In the past two weeks I have been assigned a lot of flights between Austin Texas and Salt Lake City Utah, meaning that I'm spending a lot of time in thunderstorm territory. Naturally aircraft need to avoid thunderstorms because of the destructive hail and violent wind shears, but you may be surprised to learn that "avoiding thunderstorms" is not as easy as you may believe. Yes, all modern airline aircraft are equipped with weather radar and use it as the primary avoidance tool, but interpreting the real story behind the radar picture is not as easy as watching TV. In fact, it very much reminds me of my submarine days when we used sonar to navigate through ice ridges when transiting through the Bering Strait into the Arctic Ocean. I thought you might be interested in a comparison of the two.
First of all, both radar and sonar work in the same general way; a beam of energy is transmitted (through air for radar and through water for sonar) and is bounced back or returned for processing and display. The strength of the return is proportional to the size or density of the mass reflecting the return, and the time it takes to bounce back is proportional to the distance to the target. Simple enough. Imagine that I have placed you in the middle of a dark, open room of unknown size. I then hand you a bucket of tennis balls and tell you to start tossing them at your surroundings. As you tossed tennis balls and listened to where they hit the wall and how long it took you would eventually get some idea of the size and shape of the room you were in. Now, let's make it a little easier and give you a flashlight. Same dark room, but this time I hand you a flashlight and tell you to walk around and find the dimensions of the room, but don't bump into anything while moving around. Should be easy, right? What if I only allowed you to hold the flashlight straight out and did not allow you to point it at the floor or at the ceiling? Think you could avoid tripping on a few objects laying about, or perhaps a fixture hanging from the ceiling? What if I placed some objects painted flat black in the room? Even with the flashlight you might not see them well enough to avoid hitting them. Now you understand some of the limitations of both radar and sonar.
In my day, submarines used the AN/BQS-14 sonar for ice detection and avoidance. Originally developed to detect underwater mines, this sonar underwent many upgrades and eventually became known as PFLU (pronounced Pee-Flu) or Pulsed Forward Looking Upgrade. This sonar sent a narrow beam of energy straight forward from the sail of the sub and the returned data was displayed on a screen in the control room. But wait...why do you even need sonar to avoid overhead ice? Why don't you just submerge the submarine to a safe depth and sail along safely below the ice? Believe it or not, the bottom of the Arctic ice pack is NOT flat, not even close. The Arctic is a very dynamic environment, and the ice pack is not one solid sheet of ice. Hundreds and thousands of ice islands are floating along slowly with the current, and like tectonic plates of the earth, when they collide they are forced together with tons of pressure which forces the ice both up on the surface, and down into the ocean. With a few exceptions the bottom of the Arctic ice pack looks like the upper surface of a cave, with thousands of staligtights hanging down. Naturally if the water was deep enough you would simply pass under these ice "keels", but unfortunately, the Bering Strait is very shallow and the winter ice keels can actually reach all the way to the bottom. Bad Ju-Ju to hit those. Take a look at this photo of our actual PFLU display during the Bering transient:
In this example, the submarine is operating submerged in 157 feet of water (!), and is currently only 26 feet from the bottom (!!!). The sonar is showing us a significant ice keel target ahead at 600 yards, but we need to know if that keel reaches down to our depth or if we will pass safely below it. We will know we are in trouble if the return is still present when we get within 300 yards of it, which is the magenta line you see on the PFLU display. This is because just like aircraft radar, the PFLU beams out its energy in a cone shape, narrow at the transmitter and gradually widening out with distance. Almost any ice keel will be detected at distance because the beam is very wide, but as you approach these shallow keels they will eventually pass out of the beam and their return will fade from the screen. If however you are still seeing a return at 300 yards, it is a sure bet that this ice keel is at a dangerous depth and you must turn to avoid it. Also note on the PFLU display the submarine's speed of 5 knots. Imagine being in a 5000 ton nuclear submarine, 26 feet from the bottom of the ocean, watching as you move closer to an ice keel, hoping and praying that the return will soon fade, only to realize that you are now 300 yards from it and you must now take evasive action within the next minute or you will hit it. Now THAT'S exciting! Through hours and hours of experience we became pretty efficient at interpreting the display, understanding the limitations imposed by its fixed beam, and avoiding the ice.
Yesterday Captain Tim and I were returning to Austin from Salt Lake City, and as expected there were a lot of thunderstorms brewing up between us and a cold beer on the ground. Assuming that you can see these monsters they can be easy enough to avoid, but while flying at 35,000 feet (FL350), we were in a solid blanket of cloud and were just as blind as a submerged submarine. Our first defence is to try to stay visual, so we climb to FL370 which works for while, but eventually the clouds rise and envelope us again, leaving us with our WXR-840 weather radar as our primary defence tool. Although there are some similarities, there are significant differences between this radar and the BQS-14 sonar on the submarine, not to mention the operational differences between an aircraft and a submarine. Let's discuss the equipment differences.
WXR-840 weather radar uses a flat plate "dish" in the aircraft nose that sweeps to the left and right of the aircraft's flight path. Unlike the sonar however, the 840 can be tilted up and down. Using this tilt control is absolutely critical to safely avoiding killer thunderstorms. Here's why. Without getting into too much detail, a mature thunderstorm is a highly complex system, and has several well-defined zones or areas within its boundaries. Below the clouds it typically the rain curtain and its associated wind, lightning and thunder. That's what we all experienced on the ground countless times, and depending on the storm's strength they can be scary. What's really scary is the middle section of the vertical storm, which contains up and down drafts of fantastic strength, wind shears that can instantly cause structural damage to aircraft, and most often heavy quantities of rain and wet hail. It is this middle section that reflects radar energy so well and shows up vividly on our radar display. The upper section of a storm, and remember that these devils can reach altitudes much higher that the 41,000 our CRJ can climb to, contains very little moisture. However, they still contain the wind shear, violent up and down drafts, and often dry hail, and unfortunately none of these will reflect radar energy. It is not unheard of to have pilots flying aircraft with working weather radar fly into the dangerous upper areas of a storm because they had their radar set to sweep only the upper altitudes where they are flying, not realizing that unless they tilted their radar so that it could look down into the reflective zones of the storm there stood no chance of seeing anything. These encounters rarely end well for either crew or passengers.
Take a look at the photos below taken on our flight yesterday (click on the photos to enlarge them).
In the photo on the left, we are flying at FL360 heading toward Junction City and are "painting" a moderate return about 20 degrees left of course at about 70 miles. In the upper right corner of the display you will see "T+1.0" which is our current tilt setting, which is ahead and slightly up. In the second photo taken only a minute later, you can note two things...the tilt setting has been lowered to -0.5, and that same storm suddenly looks a lot meaner. This is what it looked like from the cockpit window:
I had it easy on my leg since I was above the cloud deck and could visually avoid the storms. On Tim's return leg we were "in the goo" for almost the entire trip, leaving us to navigate around the bad Ju-Ju using the WXR-840 display, and more importantly, our experience in reading those returns. With a few zigs and a few zags we were able to get our 73 passengers safely to Austin on time without them even knowing anything about the tactical decisions being made each minute of that flight.
Once back at the hotel, Tom and I were discussing the physical and mental toll that a six hour return flight can place on a flight crew, and we both realized that from the minute the first checklist starts on the ground until we complete the final checklist at the destination, flying in this summer thunderstorm environment forces the crew to make literally hundreds of thousands of decisions, one on top of another, non-stop. There is no room for error, for just like in a submarine operating 26 feet from the ocean bottom with ice passing feet above you, navigating through thunderstorms is unforgiving, and a single mistake can prove fatal. Unlike submarines however, airline crews do this every hour of everyday, all around the world.
Here's hoping you can avoid all of the bad Ju-Ju in your life!
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