Cabin temperature depends on the weather and the aircraf's recent history
Whether the cabin feels pleasantly cool or rather warm during boarding depends primarily on two factors: the current weather conditions and when the aircraft was last adequately cooled. If an aircraft has just returned from a previous flight, the cabin interior is often still comfortably temperate. The situation is different when an aircraft has been parked in direct sunlight for several hours. Similar to a tent heating up on a summer day, temperatures inside the aircraft can rise significantly above the outside temperature. During periods of extreme heat and intense sunshine, it is therefore especially important to keep aircraft as cool as possible.

Three ways to keep the cabin cool
There are generally three ways to cool an aircraft while it is on ground:

• Pre-Conditioned Air (PCA): Conditioned air is supplied to the aircraft through air hoses connected to airport infrastructure.
• Mobile Air Conditioning Units: External air-conditioning units provide cooled air directly to the cabin.
• Auxiliary Power Unit (APU): The auxiliary engine located in the tail of the aircraft provides electrical power and compressed air for air conditioning.

From an environmental perspective, pre-conditioned air offers the best overall balance and is therefore the preferred option whenever available. However, not all airports have sufficiently powerful systems, and some airports lack this infrastructure altogether. In such cases, crews rely on the alternative cooling methods.

Air conditioning is more complex than many people think
In principle, any aircraft can be cooled on the ground as long as at least one of the mentioned systems can be used. However, air conditioning does not typically begin hours before departure, and both environmental and economic considerations play a role in determining when cooling systems are operated. The process is also technically complex. When cooled air is supplied to the aircraft, air must simultaneously be allowed to escape from the cabin. Otherwise, cooling efficiency would be reduced and cabin pressure would increase. On some aircraft types, including the Airbus A220, specific electrical systems must be powered to enable the opening of outflow valves that allow air to leave the cabin.

When environmental regulations affect cooling
Environmental and noise-abatement regulations also play an important role in ground air conditioning. These rules can vary considerably from one airport to another. At some airports, the use of the APU is only permitted during specific time windows. In Barcelona, for example, the APU may only be operated for two minutes after arrival at the gate and six minutes before pushback. Violations can result in substantial fines. To operate the APU outside these limits, airlines often need special permission from the airport authority.

Why the cabin may temporarly warm up during engine start
One situation in which rising cabin temperatures are difficult to avoid is during engine start. By this point, the airport’s conditioned-air hose has already been disconnected. At the same time, the compressed air produced by the APU is needed to start the engines and is therefore temporarily unavailable for air conditioning. An engine start takes between 50 and 90 seconds, depending on the aircraft type. When two engines are started right one after the other, several minutes can pass without active cooling. Air continues to circulate throughout the cabin, but it is no longer actively cooled. Whenever possible, crews use special procedures on hot days to restore conditioned air to the cabin as soon as the first engine has been started.

Everyone is working towards the same goal
Ground staff, cockpit crews, and cabin crews all share the same objective: providing passengers with the most comfortable environment possible. If the cabin still feels warm during boarding, those responsible are usually already working on a solution. In many cases, technical procedures, operational requirements, and local regulations must all be taken into account.

Patience usually pays off
A warm cabin during boarding does not mean that air conditioning has been forgotten or is not functioning properly. In many cases, the cause can be traced back to weather conditions, technical procedures, or regulatory requirements. With a little patience, the situation usually improves within a few minutes, as the teams on board and on the ground are already working to bring the cabin temperature back to a comfortable level.