You assume the independency of the events, which is not true in general - you need to factor in the dependency through fuel problems.
The engine can either work (P=6/7) or fail (P=1/7).
Let's look at the probability of the independent events.
1.P(W1,W2)=P(W1)*P(W2)=6/7*6/7=36/49
2.P(W1,F2)=P(W1)*P(F2)=6/7*1/7=6/49
3.P(F1,W2)=P(F1)*P(W2)=1/7*6/7=6/49
4.P(F1,F2)=P(F1)*P(F2)=1/7*1/7=1/49