The Dangers of Landing in Fog, or with a Low Cloud Ceiling, Especially When Hand Flying and Using Flight Director Guidance for a Precision or Non-precision Approach, or Hand Flying and Receiving an Air Traffic Control Precision or Non-precision Radar Guided Approach
by International Captain Paul Miller
Whenever you are planning a landing in fog, or when there is a very low ceiling below 500 feet, it is a very good idea to look at two separate windfield data sets and compare both the speeds of the wind and the direction from which the winds are coming, at both the Final Approach Fix and on the surface.
First look at the winds aloft at 3000 feet. Winds aloft are measured in true direction, flight crew will need to calculate magnetic winds from true using local variation, and remembering that East is least or negative, and West is best or positive, meaning subtract East variation from true winds aloft to get magnetic winds aloft. Note the speed and the direction from which the winds are coming. These winds will be most likely similar to the winds at the Final Approach Fix (FAF) altitude and will be the closest windfield data available
Next, look at surface winds forecast for time of landing. Surface winds are magnetic. Note the speed and the direction from which the winds are coming. Surface winds are usually measured at 10 meters, and will be the closest windfield data available for the Missed Approach Point (MAP) or the Decision Height (DH) or Minimum Descent Altitude (MDA), in terms of transitioning from instrument flying inside the flight deck to visual flying to land on the runway, if flight crew can see the runway at that point.
Why?
It is a very good question to ask “why?”
Here is why. Whenever there is fog, or a low ceiling such as below 500 feet, both the wind speed and the wind direction most likely will change significantly between the final approach fix (FAF) and the missed approach point (MAP) or minimum descent altitude (MDA), and the runway surface.
Again why is that so?
Again, very good question “why?”
Here is why this is so. Fog and or low ceilings are the meteorological result to two separate stratified air masses meeting. The fog or the low ceiling is the interface layer, which is why above the fog or above the low ceiling layer it may be clear and on the surface it may be foggy or be a low ceiling, even if below the low ceiling it is clear again. The two air masses have different speeds, directions, relative humidity, visibility, and temperature. The fog or low ceiling is the visible transition layer.
But rather than causing a problem to the flight crew members, knowledge of the two air mass characteristics can become a tool to use to ensure a stabilized approach to a smooth and safe touch down.
Because the two air masses are brushing by each other, their wind speed and wind direction will be different. That is right, the air mass between final approach fix and the missed approach point will not be homogeneous, and this is the meteorological reason that the airport is experiencing and reporting fog, or low ceilings.
The speeds of the wind in the two layers will likely be different, and may differ by as much as 15 knots or more. The direction of the wind in the two layers will also likely be different, and may differ by as much as 60 to 90 degrees, or more, historical meteorological data shows.
Why is this significant, and why is this important information to know ahead of time, especially if the flight crew is hand flying the approach?
First, if the flight crew is planning to fly an auto land Cat IIIC approach, the auto land system will make needed glide path and glide slope corrections automatically for both the changes in wind direction and the changes in wind speed. But it is a good idea for the flight crew to be aware that those changes are coming and to anticipate, monitor, and expect to see the aircraft make the needed changes, and to take over manually if needed to keep the aircraft on a stabilized approach, especially if the winds are gusty and unsteady.
However, how about the case where the flight crew decides to click off the auto land, and hand fly the instrument approach from FAF to landing? So, how can a sudden change in the wind field speed and direction challenge the success of the approach and landing operation?
One of the most important safety procedures in use at most commercial airline operations today is the stabilized approach, and both the pilot flying (PF) and the pilot monitoring (PM) will be watching the instruments to ensure that the aircraft stays within the stabilized approach criteria, or goes around.
When entering fog for instance at 500 feet, or when beginning an approach in clouds with a low ceiling for instance of 200 feet, the winds will be shifting in both speed and direction in that fog or low ceiling cloud layer, and they will be shifting just when the PF is trying to keep the approach stabilized. This shift in the windfield can make it difficult to stay on glide slope and glide path, if the flight crew had not planned ahead.
Let’s look at an example of a French airport using Runway 35 Right. The surface winds are reported out of the North at 6 knots, which is a good wind in which to land.
The 3000 foot winds aloft are reported at 270T at 18 knots. So, somewhere between 3000 feet and the runway surface, the winds are going to change from 270 degrees true at 18 knots, to 360 degrees magnetic at 6 knots. The change in wind direction and speed will challenge the flight crew to stay within the stabilized approach criteria.
In this case at the FAF, the flight crew would be using a slight left crab to keep the aircraft on localizer center-line, and there would be essentially no headwind component.
But somewhere in the fog, somewhere after the FAF, the wind is going to shift from 270 at 18 knots to 360 at 6 knots, and will do so in a short time. If the flight crew does not plan to adjust out the left crab angle, the aircraft could begin to drift left of localizer center-line and away from the stabilized approach criteria, probably approaching the DH or MDA, at the worst possible time. Add to that, the wind will have also picked up a fairly sudden 6 knot headwind, which could cause the aircraft to go low on glide slope unless the flight crew brings on a little power. These changes will happen just when the stabilized approach criteria is approaching its narrowest margins, just prior to the DH or the MDA. Without timely corrections to glide-path and glide-slope, the chances of a go around due to exceeding stabilized approach criteria could rapidly increase.
So, here is a little exercise that the PF and the PM can do while preparing for the approach briefing. The exercise consists of making a table for the wind field shifts in speed and direction that can be anticipated and how the wind and the wind shifts relate to the landing runway heading. This exercise could improve the chances for a successful stabilized approach to landing.
Draw up a table with the wind direction and speed at from the 3000 foot magnetic winds aloft for that airport. Make the true to magnetic variation corrections for your area. Next, list the wind direction and speed from the surface winds for the estimated landing time Next calculate the changes in wind direction and speed that will occur somewhere during the approach. Next brief how the wind direction will change during the approach from FAF to MAP or MDA, such as the wind will shift from a 18 knot left crosswind to a 6 knot head wind. Next brief how the PF will make the appropriate corrections, such as, “Landing RW 35R, ILS coupled approach until FAF. I will hand fly the approach and I plan to take out some or all of the left crab angle and I will be adding a small amount of power to keep us on glide slope as the head wind increases. See Table 1 below.
| Flight Level | Magnetic Winds | Cross Wind (L/R XW) | Head/Tail Wind (HW/TW) |
| 3000 ft | 267/18 kt | 18 kt LXW | 0 |
| Surface | 360/6 kt | 0 | 6 kt HW |
| Windfield Change | Loss of 18 kts LXW | Gain of 6kt HW | |
| PF Response | Decrease Left Crab | Add Power | |
Table 1. Landing on RW 35 Right, with 3000 ft Winds Aloft of 270T/18kts
Note: Flight crew are aware that the windfield weather is fluid, and changing constantly, thus expectations for Windfield Change and PF response is necessarily an approximation.
Nevertheless, the point of this part of the approach briefing is to make both the PF and the PM aware and prepared, that there is a wind field change coming during the approach, and that all flight crew members have given it your best estimate for a response, and are ready for heading and power changes, to keep the aircraft on a stabilized approach, on glide path and on glide slope, into fog or low ceilings, for a smooth and safe landing.
Safety Forecast 