Saturday, September 06, 2008

Stay Away, Stay Alive

In the summer of 1993, a family friend took me flying in his 1946 Piper Clipper. It was only my second flight ever, but I was already mad about flying; contemporary notebooks saved by my mom are filled with airplanes conceived only in my twelve year old mind. Our friend Paul kept his airplane on his own property, as he had carved out a 2000 foot landing strip in the center of his Christmas tree farm. I had just got new glasses that day; I recall the airplane being somewhat blurry as I helped Paul push it out of his hangar.

Shortly after we got airborne with the aid of a hump in the middle of the strip, we turned to the west and were confronted with a great cumulonimbus cloud that filled the sky. It was at least 50 miles off but was impressive even at that distance; lightning crackled on the horizon. "If you ever see one of those," Paul instructed the young pilot-to-be, "stay far away. Those will spit you out without so much as half a thought."

Fifteen years and 5300 hours later, Paul's words of advice have stuck with me. I give thunderstorms a wide berth. Of course, to do that you must know where they are, and that isn't always very apparent. It takes a mixture of technical skill and hard-won experience to stay a safe distance from storms at all times.

I learned to fly in Minnesota and earned my advanced ratings in North Dakota, but most of my formative experience since then was in the western US. They do have thunderstorms out there, particularly in the Rocky Mountain states, but they're nothing like the monsters spawned in the Midwest, and they seldom form lines that pose a problem for circumnavigation. Therefore I was somewhat nervous about this summer's thunderstorm season, my first in the Midwest - and as a Captain, no less. It turned out just fine. I didn't have any days flying in absolutely horrendous conditions, but there was enough ugly stuff to build some valuable experience.

In an ideal world, one would avoid thunderstorms by visual means only. This is a foolproof way to stay out of dangerous weather, and to avoid harmless but annoying bumps as well. Unfortunately, visual avoidance isn't always an option. Nasty storms often continue long into the night, when it's difficult to gauge distance from lightning. Beyond that, storms aren't always well-defined, and can occasionally be obscured by widespread cloudiness not related to the storm itself. When picking your way through a line, you might have to penetrate an area of moderate precipitation between heavy cells. For all these reasons, the FAA requires airliners to be equipped with operable weather radar with few exceptions.

The theory behind radar is simple: radio waves are sent out, and precipitation sends them bouncing back to the antenna. The radar unit measures intensity by the number of waves it receives back, and distance by the time differential between transmission and reception. The result is displayed either on a dedicated screen or - like the JungleBus - on a Multifunction Display. Light precipitation is depicted by green shading, moderate by yellow, heavy as Red, and extreme precipitation is shaded magenta. The pilots can adjust the angle at which the radio waves are sent out from the aircraft; this is called tilt. They can also adjust the receiver's sensitivity; this is called gain. Advanced units may have some other features, but these controls are the pilots' primary means of controlling the radar.

The use of these controls, and the interpretation of the results, is considered a bit of a black art in the aviation world. The problem, especially in the flight levels (> 18,000'), is that you are above the freezing level. Frozen precipitation doesn't show up very well on radar; the most detectable part of the storm may be 20,000 feet below you. Therefore, at altitude you must aim the tilt somewhat downward. This is mostly problematic because terrain reflects radio energy even better than precipitation, and the resulting "ground returns" can make it very difficult to distinguish between the ground and actual storms. It's worth noting that radio waves aren't sent out at the exact angle you set the tilt to, like a lazer beam, but scatter somewhat above and below that angle. At greater distances, this results in a great range of altitudes that may return echoes. It also varies according to the size of antenna; a small antenna scatters the radio waves more widely. At 50 miles range and a slight downward tilt, a small antenna could be returning everything from ground level to 50,000 feet.

Therefore, the usual practice is to adjust the tilt downward until the ground return is readily distinguishable. At you decrease tilt, the returns come steadily closer on the scope; this is how you know it's the ground, and not actual precipitation. Then you increase the tilt until the ground returns are at 3/4 of the set range. In other words, if you have the display set so that the top edge represents a distance of 200 miles, the ground returns should start around 150 miles away. If you decrease the range, you need to readjust the tilt so the displayed area will show echoes below the freezing level.

The use of gain depends on the unit, but a common technique is to increase the gain until the ground returns at the furthest displayed range are yellow or red. This will ensure the unit is sensitive enough to display even light precipitation at closer ranges.

The Q400, as I've mentioned before, had a terrible radar. In retrospect this was excellent training as it forced the pilot to adjust the controls exactly right to get any useful information. The JungleBus' radar, by comparison, is a dream to use. In cruise with no storms nearby, I'll set the range to 300 miles with the tilt around zero; it regularly picks up storms at 200+ miles away in this configuration, which gives me enough time to query ATC or dispatch and make a plan. The automatic gain mode is smart enough that I seldom need to set gain manually. It even has an automatic tilt mode that changes the tilt as you adjust the range, although it usually needs some fine-tuning.

The usual advice is to avoid moderate precipitation when possible and stay out of heavy and extreme returns, and to avoid by twenty miles any storms classified as severe. The rub, of course, is that one must decide what exactly constitutes a severe storm. The issuance of a convective SIGMET is a strong clue, although this can be done for merely obscured cells. On the radar, you look for large areas of extreme precipitation, oddly shaped cells, or strong gradients (where precipitation goes from light to heavy in very little distance). Visually, you look for cumulonimbus that greatly exceeds your altitude.

Avoidance technique depends on the type of storm. If it's a simple airmass thunderstorm lying along my route, I'll just request a simple deviation upwind of the cell. If it's a widespread area of storms or a line connected to a larger weather system (such as a squall line), I need more information than my onboard radar can provide to make strategic decisions. Having looked at radar, prognosis, and surface analysis charts before flight definitely helps me get the big picture, but I need more recent data to make decisions from. This is where air traffic control and dispatch come in. ATC can display rudimentary radar data on their scopes, but the onboard radar is usually better for tactical maneuvering. ATC's real value is in letting us know what other flights have been doing. It's unlikely that anybody else flying along our route would've penetrated severe weather without telling someone; the holes that they used will be used by many other flights after them. Our dispatcher has access to the latest radar image similar to those your local TV meteorologist displays, and can give us routing suggestions to keep us in the clear. Areas of weather are generally described relative to VOR navigation stations; if we are unfamiliar with any identifier, there is a national reference chart in our Jepp binders.

Squall lines can be tough to deal with; they may stretch hundreds of miles long. Jets have a distinct advantage over smaller aircraft here, because we can deviate hundreds of miles without adding a ton of flying time or fuel required. The major thing here is to ensure that your dispatcher's route planning reflects any likely major deviations, and you therefore have the required fuel on board. Then it's simply a matter of flying around the line, or if you must, finding a gap to sneak through. Again, ATC is a huge help in finding these gaps. Onboard radar isn't as reliable because you often can't tell what lies beyond visible gaps due to attenuation. This is when a return is so intense that it reflects all the energy your radar unit sends out, so you can't tell what lies beyond it. You really don't want to go through what looks like a nice gap only to find a solid wall of red or purple beyond it.

Honestly, truly ugly weather isn't the greatest challenge for most. If it's red or purple, you simply don't go there. Only an idiot would penetrate a severe storm. The harder call is when it's not that bad: mostly moderate returns with perhaps just a touch of red, or the occasional flash of lightning. If this is the only stuff in the sky, it's a no-brainer to avoid it. The tough call is when it's potentially the only gap through a severe line, or is on final approach or an ATC assigned vector. In reality the tiebreaker is often TCAS; if you can see a proceeding aircraft go through without complaint, it's probably OK. This approach surely has its limitations; an extreme case was Delta 191 deciding to continue their approach to DFW through a severe cell because nobody else had gone missed approach.

Severe weather avoidance is as much an art as it is a science. Technical knowledge is surely a baseline requirement, but experience and technique are ultimately just as important. I'm glad I had the Q400's crappy radar to train on, and relatively benign scenarios this year to prepare me for future battles. And quite honestly, if I had to choose one or the other, I'm glad I got to experience a Minnesota winter as a first officer before going out and doing it as a Captain. Speaking of which, it won't be long. Dawn returned to school on Tuesday, and right on cue, the sky clouded over and the temperatures dropped twenty degrees. I saw changing leaves for the first time today. Snow will fly before long, for sure.

2 comments:

PlasticPilot said...

Great post ! This remember me a London-City to Geneva flight in a BAE146 "Jumbolino", that I took as a passenger, on a summer evening.

For a while, we flew at sunset parallel to a series of CBs, spread from Zurich (LSZH) to Lyon (LFLL). It was nice to observe the buildups, and to look at the lightning strikes.

It became much less funn when we turned towards it. Few minutes later, the captain made a call which I will always remember: "Cabin crew, be seated... quickly !"

I did tighten my belt a bit more. I did not noted the times, but for a couple of minute we went through severe bumps. No luggage compartment got open, but if the hostesses had not been secured, they would probably have hit the ceiling quite badly.

The crew probably spotted a hole through, and decided to go, as the sole other option would have been Paris (LFPG), and arriving there at midnight, with a full load of passengers to host was not a good idea.

As I knew about the 20 NM avoidance rule, I asked the captain, when we left the aircraft, how close we were. The answer surprised me... 5 NM.

Definetly not my best airline souvenir...

Sam said...

Well, the 20nm rule - and it's more of a rule of thumb than a hard, enforced rule - applies to "severe" storms. Again, that's a pretty subjective thing. I suspect that the desire to complete the mission has colored more than one pilot's perception of whether the storm is severe or not.

In June I deviated around one line of severe storms by a good 50 miles, about the closest we got to any returns was 30 miles - and the lightning was still pretty dang impressive, it looked like we were right up against it. That line spawned a number of tornadoes through Kansas & Nebraska.