Aircraft modifications and Airworthiness

Fearless Tower

Fearless Tower

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Fearless Tower
Trying to get my head around the legality of aircraft modifications.

As part of my research on a potential purchase, I obtained the FAA 337 file for the airplane and have been reviewing it. The aircraft is a '48 Cessna 170 that had the wings 'metalized' in 1961. The work on this plane was documented on a 337 and was done in accordance with a "Skycraft Design, Inc. Drawing") The specific 337 does not mention any STC (Skycraft had an STC for the leading edge landing light, but not the metal wing).

Here is the full writeup:

"Metal conversion of wings in accordance with Skycraft Design, Inc DWG #5C-170-10000 titled conversion metal skin-Cessna 170 wing. Dated 3-9-56. Approved subject to inspection by Wm. A. Welch, DER 1-59, dated 3-11-56.

Above substantiated by attached letter dated April 3, 1956."

Below that is the updated W&B.

The FAA 337 file does not include the 'attached letter'

The owner is currently out of town, but I have asked him to see if there is any additional documentation, such as the letter, in the aircraft records.

So, was this conversion even legit? I realize that things were done a little more loosely back 50+ years ago, but is this going to be a problem in the future? The metalizing of '48 wings does not seem too uncommon - I personally know of 4 (3 are currently for sale), but the detailed info on the process seems to be lacking. I have also posted this on the 170 assoc board, but wanted to get some opinions from A&Ps here as well.
 
Fearless Tower said:
"Metal conversion of wings in accordance with Skycraft Design, Inc DWG #5C-170-10000 titled conversion metal skin-Cessna 170 wing. Dated 3-9-56. Approved subject to inspection by Wm. A. Welch, DER 1-59, dated 3-11-56.

Above substantiated by attached letter dated April 3, 1956."
Click to expand...

I'm no expert on this, but I think that since Mr. Welch was a DER (Designated Engineering Respresentative, I think), there is no need for the STC. He is certifying for the FAA that the modification is airworthy, and conforms to the aircraft design regulations that were applicable at the time.
 
Fearless Tower said:
Trying to get my head around the legality of aircraft modifications.

As part of my research on a potential purchase, I obtained the FAA 337 file for the airplane and have been reviewing it. The aircraft is a '48 Cessna 170 that had the wings 'metalized' in 1961. The work on this plane was documented on a 337 and was done in accordance with a "Skycraft Design, Inc. Drawing") The specific 337 does not mention any STC (Skycraft had an STC for the leading edge landing light, but not the metal wing).

Here is the full writeup:

"Metal conversion of wings in accordance with Skycraft Design, Inc DWG #5C-170-10000 titled conversion metal skin-Cessna 170 wing. Dated 3-9-56. Approved subject to inspection by Wm. A. Welch, DER 1-59, dated 3-11-56.

Above substantiated by attached letter dated April 3, 1956."

Below that is the updated W&B.

The FAA 337 file does not include the 'attached letter'

The owner is currently out of town, but I have asked him to see if there is any additional documentation, such as the letter, in the aircraft records.

So, was this conversion even legit? I realize that things were done a little more loosely back 50+ years ago, but is this going to be a problem in the future? The metalizing of '48 wings does not seem too uncommon - I personally know of 4 (3 are currently for sale), but the detailed info on the process seems to be lacking. I have also posted this on the 170 assoc board, but wanted to get some opinions from A&Ps here as well.
Click to expand...

Metalizing the a 48 rag wing destroys the aircraft, the 48's claim to fame was its ability to be light and fast. replacing the rag with metal makes the wing heavy.

but to answer your question, the paper work is legal.

the only problem you will encounter in the future will be your disappointment in the aircraft's performance. my 48 would show 132 MPH in level flight at 2500RPM. it weighed 1050 pounds empty.

don't forget to join the 170 association
 
I am no A&P but a 337 is an application form for a Major Alteration.

To support the 337 you need Approved Data.

I don't know about 1961 but today there are many categories of what constitutes approved data. An STC is the best type sure enough, as would be an 8110-3 from a DER, as would be a pile of other things e.g. repair manuals, SBs, etc. AC43-210 lists today's acceptable categories. None of these things are (usually) mandatory individually, even though a lot of people like to say "can't do this without an STC" about great many things. There are merely categories of approved data, some of which are worth more than others.

If your 337 is FAA approved then you should be covered.

Some stunts have been done e.g. a 337 handed to a customer but it was never seen by the FAA, not even for filing after the job.

The above is my understanding, having just gone around this block on a little avionics project :)
 
peter-h said:
I am no A&P but a 337 is an application form for a Major Alteration.

The 337 is a method of placing data into the aircraft history records, that include changes in the aircraft design, major repairs, and any alterations to the flight manuals, such as directed in STCsTo support the 337 you need Approved Data.

The 337 is the approved data on field approvals as soon as the ASI stamps and signs block 4 of the form.

I don't know about 1961 but today there are many categories of what constitutes approved data. An STC is the best type sure enough, as would be an 8110-3 from a DER, as would be a pile of other things e.g. repair manuals, SBs, etc.

not much has changed in that respect since I started repairing aircraft, the last change in policy was where to send 337s that have approved data already in place.
AC43-210 lists today's acceptable categories. None of these things are (usually) mandatory individually, even though a lot of people like to say "can't do this without an STC" about great many things. There are merely categories of approved data, some of which are worth more than others.

If your 337 is FAA approved then you should be covered.

to know if it is, get the aircraft history records, and look for the 337 and the stamp in block 4

Some stunts have been done e.g. a 337 handed to a customer but it was never seen by the FAA, not even for filing after the job.

That's a violation on the A&P-IA who signed off the return to service.
The above is my understanding, having just gone around this block on a little avionics project :)
Click to expand...

I am an A&P-IA and deal with 337s almost dailey, and have owned 3 C-170s 1 of which was a 48 (2623V)
 
There was an STC for the 120/140 planes to metalize the wings. If you would like to know the addresses used by Skycraft Design and the number of the 120/140 STC, let me know.

For years, I tried to chase the metalization STC's but found that the FAA kept NONE of the documentation. Moreover, I have tried for the same years to find a set, any set of the STC's which an FBO or a plane owner on whose plane the deed was donee. No luck.

With respect to the "heavy wing" comment. Yes, of course it adds weight but HOW MUCH????

I would like to know if there is a mention of the weight change, metal, less the fabric and treatments??

Neal
 
Cess120/140 said:
There was an STC for the 120/140 planes to metalize the wings. If you would like to know the addresses used by Skycraft Design and the number of the 120/140 STC, let me know.

For years, I tried to chase the metalization STC's but found that the FAA kept NONE of the documentation. Moreover, I have tried for the same years to find a set, any set of the STC's which an FBO or a plane owner on whose plane the deed was donee. No luck.

With respect to the "heavy wing" comment. Yes, of course it adds weight but HOW MUCH????

I would like to know if there is a mention of the weight change, metal, less the fabric and treatments??

Neal
Click to expand...

The weight gain should be documented on the 337 when the conversion was done. The airplane I am looking at gained 46 lbs in the process.

What Tom said about speed is true - this airplane cruises around 95 kts....but I don't mind the speed - I'm looking at it as a tailwheel timebuilder and something I can do the occasional hamburger run with my wife and daughters (at 3 and 1 month, the girls won't put us overgross any time soon).

What is funny about the drawings is that while alot of 120s, 140s and 170s were converted, no one seems to have the orginal drawings. You'd think some meticulous sort would have kept it in the airplane records, but no one on the 120/140 forum or 170 forum has ever seen it.
 
Fearless Tower said:
How'd you get it so light? This 170 in question weighed 1248 BEFORE the metal wings.

Oh I did join - Lots of good info!
Click to expand...
Good get the 170 service notes, and remember the 170 association is full of self proclaimed experts with their own ideas of what is right and what is not.

WoW 1248 that is a fat one..... no wonder the 95Kn, I'll bet the pitot tube is bent wrong too. tie a piece of twine to the pitot, and fly, the twine should be parallel to the tube in level flight. If it isn't bend it until it is, and be sure the pitot tube is long enough to be in clean air in front of the wing, some have been trimmed.

to get mine light I:
I removed the old Cotton and the 12 coats of dope and replaced it with ceconite and poly fiber.
Removed the heavy wool interior and replaced it with nomex, the seats were done in comfort foam.
Removed all the paint, and painted it with 1 coat of Randolph process.

Plus I removed all the radios and installed 1 VAL COM 760, and a transponder mode C,
Removed all the old cotton/rubber wires and rewired with teflon coated wires.
Removed the old AN 1 gyros and replaced them with new.
Removed the McCauley wheels and good year brakes and replaced them with Cleveland kit.

I also removed the cabin floor skins and rebuilt the gear boxes, removed all the black yuck from under the floors and rear fuselage rudder pedals.

anywhere I could save a ounce of weight I did.

Don't be surprised when the metal wing stalls at a much higher airspeed than the book says it should.

Do not spin the metal wing, unless you like being a test pilot, once the heavy wing picks up momentum, it is difficult to stop the spin.

Get the 170 service notes, it is the only good MM for the aircraft.

Do not slow flight the 170 rag wing at high throttle settings, it will cook the back two cylinders. that dumb trick cost me a set of new superior cylinders.

Have fun fly safe.. But I'd look for a "B" model. they are sweet.
 
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