A mountain flight to Lukla is included on Day 3 of this tour, and back from Lukla on Day 14. Although we schedule departure dates outside of the main monsoon season, weather conditions in any mountain region can be unpredictable. It is not uncommon for flights to and from Lukla to be delayed or cancelled during the trekking season. If your flight to Lukla is cancelled then your host will revert to 'Plan B', as follows:
You will spend an extra night in Kathmandu or Manthali and attempt to fly the next day. If you reach Lukla on Day 4, you will skip a night in Namche Bazaar but otherwise follow the normal itinerary.
Alternatively, it may be possible to fly part-way (to Phaplu), then drive on a dirt road to Kharikhola (6-7 hrs drive), and begin trekking from there. In this case, it is still possible to reach Everest Base Camp in the remaining timeframe by skipping a night in Namche Bazaar. This option is available on Day 3 or Day 4.
There is one further option in the event of flight cancellations to Lukla: to try to charter helicopters for the group (subject to availability). Helicopters are less hindered by poor conditions and can often fly when planes cannot. Anyone taking this option will need to pay roughly $350-500 (USD) extra, per person (the full helicopter cost is more than this, but your cancelled Lukla flight will be refunded and deducted from the total price).
If there are more than two consecutive days of Lukla flight cancellations and it has not been possible to reach Lukla/Kharikhola by the alternative means described above on or before Day 4, then it is no longer feasible to safely reach Everest Base Camp in the time available. Your host will offer an alternative trek in a different region. Usually, the alternative trek offered is the Annapurna Circuit Trek from Chame to Jomsom (via the Thorong La Pass).
Please note that if you spend extra nights in Kathmandu as a result of Lukla flight cancellations, your host will ask you to pay the difference (of around $45 (USD) per room, per night) as the hotel in Kathmandu is more expensive than glamping in Manthali or a night in a teahouse.
See also 'When should I book my return flight?'