If instead of aviation fuel you pour gasoline or water in to the tank of an airplane, the plane would never be able to take off.  The prayers and supplications of one whose prayers are answered does not have any “haram food”([1]) in their stomach.

The hadith that states “من سره أن يستجاب دعوته فليطيب مكسبه” – Who ever desires to have their supplications answered should purify and make halal their income and their food. ([2])

Notwithstanding that a supplication is intended for requesting what is good, however most of our requests are not for good, but rather we imagine that what we are requesting is good.

[1] This refers to anything that may be eaten, from non halal food such as alcohol, pork, and meat slaughtered in a non-Islamic manner, through to halal food that may have been purchased with money acquired through haram means, for example money earned through gambling, through the trade of liquor etc.  Of course, it is not restricted to food, of one has money in their accounts which is non-halal acquired, they are considered to have consumed “haram bites” in to their stomach.  See item 90 and the consumption of Riba as an example.

[2] Bihar Al Anwar V90 P373