The other night we left Louisville for the quick half-hour flight to Knoxville, TN. It was a fabulous clear night in KY (and had been all week), but Knoxville was fogged in. Fog is not a difficult concept to grasp, but the difficulties it poses for aviation might not be something the average person has pondered. How does an airplane land in fog? How does it find the airport? The runway? How do we know where the ground is as we descend? Even apart from the critical phases of takeoff and approach and landing, an airplane spends much of its time in clouds where there is no visibility whatsoever. How does this work? Before I began flying, I hadn't the foggiest idea (sorry).
Some months ago I put up a post about flight instruments, this more-or-less standardized suite of instruments which enables us to fly when we can't see. This, plus various mechanisms and addenda to the flight instruments, is what gets us on the ground in inclement weather. Given airlines' safety records, it's obviously a pretty good system. It can enable us to take off from a given airport and enter the clouds immediately and not see the ground (or anything else) again until, say, two hours later when we're lined up with the runway and 10 seconds or less from touchdown. With a 200 ton machine traveling at 500 mph, that's a pretty cool thing. After some acclimation and experience, a pilot comes to prefer these instruments to looking out the window for information, since the instruments just give you a much better idea of what's going on. Supplement these flight instruments with the stuff we need to navigate from one place to another (and with radio equipment for talking to Air Traffic Control, etc., etc.), and one begins to see why cockpits are so cluttered with dials and knobs and switches.
Newer jets, especially those designed to fly long (trans-oceanic) distances, have the ability to land themselves. This is a sensible development, since an airplane that is about to cross an ocean needs to know with some certainty that it will be able to get on the ground at or near its destination some 8 or 10 or 12 hours hence. The time it takes to traverse these long distances is enough for weather to change radically, for forecasts to turn out quite wrong--it happens. Domestic flights are shorter and so do not rub up against this limitation so often, though a diversion to an unscheduled destination is an expensive hassle for everyone. This "autoland" technology can enable an airplane to land safely in conditions as bad as zero / zero (ceiling / visibility).
Anyway, in spite of it being able to carry enough fuel for a10-12 hour flight, my beloved DC-8 is too old to have this autoland technology (an old airplane can be updated--ours have much newer, more efficient engines and modernized flight instruments from the original specification--but things like an autoland system require an almost impossibly expensive recertification of the aircraft; so after a point an aircraft will live out its life pretty as originally designed). And so, like with many other realms in aviation where automation has crept in and left the old girl behind, in our DC-8 we must accomplish the low-visibility landing manually. No big deal, but in weather conditions like this it's an experience. On this night, as we approached Knoxville we could see that the fog was a several-hundred-feet-thick cloud layer sitting on the surface, with clear skies above. One could see occasional breaks in the clouds where surface detail was visible, and the lights of the towns and major features glowed dimly through the cloud blanket. But to get on the ground we needed to descend into the soup where one could see almost nothing and land.
The details are too technical to be interesting (though I'm convinced that some good video footage of the experience would be interesting to many people), but I can summarize a bit: we are very precisely guided in our course and descent by a couple ground-based radio signals specific to each airport & runway, the specific details charted on a published instrument approach procedure. This is the same procedure we follow on every other landing in any other kind of weather, but in low visibility it's all we have to fly by. This approach procedure takes us down to a certain minimum height above the ground (in our case, 100' above the surface), where we pick out the extremely powerful approach lighting system and use that to descend visually the final hundred feet to a normal touchdown and rollout, guided the whole while by in-runway lighting which kept us on the center line. These lights are present and turned on for every night landing, but in good weather their intensity is turned WAY down. For landing in these soupy conditions, they apply some serious wattage to the lights (so they can be seen thru the fog), and the effect is a bit breathtaking. As you flare for the landing, the lights are whirring below you at 170 mph, while every other direction shows stationary, featureless gray. Spooky.
Ironically, the taxi back after the landing was more challenging than the landing itself, as the taxiway lighting is much less elaborate. That makes sense, as it's a less critical phase of flight. There are lights and signs designed to keep one from taxiing onto a runway or into an area where one is not wanted, and we just proceed at a snail's pace and ask questions of the controller if there is the slightest question.
Even cooler yet, maybe, was for us to be descending the stairs at the end of the flight, the airplane parked on the ramp in the fog, and to hear FedEx's 727 pass close by, only a few hundred feet away but invisible in the murk, with a noise that actually shakes the body a bit, and land on the runway next to us, after following the same instrument procedure to a landing. It sounds absolutely terrifying when you hear so much and see so little (i.e. nothing).