Outright aircraft strength

[Shaft109]

Mods please feel free to move this to a more relevant area if appropriate.

All airframes are designed and built to be strong where necessary but as light as possible with a safety factor built in - but given certain job descriptions some are 'more equal' than others i.e. the A10 is specifically built to be 'tough' and I expect all carrier aircraft are that bit overbuilt to take the pounding of deck landings.


So my question is are some air frames simply tougher than others far in excess of the normal limits or are they made from stronger materials or what?

Sorry if it's a bit vague I'm struggling to be specific in what I mean - on the Bucc thread running Beagle refers to the Buccaneer as 'immensely strong' which i can understand given it's original role but would a Tornado or Hawk et al. survive the same maneuver in the same circumstances?

Or in the Second World War were certain bombers more likely to get you home missing a few bits and bobs?

The analogy I have in mind is the Toyota pick up on top gear - beat to sh!t but still intact where some other cars would be toast in minutes.

[Roadster280]

Hence the T-45 Goshawk vs the original. Among other naval-specific mods, it has a strengthened airframe, I believe.

I don't suppose you want to incur the cost of implementing those extra features if they won't be required in the aircraft's intended role. You don't get cargo doors on most airliners. You can have them, but you pay for them, and floor strengthening etc.

There's a vid out there of an old Airbus (A310, I think) being stripped of passenger fit and converted to a freighter.

[Pontius Navigator]

The F4 was a naval aircraft of the 50s and no doubt stronger than needed as a land fighter.

The F4K was procured as a carrier aircraft whereas its stablemate, the F4M, was only a land plane. I doubt it was made less strong though it lacked hydraulic wing fold and several other mods need for RN carriers.

[Bigbux]

SU25 has a titanium cabin that can take direct 30mm hits at close range. You just wouldn't expect it on an aircraft - but that's design for you.

[TBM-Legend]

The A-10 has a titanium cockpit tub for pilot protection.

One of my friends flew the A-10 and he said pilots always asked where is the titanium canopy? Rolling in on a target and taking a round or two through there might mean you're in a blender while the rounds try to 'escape'....:}

[Fantome]

Or in the Second World War were certain bombers more likely to get you home missing a few bits and bobs?


see Reg Levy's NIGHT FLAK AND HIJACK PP 42-43. His description of the punishment his Mosquito took might put your hair on end.

[sandiego89]

Shaft, there are so may variables that this is tough to answer in a quick post.

Yes some airframes are stronger than others, and there are a whole range of compromises and tradeoffs that go into the design of an aircraft. Military specifications calling for ballistic protection and crash survivability will generally result in beefier, heavier airframes. Aircraft designed to operate aboard aircraft carriers are more robust to cope with the stresses of catapult launches and arrested landings.

Most carrier aircraft have a good reputation for being strong and include the F-4, A-4, Hellcat, etc. Besides the fatigue issues in later life (land based), the Bucc was indeed known as a robust aircraft.

Some aircraft later served in roles that they were not inteded to serve, such as most of the cold war fleet going to low altitude penetration and later suffering fatigue issues or limitations (Valiant, B-52, etc).

I have some knowledge of the H-60 airframe, and can atest it is a robust airframe, mostly because of the initial design requirements for a battlefield helicopter. Other civil helicopters of similar size may have less stringent requirements. I know which one I would want to be in a rollover or crash.

A common rule of thumb is many modern aircraft are designed for a certain load limit, and then a 150% failure limit is built into the design. That is the structure can survive 150% of the normally anticipated highest load limit it will see in normal service. Structures, or even whole aircraft, can then be tested, and if they fail before that point, it may be time to go back to the drawing board. Some more weel-knowm examples include the the C-17 and C-5 that suffered wing issues and needed to be re-worked, and the C-133 and Comet suffered fuselage cracking.

WWII provided some great examples of different design philiopshies. The Japanese Zero was designed for optimum range an manueverabilty, but did so by sacrificing armor protection and had a less robust structure. The Hellcat was designed to take a beating, and did so by having a beefier structure- which had a weight penalty.

Manufactures use a a variety of materials to meet weight, cost and strength requirements. Through history we have seen different materials introduced to meet ever increrasing requirements: wood and fabic was replaced/augmented by aluminum, then alloys, tintaium, composites etc. Titanium is a great example as it was lighter and stonger than earlier material, and had better heat properties, but was quite expensive- it is all a trade off.

Some manufactures had or have a reputation for strong aircraft. Grumman was known as the "iron works" for good reason. They built strong aircraft, and needed to as most were intened for naval use.

The B-17 had a good reputation for taking damage. It was well built. Many bush planes are quite robust.

[Fareastdriver]

In the 30s and 40s one didn't have computer calculated stress patterns or ultrasound inspection procedures, the aircraft had to stand up by itself. The designers and the people who were going to build them used to look at something like the main spar, suck their teeth and then add on a few layers.
This attitude gave birth to aircraft like the Douglas DC3 which has never suffered from fatique problems

[tartare]

Carrier based aircraft would primarily be excessively strong in the fuselage barrel area - right?
I seem to remember some discussion of this when there were cracks found in Hornet fuses.
I assume you essentially build a very strong `centre bit' of the fuse where the gear attaches... to absorb the loads... and then similarly the spine of the aircraft from the barrel to the tail is excessively strong to hang the engines off, and attach arrestor hook?

[Pontius Navigator]

In WW 2 the Halifax was reputed to be more survivable than the Lancaster.

At one point, I think on the Lancaster, the engineers examined badly damaged aircraft with a view to strengthening. It was pointed out that these aircraft were fine. What needed beefing up were the undamaged areas as damage in those areas may have resulted in the missing one's loss.

[Pontius Navigator]

The Valiant losses were not due to an operational change from their original requirement but through metallurgy and the premature failure of the advanced alloys in the spars, spars never installed were found to exhibit the same stress signs. Aircraft assigned to RRE were very low hours and similarly afflicted.

The RAF knew in at least 1956 that there was a problem with DTD 683, the alloy the Valiant centre plane spar was made out of, but the materials low fatigue resistance was known from the very begining of the Valiant project in 1947, it was hoped that new techniques in manufacture would solve this inherent weakness. The other problem for the Valiant was that it was built to a 'safe-life' strategy, the 'safe-life' strategy of A/c design was abandoned in around 1956 as it could not ensure safety in a catestrophic failure. Also in 1956 this article:-
1790 Structural Changes Caused by Plastic Strain and by fatigue in Aluminium-Zinc-Magnesium-Copper Alloys Corresponding to DTD.683 (Broom and Mezza)
appeared in
The Journal of the Institute of Metals (JIM) Vol 86, 1957-1958, (written in November 1956)
The President of the Institute of Metals was Lord Tedder, Marshal of the RAF .

Lord Tedder

[jimjim1]

I was shocked when the 1998 jet hits cable car story was reported. I had no idea that such an aircraft could conceivably sever the steel cables like those supporting a cable car and fly on. I guess perhaps that the cables struck the landing gear or a similar tough bit.

Wiki says:
"The cable was severed and 20 people in the cabin descending from Cermis plunged over 80 metres (260 ft) to their deaths. The plane had wing and tail damage but was able to return to its base, "

The cables supporting cable cars are not flimsy.

A couple of poor images of the damage to the aircraft at starboard wing root.

Aircraft was EA-6B Prowler.

BBC News | Europe | Cable car deaths - aircrew charged (http://news.bbc.co.uk/1/hi/world/europe/70394.stm)
Damage to prowler jetgoogle for images [Damage to prowler jet cable car "archive.militarytimes.com"]
Site is subscription only, and I don't have a subscription, so any link I post may be no use.
The returned image may be a reduced resolution preview, not sure.If anyone has a link to the US official report which has apparently been released to the Italian newspaper La Stampa it might be interesting.

Exclusive: Classified Documents Show U.S. Full Responsibility For 1998 Italy Ski Gondola Disaster (http://www.worldcrunch.com/eyes-on-the-u.s./exclusive-classified-documents-show-u.s.-full-responsibility-for-1998-italy-ski-gondola-disaster-/c5s3448/)

[Pontius Navigator]

JJ,http://en.m.wikipedia.org/wiki/Cavalese_cable_car_disaster_(1998)

[Rigga]

A Group Captain at Laarbruch (cant remember his name) once told us on our arrivals course "...the Buccaneer is made of Ships Plating and Girders while the Jaguar is made of resin-impregnated paper, tin-foil and locking wire".


...if that's any help?

[smujsmith]

Perhaps drifting away from the OPs intent, if it related solely to Military aircraft. I had some 500 hours solo experience of flying gliders over my time in the RAF. By basic trade I'm an Airframe tradesman, and was somewhat surprised when our CFI told me that modern gliders, spar wise, are stressed to take more than 20G loading, on account of spending most of our time in rough air and turbulence. His assertions were put to the test not long after when a fellow club member managed to fly into the blackest cu nim she could find that day. Her exit from the bottom of the cloud, minus wings, cost her her life. It appears she had found a way of exceeding whatever the true limits were on that aircraft, assisted by the nature of a violent piece of weather. As a C130 Ground Eng I never doubted the integrity of the airframe, yet could never forget seeing the wings fold on the firefighting Herk lost a few years after I left the service. i suspect though, that as is mentioned with the A10, designers "build in" strength based on the intended use of the aircraft. Who knows what happens when that aircraft is taken out of its designed environment. As a starter, the"Australia" patch on the underwing of the Vulcan, a direct result of its new low level role. I'm sure many others can offer input on the subject, it's certainly a "meaty" subject.

Smudge:ok:

[Pontius Navigator]

Smuj, not heard it called that before but it certainly contributed to the weight growth. IIRC we had zfw iro 96-98k when I started and near 110k 10 years later. Obviously some related to extra kit such as RS 2, HRS, TFR, Swivel Seats although the PTR175 and Collins may have been lighter. And of course polyurethane paint.

[GreenKnight121]

jimjim1 - the landing gear was retracted, and to get to it the cable would have had to cut through the radar and radar-mounting bulkhead for the front landing gear, and through all that and the bottom of the cockpit and front of the engines to reach the mains.

[India Four Two]

when our CFI told me that modern gliders, spar wise, are stressed to take more than 20G loading,smuj,

Not quite 20 G, but still very impressive. Here's a video of a DG-1000 wing, being tested to destruction. The initial test was to the maximum certified flight load (6.4 G, J = 1, force applied by the crane 3240 lbs), with a cold wing. The wingtip bent upwards by 2.3 m!

They then heated the wing to 54° C (to simulate central Australia) and bent it until it failed at J=1.95. I calculate that is 12.5 G and a force of 6318 lbs! The certification requirement is J=1.725 for 3 seconds.

N1oduHSFPZQ#

zeuPLms36mA

I was surprised by the lack of safety equipment and the close proximity of the testers to the wing.

There is a very good description of the test here:
DG Flugzeugbau: Destruction of the Wing (http://www.dg-flugzeugbau.de/Data/TM-DG/dg-500/dg-500mb/843-17/index.php?id=bruchversuch-e)

although when I went to the site today, the pictures were missing.

[9 lives]

Happily, the "strength" of an aircraft structure is complex and multi faceted. There are many design opportunities to optimize the design. Strong is nice, but it's usually either very heavy, or very expensive. But too strong may eventually break, either by overload, or fatigue. As India Two Four shows, "strong" is better when it is carefully balanced with flexible, as the flexing will relieve the need to carry the load somewhat.

Changing an airframe by "strengthening" requires a lot of applied knowledge. Just adding a gusset or doubler here and there can have the effect of shifting the load to be carried to another part of the airframe, which is perhaps even less able to sustain those loads, and possibly eve concentrating the load at that more vulnerable place.

The change to the structure can also cause the intended flexing to happen differently, and disastrously. I recall reading of an early German monoplane, which suffered wing failure during test flight. The designer strengthened the wing spars, and failed at even a lesser load. By strengthening the spar, the designer unwittingly changed changed how the wing flexed under G load, and now, as G was pulled, the wings flexed so as to increase their angle of incidence near the tips, and ripped them off. The spar had been strengthened behind the flexing axis of the wing. So if you're going to stiffen a wing structure, the leading edge is probably the place to do it.

The "get you home" properties of an airframe are often as much about multiple load paths, as a "strong" or "tough" structure. It's nice to have a wing with many spars (Lear Jet, for example), so if a spar fails, the rest will get you home. COnsidering the durability of the dH Mosquito. it was wood, with so many load paths that if you shot away some, the rest would carry the load, hoping of course that the pilot then recognized the need to handle compromised airframe with a gentle touch.

This nice characteristic of a well designed aircraft is found in many good designs as layered structure, so one layer can fail, and the others carry the load. "Built up" wing spars are generally an example of this, where multiple pieces make up the spar caps, so if one is broken the others carry the load until you get home.

The back side of this is the possibility of a "latent failure" such that an element of the structure has been damaged (fatigue/corrosion/overload) but the associated structure is still carrying the load, so the failure is undetected. Good inspectability is vital.

Aircraft structure is a fascinating subject to learn, and there is seemingly endless material.

[Krystal n chips]

" Her exit from the bottom of the cloud, minus wings, cost her her life. It appears she had found a way of exceeding whatever the true limits were on that aircraft, assisted by the nature of a violent piece of weather

As I understood it, she had entered cloud and the accident report surmised she had entered a spin with the wings parting company at about 11g.

I knew the lady concerned.

[glad rag]

" Her exit from the bottom of the cloud, minus wings, cost her her life. It appears she had found a way of exceeding whatever the true limits were on that aircraft, assisted by the nature of a violent piece of weather

As I understood it, she had entered cloud and the accident report surmised she had entered a spin with the wings parting company at about 11g.

I knew the lady concerned.

Sobering stuff K.





As for the A10, it's "strength" comes from it's simplicity and that it's designed to fly with significant bits missing here and there...

As for the wingless F-15 who would have believed that?

[Genghis the Engineer]

Mods please feel free to move this to a more relevant area if appropriate.

All airframes are designed and built to be strong where necessary but as light as possible with a safety factor built in - but given certain job descriptions some are 'more equal' than others i.e. the A10 is specifically built to be 'tough' and I expect all carrier aircraft are that bit overbuilt to take the pounding of deck landings.


So my question is are some air frames simply tougher than others far in excess of the normal limits or are they made from stronger materials or what?

I'll give this a shot, as I used to teach aspects of this stuff.

Firstly, let's be clear about "strong". There are several ways you can define this where an aeroplane is concerned.

The simplest is the V-N diagram; the speed-g envelope which applies to any aeroplane, albeit with different values depending upon aeroplane role. A light aeroplane will have something like +3.8/-1.5g and 170kn. From memory the Hawk was around +8/-4 (a bit less in a training role) and 350kn/M=1.05, a modern fighter will be around +9/-4, 650kn/M=2.2, and a modern transport around +2.5/-1.25/350kn/M=0.85. Typically you design to that plus an appropriate safety margin, which is usually about 1.5 (i.e. add 50%) for metal structures, with another 20-50% over that for composite structures.

Additional to that will be a requirement for damage tolerance. For most civil or training aeroplanes, that's fairly trivial - requirements for a certain amount of hail / corrosion damage, or manufacturing defects. So, the V-N diagram requirements have to be met with predetermined levels of damage. Where you are dealing with aircraft that have any kind of combat role, you add in additional requirements there - in simplistic terms, it's still got to meet the requirements when it's been shot at a bit. That means inevitably a fair bit of redundant structure - Barnes Wallis' geodetic constructed Wellington did that really well - but any aeroplane can be designed with redundant structure. This is, incidentally, one reason why even the aerobatic variants of the various Spitfire replicas can afford ot be massively lighter than the original - they might be thrown around a great deal, but they're unlikely to ever get shot at. On the other hand, anything design for a low level surface attack role, such as the A10, will have a huge amount of structural redundancy built in.

Undercarriages are pretty straightforward for the vast majority of flying machines, as the task of landing doesn't vary much between a civil or military trainer, ditto transport, or even a fighter. There are known formulae for determining worst case impact loads, and they pretty much design the undercarriage for you.

The big exception is landing on a ship, whether that's a fixed or rotary winged arrival - where the requirement for a distinct "arrival" has to be designed in, combined with the obvious fact that ships keep moving up and down, and if that is mis-timed, the loads go up a lot. However, it is also important to realise that this isn't really about the forces, it's really all about the energy of impact. So, the best way to deal with lots of energy, is to make an undercarriage with a lot of travel, and within that plenty of damped energy absorption. That in itself brings the peak loads in the undercarriage down, and makes an undercarriage which will take a huge amount of abuse. Note, that this makes the way to build a maritime undercarriage long wth lots of travel, not just beef it up, as making it stiffer will actually just make things worse.


Control surfaces, linkages and actuators are a whole additional game, as you need to consider simple (only in one direction or centralising), control reversal, and then the speed at which they'll be operated. You pretty much want a fighter to be "carefree" as in full deflection up to Vne, which is a big ask structurally, and would mean than an aircraft in that class will be designed to a much tougher set of requirements. Its an interesting point there with civil transports apapted for military use where a certain amount of high speed severe manoeuvring may be required, so you're likely to need to beef the control runs of such an aeroplane up before you paint it green.

Sorry if it's a bit vague I'm struggling to be specific in what I mean - on the Bucc thread running Beagle refers to the Buccaneer as 'immensely strong' which i can understand given it's original role but would a Tornado or Hawk et al. survive the same maneuver in the same circumstances?

Or in the Second World War were certain bombers more likely to get you home missing a few bits and bobs?

The analogy I have in mind is the Toyota pick up on top gear - beat to sh!t but still intact where some other cars would be toast in minutes.


WW2 of-course the design methods were much cruder than we have now, so there was an inevitable need to overdesign somewhat. Also however there was extensive use of frame + lightweight covering structures (Wellington, Hurricane...) That fabric or light alloy covering was by and large not loadbearing, so you could burn or blow it away to your hearts content, and whilst it didn't do the aerodynamics many favours, it didn't worry the structure much.



Hope that helps a bit - a few books off the shelf that cover this a bit...


Megson is the book I learned most of the basic theory from, although I see from the reviews that some of the mistakes are apparently still in there. (Megson was taught by a chap called Professor Dennis Meade, who taught me when I was an undergraduate at Southampton).

Fielding is the only aircraft design book I know which covers the military side well. John Fielding used to run Cranfield's MSc in aircraft design, as well as a lot of industrial experience.

Gratton has just come out, and is probably the only textbook out there covering the practice of airworthiness assessment. Not too much maths in it either and written by somebody who has a fairly substantial background in both military and civil certification. Stupidly expensive, presumably the publisher are profiteering off the fact that there's no real competition.

G

[India Four Two]

The back side of this is the possibility of a "latent failure" such that an element of the structure has been damaged (fatigue/corrosion/overload) but the associated structure is still carrying the load, so the failure is undetected. Good inspectability is vital.

A case in point being the design of the Blanik L-13 spar root. All Blaniks were grounded following a fatal in-flight wing failure, due to fatigue, in Austria. The design of the spar is such that a potential failure is undetectable visually and the cost of a suitable repair scheme was more than the glider is worth.

[glad rag]

BCal Flight over the Andes 1 (http://www.british-caledonian.com/BCal_G-ASIX_Flight_over_the_Andes_1.html)

[Shaft109]

Thanks all - will be doing a little further reading with that Genghis.

Certainly clearing up a few things I've wanted to know.

One thing India touches on is the possibility of unseen or latent damage caused earlier in the a/c life - mulled this once after whizzing around at 3+g during hour building after getting bored with S + L practice. I was always honest about what I'd been up to but who knew what had really happened to it in the past.

Good to know it had probably been accounted for.

[portsharbourflyer]

Some very good replies so far, however the one thing that hasn't been mentioned so far is that for the static analysis there is both ultimate and proof / limit assessment.

A lot of you have mentioned the 1.5 Ultimate factor, that is only for the ultimate static load case (limit case x 1.5), in this instance the component is assessed against the material ultimate strength (that is the strength at which rupture will begin to occur).

It is also normal to consider the limit load (ie: no safety factor applied) or a reduced safety factor (proof) against the materials yield point (the point at which a material starts to deform and the deformation remains permanent).

Typically this is where the civil and military regs are different, in the civil world the proof factor is 1.0 (therefore the same as the limit load), the military have typically used a proof factor of 1.125.

So on the proof load check the aim is to show that no permanent deformation of the structure occurs.

So for aluminium structure, deformation is expected well before an out right failure in most circumstances.

[RRNemesis]

Smuj & KnC
The investigation, into the Ventus, concluded that: the failure of the glider wing was due to it being flown with the trailing edge flaps down, and possibly the airbrakes extended, beyond the flap limiting speed. Flap failure and subsequent damage to the rear spar caused a loss of torsional strength of the wings; these departed the fuselage following several violent oscillations.
There was no comment in the report regarding spinning or experienced G loads. :ugh:

[Krystal n chips]

RR Nemesis,

Given the content of this thread, and the tragedy involved, it would be inappropriate for it to descend into a slanging match.

I note the 4CGC annotation and I can say with all honesty, that the information I posted came from two very well respected and reliable sources within 4 C's, and I also suspect we know each other from that era.

Would you care to remove the :ugh:smilie please because I am not given to making comments about accidents in a frivolous or dismissive manner

There is a possibility, we could be talking about 2 separate accidents here however.

Check you PM's please

.

[dragartist]

What a great post Genghis, may I please ask a supplementary question about the use of composites in the modern era. I do understand that higher reserve factors were applied in the early days of composites because not much was known about failures. Also the inability to inspect for defects. (I am guessing during manufacture or damage/ overstress in use)


With 787 and A350 are the RFs still increased. I would have thought that with all the experience gained and modern NDT 1.1 would have been sufficient.


I am with you all the way about deflection being used to absorb energy. Local stiffening just moved the problem. When I was a student I recall a book entitled the New Science of Strong Materials. It was littered with cartoons and phrases such as "why wings bend" and the likes.


Had I still been a student I would have asked my Dad to buy me the Guy Gratton book for Christmas or my Birthday.

[Genghis the Engineer]

What a great post Genghis, may I please ask a supplementary question about the use of composites in the modern era. I do understand that higher reserve factors were applied in the early days of composites because not much was known about failures. Also the inability to inspect for defects. (I am guessing during manufacture or damage/ overstress in use)


With 787 and A350 are the RFs still increased. I would have thought that with all the experience gained and modern NDT 1.1 would have been sufficient.

Thanks, glad I could help.

Yes, SFs are still increased for modern composite aeroplanes, but not by as much as they used to be because they're now using high temperature / pre-fatigued structural data.

There's more variability in composites: with temperature, moisture absorption, invisible stress / impact damage, manufacturing quality and ageing, than there is with metals. So, the larger safety factors are being applied, but they depend very much upon how you do it. The more work to demonstrate exactly what the safety factors are due to all of these factors, the more you can bring them down.

(SF = margin between limit and ultimate, whilst RF = margin between ultimate and failure)

I am with you all the way about deflection being used to absorb energy. Local stiffening just moved the problem. When I was a student I recall a book entitled the New Science of Strong Materials. It was littered with cartoons and phrases such as "why wings bend" and the likes.

I've read that a few times, first when I was doing my A-levels IIRC; it was recommended for the Physics course, then during my degree, and once or twice since. He wrote a sequel as well: "Structures, or why things don't fall down". JE Gordon is the author.


Had I still been a student I would have asked my Dad to buy me the Guy Gratton book for Christmas or my Birthday.

I wrote it off against tax! You can read a few sample bits for free on Springer's website if you take a look.

G

[JOE-FBS]

I think Genghis has it pretty well covered but to add some colour, here are a few details from my current job certifying a composite CS-23 / FAR 23 aeroplane:

The regulatory loadings are minima. We have chosen (as have others in the same class) to certify ours to +/-10g which means we will do the certification ultimate load tests, after the fatigue test, to the loads for15g. we will do that at 72 centigrade. The airframe used for all that testing will also incorporate some tens of points of deliberate manufacturing error or deliberate impact damage. In addition, the material properties values we use in calculations for the structure are based on low expectations derived from statistics from multiple sample tests (that's not a very scientific sentence, I am trying to convey the ideas for general readers). Those sample tests are on test pieces that have been conditioned in a hot and humid environment for several weeks. In summary, for composite aircraft, there are multiple layers of pessimism in the calculating and testing of the airframe.

One last thing, for about the last twenty years, seats, which for small aeroplanes tends to mean a big chunk of the front end of the aeroplane as well, have been dynamically tested for occupant injury protection (similar to the crash tests used in car advertising). There are probably videos on the internet if you look for them. Discovery Canada did a good piece on it in the airliner context in 2013 (but I'm only saying that because I was the technical talking head :) ).

Oh yes, books. I found Cutler and Liber's Understanding Aircraft Structures to be a good introduction.

[dragartist]

Thanks Genghis, and Joe, Don't wish to steal the OPs thread but more on composites. I am currently working in crash testing. We undertook a project recently with some EU funding looking into embedding sensors in composite wind turbine blades for early failure detection. I also had a bit of a dabble into acoustic emissions with a group up in Aberdeen (Composite pressure vessels) our longer term aspirations was to embed the transducers into the windings. I don't think this happened on this project before I moved on. I do wonder if this has been applied to composite aircraft structures. We were imposing significant damage to these vessels before testing to ultimate to ensure the sensors were able to detect early fibre failure. The Design Authority had applied factors way over the top but I did understand the caution. A fine example of multiple load paths on a different level working in your favour.

[JOE-FBS]

My experience in airframes is only at the small (part 23) end. Sensors would be painful, they fail, their wires fail, their displays fail. On very simple low cost (relative to airliners and biz jets) aeroplanes, we would rather avoid them.

On our programme, the only structural sensors are for temperature. It is theoretically possible to get the airframe temperature above the safe limit for the material (it is pretty improbable, one would probably have to have an all black aeroplane parked in Death Valley in the summer then pull straight to 10g on take-off) so we are obliged to do that. Otherwise we are avoiding electrical content wherever we can.