twistedenginestarter, Aisles are generally 18-20 inch wide, dependent on how you measure. Twin aisles require double that in cabin width / fuselage cross section. A wide, single "1.5" ~30 inch wide aisle has some of the advantages of twin aisles;
- People are able to pass each other, instead of blocking each other, shortening (de)boarding times and waiting lines.
- For the larger NB segment (>170 seats) the larger cross section offers structural efficiency for the longer fuselages (753 & A346 had issues)
- It lowers the claustrophobic experience on longer MoM like flights (up to 9 hours). People dislike the TATL 757s because of it.
- It avoids the 2-3-2 cabins weight that make the associated cost per unit uncompetitive with single aisles.
- It offers more cabin options for narrow body premium cabins like introduced by Delta, JetBlue and American these days. And real big luggage bins, facilitating more people handling their own.
The MC-21's single aisle cabin is already wider than the cabin of an
Airbus A320 by 12 cm and a
Boeing 737 by 28 cm. The large MS21-400 will be significant bigger then a A321NEO.