Flying Into a Fog Is Safer Than You Think

Feb. 8, 2006 — -- So it's the end of the flight, your captain has briefed you on the PA that you're landing on time despite fog ahead, and over the seat audio channel, which allows you to eavesdrop on controller-to-pilot radio exchanges, one of the pilots has just acknowledged the flight's been cleared for something called an ILS.

ILS?

"Roger, approach, BigBird 25 cleared for the ILS runway one-three, tower at the marker."

Fact is, you'd really like to know what the devil "ILS" means, but a quick rifling through the in-flight magazine reveals nothing but guidance on airport layouts and ads for learning French the easy way. Here's the explanation you can use to dazzle your seatmate on your next flight:

"ILS" stands for Instrument Landing System, a brilliant piece of technology now found in virtually every airliner's cockpit that lets us see an otherwise invisible pathway from a point perhaps 10 to 15 miles away from a fog-bound airport to a spot some 50 feet over the threshold of a runway -- an electronic ramp in the sky, so to speak.

If you've ever been aboard an airliner large or small in marginal weather, you've sat through an approach using this marvelous device. It's a piece of technological wizardry that finally emerged from testing into operational reality in January … of 1938!

That's right. Sixty-eight years ago. (There's an excellent history of the ILS here).

The fact that the ILS is still the mainspring of safe, all-weather landing capability worldwide doesn't mean the thousands of ILS systems are archaic and out of date; it merely shows how incredibly reliable and boilerplate the technology has become.

How an ILS Works

Flying a large airplane safely to a point over the end of a runway is pretty simple in daytime with crystal-clear weather. But when weather obscures the field and you can see nothing but gray outside, landing safely requires a precise, reliable pathway to make what we call a stabilized approach.

As far as the raw guidance we follow, an ILS provides two basic pathways. The first is called the localizer, and it is represented in the cockpit by a vertical needle that moves left and right over a center dot to show whether the aircraft is left or right of the centerline of the runway (think of an imaginary line running backward from the runway at the precise same compass heading and extending for many miles in the direction from which we're coming).

If, when approaching an airport on an ILS approach, the needle begins to move to the left of the center dot on the instrument, that means your aircraft has drifted to the right of the centerline of the runway and you need to make a small, somewhat subtle change of heading back toward that needle and fly the aircraft back to the centerline. When you've corrected your flight path, that vertical needle will be right in the middle again. It's exactly like staying in the middle of the lane on the interstate but you're using an instrument rather than looking at the road.

The second part of an ILS is the sloping, vertical pathway that stretches from a precise point in the sky several thousand feet above the ground and four or more miles from the runway to a point some 50 feet over the runway threshold. That 50-foot target is a place we want to try to fly through in a stable manner, perfectly aligned for a safe landing. This vertical track is called the glidepath.

The glidepath is traditionally represented by a horizontal needle moving up and down over that same center dot. If the needle begins to move above that center dot, the ILS is telling you that the aircraft is drifting below the safe descent path you want to follow. The pilot flying the aircraft then gently increases back pressure on the yoke (or control stick) to shallow out the descent rate and bring the needle back to the middle dot, at which point the aircraft returns to a safe and predictable pathway. If the needle ever gets to the top of the instrument case (or the bottom), it's time to start a missed spproach, a very orderly procedure that will bring you back to a safe altitude and give the air traffic controller time to reposition you for a second ILS.

The two ILS pathways -- localizer and glideslope -- are typically depicted on the same instrument face as cross hairs, and radio transmitters produce them near the end of the runway broadcasting those high-frequency beams of radio waves backward. Over the decades, the reliability of that radio equipment has improved tremendously, and the precision of the ILS approach has increased as well.

But there's one more vital element in an ILS approach I haven't explained: the DH, or decision height. Even though the ILS can guide you through pea-soup fog in a very stable approach with plenty of clearance from any ground obstructions, at some point the pilot needs to see the runway in order to flare and land. In the traditional ILS approach, you can fly down the glideslope until you're as low as 200 feet above the ground (AGL, above ground level). If at 200 feet you cannot adequately see the runway environment to land, you can't legally continue the approach and must execute a missed approach.

Newer forms of the ILS (known as Category II and even Category III) incorporate incredibly redundant electronics on the aircraft as well as the ground along with specialized training for the pilots so that they can safely fly down to 100 feet (Category II) and even 50 feet (Category III) before having to see the runway. When accompanied by a marvelous device called the HUD (heads up fisplay -- essentially a transparent TV screen that lets the pilot see the instruments while looking out the window for the runway), landing in zero-zero conditions (zero ceiling, zero visibility) can be achieved with complete safety. While it's not the licensed norm, that means you could fly a Category III ILS down the glideslope seeing nothing of the runway until the wheels touched. (In fact, they've been doing just that in Europe since the 1960s).

With Global Positioning Satellite systems, or GPS, now coming into cockpits and anchoring the already legendary accuracy of ILS systems, our ability to land safely in low visibility has improved to near-perfect.

Go ahead. Wake up your seatmate and fill her in. After all, this is a real confidence builder!