APU - Auxiliary Power Unit
After yesterday's story, I got many questions about what an APU is and why we didn't just use the airport AC. Today, I'll answer those!
The APU is the third engine on my aircraft. It's located in the tail of the aircraft, but in stead of providing thrust, it provides electrical power and pneumatic pressure. This pneumatic pressure is used to start our engines. An additional benefit is that we can use the APU to provide fresh air, so we can cool down the aircraft (very important in summer, I've had to offload people because they became overheated). Yesterday one of the airports I flew to was Corfu and unfortunately they didn't have AC, which can be an issue with a temperature of 35 C. When we get on board, we switch on the batteries, the ground power and then we do the rest of our checks, including the APU fire test and eventually switching on the APU. If we don't have ground power, we can start the APU immediately, by using the battery power. The APU will then provide the electricity for the aircraft. However, if the APU isn't working, we need that ground power to be able to set up the aircraft for departure. When we're ready to start our engines, the APU will provide the pneumatic pressure that's needed for this. However, without an APU, we need an ASU, an air start unit. This unit will provide the needed pressure. We need a minimum of 30 PSI to start and minimum 25 PSI throughout the start sequence. After the first engine is started, the ASU is disconnected and we use a crossbleed start procedure to start the second engine. We get enough air pressure in this case, by increasing the thrust on the running engine. When we get back on to stand after a flight, we first need to have the ground power connected before we shut down the last engine. You can imagine that things like these delay our day! If the APU is working, we switch it off after the second engine is started and we switch it back on again after landing. The APU is also used in case of an engine failure as an extra generator. I hope this explains things a bit for you and that it's a good #mondaymotivation for the ones who are looking into going into aviation! ✈️