no more than twominute intervals in order to zero out the system.)
The HEP
Now, with "the magic radar formula" and stabilization well understood, how do you conduct a height evaluation procedure?
First perform TIP, which places the bottom of the beam level at your current altitude. Next, note the distance to any echo of interest and your tilt setting. Finally, raise tilt until the echo becomes so weak that you can just barely see it on the indicator. (The echo becomes weaker because the beam begins to overscan it.) Now, the distance to the echo multiplied by 100 multiplied by the tilt difference equals the height of the storm above your aircraft's current altitude.
Let's use Delta 191 at 1756:00 as the example. The flight was about 7,000 feet agl, 23 nm from that cell over the approach course.
At least seven degrees of uptilt from TIP would have been required to reduce the echo to just detectable (calculating 23 times 100 times seven: 16,100 feet). Adding the current altitude, 7,000 feet, yields a radar top of about 23,000 feet. A 23,000foothigh contouring storm located directly on the localizer is bad news to anybody.
A Shortcut HEP
But a reasonable question is, does a crew have time for that much knob tweaking and mental arithmetic during a busy approach?
Normally, yes. With practice, the entire TIP and HEP tilt manipulations require only 15 or 20 seconds. But even 15 seconds can be too long at times.
In that event, use the tiltup technique, TUT for short. That is, adjust tilt so that the center of your beam is angled up 10 degrees.
To accomplish that you must know where true zero degrees tilt istrue zero degrees tilt being the setting that results in the center of your beam being parallel to the earth. It's probably not at the zerodegree mark on the tilt control because of antenna installation errors and/or stabilization gyro excursions.
To find it, execute a TIP, then lower tilt onehalf your beam width (see Figure 3 to find your beam width). With a 12inch antenna (assuming Xband radar) true zero degrees tilt is at TIP 4.0 degrees; with an 18inch antenna, it's at ~P 2.5 degrees; with a 30inch antenna it's at TIP 1.5 degrees.
When using the + 10degree procedure, ignore the fact that your beam is a cone, that it has a width. Assume that radar tops are at the center of your beam, which will build in a safety cushion.
Now, with + 10 degrees on the tilt, echoes displayed at a distance of 30 nm or more have radar tops at least 30,000 feet above your current altitude. Those displayed at distances of 20 nm or more have radar tops of more than 20,000 above you. (Note: HEP and TUT give you an estimate of radar topsnot the top of the storm. For overflight planning, hazards may exist several thousand feet above the radar top of a weather system.)
Research has proven that the hazards associated with a weather system are directly proportional to its radar height. In the terminal area, any storm with a radar top in excess of 20,000 feet agl is a potential killer. In addition, any echo that contours at any distance with + 10 degrees of tilt is a nogo when operating below 15,000 feet.
From that comes three absolute, lifeanddeath, terminal area rules.
· With + 10 degrees of tilt, any echo that first appears on the display at 20 nm or greaterwhether contouring or notmust be avoided.
· Any echo that contours at any distance with + 10 degrees of tilt must be avoided regardless of how tall it is.
· With + 10 degrees on the tilt and after the gear is out, if a contouring echo is detected inside the fivenauticalmile arc and cannot be avoided, initiate a missed approach immediately. On departure, don't go.
Looking back over the string of 23 thunderstormrelated air carrier accidents that have occurred in the last quarter century, the TUT rules alone would have prevented 16. And the false alarm rate of the rules is very low. The best data available indicate that abiding by those rules will result in an average of only 16 delays per year per major airporta small price to pay, when you consider the cost of one fatal accident.
Surface Analysis
As important and useful as the + 10 degrees tilt procedure is, it must be frequently alternated with the Surface Analysis Procedure (SAP), especially in the terminal area.
The SAP requires that the tilt be adjusted so that the bottom of your beam slopes downward four degrees (TIP minus four degrees).
That tilt setting and the 50nm range setting (or as close to 50 nm as possible with your particular range selector) should be considered your normal terminal area radar configuration. Adjust tilt to TIP and TUT for several sweeps at frequent intervals, but always return it to the SAP position. Shorter or longer range selections should be used only intermittently, the exception being when within 10 nm of the runway on approach. As the airport draws near, a 25nm or even 10nm range selection is appropriate, but do not fail to go back out to 50 nm on occasion with tilt set at TUT to survey your missed approach pathjust in case.
Whenever the bottom of your radar beam is slanted down four degrees your radar display will provide you with three vital types of information:
(1) A terminal area radar mapWith the bottom of your beam down four degrees, the radar will paint a band of ground from 25 nm outward when the aircraft is at 10,000 feet agl. On arrivals, the 25 nm works inward 2.5 nm per 1,000 feet of descent so that when crossing the marker inbound, your radar will be displaying ground from about four nautical miles outward to the edge of your indicator.
Ground painting terminal areas frequently on VMC days is critical. A pilot who flies into a given terminal six or more times per year should know the radar signature of the area so well that he can fly to the center of the airport with no guidance other than radar. He should be able to point out, on the indicator, every lake, ridge, mountain peak, shopping center, tank farm and railroad yard in a 25nm radius of the middle marker. Such ability is called radar professionalism. It is often useful and sometimes lifesaving.
By knowing where the major radar landmarks are, a pilot can quickly identify storms in the airport vicinity just by glancing at the radar indicator and without touching any radar control:.
They are the strong echoes that weren't in a particular location last trip. Mountain peaks, suburban developments and tank farms don't grow overnight.
After identifying a weather echo either with TIP or by ground mapping, the wise pilot watches what happens with his tilt set at the SAP position. If the echo gets stronger or larger over a period of four or five minutes, it's a rapidly growing storm. If it gets smaller and weaker minute to minute, it's likely to be a dissipating storm.
Interestingly, during the eight minutes from 1751:19 to 1759:37 that Delta 191 was vectored straight toward DFW, a cell that was six nautical miles northeast of the airport over mostly farmland dissipated from Level 3 to nothing. At the same time, a second cell at four nautical miles southwest of the first, grew from nothing to Level 3. It was the second cell that spewed out the downburst as Flight 191 flew into it.
(2) Terrain threat warningWith a fourdegree down slant to the bottom of the beam, over flat terrain, ground should be painted from the 25nm arc outward as the aircraft passes through 10,000 feet agl. At 5,000 feet it should be on the 12nm arc. Crossing the marker it should be on about the fournauticalmile arc.
If your radar indicator doesn't agree during a climb or descent, something is not right, including the possibility that you may not have as much terrain clearance as you think.
Over mountainous terrain, a ground echo that intrudes inside the 10nm arc with tilt adjusted for SAP should command your undivided attention. You are less than 4,000 feet above a peak. Perhaps you are below it. One that intrudes itself inside the fivenauticalmile arc is cause for alarm, unless you have it visually, because you cannot be more than 2,000 feet above its peak. Within about two minutes you'll discover if you are below the peak.
(3) Radar shadow enhancementThis skill is the most important of all. Failure to identify a radar shadow is the greatest cause of convective weather accidents.
The tragedy is that they just about jump off the scope at you when you're aloft if you would just select the 50nm range and adjust your tilt so the bottom of your beam is angled down four degrees.
The radar shadow in the accompanying photo (Figure 4) is that black wedge trailing off into the distance at the 11 o'clock position. The black area on the right is a large lake.
Shadows always point directly away from the antenna. They have a feathered edge, as in the photo, and they continue outward as far as ground returns can be painted. (If the sides of a shadow come together in the distance, you are above the object casting it.)
Radar shadows are caused by a weather system so dense that radar energy cannot penetrate it just as a shadow from a flashlight is caused by an object so dense that light is not able to penetrate it).
Notice in the photo that energy is penetrating the left side of the storm; it cannot penetrate the right side.
The rule is, never, never, ever continue toward a radar shadow. Failure to recognize a shadow and abide by the rule is the cause of 90 percent of convective weather accidents. Eastern Airlines Flight 66 crashed in a shadow at JFK in 1975; Southern's Flight 242 crashed in a shadow at New Hope, Georgia in 1977; Air Wisconsin crashed in a shadow northeast of Omaha in 1980; Pan American crashed in a shadow on takeoff at New Orleans in 1982; USAir crashed in a shadow at Detroit in 1984; Delta Airlines Flight 191 crashed in a shadow at DEW in 1985; a corporate Westwind crashed near several shadows in 1986.
Evidence is abundant that airplanes cannot fly through weather that casts a shadow. Rainfall rates in a shadow producing storm frequently exceed the certification limits of the aircraft and engines. In addition, shadowing storms will contain microbursts, downbursts, large hail, extreme turbulence and very possibly, tornadoes. A storm casting a shadow can be identified with tilt up, but the clues are subtle and easily misread. With the SAP technique they cannot be missed. Those who know how to identify and read such indications are safe pilots; those who don't may as well turn off their radar. It's useless to them. (By the way, don't rely on the gimmicks of some newer radars that purport to identify shadows but often do not indicate a shadow where there is one. Put your tilt down and watch for the real shadows.)
In these few pages we have barely exposed the tip of an iceberg. We've touched on only two of hundreds of techniques, shortcuts and radar facts that can never be discovered in years of selfeducation. We hope that the NTSB will have second thoughts on their tadt agreement with the statement, .... .with any airborne radar device, written instructions and classroom academics are highly inadequate."
Meanwhile, don't let that attitude be the cause of your accident. Practice the TIP, REP, TUT and SAP techniques until they become second nature with you. Then steer far, far away from all radar shadows.
Those Confusing
Side Lobes
The fundamental truth in understanding tilt management is that radar detects and displays only those objects that fall within the limited area of the three degree to 1 degree conical beam. Objects not "Illuminated" by the finite dimensions of the beam are not detected.
But for every rule there is an exception. If the antenna installation is poorly engineered or if the radome is imperfect, ghost images appear on the indicator. Several weather echoes can be seen in the accompanying photograph. The one at 12 o'clock and two nautical miles, however, is not weather. It's a false echo.
False echoes are commonly referred to as "side lobe returns," or simply as side lobes, although in reality they are caused by refracted energy from the main beam. Metal objects in the radome cavity, water trapped In the honeycomb of the radome, a metallic paint strip. around the nose or one of those horrible plastic nose caps can generate side lobe returns.
The cause should be corrected by taking the aircraft to a firstclass radome repair shop, but almost no one bothers, so pilots must learn to live with false echoes.
In some instances a spear of refracted energy points downward, bounces off the ground and shows up as an echo at 12 o'clock. No matter where you place the tilt, there is the echo at 12 o'clock. In other instances the spear points to the side so that you see a sidelobe return, also at 12 o'clock, only when flying abeam a storm.
In still other instances, you see no side lobe return when your radome is dry; you see one only when your radome is wet or has Ice on it. Whenever you're in weather, you always have a false, contouring storm at 12 o'clock, never more than five or six miles distant.
A necessary radar skill is learning to recognize and ignore side lobe returns. There are several ways to identify them. First, just observe your radar on clear days. Execute TIP and if you see an echo, it's a side lobe. Mark it in your mind as something to ignore.
If you see side lobe returns only when in weather, however, identifying them gets a little trickier. One clue is movement. An echo that you cannot catch up to is a side lobe return because storms never move as fast as an airplane flies. An exception (there's that exception again) is when you descend. A side lobe caused by energy spilling off the antenna and pointed straight down will appear at a range roughly equal to aircraft altitude agl. Therefore, when you're descending or flying toward rising terrain, it will appear that you're overtaking a side lobewhich is something else you must observe on several flights and learn to ignore.
If all else fails, select the 50nm range (or as close to 50 nm as possible), run your tilt down until you have a solid ground paint from 10 or 15 nm outward, then back way off counterclockwise on your gain control. If a shadow appears behind the suspect echo, it's real; if not, it's a side lobe because side lobes do not cast shadows.
Last edited by Agaricus bisporus; 9th May 2011 at 20:01.
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VERY VERY VERY intresting, thanks much mates!
In fact, my question was to have confirmation about the rule tilt*NM*100 and so on..
Here the original PDF file of Airbus "Optimum Use of the Weather Radar" as told by Slasher:
http://www.airbus.com/fileadmin/medi...V_WX-SEQ07.pdf
Let's keep the discussion going.
Thanks again!
Moto
In fact, my question was to have confirmation about the rule tilt*NM*100 and so on..
Here the original PDF file of Airbus "Optimum Use of the Weather Radar" as told by Slasher:
http://www.airbus.com/fileadmin/medi...V_WX-SEQ07.pdf
Let's keep the discussion going.
Thanks again!
Moto
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Location: USA
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Graybeard is correct....
Gentlemen (and Ladies),
Airbus has excellent documents that give a nice explanation of
how to use the RADAR. One, Optimum Use of Weather RADAR is one.
Another is Getting to Grips with Aircraft Surveillance.
Airbus also has a nice PowerPoint on the subject, as well.
I have to chuckle when people go through the math to determine the
tops of the storms. My response is: Why bother? Unless you're at relatively high latitudes amd storms are relatively low (low tropo. perhaps),
trying to determine the tops is a nice academic exercise, but has little practical use.
Yep, there it is....I said it!
Most major carriers have an SOP regarding 'topping' TRW. My old company did....still does, I'm sure....but, it's been several years since
I retired....don't know the current SOPs.
But, the old SOP....You have to top a storm by at least 5000 feet. OK, fair enough. My A-320 series has a max altitude of FL390. So, in order to 'top' a storm and still be in compliance with the SOP, the storm tops had to have max of 34,000 feet.....and this is assuming you're at FL390. If you're at FL350, the max storm tops can not exceed FL300.
So, even if you do try to top the storm, in many cases you may have a smaller-than-comfortable buffet margin....a place you don't want to be if you get into some rough air.
My point: For the most part, you go around the storm, not over it.
Agreed, tilt control is arguably the most critical, yet least understood and most misused control on the RADAR unit. A close second is the Gain control.
And, agreed, many airlines provide poor training in the use of RADAR. On one hand, you have United, where the training is typically good. On the other hand, you have XXXX, where even the instructors lack knowledge, training, and experience in this area. At my old company, all new hires went through two days of RADAR school, taught by an avionics guy....not a pilot. (Yes, that was tough....this guy was brilliant....but, he thought most pilots were stupid. Maybe he was right.)
So, my post is mostly bullXXXX, take it for what it's worth. All I can say is, read the documents, read your company's SOP, practice using the RADAR in the daytime in relatively good weather....where you can correlate what you see on the screen to what you can see outside the window. Practice, practice, practice.... And, you'll learn what the RADAR can and cannot do for you....depending on latitude, season of the year, type of storm system, etc. Do this over a period of 30 years, and you'll get really good at it.
Then, it's time to retire....
Fly safe,
PantLoad
Airbus has excellent documents that give a nice explanation of
how to use the RADAR. One, Optimum Use of Weather RADAR is one.
Another is Getting to Grips with Aircraft Surveillance.
Airbus also has a nice PowerPoint on the subject, as well.
I have to chuckle when people go through the math to determine the
tops of the storms. My response is: Why bother? Unless you're at relatively high latitudes amd storms are relatively low (low tropo. perhaps),
trying to determine the tops is a nice academic exercise, but has little practical use.
Yep, there it is....I said it!
Most major carriers have an SOP regarding 'topping' TRW. My old company did....still does, I'm sure....but, it's been several years since
I retired....don't know the current SOPs.
But, the old SOP....You have to top a storm by at least 5000 feet. OK, fair enough. My A-320 series has a max altitude of FL390. So, in order to 'top' a storm and still be in compliance with the SOP, the storm tops had to have max of 34,000 feet.....and this is assuming you're at FL390. If you're at FL350, the max storm tops can not exceed FL300.
So, even if you do try to top the storm, in many cases you may have a smaller-than-comfortable buffet margin....a place you don't want to be if you get into some rough air.
My point: For the most part, you go around the storm, not over it.
Agreed, tilt control is arguably the most critical, yet least understood and most misused control on the RADAR unit. A close second is the Gain control.
And, agreed, many airlines provide poor training in the use of RADAR. On one hand, you have United, where the training is typically good. On the other hand, you have XXXX, where even the instructors lack knowledge, training, and experience in this area. At my old company, all new hires went through two days of RADAR school, taught by an avionics guy....not a pilot. (Yes, that was tough....this guy was brilliant....but, he thought most pilots were stupid. Maybe he was right.)
So, my post is mostly bullXXXX, take it for what it's worth. All I can say is, read the documents, read your company's SOP, practice using the RADAR in the daytime in relatively good weather....where you can correlate what you see on the screen to what you can see outside the window. Practice, practice, practice.... And, you'll learn what the RADAR can and cannot do for you....depending on latitude, season of the year, type of storm system, etc. Do this over a period of 30 years, and you'll get really good at it.
Then, it's time to retire....
Fly safe,
PantLoad
