Hello Mr. Murthy,
Thank you very much for your suggestion. The Boeing 767-400ER is indeed a great aircraft. However, the MD-11 is leased by Arabian, and not owned. Our lease on the aircraft will expire in another two years. Thus, replacing the MD-11 would be a little more expensive for the company. We'd rather operate it until the engines don't spin

. Moreover, the MD-11, while heavier, has at least 1,000nm more range on its standard variant than the -400, even in the ER configuration. Our operations staff are very adapted to the MD-11 since the company used to have at least 4 at one point, all of which were serviced in house.
If in the future we consider a replacement charter aircraft, we will look into our options, including the 767-400ER. However, we will most likely look to the 777 for a replacement to bring our fleet in line with our current operation's scheme.
Nevertheless, than you for your thoughtful suggestion! We will keep it in consideration.