Aircraft inspection

Member for

8 years 6 months

Posts: 6,467

I don't know if there are many who share my concern about the lack of inspection apertures or hatches incorporated into the structure of light aircraft. I cannot ever recall any a/c I've flown having what I would regard as a number of inspection apertures sufficient to give access to all parts of the airframe that otherwise remain inaccessible. I'm prompted in this by our last inspection for the issue of a Permit on our Zenair 701. The design of the 701 does permit a reasonably good access to the fuselage via a large inspection panel in the belly of the a/c. But that is about it. Using a lighted fiber optic probe of some length it might be possible to check some parts of the wings and tail assembly but only through gaps where surfaces are designed not to meet. The 701 has an aluminium airframe, therefore it's desirable to have an occasional look around parts of the structure not readily visible. Its much the same case with 'tube and fabric' and wood based airframes. I've recently had a walk around various types at my airfield and none of the airframes I looked at had what I would describe as an adequate number of inspection hatches. On a typical light a/c I think that the minimum numbers would be three hatches or apertures per wing - root, middle and tip - four per fuselage and say three for the tail assembly. Apart from any other consideration, this would certainly convey greater peace of mind knowing that every so often off come the hatches - exactly as it is or should be with the engine cowlings -enabling an exacting scrutiny with the help of a torch and mirror in the absence of any more advanced tools. Taking a liberal dose of my own medicine, I've just finished a total bottom up refit of a small sailing boat. I've sprinkled inspection hatches everywhere I thought necessary to gain access. There's hardly a bit of the boat that I can't poke and pry into. John Green
Original post

Member for

13 years

Posts: 8,826

There was a spar inspection on a Robin I think that involved drilling out a rivet and looking through that! agreed some early ones were bad to access, something like a cessna 152 though you can add up to 5 extra wing access panels per wing if you follow certain criteria... one thing that i have been pondering buying is this.. http://www.maplin.co.uk/inspection-camera-with-colour-2.4-inch-lcd-monitor-259804 http://www.maplin.co.uk/handheld-video-optic-inspection-tool-with-lcd-display-286512 http://www.maplin.co.uk/handheld-inspection-camera-with-sd-recording-454480 AND NO, I DON'T WORK FOR THEM... most of my aircraft I have treated internally with Dinitrol.... APS 50 etc As they say, Aircraft engineering is a bit like Gynacology, you end up peering through small holes at distant objects, though on the whole it tends to smell better ;)

Member for

8 years 7 months

Posts: 1,026

Austers had zip fastened access patches on the rear fuselage and rings in the wings, that were cut out for inspections and then fabriced over until the next scheduled inspection.

Member for

10 years 5 months

Posts: 90

John, I think you'll find the answer in the term "stressed skin construction" and it may not be a good idea to go poking holes where it's not necessary. The design organisation will have given full consideration to this when they've drawn it up and as long as no manufacturing defects come along, (like the Eurostar Mainspar) then you can rest assured that the designers have done their stuff. (builders can add as much anti corrosion treatments as they like during build stages) As for the 701, well they're built like brick outhouses as you know, so I'd say don't worry old chum. Wooden aeroplanes are a different prospect as they're affected by changing weather conditions and glue joints can suffer as a result. GRP has it's own bag of problems but I'm sure any inspector with a bit of experience will know what he's looking for there as well. Trust in your inspectors work goes a long way to alleviating fears of airborne failure. I'd be happy to let my son fly in any of the aircraft I inspect.......it's the pilots which are my only concern. However, if you would like a second opinion or a fresh set of eyes for next year, I could be available.. Sandy Hutton Inspector LAA372 :D

Member for

8 years 6 months

Posts: 6,467

Sandy, Thanks for that and your kind offer. Yes, I'm aware of problems if an a/c is of stressed skin construction but, I don't know of many of that type in GA. I tended to put the lack of airframe access down to the predictable desire of the manufacturer to reduce or contain costs - it seems that way in the boat construction industry. John Green

Member for

10 years 5 months

Posts: 90

John, I think you may have misunderstood me.. Most metal aircraft are of stressed skin construction, whereby the loads taken on the primary structure (Main and rear spars) are distributed throughout the rest of the framing. ie, the ribs and the wing skinning. There is no problem with this method of construction and you should be able to rest easily on this thought. Aluminium 2024 and suchlike has known strength properties and therefore the desginers can work out the loads concerning the factor of safety limitations, this safety margin is proof tested by loading with sandbags at the approval stages.. If you really want to find out more, the "The Aeroplane Structure" by A.C. Kermode is one of the trade references. Not sure if it's still in print but I'm sure someone can come up with a copy for you. You could also have a scan in CS-VLA for some great information erm. just how many boaty things are made of Alclad because I wouldn't want to be on the oggin in one. Maybe best not to compare eh? ps, you didn't mention inspection hatches for the undercarriage but I guess that doesn't matter as it's not really "structural"..is it? (yeah, I know you can see it through the belly hatch):diablo:

Member for

9 years

Posts: 919

I'd agree with Sandy on this one - the lack of access panels is nothing to do with cost, but rather increased structural integrety by not having them. If there's no panel there, you don't need to be in! Your inspector will know the aircraft well enough to peel back skins if significant corrosion is suspected, and the aircraft manufacturer will have designed in access to the main structural elements if appropriate. Don't worry.

Member for

8 years 6 months

Posts: 6,467

Mmm, It shouldn't be a question of either or. We're constantly lectured on matters to do with aviation safety and I think that's as it should be. There should be no contradiction between accessing the internals of the airframe and sound design engineering practise - the two should be synonymous. Every time I'm proposing to fly I wish I could have a poke and a prod at the invisible bits. I am aware that there are tell tale signs visible from the outside which will give clues as to something nasty developing internally and I look for these on almost a daily basis - tell me I'm not paranoid ! Sandy, my interpretation of 'stressed skin' were skin panels that were placed and secured under tension thus adding to the rigidity and strength of the entire structure. Your explanation points to something markedly different. There are a surprising number of both sailing and power boats constructed from aluminium. Ally is prized for its strength and lightness and durability under marine conditions. It does though respond differently from GRP and timber and steel when considerations of electrolytic action arise. John Green

Member for

10 years 5 months

Posts: 90

John, I don't believe for one moment that you have every access panel off before every flight. I'm not in the business of lecturing, you need JT442 for that and as far as I'm concerned, I've finished my piece here. What I will say is that during my 37 years in the trade, there have been very few structural failures come out of the accident stats. The ones I have been aware of are where an owner has flown the aircraft beyond the limits laid down in the Aircrafts Operating Limitations Document, or has ignored them altogether. I said in a previous post that the Zenair 701 is built like a brick outhouse and if you had had a hand in the building of it, you would realise just what a strong aeroplane it is. Thanks for the heads up on ally boats though.

Member for

15 years 6 months

Posts: 1,616

Just out of interest, or not, there is an AD on the Airtourer (low wing metal construction) which needs the wing (at 3000 hours) to come off to inspect the area around the wing mount bolts. There is also an approved mod to cut an inspection panel to facilitate this and avoid taking the wing off. There was also a flutter on another forum when Aldi were doing the inspection fibre optic light for £49 but nobody found one. Looks like the same model as well.

Member for

9 years

Posts: 919

Stressed skin is defined by the fact that it is structural, ie. it is under stress. If you remove upper wing stressed skin, or even stressed panels, without the correct (according to the AMM/AP) supports, the wing geometry will change and you will have killed your aeroplane.
Profile picture for user BlueRobin

Member for

19 years 9 months

Posts: 2,606

Good post. Just as an amusing aside, the father of an ex-girlfriend worked on the Vulcan. When he built his shower at home, he placed an access panel underneath the tray, just in case! :cool:

Member for

8 years 6 months

Posts: 6,467

I didn't intend to give the impression that I remove every inspection hatch before each flight - paranoid I am but not that paranoid ! The shower tray inspection panel mentioned in the last contribution raised a wry smile. Many sailboats have showers but, few have access panels in the tray. leading to all kinds of problems - eventually. On my recent refit, I've placed access panels on all the above and below waterline seacocks and especially around the lavatory pan - deep joy ! I stick to my original point, not out of stubborness but because it must be generally a sensible idea, all things equal, to have access to as many hidden parts of the airframe as possible. John Green

Member for

9 years

Posts: 919

A sensible idea unless you compromise hull integrity by having holes all over the damn thing, or double the weight by using stressed access panels....

Member for

8 years 6 months

Posts: 6,467

Coincidentally, the very respected aviation engineer, Francis Donaldson of the LAA has written a short piece concerning inspection panels in the current issue of Light Aviation. He mentions various caveats concerning their use but seems generally to be in favour. John Green

Member for

10 years 5 months

Posts: 90

Page 46.. "Access Panels" Yes Mr Donaldson wrote this piece but I do not detect anywhere in the text that he is particularly in favour of them on anything other than fabric covered aircraft. If you are hell bent on extra inspection panels then please go ahead and speak to Engineering about it. I would hazard a guess that after the holes are cut and closing panels fitted, that they will require proof loading with Sandbags, since the designer isn't going to go to any expense on your behalf. Are you willing to take that risk? Before you begin though, speak to your Inspector because he is the one who will be putting his signature to the Modification proposal form. Give Mr Donaldson my regards when you speak to him though, he'll remember me since I was employed by him for 2 1/2 years at Turweston. :D Sandy Hutton

Member for

8 years 6 months

Posts: 6,467

I'm not hell bent on cutting holes, I'm not using sandbags, I'm not speaking to Francis Donaldson, I'm not speaking to Engineering, I'm not paying a designers expenses. That just about takes care of all your wholly un-necessary rant. What I had was an idea. Not a new idea but, one that would I believe enhance flight safety. I still believe that and you've written nothing that would persuade me otherwise. It is an idea for discussion and some thoughtful debate not for some one to try to bulldoze their opinions. You might be an LAA Inspector, you might for all I know be an Air Vice Marshal or the head of design from Boeing, that still doesn't make your opinion more valid than mine. In that specific connection, I don't why you should see fit to mention that you were employed by Francis Donaldson for two and one half years. Was this supposed to impress ? Try reading more slowly and therefore more accurately. John Green

Member for

9 years

Posts: 919

I don't see why anyone would want more holes in their aircraft than was absolutely neccessary. My Piper Aztec C has a total of 78 access panels - more than enough! Regards, John Applegarth, Airframes and Engines lecturer (Time-served maintenance mechanic.... damn I wish I was an inspector....:D .)

Member for

10 years 5 months

Posts: 90

John, you posed a question on an open forum and didn't get the support for making holes that you were looking for. Two of us have given you the very simple technical reasons why it isn't a good idea. Why can't you just accept it from two experienced people who have been in the trade for many years? I don't think I've misinterpreted any of your posts, so reading them again more slowly isn't going to make a jot of difference to the information you've been given quite freely by myself and JT442.

Member for

10 years 5 months

Posts: 90

But the biggest difference JT, is that the Aztec isn't a Microlight Aircraft which is built so lightly that it is reliant on the wing skins continuity to maintain structural integrity. I haven't worked on an Aztec for many years but the Cessna 310 can't be far behind in the number of access panels. Come to think of it, the Anson must have had a few as well.

Member for

9 years

Posts: 919

...but it is a 'light' aircraft as defined in the OP's first post. :) I'm building a Thorpe T2-11, and that has a couple of dozen panels built in too... I'm going to stick with the manufacturer's recomended number of holes in order to prevent flying directly to the scene of the accident. edit 1: I've just had a look at the zenair 701 schematics on the company website just to see what the aircraft actually looks like.. so here goes.. Fuselage inspection - one assumes that the elevator and rudder controls pass into the lower section of the rear fuselage - there must be a way of accessing the turnbuckles for the cables - always a good place to inspect from since the hole will be at least hand-sized... Wings - once built, they should be well sealed from the elements and should not require inspection other than periodic (1000 hours?) in-depth inspection, which could involve removal of skins... (just a guess). The same applies to the tailplane, and as with the fuselage, there should be a couple of panels to allow for inspection of the cables. edit2: just found this: http://www.701builder.com/06AccessPanels.htm which areas exactly are you wanting to get into? It seems that the 701 has quite a few accessible areas including the 2ft square hole in the bottom of the rear fuselage... you can access everything through there!