The environment in cargo compartments of commercial and freight airliners is of concern to many shippers. First among them are those shipping live animals such as beloved pets, valuable exotics and laboratory animals. Others who ship perishables such as food baskets, fresh flowers and expensive wines also have an interest in the climate control in-flight, as well as before take-off and after landing. Heat is most often the prime concern, but extremely low temperatures may also affect live animals or perishables if they are misloaded into uncontrolled cargo holds.
Aircraft cargo compartments
In aircraft manufactured by Boeing and Airbus, only one of the two main cargo holds is heated/air conditioned. Live animals in transit and most perishables are placed in that cargo hold. Some airlines have also reserved space for live animals in a small, climate-controlled hold located directly beneath the cockpit.
Most commercial passenger airlines pledge to maintain a temperature ranging from around 10 degrees C to 21.1 degrees Celsius (50 degrees to 70 degrees Fahrenheit) in the controlled cargo hold during flight. One large overnight freight shipper reports a range between 18.3 degrees and 32.2 degrees Celsius (65 degrees and 90 degrees Fahrenheit) in the cargo hold. A study of temperatures affecting laboratory mice shipped by various air carriers showed that nearly half were exposed to temperatures above 28.9 degrees Celsius (84 degrees Fahrenheit) at some point in the flight and 14% to temperatures as low as 6.67 degrees Celsius (44 degrees Fahrenheit). This 2008 study, by Eric Syversen, Fernando J. Pineda and Julie Watson pointed out that despite these temperature variations, "the mice almost always arrived safely."
Unfortunately, there is little to no temperature control in cargo compartments of aircraft parked on the ramp or at freight depots before the engines are started. During those minutes, temperatures in summer can soar and winter temperatures plunge. Most airlines follow a "last on/first off" rule for loading live animals to protect them from exposure to threatening temperatures in the span before take-off and after landing. During the hottest months of the year, many airlines will not accept animals as cargo to destinations of extreme heat like Phoenix. Inanimate perishables which are not protected by ice or cold packs in the shipping container may be damaged in conditions of excessive heat. Freezing may also occur during winter.
There are no unpressurized compartments on modern airliners or freight aircraft except the wheel wells in the nose and wings. All cargo compartments are pressurised to the same life-supporting level as the cockpit and cabin. Maintaining isolated zones of varying atmospheric pressures within the same fuselage is not feasible from an engineering standpoint and would result in internal structural stresses.