The Gaon Rabbi Moshe Feinstein discusses this subject in Iggrot Moshe (Yoreh De’ah 164:276 ). He considers an airplane to be one large keli, that is, a large vessel or utensil. If a vessel or container is constructed out of the following metals: gold, silver, copper, iron, tin and lead, it is susceptive to ritual impurity. Thus, if a deceased person is in such a vessel, the uncleanness is passed on to every part of it. He points out, however, that this applies only to these six metals which conduct tum’ah (uncleanness). Since the Torah itemizes only these metals, the implication is that a utensil consisting of other alloys (such as aluminum, plastic, etc.) does not acquire or conduct uncleanness. Today’s airplanes are made out of steel, which is a product of iron, and the floors separating the passenger compartment from the cargo compartment are covered with carpeting. The carpeting is usually made of nylon, which is a product of coal and other ingredients.
If there are two separate entrances in an airplane, one for cargo and one for the passengers, and it is impossible for the passengers and the cargo to cross paths, there would be no problem according to the view of the Rema (see also Gesher Hachayim 6:2).
According to the management of EL AL, the cargo compartment on its planes is a completely separate unit, with its own separate entrance. It is also totally sealed, the passenger compartment having compressed air while the cargo compartment does not. This could be compared to two separate houses, attached to each other, each with a separate entrance and sealed off from each other; this would be permitted to a kohen.
Also, not every flight carries a casket. According to an EL AL official, approximately half of its flights do not carry a deceased person. Therefore the situation becomes a safek, a doubtful case. Moreover, today we classify kohanim as ‘safek kohanim,’ doubtful kohanim. Therefore this becomes a ‘safek sefeka,’ a doubtful doubt – which makes it permissible.