The cut in the 737-8
In Part 2 we used the seating needs of the important 737-7 customer, Southwest Airlines, to size a six window frame shortening of the 737-8 fuselage as a plausible size for the 7X. This will yield a cabin of the right size for Southwest, where it can configure a one class economy cabin of 150 seats with its customary generous seat pitches (31-32 inches) and large galleys.
It will also give the other large 737-7 customer, WestJet of Canada, a good two class cabin seating just below 150 seats. The resulting aircraft is described in Figure 2, where we have put the 737-7X between the the popular 737-8/800 and the original 737-7/700.
The 737-7X as defined would be a more natural product companion to the 737-8, being around 20-30 seats smaller dependent on cabin configurations.
The table in Figure 3 shows the key data for the 737-7 and the projected 737-7X. The 737-7X will be heavier as it’s a longer aircraft with more seats. But it will also be heavier as it’s based on the larger and heavier 737-8, with the principal change that two fuselage sections of three frames each have been removed fore and aft of the wing.
Maximum takeoff weight of our 7X has been kept the same as the 737-8. This will give companies like Southwest a long range for demanding transcontinental flights. It will also give the Boeing Business Jet variants (BBJs) an ideal base for long range business jets.
The 7X is a heavier and longer aircraft than the 7. This increases the drag due to weight (induced drag) and the drag due to size (parasitic drag). It will therefore take more fuel when flying the same missions as the 7. As can be seen in the table, we predict the trip fuel cost to increase by 9%. This is a negative for the 7X if it carries the same passenger number as the -7 on the missions.
As we have described, the passenger traffic increases year on year, thereby adjusting optimal cabin sizes upwards by 2-3 seats per year. During the lifetime of the 7X, a capacity increase of 13%-16% will be a good thing to have over a route network with a normal passenger traffic evolution.
The important fuel parameter for the airlines is the per seat mile fuel costs and these are 6% lower for a 7X over typical mission distances. This means that a 7X will be more economical to operate as long as the aircraft can be filled with more passengers than a 737-7.
Direct costs and revenue potential
The exact cross over point in passenger numbers for the two variants will be dependent on the total operating costs for the aircraft and the revenue generating potential. This will be the subject for our final article in the series.
In this part, we will also check that the demanding field performance that Southwest has for the 737-7 (flying transcontinental missions from Midway and Burbank) can be achieved with a 737-7X.