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The India-Pakistan feud has heated up again, with Pakistan closing its airspace as a result.

I know many international routes fly over Pakistan, and that the closure would cause issues, but this report surprised me:

Thousands of people were also stranded by affected airlines that not only land in Pakistan, but fly over its airspace -- one of the major routes from Southeast Asia into Europe.

Thai Airways announced that all its European routes "departing near midnight of 27 FEB through early 28 FEB" were canceled "due to sudden closure of Pakistani airspace as a result of tension between India and Pakistan."

Why would the airspace closure force flights over it to cancel entirely? I would think you would just reroute them around it.

To give an example of how flights can be affected by this in ways to make them impossible, Iranian airspace is closed from sunset to sunrise (unless things have changed recently).
Any aircraft that due to the closure of Pakistani airspace would need to cross Iranian airspace and be unable to do so because of that closure now needs to be cancelled or rescheduled.

Also, if the diversion around Pakistani airspace would stretch the endurance of an aircraft beyond what fuel it can carry, it cannot fly the route.

Or the diversion would cause such a delay in the schedule that it causes too much problems with the overall scheduling of the airline. Such things tend to have a ripple effect. One flight gets delayed by several hours, dozens of others get delayed as a direct result, hundreds more can suffer delays in the end.

Also, some airports only operate during daylight hours. A diversion around Pakistan might mean the flight can't make it to one such airports before sunset and thus can't land at its intended destination. Now the airline has 2 options, divert to another airport, arrange for bus or train transport for all the passengers, and in the morning yet another hop to the intended destination to pick up the new passengers, or cancel the flight.

Not mentioned in the other answers is simply logistics coordination. If you can't fly over Pakistan, that suggests that maybe you have to fly somewhere else. Perhaps flying around means an overflight of China or Kyrgyzstan. Do they charge overflight fees? Do they require prior permits? Even if the money involved isn't huge, starting up a new route may mean the first time your airline had to work with that country. I'd expect that could take a few days.

Even if you already have a relationship with the country from other routes, adding permits and having the accountants approve the route choice would still be required.

https://www.jetex.com/overflight-fees-in-asia-pacific/

There are multiple reasons for cancelling instead of rerouting:

• Take-off & landing slot assignments
  
  
  • You may have a pair of slots available and your regularly scheduled flight uses them. Rerouting the flight will take longer and you may no longer make your destination landing slot
  
  


• Departure & arrival gate assignments
  
  
  • Likewise, you may no longer have a gate assignment available at the destination airport because the reroute takes too long.
  
  


• Aircraft range.
  
  
  • If your flight is near the maximum range by overflying Pakistan, rerouting around it could push the plane past its maximum allowable range.
  
  

There was an extensive report on the grounding of Thai Airways flights in The Guardian today. To quote:

Thai Airways said later on Thursday it would resume flights after gaining permission from China to use its airspace for nearly a dozen flights to Europe set to leave on Thursday afternoon and Friday morning.

Quite simply, Thai lacked the necessary permissions.

The Chicago Convention on International Civil Aviation established a general right for overflight by foreign aircraft, but many states nonetheless require airlines to acquire permission in advance – including China.