APU Expertise: The improved APS5000 performance, in harsh environment as well

Foreword

The APS5000 Auxiliary Power Unit has been running as the Boeing 787 Dreamliner APU for about ten years now. At first, despite its innovative qualities, performance was generally regarded as suboptimal due to operational challenges. During the following years various solutions were implemented. Performance improved considerably. The engine shows impressive performances all over the world. Operators in harsh environments however may experience specific issues, due to fine dust clogging the cooling system. This is a commonly known issue for any gas turbine operated in a harsh environment. An APU that spools up in a sandstorm will wear due to ingestion of dust and sand.

Amsterdam based EPCOR ? one of the world’s main suppliers of APU maintenance, repair and overhaul ? offers Hot Section Cleaning to mitigate the consequences of these issues. Hot Section Cleaning is based on a scheduled maintenance plan by Pratt & Whitney Canada (P&WC) and is modified and improved continuously. Worldwide, only three shops are licensed to carry it out. Being one of them, EPCOR is now an experienced supplier in this highly specialized field of APU maintenance.?

Lead Engineer APU Casper Klerk of EPCOR shares his APS5000 experience and explains how to mitigate the consequences of operating this APU in harsh environment

How would you rate the APS5000?

Essentially, as a very good product. It features some unique strengths. For instance, it takes only 60 seconds to start up, delivering electricity almost immediately. In addition, the APS5000 has a built-in solidity that actually exceeds the capacity needed. This means that in case of wear, this APU will fail at a later point in time than an APU normally would. So in fact it lasts longer.

What can you tell about its improved performance?

As a subsidiary of Air France Industries KLM Engineering & Maintenance, and as a maintenance partner of many Boeing?787 operators, we know all about this APU from its introduction in the first years till the product it is right now. Actually, I act as a speaker at the AFI KLM E&M 787 Operator Experience Conference to share about this evolving APU.

Can you give an example?

One problem was the failure of the bearings supporting the gear shaft. A major modification became available to solve this. But at EPCOR we also succeeded in preventing bearing stress by monitoring gearbox oil pressures with our predictive maintenance system PROGNOS? for APU. By doing so, we were able to execute just in time removals. Another problem that many operators encountered, was hot section degradation. We managed early detection for hot section degradation with PROGNOS? for APU as well, by checking for sudden shifts in the EGT (exhaust gas temperature).

What do you mean by harsh environment?

The brief description would be an environment with desert storms, a lot of heat and a lot of dust. Think of countries in the Middle East, such as Qatar, Oman, Bahrain and Saudi Arabia. Other countries in that region, such as Egypt, pose less of a problem.

An example of harsh environment? Imagine desert storms, a lot of heat and a lot of dust…

So now all is well with the APS5000?

Generally speaking, it is. This APU performs as the superior product it was designed to be. However, there is an exception. All operators in a harsh environment will recognize the wear of APUs, for operators of Boeing 787 that is no exception.

What is the problem with harsh environment for operators of Boeing 787?

The main problem is that, like with any APU, the cooling section gets clogged with fine dust particles. Consequently, the APU gets less cooling and heats up. This may cause blades to start shifting. A so-called blade walk could result in an APU seizure or oil leakage. What we see now, is that this occurs with each APU operated in this harsh environment, in an early stage of operations. The exact moment is hard to predict, but it is usually somewhere around 5,000 operating hours. Once this happens, the APU is damaged seriously, often beyond economic repair (BER). We had these damaged APUs coming into our shop and the costs for repair were really high. We don’t want that for our customers. We want to help them lower these costs.

APS5000 improvements over time

2015: APS5000 retrofit program with improved riveting process per SB 24 to mitigate blade walk events / Solution to blade walk events with new rivet design per SB 22/23.

2016: Improved start reliability with new Exciter Box per SB 500055.

2017: Improved Impeller/Shroud clearance per SB 500058 to prevent Impeller rub / Improved T1 & T2 disk seal arm design per SB 500065 to prevent seal arm fracture.

2018: Improved Gearbox Gear and Bearing design per SB 500077 to prevent roller bearing failure / Improved rotor assembly design and procedures per SB 500078 to lower vibration level and ease of APU assembly.

2020: Improved Compressor Area per SB 500082 to reduce wear on compressor parts.

Is there a solution for this?

P&WC has come up with a program called Hot Section Cleaning. We are one of the three shops in the world to carry out this scheduled maintenance plan. It consists of taking apart the APU, clean it, replace parts if necessary and put the thing back together again. By doing so, all dirt and clogging are removed and the lifecycle of the parts and the APU itself are prolonged.

We are one of the three shops in the world to carry out this scheduled maintenance plan

Have you done a lot of these Hot Section Cleanings?

We did 16 of them. Less than expected, due to the pandemic induced flight reductions. But we expect a lot more of these specific shop visits once things get back to normal again. Our findings varied. The condition of some APUs was relatively good, but with others we were surprised to find cracks in the turbine section and the combustion chamber. This variation means we need to finetune the program. At this moment, this inspection has an interval between 2,500 and 3,000 hours with a brand new APU and 3,000 to 4,000 hours for the second run. Together with the manufacturer we want to find out what the best interval would be per customer. You don’t want to have customers send in their APUs before it is really necessary. On the other hand you don’t want to wait until the serious damage is done.

How does this help operators with APUs operated in harsh environment?

The big advantage is that the removals for these specific inspections are planned. So operators are not confronted with unscheduled removals and all the unpleasant and expensive consequences. In general, customers want predictability. Not only in terms of time, but in costs too. Actually, we are getting there as well. We think we will be able to predict the number of parts that will have to be replaced at these inspections. This means predictable and relatively low costs as opposed to the high costs of an unplanned removal.

Would you rate this as a successful program?

Certainly. Of all the APUs we performed the Hot Section Cleaning on, none failed. So we have faith in this program. As I said before, it needs finetuning. Together with the OEM we are working on that. In addition, monitoring vibrations of the blades might also contribute to preventing damage by dust clogging. So this is also part of the solution.

What is your personal take on this issue??

Of course, I think it is too bad for operators in harsh environment that they are confronted with this problem in the first place. But I am happy mitigating solutions are in place now. I learn a lot of working on the program with both the manufacturer and our customers. And I am glad that at EPCOR we are transparent both during inspections and in our communications. We share our expertise and show our customers everything, so they know what is happening to their APU. I see it as a constructive way of working together on further improvement.

Of all the APUs we performed the Hot Section Cleaning on, none failed

EPCOR is one of the three shops in the world to carry out this scheduled maintenance plan.

Pratt & Whitney Canada about APS5000

The APS5000 is the industry’s first all-electric APU for large commercial aircraft. This single-shaft, variablespeed gas turbine APU develops 450kVA of electrical power at sea level that starts and operates up to 43,100 feet.

The APS5000 APU was designed for the Boeing 787 Dreamliner. It is the quietest APU in its class with the lowest emissions in the industry. The APS5000 air system, like any other gas turbine engine, provides cooling flows to maintain metal temperatures within design intent throughout operational envelope of the APU. The hot section parts of the APS5000A are built with the most efficient thermal barriers and coatings in the industry.?

The ingestion and accumulation of the sand/dust may progressively starve the hot-end components from the required cooling flows. P&WC has been working with the operators in harsh environments on developing customized intervals for scheduled removals, striving for maximum APU TOW, minimum overall repair costs and minimizing operational impact. Three years of applying this approach have demonstrated the efficiency of the program by significantly reducing unscheduled events and minimizing impact on operation.

About EPCOR

EPCOR is an Air France Industries KLM Engineering & Maintenance subsidiary and a centre of excellence in maintenance, repair and overhaul of APUs and pneumatic components. In 15 years, the 190 employees of EPCOR performed 2,100 APU shop visits for 94 airlines from around the world. Additionally, 1,000 APUs of some 30+ customers are monitored daily in real time. All in all, EPCOR has gathered an enormous amount of experience and expertise. EPCOR features capabilities for 5 types of APUs from Honeywell and for 3 types of APUs from Pratt?&?Whitney?Canada. More than 20% of the worldwide 777 APU repairs are performed by EPCOR and over 30% of the worldwide 787 APU are serviced in the EPCOR shop. EPCOR is one of the 3 licensed shops of 787 APU repair and overhaul and exclusively licensed for all Honeywell GTCP331- 350 APU repair and overhaul. EPCOR offers Engineering as a Service. For a modest monthly fee, EPCOR specialists proactively offer customers advice on the operation of their APU. This advice is based on data and findings generated by monitoring through predictive maintenance system PROGNOS? for APU. EPCOR ensures quality and offers a flexible scope to meet customer demands.

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