When an aircraft has completed a long flight, does it have to "cool down" before the next flight?
It depends on what is meant by "cool down". Jet engines must be cooled after they have been operated at high power. When we use full reverse after a landing, a three to four minute cool down is required from the point the reverse has been used. This ensures proper thermal stabilization in the hot part of the engine. The loading and unloading of the aircraft and the transfer of passengers by bus normally cover this time. The engine cooling period is therefore unimportant. Brake cooling comes next. The brakes need to cool to ensure they are effective in the event of a refused start. Brake cooling is also important to prevent a possible wheel arch fire. This is so important because when the landing gear is retracted after take-off, the brakes rest in the wheel arch, which contains hydraulic tanks and hydraulic lines. If there is a leak and something falls on the hot brakes, a fire can break out in the wheel arch. Airbus A320, for example, has a temperature limit for the take-off brake to 300 degrees Celsius. This temperature limit is there to protect the aircraft from a possible fire in the wheel arch. It can take a long time for the brakes to cool down depending on how hot they are, what winds are prevailing and how high the outside temperature is. Some aircraft have built-in brake fans. If these aren't there, you may need an external cooling device. The flight time has no influence on the cooling times of the engine and brakes. However, the longer the flight, the more lubricating oil the engines use to reduce the friction between the bearings and thus the heat. So after a long flight you may need to top up the oil. There is a minimum amount of oil that must be carried on a flight. When the amount of oil is below a certain level, engineers always refill it during the turnaround or stopover. This is particularly critical if the follow-up flight falls under ETOPS (Extended range operations) regulations.
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