L-19 Leaf spring installation

[flyingredyeti]

Del, while I appreciate your effort to actually consult the regulations while discussing them, I'm afraid full analysis of the copied (and annotated), definitions is not entirely necessary. I have yet to verify this, but the one of the very points you put in bold print:

f. Changes to landing gear, such as internal parts of shock struts, length, geometry, or brakes and brake
systems,

is listed under the category of:

404. ALTERATIONS REQUIRING ACO ENGINEERING APPROVAL
Items (a) through (o) in the following list of typical alterations will most likely require engineering
approval, and often require an STC. Items in (p) do not normally require an STC but will usually require
ACO engineering assistance.

The fact that you highlighted this in order to bolster your claim demonstrates that perhaps you read what you wanted to see, not what it actually says. This is not meant to offend, we all do it. But, this is clearly stated in your own copy of the text.

David, as I said, this discussion is not meant to change your mind. Do what you want to, however no matter how strong your opinion on the matter. The statement you've made:

I would argue the tailwheel main spring and 'eye bolt' are sufficiently similar in type, shape, materials and dimensions to the L-19 to have NO appreciable affect on weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness. The only difference is a slight increase in thickness that has no affect on performance.

is in fact groundless unless you have some data to back it up. Nor does it change the manufacturer's specifications or the FAA definition of a major alteration.. I know it's good enough for you, but that doesn't make it the best practice. Engineers don't determine these standards based on personal opinion; they use the laws of physics, established methodology, and extensive testing.

Bruce, are you implying that because the part is PMA'd, it is therefore safe to install on the aircraft of our choice? If so, why can't we just plant any piece of equipment on our aircraft if it has a PMA? Why bother with so many STC's? You are probably right about the metallurgy of springs allowing for equivalent strengths at different thickness. Typically the goal is to provide greater strength in a smaller spring, though, for dimensional limitations of the installation. What sense would it make to put a larger, heavier spring of the same strength on an aircraft? The name of the game is weight savings, so by what logic could one assume that a PMA'd replacement would be designed to be heavier with no strength gain, nor (as you said), any appreciable increase in life span?

[flyingredyeti]

I have also found the "Northwest Mountain Region, Guide to Aircraft Alterations" a very good read, tho it looks like the FAA took it down off there site - I just happen to have a pdf version.

Thanks for that Karl, that certainly provides some useful data. Given that it was produced by a FSDO, it may meet the definition of approved/accepted data, don't you think? Worth Checking out.

As to our discussion, I found the following paragraph from that publication to make an interesting statement:

303. DATA NOT APPROVED OR ACCEPTABLE TO THE
ADMINISTRATOR
a. General. There is much information available to the aviation public that has had no evaluation of
acceptability or approval. This information is not FAA approved, and it usually has not been put to the
test of practical experience. Some alterations that have been installed may appear to have undergone
practical experience. But in reality, the aircraft involved have never approached the edge of the aircraft
safe operating envelope, and the effects of the alteration evaluated. Information of this nature is found in
sales catalogs or brochures, newsletters, magazines, etc. Additionally, manufacturer’s parts lists,
illustrated parts catalogs, and similar documents, while useful, are not approved and do not provide data
acceptable to the Administrator.

As I read it, another way of stating this is that just because an unapproved part hasn't failed yet, doesn't mean it won't if pushed close to the limits the aircraft was designed for.

[ghostflyer]

These aircraft are over 60 years old and most parts are not not available through the normal channels . So when replacing parts regards where they come from or the certification of these parts some thing called common sense has to prevail . Even some of the regulations are old and need up dating . Technology is moving faster than the FAA can adjust to the changing environment . It's moving faster than some A/P ,s and IA,s and DER,s can cope. I am not saying we shouldn't comply with the regulations , but some regulations do not comply with common sense so we shouldn't shut out or ostracize the officers of the FAA but try and work with them . Whilst it's nice to have a 'original' 170 it's impossible . For example the original aluminum wasn't 2024 T3 so do we get a DER endorsement for the replacement owner manufactured inspection panel . A suggestion of mine is if we are having issues with the FAA regulations and the fitment of parts or procurement of these parts legally let's have a member be a sole representative of the 170 club to negotiate with the FAA directly . He reports directly back to the committee. Now to throw some petrol on the bar b que to really burn the meat .

[bagarre]

Best of luck in becoming a mechanic.
Like I said in my last post. I don't care if you agree.

[flyingredyeti]

These aircraft are over 60 years old and most parts are not not available through the normal channels . So when replacing parts regards where they come from or the certification of these parts some thing called common sense has to prevail . Even some of the regulations are old and need up dating . Technology is moving faster than the FAA can adjust to the changing environment . It's moving faster than some A/P ,s and IA,s and DER,s can cope. I am not saying we shouldn't comply with the regulations , but some regulations do not comply with common sense so we shouldn't shut out or ostracize the officers of the FAA but try and work with them . Whilst it's nice to have a 'original' 170 it's impossible . For example the original aluminum wasn't 2024 T3 so do we get a DER endorsement for the replacement owner manufactured inspection panel . A suggestion of mine is if we are having issues with the FAA regulations and the fitment of parts or procurement of these parts legally let's have a member be a sole representative of the 170 club to negotiate with the FAA directly . He reports directly back to the committee. Now to throw some petrol on the bar b que to really burn the meat .

I found an advisory circular on this very point. I'm surprised no one else presented it first:

http://www.faa.gov/documentLibrary/medi ... _23-27.pdf

It's advisory only, but offers guidance on the best way to get around the problem you mentioned above, for the best success at approval.

[wingnut]

Joshua,
Paragraph (f) was highlighted to show that landing gear is one of many groups of item that DO NOT fit in with your previous suggestion that "ANY" alteration is major. If that were true, then why give examples of specific alterations to be considered major? Why not simply say "ANY alteration to landing gear components is major". Clearly the FAA agrees there is a difference between major and minor, and they provide much guidance for the A&P, and allow them to make that call. The particular alterations you've chosen for this discussion are minor. It is easy to follow the evolution of the 170 series and the Bird Dog.
Univair has a main leaf spring 0642101 eligible for both the L19 and the 170. The L19 used the 3200 tail wheel, the 170 did not (didn't leave the factory with them). If you install a 3200 on a 170 you must change to a longer bolt (the AN7-10A call out in the IPC will not work and Scott never addressed the length/type bolt because there are so many applications). To choose the proper replacement bolt and to install same is less than elementary.
So this a great time to reference a Cessna product, L19 IPC. Because the 3200 is already approved on the 170 series, and is installed by OEM on the L19, and both are eligible to use the same main leaf, it would be reasonable judgement to conclude that method of attachment of Scott 3200 tail wheel assy to the main leaf using the eyebolt referenced in the L19 IPC is a minor alteration. Also note the L19 call out for AN960-716 washers is "AR" (as required). I'll add the IPC Pages shortly, along with some supporting measures and symmetry I verified this morning.

[wingnut]

I've done all this before, but didn't take notes. To satisfy my curiosity, this morning I measured the weight on the tail wheels and compared the symmetry of both planes. The 170B tail wheel had 145 lbs on it

The L19 tail wheel had 152 lbs on it

The following 2 pictures to show the azz end is structurally identical. Ironically, the L19 has the standard AN bolt, and the 170 has the L19 bolt (the AN bolt that Cessna welded a ring on)

I also compared the geometry of tail wheel in relation to the world, and they were both within spec for each plane and also matched each other. I had always wondered about this, because the main leafs are identical in every way except thickness, and I knew the L19 sets taller at the main gear. Because of the much heavier O-470-13, Cessna canted the main gear forward to keep the weight on tail in the same desirable range as the 170 series. In doing so, they had to raise the front end of fuselage (lengthen the main gear legs) to keep the desired and proven camber on the tail wheel assy.

[wingnut]

I tried to get a 6th picture in that last post. Here it is, just to show those who might not be aware the L19 and 170 are not cousins, they're twin brothers with regard to aft fuselage and empennage structure.

Sorry about poor quality of pictures

[wingnut]

Wow Del, you have the patience of a saint.

A saint I am not. I don't claim I'm right. I admit when I'm wrong. In fact I've learned a hell of a lot more being wrong than I have when I've been right.

[flyingredyeti]

Wow Del, you have the patience of a saint.

Perhaps, however I'm one of the few that has been able to continue this discussion with out lashing out at or insulting other members.

Del, now we're getting somewhere. Thanks for the data, although I cannot say I'm fully convinced. Firstly, I disagree that because the regional guide offers examples of landing gear alterations, that those are exclusively binding- they are simply examples under the header of Landing Gear Alterations. Yes, I have seen a couple of the springs sold by univair cross-listed between the L-19 and 170, but my focus of this thread is the heavy duty spring in particular, which is only listed as applicable to the L-19. Honestly, I would like to know for if there really is a fit issue or not, and if the tension is the same.

I have read elsewhere on this forum (perhaps it was one of George's post), that the L-19 spring does not really fit properly, is that correct? Per 23-27 (which is advisory only), it states that if there is an issue with fit of the replacement part to the aircraft, it may not be approved and would need further review by an engineer. I have written to Univair and asked for dimensional comparisons between the heavy-duty L19 spring and the Cessna 170 spring, and am awaiting a response.

While the empty weight on the tail between both aircraft appears to be similar, the military tend to design their equipment to suffer some serious abuse, so it is not a far stretch of the imagination to see the use of a stiffer spring on the military version of an aircraft. Stiffer spring = more stress transmitted to mounting bracket = stress concentration.

I think we're getting somewhere here, but like I said, I'm not certain we have an answer yet. Thanks for all the attachments.

[wingnut]

Josh,
It's getting late, so I'll only address the idea that the slightly thicker main spring could impart additional stress to the airframe.
First, the airframes are identical in the area of attachment; the 170 and the L19 (305A) are identical from FS 158 aft.
Second, the most common type damage observed in the area of the leaf stack assembly is the 'alligator' mount bracket. This is due to torsion load on the leaf when steering, or handling during a crosswind.
The thicker spring does not twist as much, and therefore transfers the torsion load to the forward attachment bolt/bushing rather than the area at the aft end of the torsion box where the "U" bolt clamps everything together. This is difficult to explain. You need to be here to see it.
You make it to KMEZ, I'll put you up in our finest, feed you, and most importantly I'll be your friend. Invitation is open indefinitely.

[flyingredyeti]

Del, thanks for the invitation, that's very generous. As for the imparting the load, whether it be compressive or torsional, what you are saying in fact reinforces my point. The stiffer spring, then, transmits more stress forward. I know you said the damage occurs in the mounting bracket, but as aircraft are required to be designed to distribute loads across the structure (to prevent stress concentrations), this implies that the stress is transmitted beyond the bracket, which may simply the first to reveal damage. Not being an engineer, I obviously couldn't say to what factor of stress may be delivered, if this is true, but I can say that the greater number of landing cycles means the greater the total stress delivered. Aluminum work hardens. With enough, moderate stress over time, it breaks.
With that theory in mind, see the following:

§23.235 Operation on unpaved surfaces.

The airplane must be demonstrated to have satisfactory characteristics and the shock-absorbing mechanism must not damage the structure of the airplane when the airplane is taxied on the roughest ground that may reasonably be expected in normal operation and when takeoffs and landings are performed on unpaved runways having the roughest surface that may reasonably be expected in normal operation.

It very well may be that the military added a heavier spring just to reduce time spent on broken springs in the field, and it could be that operation of the 170 at it's limits will not cause failure in the primary structure. However, it may also be that the military designed the Bird Dog with a much shorter operational life in mind than the 170 experiences, or perhaps there is another reason for the difference. I don't know if any of these are true, and that's the point. Without data confirmed by research, engineering and repeated load tests, we just don't know if this modification or any other in the same light, is hazardoust to our vintage aircraft or not.

Maintaining an aircraft without conforming to the prescribed data is legally limited to experimental aircraft, and I think this is for good reason.

[flyingredyeti]

Funny, after all of this discussion of major vs. minor alterations, no one has noticed this, or at least no one has mentioned it:

Appendix A to Part 43—Major Alterations, Major Repairs, and Preventive Maintenance
(a) Major alterations—(1) Airframe major alterations. Alterations of the following parts and alterations of the following types, when not listed in the aircraft specifications issued by the FAA, are airframe major alterations:
(i) Wings.
(ii) Tail surfaces.
(iii) Fuselage.
(iv) Engine mounts.
(v) Control system.
(vi) Landing gear.
(vii) Hull or floats.
(viii) Elements of an airframe including spars, ribs, fittings, shock absorbers, bracing, cowling, fairings, and balance weights.

So the question then remains; is the replacement of the stock spring for the L-19 spring an alteration at all?

[flyingredyeti]

Okay, digging further, here is what I found.

First, while the TCM for the L-19 does include the Scott 3200 assembly, which is also approved for the Cessna 170, this assembly does not include the leaf spring, as shown in Scott SBI-168. It shows attachment to the springs, sure, but does not include the spring as part of the assembly. Hence, the spring is a Cessna part, exclusive of the 3200 assembly, which also allowed for two bracket options (on the tailwheel), to accommodate different size springs. So, it seems to be presumptious to assume that just because the L-19 and the 170 both were approved for the 3200, this also means they both use the same springs.

However, once again, Del is correct that Univair is listing these springs as applicable for both, so were getting closer to an answer on the topic. On the other hand, several facts that must be considered are:
a)The 06 series springs sold by Univair are not listed in the 170 IPC with the same number, although this is not conclusive in itself
b) While the 0642101 CESSNA MAIN LEAF SPRING, which shares the L-19 IPC#, is cross listed for the 170, the 170 equivalent U0542106 CESSNA TAILWHEEL SPRING - MAIN LEAF, is not cross-listed for the L-19, so they may not be identical.
d)There is a reported fit and dimensional difference between the factory 170 springs and the L-19 spring (as mentioned numerous times on this forum.
e)The following spring: U0442107 CESSNA TAILWHEEL SPRING is listed as applicable to the 120, the 140 series, & the 170, but it is not listed in the 170 IPC, despite using the same P/N series; i.e. 044210- There is also a difference in applicability listings between the 170 series spring sets, and the L-19 spring set

Once again, none of this is conclusive, but it does suggest, to me, that Univair is somewhat indirectly marketing springs as applicable to both the 170 & the L-19, although they might not actually be equivalent to the stock 170 series springs. As a vendor's catalog is not data acceptable to the FAA, it is the A&P's responsibility to determine if the springs to be installed are in fact the correct part.

[johneeb]

Why are the two washers on the bottom the 7/16" mounting bolt for the Scott 3200 tail wheel 7/16" (AN960-716) and the one on the top of the tail wheel on the same 7/16" bolt 1/2" (AN960-816)? Further why does the 170 IPC Page 155 item 94-7A show 12 units (washers) per ass'y?