Academic no longer

I'd always considered the answer to the question “How low can you descend on an instrument approach if you have just the approach lights in sight” to be an academic one.

Until yesterday.

Usually, this trip is a piece of cake. Wheels up around 6:45 a.m., land at Fargo around 7:45, sit around for a bit and back home sipping a beer by 5:30 p.m. It's one of my favorite routes because it's the only one, aside from the occasional ad-hoc charter, that offers a semblance of a “normal” work day.

Yesterday though, the weather had me sitting up straight in my seat.

The forecast had been decent enough, calling for 300-foot ceilings and 3-mile visibilities. Low to be sure, but doable and legal. In the Part 135 world we can't legally depart if our destination is forecast to be below minimums for an approach when we arrive. This is a good thing in that it takes the temptation to “take a peek” out of the equation.

We also can't continue an instrument approach beyond the final approach fix if the reported weather is below minimums when we arrive. This is also a very good thing in that the decision is made for us and there is no question about flying an approach in below minimum conditions and perhaps going below the minimum altitude, illegally, to try and get in.

If we show up and the weather is below minimums we don't fly an approach. Period. We either hold and wait for the weather to improve or go someplace else.

Under Part 91, the rules that govern most general aviation flying, there are no such restrictions. The accident reports are also filled every year with pilots who descended below minimums and hit something solid. It happens in the Part 135 world as well, but if you play by the rules and fly a good approach it should, in theory, end in a successful landing virtually all the time since the weather is above minimums.

Yes, the weather can change in the few minutes between the final approach fix and reaching the approach minimums but for the most part, if it's above minimums when you start it'll stay that way long enough to find a runway.

Which brings us to a second favorite question of instrument instructors, check airmen and instrument flying aficionados: “What determines if the reported weather is below minimums?”

The answer, by the way, is “visibility.” If the reported visibility is at or above the value shown on your approach plate you're good to fly the approach. Visibility is controlling, ceiling (which at first glance would seem to come into play) is not.

I'd always considered the “visibility” question to be somewhat less academic than the “how low can you go” question but still pretty far out there for most flying.

Both of those questions wound up coming into play around 8 a.m.

Fargo was reporting 100-foot overcast with 2 1/2-mile visibility in rain and mist and winds out of the northwest at 12 knots when I checked on with the approach controller.

In a nutshell, that meant the visibility was above minimums and I could legally fly the approach but the reported ceiling was 100 feet below the decision altitude, 200 feet agl, for the approach.

Or, to put it bluntly, I'd still be in the clouds when I'd descended to the decision altitude.

This is where that marvellous, and often under-appreciated, component of the Instrument Landing System came into play: The approach lighting system.

“The lights,” for short, are a godsend. They're bright enough to show through all but the worst ooze and are arranged in such a way as to give a poor, tired pilot descending out of the clouds both a horizon line with which to keep the wings level and an extended runway centerline to aid in their line up.

It's a brilliant invention and one that allows the answer to the question “How low can you go with just the lights in sight?”

The answer: 100-feet above the runway touchdown zone elevation, a value that's shown on every approach chart.

So, with all those pieces of the puzzle assembled my plan was simple enough: Fly the approach, look for the lights through the clouds, continue down another 100 feet then hope I spotted the runway.

All those questions, rules and training I'd long considered as fairly academic were suddenly very real indeed.

For the first time in a very long while I managed to fly a decent approach. Amazing how the prospect of “maybe, maybe not” aids ones focus.

At 200 feet above the snow I looked up and there were the lights, ever so dimly burning though the clouds I was still inside. No runway to be seen but the lights were good enough.

At 100 feet the runway oozed into view so I kept going, touched down and taxied in to drop my cargo. If the clouds had been 10 feet lower I wouldn't have made it in.

As I sat inside sipping coffee I thought back to my days as an instrument student, struggling mightily on every approach and overwhelmed by the workload.

I'm a decidedly average pilot and it was so tempting to just fly “ok” and not perfectly because I honestly never planned to fly an approach that truly demanded perfection and knowledge of a few arcane rules to complete.

In the end though the hard work demanded by my instructors paid off. For those of you reading this who are working on that instrument rating hopefully it'll give you a little extra motivation. It's a brutally difficult rating to earn, the toughest one in my opinion, but the sacrifice and hard work does pay off.

You might want to work up to flying approaches to 100 feet, to do so immediately isn't wise, but know that it's within your capabilities and one day you just might have to fly one for real.