You leave from London Heathrow and on the screens (and at the terminal) you see Riyadh Air but you don't know how to book it
On the arrival and departure screens at London Heathrow Airport, RX 401 and RX [...] flights have been appearing for weeks.

On the arrival and departure screens at London Heathrow Airport, flights have been appearing for weeks. RX 401 and RX 402 To and from Riyadh. They are operated by Riyadh Air, the new Saudi company that will have the Kingdom's capital as its hub.
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Those who may have passed through LHR in recent times may have noticed the flight. And, perhaps, taxiing early in the morning after a landing or in view of a takeoff from runway 27L/9R will also have seen the Boeing 787-9 in the Saudi carrier's stunning livery docked at a finger in Terminal 4.

However, if that somebody were to try to book that flight, they wouldn't be able to do it. Neither from the airline website, nor on flight search engines such as Google Flights or Skyscanner. And this because Riyadh Air, despite having a 'live' Dreamliner that has been connecting Riyadh and London daily for months, is still a 'paper airline'.
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He flies to LHR only to avoid losing the slot he obtained at the London airport, which has strict rules on the actual use of slots by carriers, so after a certain number of months of inoperability, the slot is reassigned. And on board are (when there are), Only company staff members and special guests invited by the company itself, which is using the connections to familiarize as much of its cabin crew as possible with the Dreamliner (of which it has ordered 39 examples in addition to 33 options) and with in-flight service.
All consistent with the fact that the 787 has only Riyadh Air's livery, but being a plane leased from Oman Air, it has the Omani company's two-class interior (Business and Economy), quite different from his, which will be four-class with 4 seats in First, 24 in Business, 39 in Premium Economy and 223 in Economy.

In recent hours, the Saudi Arabian carrier announced that, alongside London and Dubai, Cairo will also be part of its first-hour network. But, while saying that his first real plane is expected "in the coming weeks." still has not provided a date for the actual start of bookings.
Deliveries should indeed be imminent, as three Dreamliner (two of which are currently parked in Charleston, where one of the two 787 assembly lines in the U.S. is based) already have the brands. The actual start of operations with passengers on board is different since, as we said, reservations have not yet been opened.
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