The APU is located in the tail cone, seen here with the access panels open on the bottom
It occurred to me recently that, while I’ve been writing on here for almost a month now, I haven’t written about what exactly I’ve done at work at United Airlines, especially since I only have about a week left in my internship.
I was lucky enough to nab the internship with the company at their San Francisco Maintenance and Overhaul Facility in their Powerplant Engineering department. By “powerplant”, I don’t mean those huge facilities that power your homes. That’s a completely different field of study that I’m in no way suited for. Instead, “powerplant” here refers to the engines that United operates.
Before you jump to any conclusions, you probably have never heard of the type of engine I work on. No, the majority of my work does not comprise of those jet engines that produce tens of thousands of pounds of thrust. Rather, most of my time is taken up working on Auxiliary Power Units, or APUs.
I told you that you’ve never heard of it.
There’s good reason you may have never heard of an APU. When you look at a modern airliner, it’s not exactly something that sticks out from those huge turbofan engines. On most airliners, the APU is tucked inside the tail cone.
So what does an APU do? First, you must understand that an APU does not produce thrust. It’s sole purpose is to provide auxiliary air and electricity to the aircraft while the engines are not running, like while the plane is sitting at the gate and passengers are boarding. They are also used to provide air pressure to the engines during the engine start sequence. They are classified as aero-engine derivatives and industrial gas turbines. That is, they incorporate the same concepts and operation as a turbine engine to generate power.
How does it work? An APU is split into three sections: the power section, load compressor, and gearbox, which I’ll explain here.
A Sundstrand APS3200 APU, used on the Airbus A318/A319/A320/A321 aircraft (click for larger version)
As the name implies, the power section is the module where the APU gets its power. This is the part of the engine that most resembles a jet engine, since it has an inlet, compressor, combustor, turbine, and exhaust. For the readers who aren’t familiar with how turbine engines operate, it’s not too hard to learn. All a turbine engine does is pull in ambient air and increase its potential energy by compressing it, adding fuel, igniting it, and then shooting it out through the exhaust. The turbine then converts the energy from the high-velocity and hot exhaust gas into rotational energy, which turns a common shaft.
Here, the common shaft turns another compressor that pulls in ambient air and compresses it, but this time the air is directed to the air conditioning system of the aircraft. This air from the load compressor is also known as “bleed air”. The load compressor matches the demand for bleed air through an inlet guide vane, which regulates the amount of air that the load compressor pulls in.
The gearbox also receives its power through the common shaft from the turbine. This module consists of an assortment of gears that are turned by the common shaft and power various accessories, such as the generator (which produces the electricity for the aircraft), fuel control unit, lubrication system, and other APU subsystems.
The gearbox module after being removed from a Boeing 757 APU
So what exactly do I do? Like any engine, an APU requires regular maintenance and support. I don’t work with line maintenance, which deals with tasks that can usually be resolved without taking the aircraft out of service. Instead, the cases I deal with are a little more serious, ones that require that the APU be removed from the aircraft for further inspection. The reasons for removals are vast, anything from an auto-shutdown to high oil temperature. The bottom line is that if line maintenance can’t resolve the issue, they’ll send the engine to us in SFO.
Once the engine arrives in SFO, it will be reviewed and the engineering department will decide what the best course of action is. It may go through diagnostics in the test cell to narrow down the problem, or it may be inspected further by a technician if it cannot be run. At the end of the day, the goal of the process is to isolate the fault and then implement a solution that addresses the root cause of the issue. It may seem easy to do in writing, but in fact it can be rather difficult. There’s never a smoking gun to an engine problem, but rather a series of failures and issues that cause the problem. Therefore, it can be challenging to determine in what order those problems occurred and how they led to the ultimate failure mode.
Unlike the main engines, APUs are unique in the sense that they are not always flight critical. The only exception is for ETOPS flights, which stands for Extended-range Twin-engine Operational Performance Standards. ETOPS regulations govern flights on aircraft with two engines (i.e. 757, 767, 777, A330, etc.) that operate on routes which are more than 60 minutes from a diversion airport (i.e. transatlantic, transpacific, and polar routes), and the rules state that the aircraft operating in these parts of the globe must have an operational APU in the event of an in-flight engine failure or main engine generator failure.
If a flight is not governed by ETOPS, an APU is really just extra equipment. If it fails for some reason, maintenance on it can be deferred up to 10 days. Furthermore, APUs are being utilized less by airlines nowadays since they burn between 500 and 900 pounds per hour of expensive jet fuel. Instead, the operators are connecting their aircraft to external power ports at the gate to keep the plane powered and cool.
In fact, the overall need for APUs is on the decline. For example, the new Boeing 787 has a significantly simplified pneumatic system, one that does not use bleed air and uses electrically driven motors, pumps, and compressors instead (see this website for an expanded explanation). Therefore, the APU on the 787, the Sundstrand APS5000, is used only to supply electrical power, not electricity and air like current models. Also, as engines like the GE90 and GEnx become more reliable and their in-flight shutdown rate is reduced, I will not be surprised if ETOPS rules are eventually modified to eliminate the need for an APU.
APU start switch on the overhead panel in a Boeing 767 flight deck