Honeywell 36-100 APU

In a conversation with Mark Russo, national sales and service manager, APU program, for Dallas Airmotive, he shared that in his conversations with fellow aircraft mechanics, a common misconception is the concept of on-condition maintenance. In this article we will discuss several topics that Russo shares to help clear the muddy water including defining the on-condition maintenance requirement, looking at common misconceptions when it comes to on-condition maintenance, and discussing on-condition maintenance and how it applies to the Honeywell 36-100 auxiliary power unit (APU).

Origin and Background

So where does the concept of on-condition maintenance come from? Well, we can take a look at Advisory Circular (AC) 120-17A Maintenance Control by Reliability Methods. Appendix 1 of that AC is titled Airline/Manufacturer Program Planning Document – MSG-2. Within Appendix 1, Chapter 14 discusses the three primary maintenance processes used by maintenance programs. They are:

1. Hard-Time,
2. On-Condition, and
3. Condition-Monitoring.

Hard-Time (HT). This is a preventive primary maintenance process. It requires that an appliance or part be periodically overhauled in accordance with the carrier’s maintenance manual or that it be removed from service. An example of this would be a part that must come off the aircraft at X amount of hours or calendar years for an overhaul (or disposal if a non-repairable item). It is hard time.

On-Condition (OC). This is a preventive primary maintenance process. It requires that an appliance or part be periodically inspected or checked against some appropriate physical standard to determine whether it can continue in service. The purpose of the standard is to remove the unit from service before failure during normal operation occurs.

The thing to keep in mind with on-condition monitoring is that just like hard-time inspections, on-condition maintenance still has an inspection requirement (time or calendar based). But instead of an overhaul being accomplished at that time, an inspection is performed. If the part meets the inspection standards, it remains in service.

A misconception that many mechanics have is the definition of on-condition maintenance. It is not a “fly it until it breaks” concept. Rather, based on information from periodic inspections, a part under an on-condition maintenance program can be removed from service before the part fails.

Condition-Monitoring (CM). This is a maintenance process for items that have neither “hard-time” nor “on-condition” maintenance as their primary maintenance process. CM is accomplished by appropriate means available to an operator for finding and solving problem areas. The detailed requirements for the condition-monitoring process are included as Appendix 1 to this circular.

Condition monitoring can include items such as trend analysis, BITE, or spectrum analysis.

The 36-100 APU

Although not a powerplant, the Honeywell 36-100 APU is a complex turbine engine. It has a radial and flow compressor with a remote combustion system and hot section. It has a planetary gear system and a starter clutch system. Needless to say, all of the APU’s components and accessories must be considered in the maintenance process at some point. In the introduction of its maintenance manual, Honeywell notes:

A. On-Condition Maintenance Recommendations

Note: The manufacturer recommends the on-condition maintenance concept for the 36-100 APU in lieu of a firm overhaul time period.

On-Condition Maintenance

In the past, mechanics who worked on Honeywell 36-100 APUs were used to performing scheduled visual HSI inspections that involved a borescope inspection of critical hot section components to determine condition. This inspection requirement was removed by Honeywell (although a fiberscope inspection may be required if there is damage suspected to the hot section).

Russo shares, “When Honeywell removed this requirement, it ruffled some feathers in the maintenance community. Many mechanics felt that a “fly it until it fails” maintenance schedule was not appropriate for an APU.”

Russo stresses that we need to change our mind set about on-condition maintenance. On-condition maintenance is not a “fly-to-fail” concept. On the contrary, on-condition maintenance involves monitoring the component through scheduled inspections and taking corrective action as necessary based on the results of those inspections.

If we take a look at the 36-100 inspection requirements, we see that the APU is being inspected at regular intervals (300, 400, 450 hours) that involve inspection and minor maintenance. Corrective action is taken if the APU doesn’t meet inspection requirements. If we take a look at all the maintenance and inspection requirements that are in place, most of the APU system is covered, and any deficiencies or anomalies can be detected long before the unit actually fails.

There are a few components that are not specifically inspected during scheduled inspections. These areas include gears, shafts, bearings, oil pump, and fuel control unit. Although not addressed specifically in visual inspection requirements, these components are ultimately being monitored through other inspections such as oil filter inspections, SOAP analysis, etc. So ultimately, the health of the APU is being monitored effectively without the visual HSI requirement.

Russo offers three R’s to help you remember how to get the most out of your APU.

  • Remember scope and limitations. Inspection does not mean overhaul. Keep in mind that the individual status of each APU could be a little different between each APU.

  • Research critical components. Check the logbook for high time LRUs. For example, starter motors and load control valves will naturally be subjected to wear over time.

  • Request optional or special actions. Seek advice from those who know. For instance, repair stations that do a lot of work on APUs can often offer recommendations on additional inspections that could help identify any problem areas that may not be addressed in the original on-condition work scope.

This article has touched on the topic of on-condition maintenance and how they affect the Honeywell 36-100. In an upcoming issue, Russo will discuss specific inspection tips when working with this APU. Do you have a 36-100 question? You can contact Russo at (973)728-3002 or e-mail at