LY744 From Canada, joined Feb 2001, 5536 posts, RR: 10 Reply 1, posted (5 years 4 months 1 week 2 days 11 hours ago) and read 2845 times:
More doors = more seating capacity possible, because max seating is limited by the number and type of doors, as they govern how fast an aircraft can be evacuated.
At first Boeing didn't offer the option of more/bigger doors, and 739s were limited to the same maximum seating capacity as the 738s (189 IIRC). This wasn't much of a problem for mainline airlines, since they were going to have a 2 class configuration with less seats, but holiday/charter airlines could be scared off from buying the -900. Now that the 757 is no longer in production, having an aircraft that has the same max seating as a A321 was more important for Boeing I guess.
Different door configurations is an option on many other aircraft, including the 757-200, 767, A330 etc.
XT6Wagon From United States of America, joined Feb 2007, 3276 posts, RR: 4 Reply 4, posted (5 years 4 months 1 week 2 days 10 hours ago) and read 2806 times:
The 737-900ER has the extra rear door. This is to provide extra seating capacity since most of the plain 900 in service were hitting the max seats allowed due to the exit capacity. If an airline does not wish to use the doors they can deactivate them and panel over the inside.
I am uncertain if you can still order a 900 without the extra rear doors.
Yes the 739ER has winglets as a standard feature. CO must take them with winglets preinstalled. Interestingly enough, they take their new 738s without winglets and have them installed later on because they got a good aftermarket deal to get them installed which apparently is less expensive than getting them originally installed by Boeing.
If you have never designed an airplane part before, let the real designers do the work!