designer’s secrets - steve swift pty...

Designer’s SecretsMaintainers should know

Steve Swift Pty LtdAirworthiness consulting and training

Good morning everyone.

I want to show you a presentation I gave to a group of maintainers in Hong Kong, in November 2015, for the International Federation of Airworthiness.

I want to tell you some of the secrets I think we, as designers, should share with maintainers. For safety. For HOLSIP.

Designer’s SecretsMaintainers should know

Steve Swift Pty LtdAirworthiness consulting and training

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© Steve Swift Pty Ltd 2016

http://steveswift.com.au

I invited the maintainers to peep through the keyhole into my world as a designer. I showed them this photo of my fifirst job at Douglas, in 1977, designing structure for the DC-9 Super 80, which is now the 717. (Yes, that's me in the middle!)

Then, in 1979, I went to Boeing, where I designed structure for the 757.

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In 1984, I joined Australia’s Civil Aviation Safety Authority, CASA. There, instead of designing structure, I regulated its design, and maintenance, for 26 years.

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Steve Swift Pty Ltd

Airworthiness consulting and training

In 2010, I started my airworthiness consulting and training company. My clients include designers, maintainers, regulators, researchers and others.

I asked the audience at Hong Kong if any were designers. There were none.

Steve Swift Pty Ltd

Airworthiness consulting and training

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www.steveswift.com.au

http://steveswift.com.au

Maintainers?Part 66

A&P

LAME

CAMO

AMO

MRO

Repair station

They were all involved in maintenance, one way or another.

Maintainers?Part 66

A&P

LAME

CAMO

AMO

MRO

Repair station

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5 Secrets

Care

Certainty

Chemophobia

Calcs

Change.

I told them I wanted to let them in on fifive designer’s secrets, for safety and economy.

5 Secrets

Care

Certainty

Chemophobia

Calcs

Change.

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Care

First was the secret of the designer’ care about maintenance.

For a maintainer, it’s no secret that for a long while designers did not care about maintenance.

Care

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They all nodded when I showed them this photo: maintainers scratching their chins over a design that's hard to maintain.

I told them how it was once just up to the designer. Some were interested in maintenance; some were not. I told them two stories from my times with Douglas and Boeing.

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Designers design maintenance

FAR 25.571Airworthiness

Limitations

I then explained how, in the late 1970s, new rules forced designers to design the maintenance as well as the aircraft. Rules like FAR 25.571, from which we get Airworthiness Limitations, moved the design of the maintenance program from a black art to a science, and from maintainers to designers.

I explained to the maintainers that they should not be surprised if designers are now showing an interest in maintenance. It is now some of their business. Some maintainers resent this. We are creeping into their domain.

Designers design maintenance

FAR 25.571Airworthiness

Limitations

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Certainty

Second, there’s the secret of certainty, which is really the secret of uncertainty. Despite all the science, the design of maintenance programs is still an uncertain business. I admitted that we, as designers, are often not as confifident as we appear about our predictions.

Certainty

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Predict

Site

Scenario

Growth

Missable

Critical

For structural inspections for fatigue, corrosion and the like, here’s the fifive things designers need to predict.

(Mechanics can help designers, especially for the fifirst two. They can help us ask more 'what ifs', because they have more practical experience.)

Predict

Site

Scenario

Growth

Missable

Critical

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missable

time

crac

k si

zecritical

growthinterval

For the last three, I showed them this graph, of crack size v time, with which we are all familiar.

First, we need to predict growth. Then, for our proposed inspection method, we need to predict when a crack will be so small that it’s missable. Then, we need to predict when a crack will be so big that it’s critical - the structure could break. We can now work out the interval. Any longer and a crack missed by the last inspection could grow to critical before the next.

How do we predict these things? 'Test is best', so I showed a video of a fatigue test. But, I admitted that even tests are not perfect. One reason is that they are not 'holistic’. For example, few simulate the environment, for corrosion, and small loads, for fretting.

missable

time

crac

k si

zecritical

growthinterval

12© Steve Swift Pty Ltd 2016

Quiz

Years in service?

EASA ADs?

For structure?

10

6733

So, even the best fatigue tests, like the one for the Airbus A380, don't predict everything.

I asked the three questions above. The answers are: 10; 67; 33. Airworthiness Directives (ADs) are mandatory corrections for unexpected safety problems. So, there have been many surprises, about half involving structure.

Quiz

Years in service?

EASA ADs?

For structure?

106733

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For example. The movable flflap track fairings cracked during certifification flflight testing! It’s no surprise, then, that EASA’s ADs require inspections as early as 100 flflights.

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Then there was the cracking of the wing ribs. And, that was only a lucky discovery after the cracking in the next slide...

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– 63 –

Figure 46: Offset oil feed stub pipe counter bore

Figure 47: Diagrammatic representation of the offsets produced during manufacturing operations

Insert document title

Location | Date

ATSB Transport Safety Report[Insert Mode] Occurrence InvestigationXX-YYYY-####Final

Investigation

In-flight uncontained engine failure Airbus A380-842, VH-OQA

Investigation

overhead Batam Island, Indonesia | 4 November 2010

ATSB Transport Safety ReportAviation Occurrence InvestigationAO-2010-089Final – 27 June 2013

…in a small oil pipe. It was nearly a disaster, as you know.

I reminded the audience at Hong Kong that none of this is a criticism of Airbus. Airbus did a good job by industry standards. I even helped Airbus. The point is that even when we do our best, there is still uncertainty.

What’s the lesson, then, for maintainers? It’s still to trust us, but also be alert to the unexpected.

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Chemophobia

The next secret I explained is that many designers suffffer from chemophobia.

Chemophobia

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It’s the fear of chemistry, especially electrochemistry. Structures engineers prefer physics.

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missable

time

crac

k si

ze

interval

critical

growth

corro

sion

So designers don’t like analysing corrosion. They’d rather leave that to maintainers in the Maintenance Review Board (MRB).

Designers prefer physics. They can generate nice graphs with their computers. However, can’t we now do the same for corrosion? (Better than analysing corrosion growing, as above, would be analysing the coating deteriorating.) Even so, the chemophobia persists.

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CP

So, manufacturers and regulators still analyse fatigue and corrosion separately, like above. While designers analyse fatigue on the left, to produce the Airworthiness Limitations, maintainers separately analyse corrosion on the right, to produce the Maintenance Review Board (MRB) Report and Corrosion Prevention and Control Program (CPCP).

It’s as if fatigue and corrosion don't interact, which we know they do. For example, pitting corrosion can cause cracks to start 10 to 100 times earlier. So, designers and maintainers should analyse fatigue and corrosion together, as HOLSIP recommends.

AIRCRAFT CERTIFICATION

FLIGHTSTANDARDSCERTIFICATION

MAINTENANCE REVIEWBOARD

FAR 25.271 MSG-3FAR 25.571 MSG-3Airworthiness Limitations

MRB ReportsCPCP

Certification vs MRBAircraft Certification vs Flight Standards

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CalcsCalcs

Is uncertainty and lack of confifidence why designers keep their calcs secret?

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Does it matter? One reason is reporting. Regulators require maintainers to report things they think could be unsafe for the flfleet.

But, how do they know? For example, which of the above cracks is more important to report? Is it the small crack on the left or the big one on the right? Naturally, most maintainers would think the big one. As designers, you know you need more information, right? Ok, which information?

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ComparePrediction

Site

Scenario

Growth

Missable

Critical

Practice

Site

Scenario

Growth

Missable

Critical

ComparePrediction

Site

Scenario

Growth

Missable

Critical

Practice

Site

Scenario

Growth

Missable

Critical

This is the information. Remember it? Why do we want maintainers to report things they fifind? Isn't it so we can check our predictions, to see if any need correction?

The small crack could threaten flfleet safety more. For example, if we never predicted it, so it has no Instructions for Continued Airworthiness (ICA). With no ICA, no one will be looking for similar cracking on other aircraft.

How will maintainers know this? They won't, because they won't have our calcs. As designers, we should understand this and try to dispel the popular myth that you only need to worry about the big stuffff. We should ask maintainers to report everything, and explain why.

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Maintenance = Mission

ICA?

Another reason maintainers should know the designer’s calcs is for the maintenance program. The maintenance must match the mission, right? But, how can the maintainer do that if the mission in the designer’s calcs is secret? Most ICAs won’t tell you what they cover. For example, will the ICA cover aerobatics, like above? Will the ICA for a military transport or a cropduster cover fifirebombing? Will the ICA for a business jet cover coastal patrol?

The maintainers who approve maintenance programs won't know unless we tell them, so they know when to ask. Militaries do this better than civil aviation.

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ChangeChange

My fifinal secret affffects when maintainers want to change the maintenance program by escalating the time between inspections. And, this really is a secret. It’s a secret to the International Maintenance Review Board Policy Board (IMRBPB). It’s even a secret to most designers, because few have experience developing structural airworthiness limitations to FAR or CS 25.571.

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I told a true story, from when I worked for CASA.

An airline called to tell me that they’d inspected the propeller blades on this aircraft many times, as an Airworthiness Directive (AD) required, and not found a single fatigue crack. ‘Nil fifindings’. The airline asked if I would approve escalating the inspection interval.

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missable

time

crac

k si

ze

intervalgrowth

missable

time

crac

k si

ze

interval

critical

growth

I thought of the above, which is the secret behind the inspection interval in the AD. If you want to change the interval, you’ve got to know more about the above variables than does the AD. What does ‘nil fifindings’ tell you about any of them? Nothing. So, I said, ‘no’.

However, a few months later, I saw on the news that a propeller blade had broken offff the aircraft, soon after take-offff. Despite my advice, the airline had applied to CASA, to someone else, who didn’t know this designer’s secret, so they approved the escalation. Fortunately, the propeller blade missed the aircraft, which landed safely on one engine. All on board were lucky to be alive.

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66 BEng

Well, that’s the fifive secrets. If you are a designer, please share more of your secrets with maintainers. And, invite them to share their secrets with you. We need each other.

Such teamwork is another way in which we can be more holistic in the way we manage structural integrity.

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