Charging an apple iPad Pro on very long flights

While many airlines offer in-seat charging facilities, either 110VAC @400Hz or 5V USB, some older aircraft may not be equipped with charging facilities - or may only offer the facility in premium seating.

As such, you might be well advised to carry both your Power Adapter and charging cable - and a high capacity USB PowerBank. So equipped, you should have covered all likely eventualities.

International Airline restrictions limit the maximum capacity of Li-ion batteries that can be carried on board - and are then only permitted in hand luggage - the maximum permitted capacity being 100Wh. For a USB PowerBank, this equates to ~27000mAh.

One example of a suitable high capacity PowerBank, that can be legally carried onboard a commercial passenger aircraft is the Mophie PowerStation USB-C 3XL.

In most cases: nothing. Most planes will have household style outlets that you can connect your device to for charging. Some may have USB ports.

To be certain, you would need to contact the airline and inquire about specific charging options on your flight.

United Airlines for example offers 110V charging on all seats in most of their airplanes.

https://www.united.com/ual/en/us/fly/travel/inflight/power-outlets.html