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I'm stuck in Frankfurt. I missed my connection due to a delay. I was rebooked, but this new flight was also cancelled due to bad weather.

About 30 percent of all flights were cancelled today. However, if the weather is that bad, shouldn't all departures have been cancelled? Why was the weather bad for my flight, but not for other flights that actually departed?

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    $\begingroup$ Are all the flights going to the same place you're going? $\endgroup$ – Harper - Reinstate Monica Dec 11 '17 at 20:34
  • $\begingroup$ no, it was different places but they announced the problem in Frankfurt airport, not in my destination $\endgroup$ – Vitalii Dec 13 '17 at 11:08
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    $\begingroup$ This is the magic that the schedulers have to execute for every flight. You can't get an airplane from B to C if either the airplane or the crew couldn't make it from A to B. And then it dominoes. Plus, if an airplane can't make it to C for whatever reason, why make it leave B? $\endgroup$ – Shawn Dec 13 '17 at 14:33
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In general, flights get cancelled when arriving flights are delayed or themselves cancelled. Your flight needs three critical things to depart on time, or at all:

  1. an airplane,
  2. flight + cabin crew, and
  3. weather that meets requirements

Aircraft may not arrive due to weather diversions (couldn't land with fuel on hand, etc). If there's no airplane, your flight obviously can't depart. Same goes for bad weather at your departure airport - if it's too bad, you can't go. If the weather at your destination is bad, the flight may also be delayed because the airport can't handle as many aircraft as normal. That's fairly straightforward. Where it gets most complicated is the crew.

Crew have what are referred to as "duty and rest requirements" (at least in the US; other jurisdictions have similar rules, possibly with a different name). A flight crew is "on duty" when they're first scheduled to fly, whether the flight happens or not, and the number of "duty hours" they can have on a given day are limited. The FAA last revised these rules in 2011, as far as I know, but I'm not an airline pilot. With the 2011 rules, "Flight Duty Period limits under the new rules range from nine to 14 hours". These rules exist to prevent fatigue, a major cause of aviation accidents.

What this means is that if weather delays cause crews to overfly their limits, they may be unable to operate the outbound flight. With your example of Frankfurt, there is weather causing delays to arriving flights.

For example...

If a hypothetical crew with a 10 hour duty limit was scheduled to fly, say, three LHR-FRA round trips (call it 3 hours of flight duty time each round trip; this may not be a valid example), but were delayed on their third FRA arrival by 2 hours, they would exceed their duty limits by 1 hour on the return flight (1.5 * 2 + 1.5 * 2 + 1.5 + 2 = 9.5 + 1.5 = 11 > 10). As a result, they would not legally be able to depart.

Normally, airlines plan for this with "reserve" crews that are on call to fill in for missing, sick, overflown, or otherwise unavailable crews so that flights are not seriously delayed. When a huge storm hits, this falls apart as there are not enough reserve crews to operate all the flights, and the situation snowballs out of control.

Our hypothetical crew would be replaced by reserve pilots (and cabin crew) if they were available, and the flight might be delayed by more than the 2-hour flight delay they experienced. If not, the flight would be canceled.

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    $\begingroup$ ATPs, please feel free to correct any issues with this answer. I'm a lowly CFI with no direct airline experience and I'm sure there are holes or inaccuracies! $\endgroup$ – egid Dec 11 '17 at 0:17
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    $\begingroup$ It's not just bad weather at the departure airport. Two weeks ago, I was flying from Frankfurt to Brussels and we were parked near the runway waiting for departure clearance for longer than the actual flight time! The reason was that due to unfavorable winds in Brussels only about half of the runways that are normally used were available, so we weren't allowed to take off until we had a confirmed landing slot in Brussels. Note that "half the runways can be used" pretty much means "half the flights need to be canceled". Even if it had been the most beautiful weather in FRA, we wouldn't … $\endgroup$ – Jörg W Mittag Dec 11 '17 at 2:43
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    $\begingroup$ … have been allowed to take off due to "bad weather". $\endgroup$ – Jörg W Mittag Dec 11 '17 at 2:43
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    $\begingroup$ Also, in bad weather minimum separation between aircraft can be increased and that too decreases capacity. Or in snowfall having a runway closed every so often for the runway to be cleared of snow very obviously decreases capacity. $\endgroup$ – gsnedders Dec 11 '17 at 10:26
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    $\begingroup$ @Notts90 Probably not. The reason is that plane will fly different passengers once it reaches its destination. That's why cancellations have a ripple effect. I'm willing to best it would cost the airline more money to leave an empty airplane where it is, than to fly it empty to the destination. Easier to juggle one set of passengers than two. $\endgroup$ – Machavity Dec 11 '17 at 15:41
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If the bad weather is a hurricane or large thunderstorm cell right over the airport, yes you would expect all flights to be cancelled rather than just some. But if the weather brings low visibility, like parts of Europe have experienced this week, it is a slightly different story. The main reason for cancellations is that the airport cannot operate at full capacity.

Usually, aircraft can takeoff in very low visibility (such as fog), but to land the visibility requirements are higher. So if your aircraft is already on the ground before the bad weather hits, it still might be able to leave, whereas another flight is cancelled because the aircraft operating it can't land.

However big airports with frequent visibility problems usually have a category 3 ILS (Instrument Landing System). This category allows an appropriately certified aircraft and crew to land in virtually zero visibility using Autoland. But this system has limitations. The ground-based transmitters are sensitive and need large protected zones - this means that certain runways and taxiways cannot be used at the same time as an Autoland.

This ties in to Low Visibility Procedures (LVPs). Every airport and airline has their own, but the common theme is that everything slows down. The cumulative effect of both Autoland operations and LVPs is that the amount of movements an airport can safely handle per hour drops dramatically.

You can imagine that if there are 50 scheduled flights per hour, but weather reduces capacity to 30 per hour, 20 flights will spill over into the next one. You'll then have 70 flights fighting to use 30 slots, then 110, and so on as the day progresses. For busy airports, mass cancellations are the only way to get things back on track - but 30 flights per hour can still go ahead. In reality the airlines will be told to cancel a certain number of flights and yours might be one of the unlucky ones picked.

Another thing tor remember is that the bad weather in question might not be at the airport you are at, but rather the destination or en-route. Rather than being stuck in a holding pattern for a few hours waiting for the destination airport to clear, it is preferable to wait on the ground - so even if the weather is sunny at the originating airport, a flight could be delayed or cancelled due to weather at the destination.

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    $\begingroup$ Autoland, not autotaxi... as a pilot, you still need to be able to safely get your aircraft off the runway, and ideally to a gate. $\endgroup$ – a CVn Dec 11 '17 at 10:49
  • $\begingroup$ @MichaelKjörling yes there is no such thing as autotaxi. A category 3C ILS allows for landing in no visibility whatsoever, but because a little bit of visibility is needed for taxi, no airport has bothered with this certification yet $\endgroup$ – Ben Dec 11 '17 at 21:25
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As @Ben correctly guessed, Frankfurt airport was (and still currently is) reduced in capacity due to snowfall. I heard about it yesterday and this news article had more details (translation below):

Zwischenzeitlich stand an Deutschlands größtem Luftfahrt-Drehkreuz nur eine Piste für startende und landende Jets zur Verfügung. Eine Sprecherin des Flughafenbetreibers Fraport bezeichnete die Situation als «dramatisch». Selbst auf den geräumten und gestreuten Bahnen seien wegen des anhaltenden Schneefalls nicht die vorgeschriebenen Bremswerte erreicht worden.

Translation:

Temporarily, Germany's largest hub airport only had one runway available for starting and landing jets. A spokeswoman for airport operator Fraport called the situation "dramatic." Even the ploughed and gritted runways did not reach the required deceleration values because of the continuing snowfall.

This might have been a major reason that a major part but not all flights were canceled.

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    $\begingroup$ Welcome to av.se - nice answer! $\endgroup$ – Ralph J Dec 11 '17 at 11:01
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    $\begingroup$ "Not reach the required deceleration values" - i.e. due to the icing, planes braked slower and would need more runway length. This would explain why some (heavier) planes would be unable to land while others could still operate. $\endgroup$ – MSalters Dec 11 '17 at 11:09
  • $\begingroup$ If an airport is running at reduced capacity due to bad weather (or a closed runway etc), then airlines will generally cancel short-haul fights first. After all, Frankfurt -> Munich is almost as quick by train as by air. Frankfurt->Paris isn't hard by rail. Frankfurt->London is doable for the return leg (takes about 15 hours). But Frankfurt->New York? Forget it. $\endgroup$ – CSM Dec 13 '17 at 18:41
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Another requirement for flights is weather minimums at the destination airport. If the destination airport is forecast to have within +/- 1 hour of ETA ceiling and visibility less than 2,000 ft or 3 sm visibility, than a suitable alternate airport with certain weather criteria are needed.

Therefore airplane "A" can not leave because the weather at the destination is too bad or an alternate is not cost effective to divert to if needed. At the same airport, other airplanes are coming and going and airplane "B" will leave because it is headed towards good weather.

This is often the case for long international flights. Say you are planing a 12hr flight from Frankfurt to New York City, USA. If the weather is bad for 500mi around NY, the air carrier may not want to risk being stranded some where else. If the weather is expected to worsen, a lot of bad things can happen during the 12hr flight. But other closer flights, or flights to other areas of the USA my be landing and taking off.

Also two carriers with the same departure and destination airport may have a different risk assessment. One carrier may have attractive alternate flights with good connections later and choose to take the risk. The other carrier may not have good options. Therefore one will be willing to depart and the other will delay "due to weather".

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  • $\begingroup$ If a plane is scheduled to fly from A to B to C to D, and it does not appear that weather at B is going to be suitable for flying any time before the plane is scheduled to arrive at C, would it be possible for the airline to fly to C (skipping B entirely) and either have passengers who were going to B either disembark before taking off for C, or else fly such passengers to C and then book them on a flight from C to B? Do airlines ever do that? $\endgroup$ – supercat Dec 13 '17 at 18:23
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Additionally to all the other answers, there is one factor that could also explain a difference of treatment between two different flights: the capabilities of the aircraft.

Some aircraft are better equipped than others to face bad weather conditions. It depends mainly on their equipment, size and design.

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    $\begingroup$ Welcome to aviation.SE! $\endgroup$ – Pondlife Dec 12 '17 at 21:29
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    $\begingroup$ smaller aircraft are more fragile - is that actually true? I mean, aerobatic aircraft are quite small compared to a 777, but they seem quite robust! :) $\endgroup$ – egid Dec 13 '17 at 0:57
  • $\begingroup$ To be fair, you have to compare all things equal otherwise, you can’t compare aerobatic aircrafts with commercial aircrafts :) $\endgroup$ – KPM Dec 13 '17 at 11:12
  • $\begingroup$ Okay, so, what are you suggesting we compare? Is there anything that actually suggests that a smaller aircraft is more fragile? What I understand about the challenges engineering large aircraft, the opposite could be true. $\endgroup$ – egid Dec 13 '17 at 21:03
  • $\begingroup$ Whatever. I’m removing that detail that seems to get all your attention. $\endgroup$ – KPM Dec 13 '17 at 23:23
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Another reason capacity can go down, one that's given me some close calls: The rules for how far apart aircraft must be is higher when they can't see each other. Take two runways a bit too close together at a busy airport, add fog. Oops, while both runways are usable the airspace can only handle one runway worth of planes. Half the flights get delayed/cancelled due to weather even though the airport is still open. The trip to visit her family used to connect through an airport notorious for this problem.

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    $\begingroup$ "The trip to visit her family"? Whose family? $\endgroup$ – Fabio says Reinstate Monica Dec 13 '17 at 10:06
  • $\begingroup$ @FabioTurati "Her" = my wife. Don't be fooled by my name, I'm male. $\endgroup$ – Loren Pechtel Dec 14 '17 at 4:44
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    $\begingroup$ Ok, but that's not the point. The point is that you talk about "her family", but you never mention who she is. To make it understandable you should edit that sentence to "The trip to visit my wife's family". $\endgroup$ – Fabio says Reinstate Monica Dec 14 '17 at 10:11
  • $\begingroup$ @FabioTurati I would think most people would assume that an unspecified "her" that I was traveling with would be a wife or significant other. $\endgroup$ – Loren Pechtel Dec 15 '17 at 0:27

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