Cloud Phase

As a rule, I don't like to write about accidents; I prefer to write about preventing them. Still, I read about accidents, and I was interested in Scott Dennstaedt's analysis of the weather factors in last week's tragic crash of a TBM-700 in New Jersey. I have a little TBM-700 time, working with an owner who wanted to get his ATP who quickly realized that it was far more cost effective to do so in a simulator. It's an amazingly capable airplane.

But, as I said, I don't write about accidents.

Dennstaedt's analysis of the weather factors included a weather "product" (as the National Weather Service people call it) that's new to me, the Cloud Phase chart. It took me a second to realize what this means: in chemistry we learn that matter is generally in one of three phases, solid, liquid, or gas. So the cloud phase refers to the water in the cloud. If it's solid, that's ice, which is less likely to stick to an airframe; if it's liquid, though, and cold enough, then it is more likely to stick to the airplane and freeze.

Here's a recent example, showing the cloud phases for the Eastern USA. The most important color is baby blue, indicating cold liquid water. The Ohio Valley, Michigan, and northern New England have cold liquid water clouds.

Compare that with the "Maximum Icing Severity Chart" available from aviationweather.gov. This shows the potential for heavy ice throughout the Ohio Valley, Michigan, and northern New England, with SLD (Supercooled Liquid Droplets) potential in northern New England. This seems to be associated with an occluded front that runs from a low centered near Kingston, Ontario (CYGK) that runs to New York City (KJFK), which becomes a cold front that seems to be forming a wave off the DelMarVa peninsula off Dover, Delaware (KDOV).

It should be obvious that low pressure over a Great Lake will form ice, but in cases where the situation is less obvious the cloud phase chart could be a useful aid in making a go-no go decision.

Now here's the trick: it's really hard to find the chart. I finally found it at a NASA-Langley website. Click on the Conus links under GOES EAST or GOES WEST, then use a drop-down menu to access the Cloud Phase chart.

Not easy, but nothing about icing is ever easy.

Heat on Pitot Tubes

There's an article about pitot tubes in the current (January 6, 2009) issue of Aviation Week and Space Technology. We start to learn about pitot tubes during Flight Lesson One. Learning about pitot tubes changed my academic career, at least a little: in pure Mathematics, we don't need instruments, and we certainly don't learn that instruments need to calibrated.

Like any instrument, a pitot tube is designed to work under specific conditions. AF447, which crashed into the Atlantic Ocean last year, had pitot tubes that were certified to handle temperatures as low as -40C, although it is not unusual for turbine airplanes to fly in colder air. For example, the King Airs I flew were limited to outside air temperatures above -53.9C; certification down to -40C is not enough. Now, some are calling for pitot tube certification down to -70C, because temperatures that low occur in the very high thunderstorms of the Intertropical Convergence Zone. Nobody seems to know what shapes ice crystals take at those temperatures, and, besides, it's hard to keep something warm when the ambient temperature is that cold.

The scandal is that this deficiency was first identified in the mid-1990s, but the airplanes still have inadequate pitot tubes.

I am reluctant to confess that I have never had a good attitude toward pitot heat. That overstates the case, because my attitude is consistent with whatever airplane I'm in. In something like an Archer, that is, a single without deicing equipment, I don't think about pitot heat much, because I am going to work like crazy to stay out of any icing conditions. (The few times I've had airplanes like this in the ice I turned the pitot heat on.)

The first airplane I flew that could handle ice was the Piper Seneca II. We had four of them, flying three scheduled night IFR routes in Idaho and Utah, and I was the check airman. We all knew that the pitot heat didn't work well, and expected it. You would be flying along at 25" manifold pressure (which was good for about 130 KIAS in level flight at our altitude), but the airspeed needle would hover around 60 or 70. We joked about it, but as check airman I made sure that everyone knew the target power settings for each maneuver. When the airspeed indicator failed you were pretty sure that you could put the gear down as long as you were flying level at the right power setting.

This kind of thinking might have helped the crew of Birgenair 301 in 1996. The airplane left Puerta Plata, Dominican Republic, at night. Their's was not a pitot problem -- the static ports were taped over -- but the pilots (both the human kind and the auto- kind) were confused by the false airspeed readings. They are the reason my Seneca pilots knew that if they were 8 degrees nose up and had 31.5" of manifold pressure at 2450 RPM then the indicated airspeed was about 120.

It's an early instrument lesson: PITCH + POWER = AIRSPEED. Keep this in mind and a pitot tube failure is easier to handle.

I never flew a King Air with the pitot heat off. Not once. We turned on the "hot five" after cleared for takeoff: both fuel vent heats, both pitot tube heats, and the stall warning heat. Look at the picture: you can see that ice protection is very important in King Airs. And, the King Air has fantastic instrument redundancy: it has at least two of everything, powered differently, with redundant power sources and redundant buses ("dual-powered buses"). Not to sound redundant, but it was easy to be comfortable. Or even complacent.

I have seen a lot of pilots (myself included) chasing airspeed indications. We all know that's bad, and we know how to avoid it: use the right pitch attitude. One place that happens a lot is with engine failures. Best glide speed! people say, and raise the nose until the airspeed says, say, 60. The problem is that because of instrument lag, when the airspeed indicator says 60 the airspeed is really more like 50. Too slow! they say, and lower the nose until the airspeed says 60. Only now the airspeed is more like 65. There might be a number of oscillations. The pilot is focused inside instead of outside, and is generally behind the airplane. It's easier to set up the right pitch attitude, find a place to land, and only then fine tune the airspeed.

I often cover the airspeed indicator when failing the engine on a student. That's two lessons: instruments fail, and PITCH + POWER = AIRSPEED. Both taught long before instrument training. Maybe they should be taught well after, too.

The Weather Man (and Woman)

With this week's awful weather I have been spending more time at my university office, doing research and getting ready to teach a new course in the Fall. The work expands to fill the available time...

The graduate director came by to introduce a new graduate student. "He's a retired meteorologist," she said.

She turned to him "Jim is a pilot." She has flown with me to several conferences, and her husband has flown with me for fun.

And this is where it gets weird. "'Mister, I met a man once,' it always begins."¹ The man (it's always been a man) is a professional or military pilot. "The weather was awful and I said 'Do you want to fly in this?' I don't think so..."

"What was the weather?"

"Really low. 300 overcast." It's always 300. Going to an airport with an ILS. Not 100; that's below minimums, so nobody would go. Not 200; that's at minimums, increasing the pucker factor. But always 300. Now, depending on who you are and what you fly, 300 overcast may or may not be low. To a military pilot in a jet, it's no problem (assuming there's an ILS). To an ATP in a turbine airplane, it is definitely no problem. To a student pilot, or to a guy in a J-3, it's probably a big problem. But they're never briefing a student in these stories.

In defiance of all logic, meteorologists and pilots can't communicate. I got a hint of this in November, when a student and I visited the local Weather Service office. A couple of other encounters reinforced the feeling, because I keep hearing the same story.

Their point is that we don't listen to them; mine is that they don't listen to us. Conclusion: they don't know much about flying. We don't know much about weather. Yikes!

I've had lots of wacky briefings from meteorologists. There was the Base Meteorologist (I flew with an Air Force Aero Club). He spent his day briefing C-5s and B-52s. "I dunno," he shook his head, "moderate ice at Flight Level 230." But I was flying an Archer!

I told one of our local forecasters a story about a flight over town. "There was a rotor cloud, and I had my student fly along the edge of it so he could get a feel for the turbulence."

The forecaster turned white. "You flew below a rotor? You're lucky to be alive!" Well, I disagree. Pilots fly into rotors all the time (glider pilots routinely do it when towing into the wave, and that's in formation). They are very rough, but airplanes are very tough. When I was flying turboprops I would go out of my way to avoid rotors, only because it might make the passengers uncomfortable. But I least I knew where to look.

It seems like to most meteorologists we still fly "Piper Cubs" into dirt strips, and that's their idea of a G-1000 Turbocharged 206. But of course the 206 can reach that ice at 2-3-0 (that's bad), and the 206 at least has XM weather so has a chance to dodge those thunderstorms (that's good). And a proficient instrument pilot can shoot an ILS to minimums in an Archer while pouring a coffee and telling a dirty joke. I don't think they know about LPV approaches with a 250 foot decision height.

But we pilots are worse. We don't turn white when they tell their stories; we just nod off.

Whose fault is this? The fault, dear Brutus, lies not within the stars, but within ourselves.². The fact is, pilots are not demanding that the National Weather Service provide quality briefings, and we are not helping them give us quality briefings. We have not told them what we need, and what we can and cannot do.

And it's too late.

Since the privatization of Flight Service, the National Weather Service is not allowed to do pilot weather briefings. They make aviation forecasts; they enjoy discussing the weather with us; they make the observations (or, should I say, supplement the observations?). But they may not brief us!

And so we have reached the era where pilots brief themselves. We use DUATS or DUAT or FltPlan.com, supplemented with NWS's Aviation Digital Data Service. These give us the facts, but we also need that extra insight from the experienced weather guy.

Sometimes, he's right.

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¹Casablanca
²Shakespeare, Julius Caesar

I shouldn't have...

...but I did.

Back in January I whined that my wanderlust was unsatisfied. I am logging lots of time, and having lots of fun doing it, but I wanted to go somewhere. I made a day trip in March, but now it was May and my friend in California said that the fishing was good. School was out. I'd been bringing lunch to work, so I had a little money. It was time to go flying!

Our club's airplanes are IFR but not modern IFR (see this post, but it's an axiom of aviation that if you pick a day to fly VFR too far ahead the weather will turn on you. In this case, the weather turned near the Idaho-Nevada line; I was down to 7200, and the terrain ahead was at 7500, always a bad plan! I turned tail to Twin Falls where I landed, ran inside, and filed IFR. (My apologies to the student pilot who watched me do this; I hate to let a teaching opportunity pass, and he was curious to learn more, but my friends were waiting for me at the airport in California.)

I walked out to the airplane and taxied out.

"60N, I see you filed the SNAKO departure, would you rather turn directly to the airway?"

Gulp. "Um, yeah, I was in too much of a hurry to check the crossing restriction. What is it?" The departure goes straight out for 10 miles over low terrain, then follows a DME arc. It basically adds 10 miles to the flight. But, being lazy, I chose the safe route. This was silly: I had just flown over the area with a sectional open on my lap, and knew that I could outclimb the terrain.

He removed the DP from my clearance and I just turned down V293.

The MEA is 12,000 and the bases were around 11,000; I spent the morning bashing in and out of the clouds. The temperature was right at 0C, so there was a chance of icing, but I was using the "reasonable and prudent pilot" interpretation. Since the bases were high and it was warm at the surface, the probability of a dangerous icing encounter was negligible: if the ice got bad I would find a hole, cancel IFR, and descend to warmer air.

There was a little ice. By the Aeronautical Information Manual, it was somewhere between a "trace" and "light;" there was no hazard.

But the flight was quite easy. I hunkered down and watched the gauges. The key to smooth instrument flying is practice, practice, practice. I got 1.9 hours of actual IFR time.

Nevada is pretty empty, and the only time Center called traffic to me was east of Reno. "I'm IMC," I replied, when he told me of opposite direction traffic 1,000 feet above. I tried to look up into the clouds, but that was pointless. Then "Mooney XXX has the traffic in sight."

"That's a good trick," I said on the air, "since I'm in the clouds." Was my tail sticking up like a shark fin?

"TCAS," he said. Whatever.

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With nobody to talk with, I critiqued myself. One thing I noticed is that it takes a lot of concentration to keep the pitch corrections small. My students, and I, tend to jam the nose right back to where it should be after a bump, but a slow, delicate slide works better. But being delicate is doubly difficult in the clouds or under the hood, so I am going to try having my students learn this maneuver by watching the horizon. It's amazing how slowly you can move the nose up and down, and slowly is the right way.

Except when you hit the mountain wave and start sinking at 10 knots (1000fpm)! But that's another post.

Icing

Winter in the desert means really big changes in the weather, even over the course of a day. It's not unusual for us to have a morning temperature of 5F/-15C and late afternoon temps of 38F/3C. High pressure sits over the region for days, causing temperature inversions and freezing fog (the source of the rime in the picture) with really low visibility. I remember one winter a few years ago, flying an ILS into Salt Lake City when the visibility was 1 mile and finding that I was a little distracted by having so much to look at outside the airplane. When the visibility is 1/2, it takes longer to ask "Are those approach lights or touchdown zone lights?" than it takes to reach the ground.

The fog is so thin that the chance of "airframe icing" during the approach is negligible: your time in the clouds is best measured in seconds. It's also common to have a low ceiling (200 to 400 feet) with good visibility, with cloud tops only 2500 feet above the ground. That puts the tops at some of the intermediate altitudes for instrument approaches, and it's always a little eerie to fly an approach in severe clear with the wheels dragging through the clouds, especially at dawn.

In these conditions it doesn't take long to climb through the clouds, and even if you did pick up some ice coming through it would sublimate quickly in the intense sunshine above. Still, a lot of pilots with good instrument skills hesitate to launch into these conditions, not because it is dangerous, but because it is illegal unless the airplane is certified for flight in "known icing conditions."

Now, the FAA has clarified their interpretation of "known ice" in a letter from the Chief Counsel's office. (You can read about it here.) The short of it is what will be called the "reasonable and prudent" rule: if a reasonable and prudent pilot would consider that the current conditions imply negligible risk from icing, then "known icing conditions" do not exist.

I have mixed feelings about this. On the positive side, I may now launch through the morning fog, confident that there is severe clear a few miles away, sometimes. Or consider a day with a scattered-to-broken layer at 1,200 feet AGL. We had one of these the other day, and my student and I flew a circuitous route toward clear air that kept us in class G airspace so we could fly at 1,000 feet AGL (this is impossible in much of the world, but we can do it). We flew our maneuvers in the clear, and descended for the return home. The clouds were only about 1,000 feet deep, but by the old rule that would have been known icing conditions and we would have been in violation. With the new interpretation, I would fly an instrument approach and be done with it.

On the negative side, a few years ago a friend was giving some instrument instruction in a 172 near Rexburg, Idaho. The cloud cover was far from thin, but there were no pilot reports of icing. My friend and his student did the instrument approach into Rexburg, which is based on the Idaho Falls VOR; see the chart to the right. They got no ice, and took off IFR, planning to fly the ILS 20 into Idaho Falls. It's only 20 miles or so, but they didn't make it to Idaho Falls. They had so much ice that they could not hold altitude, and stalled attempting to land in a field. They were both badly banged up, and the airplane (which, by the way, was the first airplane I was ever paid to fly) was destroyed.

I'm pretty sure that my friend (now an RJ captain, his license suspension forgiven by his company) would claim that he had been reasonable and prudent: he had just flown the route without ice. But there was still too much ice for that 172.

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So what is a reasonable and prudent pilot to do? First, use your head: you can't climb through 5,000 feet of ice in a 172. You can almoist surely climb through 500 feet, though. Maybe you can climb through 2,000 feet, and maybe not. What else?

Check pilot reports, either in your preflight briefing or at AviationWeather.gov. If there's a control tower, call them and ask what others have reported. You need to do this, because pilot reports don't always make it through the system to the pilot. I once flew an Archer from Washington National Airport to Griffiss AFB, NY, at night. I was on top and the air was smooth. At every handoff, I asked if there were pilot reports of icing, ready to make a new plan if there was going to be a problem. "No ice tonight" was the universal reply.

Finally, NY Center handed me off to Griffiss Approach. "Any ice tonight?" I asked. "Oh, yeah, everyone's had ice tonight." The controller was almost laughing. Grrr.. (Actually, it wasn't bad until I landed and found braking action nil on the ramp.)

Finally, you can check the Experimental Icing Forecasts at Aviationweather.gov.

Even with this, you need a plan. Ask yourself what you would do if there is more ice than a reasonable and prudent pilot would expect?. Knowing that there is clear weather a few miles away, or that the surface temperature is well above freezing, or that the tops are low makes launching reasonable.