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Technical Training for APU Systems

  • B727 APU GTCP85-98XX (B727-100, 200 as installed)
  • B737 APU GTCP131-9B (B737NG BBJ as installed)
  • B737 APU GTCP85-129XX (B737-100, -200, -300, -500 and others as installed)
  • A320 APU GTCP131-9A (A319 A320 A321 ACJ as installed)
  • B757 APU GTCP331 (B757 as installed
  • B747 APU GTCP660 (B747 as installed)
  • B767 APU GTCP331 (B767 as installed)
  • DC10 APU TSCP700 (DC10-10, -30 -40 as installed)

APU Specialist Troubleshooting Training

Course Background

The Instructor is a 37-year veteran of United Airlines Aircraft Maintenance. He designed and now teaches APU Specialist Troubleshooting Courses. During the time period of the 1980’s he was well known in the industry as the “APU Guru” for all UAL APU Fleet Types. The course is designed around real world APU problems and the troubleshooting of these problems that are still being experienced by today’s aircraft mechanics. He has written the following overview as a guide to what a customer can expect from his classroom.

High Level Overview

The predessessor to these courses was actually first designed and taught in the early 1980’s through the mid 1990’s for United Airlines. At that time UAL was experiencing severe APU troubleshooting issues and needed a better way to train their mechanics. It blends the elements of the Garrett (now Honeywell) GTCP85 APU Engine Training Course with pieces of the Airframe Course to meld most all associated APU problems into a single APU Specialist Class.

For example, the APU Generators ATA 24, APU Fire Warning ATA 26, APU Fuel Feed ATA 28 and of course the APU Engine itself ATA 49 is included in this class.

It is my personal experience based upon almost 38 years of all APU troubleshooting and training that many myths exist in our industry about what certain APU components can and cannot do. It is the aim of this class to curtail the very high instances of NFF (No Fault Found) components changed on the APU during troubleshooting by a mechanic.

The class is fully interactive and student participation is required. The classes at aimed at the mechanics and engineers that will actually work on the airlines APU problems.

This APU Specialist class proved highly successful not only at UAL but also numerous UAL Sales and Service Customers. This included at times both the Military T43 and the National Guard C22B.  A large part of the class emphasis is to help the students understand not only why a certain unit needs to be replaced but also why another unit cannot cause the suspected problem. The goal is to eliminate shotgun troubleshooting.

At the completion of this course I personally guarantee that an average airline mechanic will have the ability to effectively troubleshoot the B727 GTCP85 APU. For the airline the number of APU NFF units should fall significantly over the coming year.

Class Outline

As an example, the B727 APU Troubleshooting class is 3 days long. (24 hours). Typically the class runs 8:00 am to 3:30 pm with an hour for lunch. The Instructor needs a voice break every hour or so and frequent breaks are scheduled.

Class size is always an issue. Personally, I can take up to 12 students in a class but in reality getting 12 students to stand in the wheel well of a B727 or stuffed in the flight deck at one time is barely workable. Somewhere in the region of 6 students works best but I am very flexible on this point.

The first two days are in the classroom utilizing overhead slides or a laptop presentation and if the Customer has a spare APU available we will remove a portion of the shroud and do hands on training too. This is a good option as it allows me to see if the Customer has a single or dual thermostat APU, where the control air tee is mounted, if a selector solenoid is incorporated and a few other items of very important interest to the mechanic.

The third day is actual hands on troubleshooting on the APU if the Customer can supply access to the aircraft. If the aircraft is not available the hands on portion is simulated in the classroom or spare APU.

Most Customers only have access to an aircraft at night. Typically we will do 2 days classroom on dayshift and a quick turn on the second day to nightshift starting at approximately 21:00. This option is available from the instructor provided the Customer and students are in agreement with the “quick turn”.

It is this instructor’s belief that although training on midnight shift is feasible, the end result is less than a satisfactory experience for all concerned and therefore is not recommended.

Day 1

Day 1 concentrates on the meat of typical APU problems. The first few hours are devoted to APU theory, the causes of low pneumatic duct pressure, APU no starts and problems with high/low APU EGT when running the air conditioning packs. APU’s that just “die” are discussed.

It is my experience that this is what the student came for and this is what the student expects to get. During the first few hours I am able to gauge the students APU knowledge and modify the class based upon their APU experience.

You cannot fix an APU unless you first know what is normal and then it becomes easy to recognize what is abnormal. That is a major theme of my classes.

As we progress into the afternoon we get into the FCU, LCV and thermostat controls. Starter, ships battery, the importance of knowing the normal starting times and EGT along with other problems are discussed. Known issues such as Control Air Tee blockage and Control Air leaks are added. 3 Speed Switches, LOP switches, plugged fuel atomizers and other mysteries of the APU are discussed.

Known problems are introduced and real world fixes are discussed.

Day 2

In the morning Day 2 reviews Day 1 and moves into the airframe components such as fuel feed and fire warning. There are many configurations in the field for B727-100 and I try to find out exactly what options are on the Customers aircraft.

APU Generator and the generator control panel get special emphasis.

If the Customer utilizes the Garrett APU test box we step through those tools. If not, I explain how the students can make their own troubleshooting tools. The Garrett test box is not required but I modify the class based upon the Customers available tooling.

Cockpit troubleshooting begins in earnest as I am a true believer that most APU problems can be diagnosed from the Flight Deck provided the student understands what to look for.

Day 3

Day 3 is the opportunity to apply all the knowledge learnt in Day 1 and 2. To be fully effective an airplane with an operative APU gives the most bang for the buck. However I fully understand that this is always not possible and if not we will simulate the problems on the spare APU or the classroom blackboard.

The student will utilize the tools owned by the airline or those made by the student.

If an airplane is available I make minor but safe “faults” to the APU and expect the students to troubleshoot effectively to find the cause and the fix. The faults are commonly the usual problems found on this APU. By recognizing what is normal but actually seeing the abnormal I have found that this method of teaching pays back huge dividends for the airline. For the mechanic, this is the required exam. I allow the mechanics to work in teams, just as they will in the real world. This encourages team building and good troubleshooting dialogue. This is the truly fun part of the course and allows the students to understand that they have mastered the APU.


I enjoy doing these APU classes because my experience has taught me that many myths and urban legends exist concerning troubleshooting the APU in the mechanics community.

My primary training goal is to impart my APU knowledge upon today’s mechanics. My secondary goal is to save the Customer money by trying to eliminate NFF and so called shotgun troubleshooting.

I will be the first to admit my classes are “different”. I sat through too many dull and boring classes when I was a mechanic. These classes are designed to be both educational and fun. Based upon the feedback I get, I honestly believe I have succeeded in this area.

- Dave




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