ELi5 Why are planes grounded when there is fog but they can fly through fog and clouds.

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I dont understand when they have control towers and such advanced technology why fog can ground planes. Not much is done by eyesight now surely?

In: Technology

Instruments are only aids for navigation. Instruments can be wrong, instruments can fail. If instruments were truly that good we would not need pilots, drivers, ship captains etc.

We still need the human element. Humans can see and think critically about things. Electronics cannot. They only give an objective picture of something, when they work correctly (which is most of the time).

Flying and landing are not the same thing. Flying involves working hard to stay far away anything. In that system, a cloud or fog isn’t a problem. Alas landing can’t work without touching the ground, gently. That gentile touching is hard to do when you can’t see the ground.

It depends on who is flying. Most commercial airliners/pilots are rated for instrument-flight, meaning they can fly in adverse weather conditions like fog or night-time. But for private or charter planes/pilots, they may not be rated for that, so they can only fly when it’s clear skies.

Additionally, there is increased risk. Take off and landing are the most dangerous times in a flight. A plane in flight can’t really do that much to get out of clouds/fog; additionally, there’s not much else to run into at 30,000 ft above sea level. But, near the ground, there are a lot more hazards, so it’s a lot safer to wait for clear(er) weather before taking off to minimize the risks during the already-riskiest part of the flight.

Pilots need visibility to maneuver the airport. They need to be able to see beacons, runway marking, airport support vehicles etc.

The tower needs to be able to see plane activity on the runways.

Instrument rated pilot here. It is possible to land a plane in “zero zero” conditions, meaning clouds are to the ground and you can’t see anything ahead of you. Doing so requires the aircraft and runway to have special equipment and the aircrew to have special training. Check out Cat III Autoland, there are plenty of videos on YouTube. Any instrument approach will have “minimums”, a required vertical visibility to land. This is typically 200 feet, or 100 feet with certain lighting systems.

Approaches are flown with GPS, ground based radio beacons (instrument landing system), or a combination of the two (GLS).

Visual approaches are quite common in good weather. Planes can land closer together in visual conditions.

Large commercial aircraft are fitted with autolanding systems, but *moving* around on the ground is something that we have not been able to (completely) automate.

It is far too dangerous in cases of heavy fog to move a few dozen to a hundred tonnes of aluminium tube+fuel around on the ground. Accidents can and have happened that resulted in massive loss of life.

In future, when ILS Category IIIc potentially becomes a thing, it will enable aircraft to perform a fully automated landing, rollout and taxi to gate. Perhaps when that comes visibility alone may not stop flights anymore.

So if the question is why can’t a plane take off in fog but fly in fog… I think the answer is a plane CAN do both. The issue is if something goes wrong with take-off the pilot will not have an easy time turning around and trying to land. Also it may be hard for a pilot to see the way to the runway without bumping into things or the end of the runway if they have to abort take off. Like said in a different reply- special airports/licensed pilots and types of aircraft can do take offs and landings in fog- but not usually with people on board (Like many FedEx planes and pilots can take off and land in almost anything). I think extra caution is taken when there are a lot of souls onboard.

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