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Flying An ILS Into Fog, And The Visual Illusions Associated With It

Fog is hanging just a few hundred feet above the ground as you begin your ILS. It's perfectly clear above. Here's what to expect as you descend into the clouds just a few hundred feet above minimums.

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Video: CAT IIIb Approach Into Auckland, New Zealand

This video shows the illusion of being far above the clouds on final approach as the aircraft flies into a fog layer. CAT III ILS approaches like this can get you down to nearly zero-zero visibility and ceilings when an aircraft is equipped with a HUD, autoland, etc.

But even if you and your aircraft are only capable of CAT I approaches, you could find yourself flying an ILS into fog or low clouds very much like this.

Let's Review The Basics Of Fog

Fog may be present when a narrow temperature/dew-point spread exists (usually within 5 degrees Celsius). It's most common in areas with abundant moisture, and it generally forms when...

  • Air is cooled to the dew-point (temperature decreases).
  • Moisture is added to the air (dew-point increases).

How To Prepare: Brief For The Late-Stage Transition

If you're flying a precision approach into fog (and, of course, also assuming the weather is at or above minimums), it's easy to not fully realize just how close to the ground you are.

One of the best things you can do to prepare for this is to brief your approach early, and prepare for your late-stage transition from VMC to IMC flight.

It takes even the most experienced pilots a few seconds to mentally transition from VMC to IMC flight, so spend time briefing (even if you're flying by yourself) on what to expect as you enter the clouds, and what your decision altitude is for your missed approach.

Missed Approach Planning, And Avoiding Illusions

If you don't see the runway environment at minimums, you'll perform a missed approach just a few seconds after descending into the clouds. This means you could experience a dramatic somatogravic illusion of an overly nose-high attitude as you climb back up through the clouds. With go-around power, it's easy to interpret the acceleration sensation as a feeling of pitching up excessively.

As you accelerate, your body may sense a false pitch-up motion (somatogravic illusion), and your natural reaction is to pitch the aircraft down. This is, for obvious reasons, not what you want to do at or near minimums on an instrument approach.

Plan for the illusion, and be ready to use your scan pattern to make sure you keep a nose-high, climbing attitude throughout your missed approach.

Crew Coordination

If you're flying with another pilot, or even a non-flying passenger, briefly explain the visual conditions during final approach when descending into fog (with clear skies above).

These fog-induced illusions could take them by surprise, and the more prepared everyone is in the cockpit, the better outcome you'll have, regardless if you see the runway and land, or you need to go missed.

Have you ever flown an approach like this? Tell us about your experiences in the comments below.

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