There is no getting away from the fact that United Airlines’ recent 200-unit order for Boeing’s 737 Max throws considerable – and much needed – additional momentum behind the programme, even if the airframer had to offer a significant discount to secure the business. But the carrier’s simultaneous deal with Airbus for another large batch of A321neos will be an unwelcome reminder that the European airframer is continuing to encroach on Boeing’s turf.