# Diversions because of fog: What are the reasons?

It's fog season again around my local airport MUC (EDDM), and I have noticed a number of diversions recently. Some planes were holding a bit before attempting to land, performed a go around and then diverted to some of the nearby airports.

It appears that mostly smaller planes (< 200 seats) from not-so-distant departure airports diverted and most long-haul flights made it to the terminal as scheduled, maybe with a bit of delay.

This made me wonder what the specific reasons for the diversions could have been, and I am asking mainly for answers providing some of the top reasons, including the background information.

The airport allows for ILS CAT III approaches, which, in the case of CAT III C, translates to auto-land, possible even in no-visibility conditions, if I understand the details correctly.

This, depending on the equipment installed on the aircraft, would mean that planes capable of performing CAT III b or CAT III c approaches could land when the runway becomes visible 75 ft above ground, and all others couldn't? Would long-haul aircraft mostly be equipped with auto-land, while regional jets probably wouldn't?

Would ATC have to use bigger separation, reducing the airport's capacity, and it would be the smaller planes who lose the game of who-gets-chosen? This would, however, most likely not explain the above example of a flight actually trying the approach and diverting only after having done so?

Those who are familiar with the particular airport might be aware that there's a long debate going on about whether or not a third runway should be built. In what ways would (or wouldn't) a third runway help prevent diversions on foggy days? My guess is that CAT II planes would have to divert no matter how many runways were built, but maybe separation issues (in the air/on the ground) would be less of a trouble?

• Regarding CAT III C, note that landing is not taxiing. You might be able to autoland down to zero altitude, but you are still going to need to vacate the runway (and get to the gate) after landing. – a CVn Oct 23 '18 at 14:23
• Also important to understand is that CAT IIIC requires 3 things: adequate airport equipment, adequate aircraft equipment and adequate pilot certification, Even if the airport and given aircraft are equipped with CAT IIIC, the given crew must be qualified to perform CAT III (B and C) approaches – DeepSpace Oct 23 '18 at 14:28
• From fsims.faa.gov/wdocs/8900.1/v04%20ac%20equip%20&%20auth/…: "Special flightcrew qualification and training are also required to ensure that the aircraft is operated with the required degree of precision during these operations." Also CTRL-F for "qualification" on this page for more info. – DeepSpace Oct 23 '18 at 14:43
• Just as a note, I don't think CATIIIc has ever been implemented anywhere in the world. Last year the FAA was considering removing it from the regs completely. – TomMcW Oct 23 '18 at 15:50
• @TomMcW Thanks for pointing out this detail. Re-reading wikipedia, including the footnotes below the table listing the different ILS categories, I can kind of understand the reasons... "a plane would have to be towed to clear the runway" – zebonaut Oct 23 '18 at 18:21