19th of Shebat, 5770
If one has only one oven for meat and milk, it is forbidden to bake at the same time in the same oven meat and dairy products. However, if one of the foods is completely covered -food on an aluminum tray well covered with aluminum foil, for example- then the covered food could be placed to warm or cooked in a meat or dairy oven.
That is why we can eat Kosher food that was cooked or warm up on the airplane’s kitchen, because the food is hermetically covered, preventing an exchange of taste between the Kosher food and possible particles of non Kosher food absorbed on a non Kosher oven’s walls.
The same rule would apply in a case that we have a non Kosher oven and we need to cook or warm up in it Kosher food: Using hermetic aluminum cover (Some rabbinical authorities require a double cover in all these cases).
In normal circumstances, when you need to use the same oven for meat and dairy you need to:
1. Clean the oven thoroughly
2. Leave it without use for twenty four hours
3. Leave the oven on maximum temperature for 1 hour. After this process the oven is considered Parve because all particles of meat or dairy taste absorbed on its wall, disintegrated after this process and it can be used for meat or milk.