San Francisco has some of the most unusual weather in the world, a fact perhaps first noted by Mark Twain in his now famous quip, “I spent the coldest winter of my life one summer in San Francisco.” Tourists still don’t get it, assuming that San Francisco means California and California means fun in the sun, so pack those shorts and flip flops. My wife and I regularly hike up to Twin Peaks, a 900 foot hill above our house, partially for the exercise and partially for the 360 degree views of the entire Bay area, and there are often vendors there making a nice living selling sweat shirts with Golden Gate Bridge emblems on them to freezing tourists for outrageous prices. The wind off the ocean up there is strong and cold, even in July.
The reason it is cold is because it has just crossed hundreds of miles of ocean that is 50 to 55 degrees at the surface. The reason it is strong is because the central valley heats up to 100 degrees or more every day in the summer, creating a powerful vacuum that sucks that cold air inland, dragging cold fog along with it. The gap known as The Golden Gate, the narrow opening that separates San Francisco and Sausalito (a gap that was there for a long time before a bridge was built to cross it, in fact for a long time before it was called The Golden Gate), creates a kind of venturi, aggravating the wind and fog, and creating, at times, some of the most localized stormy weather in the world. And as you can imagine, the same vendors selling sweat shirts on Twin Peaks also do a nice business at both ends of the Golden Gate Bridge.
In the winter the pattern changes somewhat. The water is still cold, but the central valley doesn’t heat up in the winter like it does in the summer, so cold wind and fog is much less common. But winter is when Pacific storms, many originating thousands of miles away and having a tremendous amount of water and massive blocks of cold air from the Arctic to support them, hit the West Coast. San Francisco is no more vulnerable than any other area along the coast, but it’s unique geography again exacerbates the conditions when one of these storms does hit, the hills along the shore lifting the winds, the Bay itself swirling them, and The Golden Gate accelerating them.
We have such a storm forecast for later today and on through the weekend, January 3, 2008 until January 5, 2008. Heavy rain is forecast, becoming snow as it hits the Sierras, up to five feet, and winds are forecast to gust to 70 miles per hour in the Bay area, up to 100 miles per hour in the mountains. It is forecast to be one of the strongest storms to hit the Bay area in several years. The last such storm hit in November of 2002. I know, because I was flying that night.
I was doing a Maui “turn”, where you leave SFO in the morning, fly to Maui, turn around and fly back later that same day. Very routine, always the same track over, the same track back, visual approach to the north at Maui, often a visual to 28 left at SFO, unless the weather is down and the wind is out of the south, when you can plan on an ILS to either 19 left or right. The only thing that varied a little bit on those turns was the weather.
That night the weather was forecast to be quite stormy coming back to SFO, but nothing that would indicate anything other than maybe some arrival delays and a rough approach. As I remember the forecast was for rain with ceilings of 1000 feet or so, visibility a mile or two, occasionally down to ½ mile, with winds of 25 to 35 knots from the south, right down 19 left and right. Oakland, just across the bay, had the same forecast, so it wasn’t a good alternate that night, but Sacramento was forecast to be much better, the storm really wasn’t supposed to hit it hard at all. So despite the somewhat adverse weather, not going was never really a consideration: we would probably have arrival delays—you always did when the weather went down to instrument conditions at SFO—but we had plenty of fuel for that, and Sacramento was a good alternate if needed, and we had plenty of fuel to get there. The weather would be rough but manageable, we had anticipated delays, and we had a good out, so I was comfortable leaving Maui.
Approaching the coast all seemed well: weather was as forecast with no delays anticipated. Then, suddenly, things changed. Just seconds from PIRAT, a common holding point for arrivals from the west, center told us to hold as published, SFO was closed due to a “microburst.” “Microburst?” I thought. Thunderstorms were possible anytime you have a clash of air masses, but weren’t forecast or expected. But whatever, we quickly slowed to holding speed, programmed the FMC to enter the hold, got an expect approach clearance time, which the controller said was just an estimate, who knew how long the airport would stay closed, and starting figuring up how long we could hold, which looked like an hour comfortably, maybe a little more if we were lucky.
This is one of those times when ACARS, an automated system for communicating with the company, pays for itself many times over. I got on the ACARS and quickly sent a message to dispatch telling them we were holding, gave them our fuel remaining, asked if they had any more information on SFO and asked for the Sacramento weather. They came back that a major storm was going through the whole area, including Sacramento, and all airports were reporting winds of 50 to 60 knots, gusting to 75. The best airport, ironically, was SFO because the wind was right down the runway, but it was closed. San Jose and Oakland both had the same 50 to 60 knot winds, and both had runways perpendicular to the wind. So we decided to wait it out for awhile and hope conditions improved.
They didn’t. As we approached the end of our comfortable holding time, I told the International Officer, a young pilot named Matt Gibbs, and a terrific pilot in his own right—to contact the company directly on the radio and get updated Sacramento weather, and check to see if any other airports had improved, and to tell them that if Sacramento was still the best alternate that we were going to take our chances and divert there. While he was on the radio approach control came on and said, “Amtran 123, Oakland has opened up, we are accepting approaches, you’re number one.”
I said, “Standby, we’re checking with our company,” and he said, “There’s only one opening, you’re it, yes or no.”
I said, “Yes.” I yelled back to Matt, “Tell them Oakland’s open for one arrival, we’re going there.” And we did. It was a wild ride, the aircraft barely controllable, the FO hanging on to anything he could get a hold of, the IO wedged between the jump seat and the center console, and I was hanging onto the controls as best I could trying to keep it upright. I remember at one point glancing down at the EHSI, the electronic horizontal situation indicator, which displays an average wind vector showing the direction and strength of the wind, and it was showing 70 knots directly from the right. I thought about going around, but I didn’t know where else we could go that would be any better. I could have declared an emergency and landed at SFO anyway, a closed airport, but that was a pretty extreme measure and in any case only solved the cross wind problem, not the wind itself. So I continued.
It turned out alright, but a lot of that was just good luck. I got it on, got on the brakes, had to keep flying it all the way to the end of the runway, actually it still wanted to fly even taxiing in, bouncing and bucking all over the place, and taxied to a spot on the ramp in rain so heavy I could barely see the marshaller. It was all he could do to stand upright, but he was able to indicate where to park. (It turned out he wasn’t even a marshaller, but a United mechanic who saw we had nowhere to go and just parked us, all on his own. I wish I’d gotten his name, he was a real hero.) I was so glad to be on the ground, I took a big deep breath, turned around in my seat and all I could see were four big eyes staring at me. I guess we were all pretty scared. I could second guess what went on that night for the rest of my life, but it worked out. We were down, no one was hurt, and nothing was broken.
The most unusual thing that happened though, had to do with the way modern glass cockpits are designed. The EHSI is based on the HSI—a mechanical horizontal situation indicator that is essentially a slaved gyro compass with a course deviation indicator overlay. Because the compass is gyro stabilized and slaved to a remote magnetic reading device, it is a steady and reliable indicator of magnetic heading. With an airplane symbol in the middle, the combined device shows at all times the aircraft heading at the top, and with the proper course set it shows the relationship between aircraft heading and track: when on course, the difference between the two will be the wind correction angle.
The traditional HSI is a mechanical device, but the EHSI is an electronic device and is not limited by its mechanics to a compass rose and a single course indicator, but can be configured in a variety of ways to suit different situations and different preferences or policies. One of the ways is to configure it like the traditional HSI and this is the way many pilots transitioning to EHSIs like to configure it because it is so familiar. Because the heading is always shown at the top in this configuration it is called, logically enough, “heading up.”
But the more common configuration, the one all experienced EHSI users almost always use and the one less experienced users normally transition to as they get comfortable with the EHSI, is called “track up.” That is, instead of showing where the aircraft is pointed, or headed, at the very top, the top shows where the aircraft is going—where it is tracking. Normally the differences are slight and barely noticeable because the difference between the two, which represents the wind correction angle, is usually only a few degrees. With a wind correction angle of five degrees to the right, for instance, with heading up, the top of the EHSI would show the aircraft heading straight ahead with the desired course five degrees to the left of that. With track up, the course would be at the top and the heading bug would be five degrees to the right of that. In either case, if you were to look carefully at the ground you might be able to see that where you were headed and where you were going were off by five degrees, but in almost all cases both heading and track would be close together, one up and one to the side, which one depending on whether you had selected track up or heading up.
I was using track up that night, just as I almost always did, the normal configuration for the 757. But that night, with a 70 degree crosswind from the right and an approach speed of around 140 knots or so, my wind correction angle was 35 degrees to the right of course. I broke out of the overcast at about 1200 feet with what appeared to be good visibility underneath, five or six miles. The EHSI showed me dead on course, straight ahead, but looking straight ahead all I could see was water. I could see the city of San Francisco to the right, so I knew I was in more or less the right place, but I couldn’t find the airport: it was 35 degrees to the left of where I was looking, well outside of my peripheral vision and, in fact, hidden from view by the pillar between the front window and the side window. I was directly on course, but couldn’t see the runway. It took me a couple of heart stopping seconds to figure it what was happening and find the runway behind the pillar. The greater the wind correction angle, the greater the difference between track and heading and the greater the difference between what you see straight ahead and where you’re going. I knew that, in theory anyway, some dim memory from ground school, but after that I knew it for real.
Once we were parked, it took an hour or so for the wind to die down enough for air stairs to be safely driving up to the aircraft and we off loaded our frightened but relieved passengers onto chartered buses that took them back to SFO. An hour or so after that the storm had passed through, SFO opened up again, and we refueled and flew it back there empty for the next day’s launch. Driving home I saw trees down everywhere, and most of the city was blacked out. My wife had long since gone to bed in the darkness and cold—no heat—because she didn’t know exactly where I was but wasn’t worried because she knew I wouldn’t be silly enough to be out flying on a night like that. I read in the paper the next day that winds had hit 100 mph at SFO and a maintenance shed had been destroyed along with a lot of minor damage all around the airport. So maybe it wouldn’t have been such a good idea to have declared an emergency and gone to SFO after all. Not with a perfectly good airport right in front of me.
