The fuselage is the starting point of the design. The number of passengers, the seat width, layout and cargo/baggage distribution, volume and location together determine the fuselage cross section, and length, weight, drag, and landing gear requirements.

Passenger Capacity

The ATR9X will need to seat a minimum of 90 passengers.

Layout

There are two possibilities for the layout: 5 abreast, and 4 abreast seating. All 90 seat in-production airplanes today are 4 abreast, as it results in the least fuselage cross section, which impacts drag.

In a 5 abreast seating, only 18 rows of seats will be required, making the passenger cabin as long as the ATR72 today: 18 rows, 4 abreast. However, if the seating is to be 4 abreast, then there will have to be 23 rows of seats, resulting in a capacity of 92 seats.

Cabin Length

A very interesting cabin is that of the 90 seat CRJ 900’s. The aircraft cans seat a maximum of 90 passengers (22.5 rows) with a 31-inch seat pitch. As with all 90 seat airplanes, this aircraft has 2 lavatories. The aircraft does not have aft passenger and service doors, forcing the cabin to have 4 over-wing emergency exits: two on each wing.

With the engines on the ATR9X expected to be over-wing, there will be two sets of passenger / service doors: Forward, and Aft, hopefully eliminating the need for the mid-cabin emergency exits, due to the possibility of exiting the aircraft near the propellers. Also, dual passenger exits allow for a faster turnaround.

Since the turboprop flights are expected to be short sector (250-500NM), there may not be the necessity for two galleys. However, two lavatories will be required, in the interest of safety and comfort: The crew can have priority use of the forward lav, without risking him being attacked by a passenger as he strolls down the cabin to the aft lav.

The rear passenger and service doors may add to the length of the cabin, while the absence of mid cabin emergency exits may help reduce cabin length. In all, the cabin for a 4-abreast seating configuration with a 3 inch seat pitch for 90 seats is expected at around 71ft / 852 inches / 2.2m.

Cabin Width

The in-production ATR72s and Q400s have a seat width, inner armrest- inner armrest of 17.3 inches. As most people these days show a trend towards wider waists, this width may increase. Airbus, which owns 50% of ATR, and has launched a campaign showing none of its airplane seats are less than 18 inches wide, may influence the seat selection, although many Airbus A320s world over, including those recently produced, feature 17 inch wide seats.

In-production Embraer Ejets feature 18-18.25 inch wide seats, and an armrest width of 2 inches. The CSeries 100/300 feature 18.5 inch wide window/aisle seats and a 19 inch mid seat.

If the ATR9X goes for the 18 inch seat widths, each seat assembly, comprising two seats and three armrests, will be as wide as 18″ X 2 + 2″ X 3 = 42″ (inches). That makes two sets of dual seat assemblies in the same row consume 84 inches of cabin width.

In economy class, the Airbus A320 has an aisle width of 20″ (with 18″ wide seats), the E170/190 family of airplanes feature 19.75″ wide aisle, while the ATR42/72 feature 18″ wide aisles, and the CRJ900 has a 16″ wide aisle.

The 90 seat turboprop may need a wider aisle to facilitate a faster boarding/disembarking, both for a QTA and in an emergency. This may require atleast a 19″ wide aisle.

Adding the aisle width and the width of the two sets of dual seat assemblies gives us a total maximum cabin width of 84″ + 19″ = 103″ (inches) / 8ft 7″ /2.61meters. This is 6cm wider than a CRJ900 cabin, 13cm smaller than an E170/190 family aircraft, and only 4cm wider than an ATR72’s cabin.

Payload

ATR assumes each passenger (including baggage) to weigh 95kg. Bombardier assumes each passenger to weigh 102kg. We will stick to the more demanding estimate per passenger: 102kg. 102Kg caters to a 75kg passenger, with a 20kg check-in baggage, and a 7kg carry on baggage.

92 pax X 102kg/pax = 9,384kg payload for a 4 abreast seating/23 rows.

Baggage Volume

Aircraft	Max Seats	Total Cargo Volume (ft3)	Cargo Volume per Passenger (ft3)
ATR 72	74	374.5	5.0
Q400	86	502	5.8
E175	88	604.59	6.8
CRJ900	90	594	6.6
E190	114	799.18	7.0
Airbus A320	180	1322	7.3
Boeing 737-800	189	1555	8.2

The table above shows the cargo volume for each aircraft type, and the available cargo volume per passenger at maximum seating capacity. The ATR9X will be, in cargo volumetric requirement, similar to the E175 and CRJ900. We’ll take the average of the two volumes per passenger to arrive at the ATR9X’s requirement: 6.7ft3/pax X 92pax = 616.4 ft3.

Baggage Distribution:

An important part of the study, when deciding the fuselage cross section, is the location of the cargo/baggage compartments. The E170/E190 family have all the cargo under the cabin floor, resulting in a larger fuselage cross section. The ATR72 and the Q400 have baggage compartments located in the aft/forward fuselage: nothing goes under the cabin floor. The CRJ900-a 90 seat jet with probably the smallest fuselage cross section-has two cargo compartments: one at the aft fuselage, and the other underneath the forward cabin floor.

The ATR72’s cross section may not be adopted for the 90 seat ATR, as it will force all the cargo into the forward and aft fuselage: which will result in an unnecessarily long fuselage requiring more ground clearances. It also makes the cabin design awkward, with the forward cargo compartment between the cockpit and the passenger cabin.

The E170/190’s cross section is interesting, as it allows for the shortest fuselage length, but the largest cross section in the 4=abreast configuration, as all the cargo goes under the floor. The CRJ900 on the other hand, with aft cabin cargo hold and forward underfloor cargo hold, results in a longer fuselage but smallest fuselage cross section among 90 seat aircraft today. It  is an attractive design to adopt and/or improvise on.

In the CRJ900, 420ft3 of cargo can fit in the aft baggage hold. This represents 71% of the total cargo volume of 594ft3. The balance 29% fits under the forward cabin floor.