Air travel is quite safe for your pets. Thousands of pets travel without incident all over the world each year. However of the very few pets who did die while being transported by airplane, half of them die from the effects of sedation. One reason that sedation can be dangerous is that we really know very little about the effects of sedatives on animals that are under the stress of transportation and are confined in a cage at 8,000 feet or more, the altitude at which cargo holds are pressurized. Additionally, some animals react abnormally to sedatives, actually becoming more excited when sedated.
Sedatives reduce an animal's body's ability to cope with changes in the environment.
For instance when your pet begins to get hot he starts to pant to rid his body of the excess heat, when he is cold he shivers to warm himself. Sedatives may block or slow these reflexes allowing your pet to become overheated or chilled. Finally it is not uncommon for owners to administer excess sedative. You give a sedative on the way to the airport and your dog calms down. Now in the terminal your pet gets excited again.