Wing strength

[boofhead]

When preflighting a small airplane, especially an ultralight or small experimental, I sometimes push the wing tips upward hard to listen for any unusual noises or feel for internal damage. I always figured that I cannot put enough force on the wing to hurt it. However I have a lot of leverage at the tip and maybe it is not a good idea. I might be putting more force on the wing this way than it is designed to handle. Or maybe if it can handle this it is strong enough for flight.
Is there anybody out there in cyber land who can shed light on this subject? At least before I break something?

[Intruder]

The wing tips on many light airplanes are NOT structural at all -- they are merely plastic covers that also hold the nav lights. You may well damage them if you push too hard on them! IF you feel you must do this, AT LEAST determine where the spar ends, and push directly under the spar AFTER any tiedowns are removed!

Especially if the airplane is tied down when you push on a wing tip, you may induce loads that are beyond the design parameter of the wing. The wing is designed for a load across its entire surface, not concentrated in one spot at the tip.

[SNS3Guppy]

That really depends on the airplane. You shouldn't damage it, but without any insight into the specific airplane and it's construction, where you're pushing or pulling, if it's tied down (makes a difference for leverage and bending moments),etc, it's hard to say.

Generally speaking, if the airplane isn't strong enough for you to grab the wing and give it a shake, you might consider finding another ride. Where you grab and how you do it, however, as well as the specifics of the airplane you're grabbing, do make a difference.

[Doodlebug]

An old-timer once taught me that the integrity of the wing-spar can be checked by moving one wingtip up and down. The other tip should mirror the movement exactly. (obviously more suited to gliders than Boeings, but the same principle applies, I should think)

[SNS3Guppy]

On some airplanes, again, that may be true. On many, it's not. A glider, for example, will have substantial wing flex. I've flown on some airplanes in which it was very possible to walk out to the wingtip and bounce up and down like a diving board. Indeed, some wings flex considerably; movement of the opposite wingtip isn't necessarily an indicator of wing or spar integrity.

This is true of many light airplanes, as well as larger airplanes. You need to consider the specific airplane, it's construction, etc, when considering the question at hand.

[Doodlebug]

Yup, you can't budge a Lears tip, for instance, whereas a GLEX will flap it's wings like a bird while taxiing.

Used to use that tip-waggling nugget of info when preflighting the venerable ASK 13, i.e. wooden-spar glider.

[Piper19]

I don't realy see the need for pushing the wings up during preflight to check its integrity. As long as nobody touched the hangar doors with the wingtips, or did not overstress the aircraft in aerobatics, it will be ok.
However, on the old Cub and Super Cub, we did this wing waggle before taxi, but that was to check the bungee springs on the landing gear. The gear could not extend farther than a red line marker or your bungees were bad. I did this waggle by pulling the ring on the wing, where your tie down chord goes. I sometimes do the same on cessnas, but that's just a habit I have from my Cub time.

[boofhead]

Thanks for the input. I figured it might be bad for the wing, pushing and pulling on it at the tip, because the mechanical advantage is, it seems to me, to be greater than the normal lift force, spread out along the wing as it is. Then I wondered if I could over-stress the wing if I lifted the tip too hard... A wooden wing should let me know if it has damage, you think?

[SNS3Guppy]

A wooden wing should let me know if it has damage, you think?


No! In fact, a wooden wing is the hardest of all to inspect for damage. A wooden wing with structural cracking may or may not be detectable. A wooden wing with wood rot or other types of damage or failure may not be detectable by rocking or by visual inspection. I have seen spars which appeared perfectly intact, but when cut through the cross section were rotted and decayed inside.

Wood has excellent, desirable properties for aircraft construction. It doesn't fatigue. It's strong, light, workable, and with proper care can last a long time. It's relatively immune to vibration. It conforms to compound surfaces, and comes in all forms. It's glued in place. It's easily worked into complex shapes. However, it's also resistant in many ways to inspection techniques, including visual, ultrasonic, etc. One can't do magnetic particle, or dye penetrant inspections on it. Shaking a wooden structure won't tell you about it's soundness either. If it is physically broken, cracked, or fractured, it may not make a sound. Glue joints are nearly impossible to identify externally with respect to soundness and integrity. Many of the older glues used for aircraft construction have been found to be undesirable and even dangerous in comparison to modern choices...and often it's the glue that's both the strongest, and the weakest component in the construction of the aircraft. Some adhesives, including epoxies, can be extremely strong at lower temperatures, but fail entirely at higher temperatures...meaning you can have a seemingly intact structure which can quite literally come...unglued.

I've long been a proponent of pilots seeking some maintenance training and that includes inspection training. I've seen pilots walk around an airplane and certify it perfeclty servicable, only to be shocked to be shown some 30 grounding items that made it very unairworthy.

A pilot may look to see safety wire in place...but does he know if it should be safety wired in the first place...or if it's tied correctly, or the right type of wire is in place? A bolt is secured with a nut, but the washer is beneath the bolt head and not the nut...is this okay (no). How many threads should be showing through the nut? Is a fiberlock nut okay ahead of the firewall? The axle nut on the wheel assembly is secured with a cotter pin...is this okay, and is it okay to bend the ends outward, instead of around the nut? (The answer to that is that bending it the wrong way, outward, reduces the strength by more than 50%...I saw a wheel fall off of a Cessna 182 on takeoff due to that very thing, once).

Many pilots assume that the mechanic is responsible for these things, and simply do a walk around to make sure all the big pieces are still there. I've heard that very language, in fact, many times. "It will probably fly. All the big pieces are still there." This may be laziness, it may be a shortcoming because the instructor who taught them didn't know any better. I call that the heritage of inexperience...one instructor who doesn't know enough teaching a pilot who grows up to be an inexperienced instructor...who passes on this legacy of inexperience and lack of knowledge...the lack of understanding becomes legitimized (dangerously so) over time until it's deemed acceptable to not know. It's not acceptable...your life depends on it.

An excellent book which is directly applicable in the USA is called AC 43.13-1B/2B. It's applicable to US standards, but is an excellent guide no matter where you go to what's acceptable in the airplane and what's not...including how to properly inspect wooden structures, metal structures, composite structures, etc. I've always considered it required reading, and if you don't avail yourself of that information, it's to your detriment. Remember, it's well enough to look the airplane over...but do you know what it is you're seeing, or what you're looking for? You might be surprised.

You can purchase or download. The FAA has it for download at:

http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgAdvisoryCircular.nsf/0/99C827DB9BAAC81B86256B4500596C4E?OpenDocument

and:

AC 43.13-2B Acceptable Methods, Techniques, and Practices - Aircraft Alterations (http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgAdvisoryCircular.nsf/0/11E144125D63FE548625740A00731B4A?OpenDocument)

and you can get it in print for a nominal fee via:

AC 43.13 Acceptable Methods of Aircraft Repair, with new 2008 part 2B revision, ISBN-10 0884875040, ISBN-13 9780884875048, JS312617-004, AMT Bookstore, a division of Aircraft Technical Book Company (http://www.amtbooks.com/products/amt008/)

I'll add as well that I've long believed that you can get some diluted systems information from the pilot manuals for an aircraft...but if you really want to get to know the systems (and you should), then the maintenance manuals are invalueable. I've obtained them for every aircraft I've flown, and have found the information invaluable. This applies to light airplanes as well as bigger, more complex airplanes.

For example, the Cessna 206 pilot manual gives little information regarding the kidney sump tanks located at the base of the wing struts, inside the fuselage...all the 200 series airplanes through the 210 have them...and the information given for handling the fuel flow fluctuation problem they can create by their design is not only inaccurate in the pilot handbook, but can cause an unrecoverable engine failure situation.

Understanding the system and knowing that the pump return vapors are returning to the fuel tank via the same feed line that's serving the kidney tank, and how it's layed out, clearly shows why switching tanks before applying the boost pump and adjusting the mixture, is important, and why applying the boost pump first, as directed by Cessna, can make the situation worse and mean one may not be able to restart the engine at all. Knowing a little about the engine and propeller means that one understands the need to retard the throttle and avoid detuning the crankshaft...important stuff to know, but not covered in the aircraft flight manual or pilot operating handbook.

You're certainly doing the right thing seeking more understanding of the airplane. I encourage you to broaden your search and pick up the technical manuals for maintenance of the airframe, engine, and relevant components (brakes, wheels, etc). These all have good information that few pilots take the time to get to know. I see frequent discussions among private pilots all the time about engines that foul plugs...but few of them understand idle mixture, enrichment valves, proper leaning procedure. I very seldom run into pilots, including many owners, who understand the manufacturers practices and procedures for post flight runups (let's face it, were you ever taught to do one?). It's not in the Piper or Cessna manual, but it's certainly in the Lycoming and Continental operating instructions put out by the engine manufacturer...as well as a lot of other good information. Pilot manuals are idiot manuals boiled down to the minimal information, for the least common denominator...and I say that as a pilot, mechanic, and instructor.

There's a lot more to learn, and there's a lot more written in the airplane, than what's in the think little booklet in the cockpit. Know that booklet inside and out and live by it, certainly. But avail yourself also of the additional information that's out there. It's not going to contradict the flight manual (even the case of the fuel flow fluctuation procedure by Cessna, previously mentioned...that's put there as a pilot procedure, but understanding the system will give you a lot more illumination into what's going on and allow you to make informed judgement on how best to operate the airplane at a time when your options are rapidly dwindling), but support how you use the flight manual, and ultimately, it's going to give you the information and the tools to be a better aviator.

[411A]

Just for reference.

Maximum designed wing flex, as measured at the tip, prior to wing skin deformation, while in flight...

B707-320B (advanced cowl)

11 feet up, ten down.

Douglas DC-6B.

Six feet up, six feet down.

Wing bending relief, at work.

Source.
Senior design engineers.
In the latter case (DC-6B), chief project engineer, a very close relative.

[boofhead]

Thanks for the input.
I still come back to the force it would apply to a wing (say, a wooden wing with fabric cover) if I was to try to lift the airplane at the tip. Surely the mechanical advantage I would have would be great enough to show up any weakness? In flight the force on the wing is spread out along the span and the force on the wing, especially if it has a strut in the mid span, would not be too great (considering maybe a 2 g load factor). But if I am trying to bend the wing from the tip, it would have a lot of force applied at the strut, the force I could apply multiplied by the distance from the tip to the strut??
When I have done this, I see the fabric loosen up on the top, showing that I am bending the wing a little. How much can I do this before I damage it? Or if it has internal damage, would this bring it to my attention? I guess I am not talking about DC8's , here, or any airplane that is substantially built, but it could apply to most ultra lights, sport airplanes and Cubs.
I fly many airplanes that are of suspect provenance, and would like a sure way to see if they are not at least going to come apart on me. I can see wrinkles and bad corrosion, but cannot see inside the structure.

[SNS3Guppy]

I still come back to the force it would apply to a wing (say, a wooden wing with fabric cover) if I was to try to lift the airplane at the tip. Surely the mechanical advantage I would have would be great enough to show up any weakness?


See previous comments.

Ask the same question in differing ways...but the same response still applies.

[boofhead]

I guess you are right, but I am still not clear in the responses received, good as they are (I have stolen some of them and put them in memos to fellow pilots, hope that is OK?).
A last try to elucidate my query:
If I was to go out to the DC8 on the tarmac and put a couple of giant hydraulic jacks, one under the tip of each wing, and lift the airplane, full of fuel and passsengers, completely off the ground; would that put a load on the wings that exceeds the normal g loading/structural limitations?

[SNS3Guppy]

Jacking points are put in very specific places and to be used under specific loading conditons, for a reason.

Do you want to know about a DC-8, or a light general aviation airpalne?

As has been discussed over and over now, every airplane is different. What applies to one airplane does not apply to another.

You seem to believe that you've learned all you need to learn about the wing by lifting up on it. You've created some degree of upward bending moment...perhpas not bent the wing, but you've applied the moment. What if that's not where the structural failure is? You're putting a load which places the spar capstrips, or upper and lower surfaces of the spar undertension (beneath) and compression (above...what if the problem is exactly the opposite. What if the problem is a fatigue crack which will cause a wing failure...but won't shot up under the load you apply?

What if you put too much load on the structure and cause a weakness or a stress riser or create damage? Now you've become the defacto problem...not the wing, and you're not detecting, but introducing the problem. Suppose you're one of whom likes to push down on the horizontal stab of a 172 to lift the nosewheel and move it...or who helps turn it by pushing gently on the vertical stab, or the forward fairing. Suppose you're one of those who believes that you can't hurt a structure by operating the controls full deflection below Va, maneuvering speed. The problem is that you can...and Cessnas, especially 200 series Cessna's, are well known for the factory vertical stab attach brackets failing. I've discovered quite a number of them myself. You wouldn't hear the airplane making any noises. You wouldn't feel any give. But it's still broken.

I used to fly an airplane that got an enormous amount of maintenance attention. We had a program in place which stripped the paint on the lower surfaces of the wings regularly....once a week or more, and perfomed ultrasonic inspections. We crawled inside the wings to inspect them, and had a whole aproved maintenance program on it to back us up. Never the less, the wings came off the airplane, it caught fire in flight, and crashed, killing everyone. It had a good inspection program. Went through a full A check every day...but still came apart. Everyone on board was a qualified mechanic and inspector...with full tools and parts on board. The airplane got constant care...never the less, it crashed, and everyone died.

So, you're going to jack the DC8. Except where will you apply the load? On the center of the spar? Where? Yes, you can certainly damage an airplane by jacking it improperly...especially from a place where you ought not.