Do you worry about the wings on your plane SNAPPING off?
I sure used to. And it scared the HELL out of me.
Especially watching them bounce up and down as the plane was taxiing to the runway.
Or – WORSE – seeing them bouncing when flying through turbulence.
If you can relate to what I’m talking about, don’t miss this interview with an expert in aircraft construction.
Rainer Groh is completing a PhD in Aerospace Engineering at Bristol University in the UK.
And when he’s not studying, he shares his love for aircraft engineering on his Aerospace Engineering blog.
After you’ve listened to this interview (or read the transcript below), you’ll understand why the wings WON’T come off.
That’s because Rainer will explain:
- What the wings do (hint: its more than provide lift).
- How they’re designed.
- How they’re built.
- How they’re maintained.
- How airlines deal with unexpected technical issues.
- And much, much more.
Tim Benjamin: Have you ever worried that the wings on your plane could snap off?
For example, while you’re flying through turbulence.
Well, today’s guest is going to put your mind at ease by explaining how wings ACTUALLY work.
Hi there – I’m Tim Benjamin with the Fear of Flying School podcast.
And joining me today is Rainer Groh – an expert in aircraft construction.
Rainer’s completing a PhD in aerospace engineering at the University of Bristol here in the UK.
And in his spare time, he runs the Aerospace Engineering blog – a website where he writes about all things related to aircraft design and aviation.
Rainer – welcome to the Fear of Flying School.
Rainer Groh: Hi Tim.
Tim Benjamin: Now – you’re an expert in aerospace engineering.
And before we get going talking about wings, can you just explain to me what the field of aerospace engineering actually covers?
Rainer Groh: Well, it covers everything to do with the airframe – the airplane.
So, talking about the structural design.
And making sure that parts of the aircraft don’t fail.
The control of the aircraft.
So, controlling different surfaces on the aircraft in order to be able to steer to where we want to go.
The propulsion system – so we’re talking about the classic jet engine or a propeller engine.
And, finally, ergonomics as well – how do we design a cabin?
Or the space inside the aircraft so that the passengers feel at home and feel comfortable while they’re flying.
Tim Benjamin: Now, I mentioned in the intro that you also blog about aerospace engineering.
What motivated you to start doing that?
Rainer Groh: I just felt like online there wasn’t really a space where people could just inform themselves about aerospace engineering and what it’s all about.
Tim Benjamin: Give me an example of the sorts of things you’ve written about.
Rainer Groh: So, I’ve written about my background – from a PhD point of view – is about structures and new, light-weight materials.
For example, glass fibre and carbon fibre.
So, I’ve focused on that aspect of aircraft.
But at the same time, I’ve also written about one of my passions which is propulsive systems.
So, how do jet engines work?
And how do we maintain and cool them?
And at the same time, how do wings work?
Which is, I guess, why you and me are talking here today.
Tim Benjamin: Indeed.
Now, wings being the focus of today’s discussion as you point out.
I know from my own experiences as well as speaking to a lot of other people why have – or have had – a fear of flying – that one of their key fears has been something going wrong with the wings.
In particular, the wings snapping off at some stage of flight.
Can you just kind of give me a little bit of background about the wing?
In particular, what are the main functions of a wing?
Rainer Groh: So, obviously the major function of the wing is to provide lift for the aircraft so that it can actually fly.
There are also some secondary functions: storing fuel for the jet engines – and at the same time – also giving mounting points for the jet engines.
Tim Benjamin: So, we know that the wings’ primary function is to generate lift to get the plane in the air.
And to sustain it in the air.
Can you walk me through how it is that a wing on a commercial jet is designed and constructed?
Rainer Groh: It’s kind of like a two-year process.
And it will take a lot of manpower – and computer power – in order to design a wing from scratch.
Nowadays, the material used for most wings is aluminium because it’s lightweight, stiff and strong.
I don’t know if you’ve heard about the Dreamliner 787 by Boeing – that’s the first aircraft that will now be using composite materials like carbon fibre and glass fibre.
But on most aircraft we’ll be flying on today, they’ll be using aluminium.
Tim Benjamin: Now, you mentioned aluminium is the material used in other jet aircraft.
Can you just go into a bit more detail as to what the benefits of aluminium actually are – because for many of us – you mention aluminium and I certainly think immediately of tin cans for holding my beer and my Coca Cola.
Rainer Groh: There’s a wide range of different grades of aluminium.
And what we’re talking about is the 7000 range of aluminium which is the aerospace grade aluminium.
And it’s MUCH stronger and stiffer than the aluminium you know from your cans.
And the way that you do this is basically by creating an aluminium alloy.
Tim Benjamin: So, if aluminium is the kind of base ingredient, can you kind of explain to me the different – um – essentially how a wing is designed.
You know, we can obviously see the external surfaces of a wing.
But I’m assuming there’s a whole lot of stuff going on inside the wing that we can’t see.
Rainer Groh: Now, as I said, the wing in itself – or the profile of the wing – is not able to take these loads.
So, what we have is the OUTSIDE skin of the wing – which is the profile that you see.
And on the INSIDE, we have ribs, spars …
Tim Benjamin: Let’s take those one at a time.
What exactly is a ‘spar’?
Rainer Groh: So, a spar basically runs along the length of the wing.
And there is typically two spars.
By adding these spars, you’re increasing the bending stiffness of the wing.
Which means that it’s harder for it to bend.
Tim Benjamin: Am I correct in saying the spars run from the tip of the wing all the way through to the body of the aircraft – the fuselage?
Rainer Groh: The spars are running ALL THE WAY through – from tip to tip – in order to prevent the wings essentially from snapping off.
Tim Benjamin: So the spars in the two wings actually meet underneath the fuselage of the aircraft – is that how it works?
Rainer Groh: Basically, you have to imagine that you’ve got these two wings.
And the wings are attached to the fuselage on that wing box – that chunky bit that you can see right at the bottom.
Tim Benjamin: This is all interesting stuff – and it all sounds very positive.
But I know that somebody who’s got a fear of flying – you know – they’re up in a plane.
And the plane hits some turbulence.
They see the wings bouncing all over the place.
They’re still going to be looking out the window thinking ‘Look – I know all the theory sounds great – but that bouncing wing is really freaking me out.’.
Can you explain why – in the middle of a turbulent episode – somebody looking out the window at that bouncing wing – shouldn’t be worried about it coming apart or coming off the plane?
Rainer Groh: From a statistical point of view, it’s almost impossible.
The BIGGEST reason that would cause wings to snap off is bad maintenance.
That’s the ONE thing that will cause them to snap off.
I only fly western airlines because I know that their maintenance procedures are state of the art.
For example, in America you’ve got the Federal Aviation Administration – or the FAA.
And in Europe you’ve got a similar agency.
And EVERY SINGLE airline has to conform to those maintenance criteria that these federations set up.
And there’s different ways that this occurs.
For example, we can do non-destructive testing which means that we – literally – scan the entire structure for small faults to make sure that these faults do not grow any bigger.
And then cause catastrophic failure.
This is regularly performed.
And there are specific timelines when this has to happen.
For example, it will happen – the stress engineer will do a calculation on a computer – and find out when a specific crack is ASSUMED to occur on a THEORETICAL point of view.
Let’s say that’s 10,000 flight cycles.
After 10,000 flight cycles, the aircraft is pulled out.
And all the critical points are scanned and checked of whether one of these cracks has occurred.
The OTHER correlation point that we have is that before any aircraft enters service – the manufacturer may be Boeing or Airbus – has done THOUSANDS of flights beforehand to find out any in-service issues that may occur.
On top of that, there’s something called the ‘fleet leader’ which is the first aircraft of an aircraft fleet that goes into service.
Now, that aircraft might be 10,000 flight cycles ahead of ANY other aircraft in service.
Which means that if a specific fault occurs with that ONE aircraft – with that fleet leader – it will automatically cause the manufacturer to call the airlines to say ‘Hey – we’ve had this fault. Check your aircraft that nothing has happened to yours 5,000 flight cycles prior to that.’.
Tim Benjamin: That’s an interesting point because quite recently I got onto an aircraft that seemed to me to be relatively old – my guess is that it was probably about 20 years old.
And I wondered how much knowledge do engineers have about this particular model of aircraft at this particular age.
When the commercial jet age started in the 1960s, we didn’t quite know a lot about the tiny, minute, details of how aircraft fail.
Which meant that some aircraft – like the Comet – famously failed and crashed.
And that aircraft then had to be taken out of service.
But now, because we’ve got so much data about fatigue and experimental stress states, we can actually VERY closely tell how healthy an aircraft is.
Every single time we’ve had ONE aircraft in the world fail, there is a BIG assessment that goes into WHY this aircraft failed – and what we can learn about it.
And then the things that we learn are factored straight back into the design process of new aircraft.
And also the maintenance scheme of CURRENT aircraft.
And just to come up with another point: even if we DO find one of these cracks in maintenance – or let’s say there’s lightening strike which causes some chaffing on let’s say a wing or a door…
Tim Benjamin: Sorry – just to be clear, ‘chaffing’ is what?
Rainer Groh: Chaffing is a lightening strike – or if somebody was putting the luggage in the aircraft and accidentally bumped the aircraft with the vehicle used to transport the luggage.
That basically causes a dent or some ‘chaffing’.
As if you’ve got a burn mark on your skin – similar to that.
Then, what they do is that the airline takes a picture of the damage – sends it to Airbus or Boeing – and there’ll be engineers sitting there looking at the damage – sizing the damage – looking at their stress manuals – doing some calculations to find out is the aircraft still capable of flying the required lifetime?
Do we have to repair it?
Or do we have to scrap the entire aircraft?
And this happens on a day-to-day basis.
I can tell this from a personal experience – because I worked at Airbus doing these repair schemes – that even the SLIGHTEST damage – all the stress states will be calculated again.
Just to make sure that the aircraft is still fit for flying.
Tim Benjamin: You touched before on the issue of aircraft maintenance.
But can you go into a bit more detail about how the wings are maintained on an aircraft?
Rainer Groh: Before every single flight, critical parts of the structure in the aircraft are checked.
EVERY single time.
And then there are specific maintenance intervals – as I was saying before – say 10,000 flights.
Some of them will be 1,000 flights where the aircraft designer – say Airbus – will tell British Airways ‘Look – after 1,000 flights, you have to check for this and this. You have to check – say – for cracks at the attachment point between the wing and the fuselage.’.
And if there ARE cracks, then they’ll have a manual of the EXACT steps to follow – what to do.
And these are specific milestones in the aircraft’s life that the mechanics check.
Other ways to check is to use – as I was saying before – scanning.
There’s ultrasonic scanning where you take a probe and you run it across the wing surface in order to be able to look at the inside – what goes on actually inside the aluminium – to see if there’s any big cracks occurring there.
Or we can use things called fluorescent dye where we literally coat the ENTIRE wing with a paint.
And then run an ultra-violet light across the wing in order to detect scratches and cracks on the wing.
Or actually – when you crawl INSIDE the wing – on the inside of the wing.
Tim Benjamin: Something kind of related to maintenance – not necessarily to cracks though – that I’ve noticed a lot is – from time to time – I get on planes – and I love sitting on the window seat particularly near the wing.
And something I’ve noticed is that quite often I’ll get on and the wings kind of look really dirty – like they’re covered in this black soot material.
And that sometimes troubles me a little bit because I’ve wondered does that indicate that this plane’s not being kind of properly cleaned or somehow not being properly looked after.
Can you explain to me what’s going on there?
Rainer Groh: Soot and dirt – of itself – on the wing surface – as long is it’s not large CHUNKS of dirt – is in NO way detrimental to the performance of the wing.
So, the wings ARE cleaned.
But small amounts of dust or soot on the wing is not detrimental because it does NOT interfere with the airflow.
Tim Benjamin: Rainer Groh, this has been HUGELY interesting.
If anyone in my audience wants to find you on the internet, where should they look?
Rainer Groh: On aerospaceengineeringblog.com.
Tim Benjamin: Rainer – thanks for joining us on the Fear of Flying School podcast.
Rainer Groh: Cheers – thank you.
Tim Benjamin: Bye everyone.
Now I have a question for you…
What’s the most useful thing you got out of this interview? Leave me something in the comments.