Can Planes Land in Fog?
Yes, most commercial aircraft are capable of landing in thick fog but it can depend on the type of aircraft and the facilities associated with the runway at an airport. Planes can land when the visibility is as low as 75 metres.
For the vast majority of commercial flights, the pilot disconnects the autopilot when on final approach (typically between 1,000 and 500ft) and lands the aircraft manually. For the pilots to manually conduct the landing, the outside visibility must be a minimum of 550 metres and the cloud base must allow the pilots to see the runway when the aircraft is 200ft above the airfield. If they can’t see the runway at this point, they can’t continue the landing and must fly a ‘go-around’.
The observed visibility is referred to as Runway Visual Range (RVR) which is measured by specialist equipment called transmissometers located next to the runway. The altitude above the ground at which the pilots must be able to see the runway is termed ‘decision altitude’. It is at this altitude that the pilots decide whether to continue with the landing or abort it.
If the visibility is less than 550m and/or the pilots can’t see the runway at 200ft above the airport, it is still possible to land, but it would be conducted through what is called an ‘autoland’. This is where the autopilot completes the landing automatically with the pilots only taking manual control as the aircraft slows down on the runway.
Most commercial planes can therefore land using the autopilot in thick fog with visibilities as low as 75m and no decision height (i.e. the pilots don’t have to be able to see the runway until the aircraft has touched down).
The reason why an automatic landing is conducted in foggy conditions is down to the need for the aircraft to be flown incredibly accurately down to a very low altitude when the pilot can’t actually see the runway. It is simply much safer to allow the aircraft to ‘fly by numbers’ as it would be extremely challenging for the pilots to match the accuracy of the autopilot in such conditions. The pilots still have the challenging task of vigorously monitoring the aircraft and its systems and stepping in when things don’t go to plan.
An Autoland needs to be planned in advance. You can’t plan to do a manual landing, then get to 200ft, not be able to see the runway and let the Autoland do the landing for you; the aircraft has to be setup properly before-hand and the pilots need time to adequately prepare. Most pilots will tell you that setting the aircraft up for and Autoland, and then monitoring it throughout the approach and landing is far more laborious than landing the aircraft manually.
Having an aeroplane with Autoland capabilities is a bit like choosing to have parking assist on a new car. Autoland is normally an optional extra that the airline can choose to have installed on their aircraft but it comes at a cost. You can therefore be in a situation where the same aircraft types have different specifications, i.e. one B737-800 may have Autoland capabilities, but another one doesn’t.
Various aircraft also have different limitations regarding how low the visibility can be. As mentioned above, if the runway visibility is below 550m, the autopilot must do the landing. Some aircraft can land down to a minimum visibility of 300m whilst others might be 200m, 100m or 75m.
Aircraft are allowed to (and commonly do) fly with certain technical defects where there is suitable redundancy or the system isn’t critical. This can affect the Autoland status of the aircraft and downgrade it to not allow automatic landings until the problem is fixed. You probably wouldn’t want your flight cancelled because the Autoland system is temporarily inoperative but there is no fog forecast for the next week!
Unintuitively, pilots have to be trained to a higher standard to allow the autopilot to do the landing rather than doing it themselves. They have to undergo specific training to ensure they are ‘Low Visibility Procedure’ (LVP) qualified. If the pilots have not completed this specific training and been signed off as competent, they are only permitted to manually land the aircraft. This training needs to be completed once a year and consists of conducting a take-off and landing in thick fog as well as dealing with technical malfunctions that require the approach to be aborted at low altitude.
The airport itself must have certain infrastructure in place to allow aircraft to Autoland. This is in the form of navigation equipment such as an Instrument Landing System (ILS). There are various categories of ILS which allow landing in certain visibilities. Most of the time, a CAT 1 ILS is used which directs the pilots to 200ft above the airport at which point they disconnect the autopilot and manually land. However, in low visibility conditions where the visibility is less than 550m, a higher category of ILS must be installed to allow an Autoland to be completed, for example a CAT 2 or CAT 3 A/B or C.
The category of ILS is determined by how accurate it is at guiding the plane in. The type of approach and runway lights available that help the pilots spot and orientate themselves with the runway and the number and type of backup ILS systems there are (for example a backup generator to power the ILS) also determine the ILS category.
Why are there always flight delays when it is foggy?
When an airfield is operating close to capacity (such as London Heathrow or JFK in New York), there will inevitably be flight delays and cancellations in foggy conditions. This is because there needs to be greater separation between aeroplanes landing and few aircraft are permitted to be taxying on the ground at any given time.
The ILS sends out a radio beam to the aircraft on the approach which directs it towards the runway. As the pilots may not see the runway until the last few seconds of the approach, they are completely reliant on an accurate ILS radio beam directing the aircraft onto the runway. Therefore, the ILS radio beam (which needs to be located very close to the runway) must be protected to stop any interference occurring which may throw a plane off course.
If planes which are landing and taking off get too close to the ILS equipment which sends out the radio beam, it can cause interference. Therefore, aeroplanes landing must get a bit further away from the runway after landing than normal to ensure the next plane on approach isn’t affected by any interference.
Equally, aircraft taking off also need to wait further away from the runway before being given clearance to line up and take-off. This increase in separation between aircraft means that everything is slowed down and delays occur. When an airport is running at full capacity, so many delays to flights can’t be accommodated so airlines are requested to cancel flights.