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Shallow fog: Guidance for flight crews
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|Category:||Emergency & Contingency|
Note that while horizontal visibility is not obstructed above 2 m, the fog layer may be significantly thicker than 2 m and that slant visibility -- the visibility from air to ground from the aircraft while approaching the aerodrome -- can be affected.
Viewed from above, the fog may not obscure aerodrome layout or runway markings but on final approach visual references may be lost, particularly in the flare. The presence of shallow fog may make taxy and takeoff difficult depending on ambient lighting conditions, the clarity of taxiway and runway markings, and the intensity of taxiway and runway lighting.
Flight planning - Take-off Alternate and Contingency Fuel
The presence of shallow fog, at the aerodrome of departure, may diminish, or even negate, the ability to land again in the event of an emergency. In the event that shallow fog is present at the point of origin, pilots should consider selecting and filing an appropriate take-off alternate.
A forecast condition of shallow fog at destination, while not necessarily requiring the carriage of contingency fuel, is an indicator that there may be low visibility procedures in place that might cause delays. Since radiation fog is associated with high pressure air mass, shallow fog also may be affecting other aerodromes near the destination including normally selected alternates. Furthermore, what may initially be forecast as shallow fog may become more substantial depending on quite minor changes to temperature and surface wind strength. It is therefore prudent to carry extra fuel and to consider alternates further from the destination than might normally be selected.
Shallow fog at the departure aerodrome can present some unique challenges. As the pilot eye height, whilst on the ground in most commercial aircraft, is greater than 2 meters, level and upwards visibility might well not be affected. However, objects below eye level, inclusive of signage and both taxiway and runway markings can be obscured. This will be even more pronounced if a low sun angle results in glare.
Pushback, Start and Taxi Out
Vigilance, especially on the part of the ground crew, is required during pushback and start procedures. Shallow fog reduces visibility and may obscure low profile ground support equipment, such as belt loaders, or items such as misplaced chocks or luggage items which have fallen from baggage carts, all of which create potential pushback hazards. The fog will also obscure the vision of those personnel operating support equipment on other areas of the apron and may diminish their situational awareness. The potential for collision is increased and should be mitigated with strict adherence to established pushback procedures.
Taxiing in reduced visibility conditions can be challenging and the potential for a missed turn or a runway incursion is increased. Low Visibility Procedures (LVP) may be in force requiring appropriate crew training and specific authorisation from their National Aviation Authority (NAA). Specified, departure runway dependent, taxi routings, which at some aerodromes are issued by an alphanumeric route designator, may be in use. A thorough briefing of that expected route should take place prior to initiating taxi. Both pilots should be "heads up" during the taxi and actively monitor their position on the aerodrome against the appropriate chart or by electronic means. Wherever possible, checklists should be accomplished with the aircraft stationary.
Before attempting takeoff, the Captain must be satisfied that the required visibility exists for the runway in use. Where available, runway centreline lights should be used to provide additional guidance. If the sun is at a low angle and conditions allow, takeoff towards the sun should be avoided to reduce glare.
In flight monitoring
Pay particular attention to the movement of the temperature and dew point at the destination and alternates - fog will form when the temperature and dew point are the same.
Approach and landing
For most modern commercial transport aircraft equipped with auto-land, low visibility at the destination does not preclude a safe landing at an aerodrome equipped with an appropriate category Instrument Landing System (ILS). However, for non auto-land capable business and commercial aircraft, and for general aviation, shallow fog can represent a significant hazard.
When approaching overhead, it might appear that there is no fog or that it is more patchy than forecast; ask for an update on the Runway Visual Range (RVR). Note that whilst RVR will normally provide a good indication of actual visibility, in the case of shallow fog, the readings may be misleading. Transmissometers, dependent upon the type and installation, are typically mounted at the top of poles ranging from approximately 2.5 to 4 meters in height. This can result in the sensor actually being above the most significant portion of the shallow fog layer.
Since any shallow fog will be encountered in the flare, brief for a missed approach from that point. Note that a go-around from the flare, often termed a baulked landing or low energy go around, is a high risk manoeuvre. Some NAA's define the parameters for a baulked landing as:
- below 50' AGL
- landing gear down
- landing flaps selected
- thrust at idle
- airspeed decreasing
and consider it a mandatory training/testing item in all recurrent training.
Depending upon the aircraft and engine type, and the landing weight, a go-around from the flare may require a modified flight profile to ensure that it can be accomplished safely. Note that touchdown may occur during the go-around process. Most jet engines must be allowed time to "spool up" before they begin to produce thrust and airspeed should be increasing prior to retracting flaps to the go-around position or initiating climb. Consider also that any missed approach will commence below the usual decision height for the runway in use and there may be obstacle clearance considerations in the go-around - this is especially important in Instrument Meteorological Conditions (IMC) or at night.
Taxiing from the runway to the apron holds the same challenges and liabilities as were identified previously in the discussion on "Taxi Out". Anticipated taxi routes should be discussed as part of the approach and landing briefing so the crew is fully prepared for taxi as they vacate the runway. Where possible, checklists should be actioned whilst the aircraft is stationary and both pilots should be "heads up" and closely monitoring aircraft position when in motion. Final positioning on the parking stand may be more hazardous than usual as the fog may make it difficult or even impossible to visually clear the parking area. Elements of the Stand Entry Guidance System may not be visible from the flight deck. If any doubt exists, the aircraft should be stopped awaiting further guidance from the ground crew or, if necessary, the aircraft should be shut down and towed onto stand.
Accidents and Incidents
- C25A, Bern Switzerland, 2018 'On 2 March 2018, a Cessna 525A touched down at Bern aligned with the left hand edge of the runway and then left it completely before re-entering it after a little over 300 metres and completing the landing roll without further event. Damage to the aircraft and six runway edge and taxi lights was subsequently found. The Investigation noted that the crew stated that they had retained full visual contact with the runway during final approach and that the recorded braking action was good. It was not possible to establish why neither pilot had been aware of the misalignment'