What is the primary use of a life jacket on an aircraft?

A life jacket on an aircraft is specifically designed to aid in emergency water landings. In the event of a water ditching, life jackets serve a vital role in ensuring the safety of passengers and crew by providing buoyancy, thus helping them stay afloat until rescue can be secured.

While factors such as warmth or cabin pressure might be considerations for survival in certain situations, they are not the primary functions of a life jacket. For example, life jackets do not prevent hypoxia, which refers to a deficiency in oxygen reaching the tissues, and they are not designed to assist with cabin pressure issues. The main focus of a life jacket is its use in scenarios where there is a risk of having to evacuate an aircraft over water, ensuring that individuals remain safe and can be easily rescued.