An Air India plane's Auxiliary Power Unit (APU) caught fire after landing at the Delhi airport on Tuesday. As per an airline spokesperson, all passengers and crew members are safe. "Flight AI 315, operating from Hong Kong to Delhi on 22 July 2025, experienced an auxiliary power unit (APU) fire shortly after it had landed and parked at the gate," he said. The incident occurred while passengers had begun disembarking, and the APU was automatically shut down as per system design, the spokesperson added.
What is an Auxiliary Power Unit (APU)?
An Auxiliary Power Unit, commonly referred to as APU, is a small turbine engine usually located in the tail section of an aircraft. Unlike the main engines that provide thrust for flight, the APU is designed to provide energy for functions other than propulsion. Its primary role is to generate electrical power and pneumatic pressure when the main engines are shut down particularly during ground operations.
Functions and importance of the APU
As per details, the APU is critical for starting the aircraft's main engines especially when ground support equipment is unavailable. It supplies electricity to onboard systems -- such as lighting, air conditioning and cockpit instrumentation -- before engine start-up and after engine shutdown. It also provides compressed air to the Environmental Control System (ECS) and the engine starter.
Moreover, the APU allows aircraft to operate independently at airports which helps in increasing operational flexibility. For instance, it helps maintain cabin comfort by powering the air-conditioning system during boarding and prevents battery drain by supporting avionics when the engines are off.
What happens when an APU catches fire?
As per details, a fire in the APU can be dangerous especially if not contained quickly. A fire may start due to fuel leaks, oil contamination, overheating or mechanical failure. Because the APU is located in the rear fuselage, a fire here poses a risk to the aircraft's structure and internal systems.
Modern aircraft are equipped with built-in fire detection and suppression systems in the APU compartment. In the incident of a fire, these systems can activate automatically or be triggered by the crew to release extinguishing agents like Halon. Ground crews are also trained to respond rapidly. If not contained promptly, an APU fire could lead to smoke inside the cabin, electrical system damage, or, in extreme cases, evacuation and aircraft grounding for inspection and repair.
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