I have scheduled flights between two cities plus a return flight using Embraer 170. On the outbound flight the capacity is 64 in economy class but on the return flight it is only 30 economy class, Why?
Here is an example below, London City to Glasgow and then the return flight.
London City has a very short runway. Apparently, to be able to touchdown, your E-jet simply must be lighter. LCY is not a typical airport, unfortunately...
Data for E70: Ground roll landing 1,250-1,750 m
<a data-ipb='nomediaparse' href='http://gatow.airlinesim.aero/action/info/airport?id=1052'>London City</a>
LCY
EGLC
1,508 mI think that AS is aware of this problem. In reality, there are some modified aircraft from Embraer etc. who are able to operate with a sufficient payload from LCY. Same with Rio´s city airport Santos Dumont where modified Boeing 737NGs can fly in and out but in AS there are heavy payload-restrictions I think.
So, you just need a smaller aircraft...
As was said, your limiting factor is the runway length at LCY. The in-game performance calculator says that an E70 is payload restricted to 96.2% of maximum on the LCY-GLA sector, and 50.7% of max on the return. The E70 needs a longer runway length on landing than it does for take off.
The game applies payload restrictions to cargo first, and if equipped, then to economy, then business, then first class; rather than reducing all classes and freight by the same percentage. Most airlines cherish their premium passengers and see economy seats sold as a necessary evil. You can estimate your available seats by taking the Calculated Payload value from the performance calculator and valuing every passenger at 95kg, and unit of cargo at 100kg, regardless of the seat types that you have installed.
Since the E70 has no cargo capacity, a calculated payload of 4391kg on GLA-LCY means 46 passengers total, (46.22 actually, but you can't transport 0.22 passengers). Since you've configured your plane with 16 business seats, it leaves room for 30 economy passengers.
I used to run a succesful airline from LCY, so here's my advice.
For longer distance flights within Europe, use Sukhoi Superjets (or whatever they are called!)
For flights to the east coast of US/Canada (JFK, Toronto etc.), use Airbus A300-600 (I think the RF model). Can carry around 200 persons, and actually work from LCY! 
…use Airbus A300-600 (I think the RF model). Can carry around 200 persons, and actually work from LCY!
What? :huh: :o ;) . OK. So I want to operate MD-80s with 150 passengers from SAB. The runway is long enough with comfortable 1,299 feet :wub: .
I would recommend the CRJ 700/900/1000ER (depends on the demand of airports)with Economy/EcoPlus seats for European flights.
