Today, between one and four classes of travel are available on wide-body aircraft.Bar and lounge areas which were once installed on wide-body aircraft have mostly disappeared, but a few have returned in first class or business class on the Airbus A340-600, For example, aircraft scheduled for shorter flights are often configured at a higher seat density than long-haul aircraft.
As a result, airline manufacturers opted for a wider fuselage rather than a taller one (the 747, and eventually the DC-10 and L-1011).
By adding a second aisle, the wider aircraft could accommodate as many as 10 seats across, but could also be easily converted to a freighter and carry two eight-by-eight freight pallets abreast.
A twinjet design is more fuel-efficient than a trijet or four-engined aircraft of similar size.
The increased reliability of modern jet engines also allows aircraft to meet the ETOPS certification standard, which calculates reasonable safety margins for flights across oceans.
Due to current economic pressures on the airline industry, high seating densities in the economy class cabin are likely to continue.
In some of the largest single-deck wide-body aircraft, such as the Boeing 777, the extra space above the cabin is used for crew rest areas and galley storage.The upcoming Boeing 777X-9 twinjet is approaching the capacity of the earlier Boeing 747.Complete GE90 engines can only be ferried by outsize cargo aircraft such as the Antonov An-124, presenting logistics problems if a 777 is stranded in a place due to emergency diversions without the proper spare parts.Engineers were faced with many challenges as airlines demanded more passenger seats per aircraft, longer ranges and lower operating costs.Early jet aircraft such as the 707 and DC-8 seated passengers along either side of a single aisle, with no more than six seats per row.The trijet design was dismissed due to higher maintenance and fuel costs compared to a twinjet.