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are nai yaar…ham sab yahaN dost ahiN aur dost ko TUM kahne se lagtaa hai ham qareebii dost haiN 
hai na? i wud prefer if u call me tum
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i think they increase the thrust on one of the engines on the other wing equal to the drag caused by the extra engine…this fith engine is non-functional…its just a spare repaired engine being transported to its destination :)**