Performance Data or POH for engine conversion

[GAHorn]

I received and replied to the following inquiry thusly, and am posting this info the use of whoever may be interested.

hey george-
i have a 1951 Cessna 170a with the lycoming 0-360/180hp engine in it. I'm looking for a POH with all the performance data(landing on grass, upslope landings, weight & balance graphs etc.,etc.).my POH is thin, and has lots of scribbles over critical numbers...or the numbers are worn clean off.

i'm a relatively new pilot, and just bought my c170 last february and put on 200 hours so far. it's been going really well. but i want to get to know my plane better, and get very comfortable with working calculations(such as weight and balance). I need to get a new POH for that make/model with that modification-i.e. bigger engine.

thanks for any info(p.s. i tried searching this on the site, and wasn't successful).

Reply:

Hello!
Well, the POH situation has an interesting background.
Firstly,...the C-170 never had a POH. Cessna's marketing dept. issued an "Owner's Manual" which has been reprinted by Univair and sold both by themselves and thru Aircraft Spruce. It is not "approved" data, and carries no official status, although it is worth owning if you haven't one. (I suspect that is actually what you have that is written over.)
Secondly, the only performance data that is "officially blessed" is a duplicate copy of that printed in the Owner's Manual and published in the Airplane Flight Manual (AFM) which is required to be carried in the aircraft. (See the requirement posted in the Aircraft Type Certificate Data Sheet.) If a knowlegeable FAA Inspector gives you a "ramp check" and he finds you without that AFM, (a one-page document, printed front/back) then you will be in violation. (Cessna will sell you a replacement if you haven't got one, for about $50. As a member, TIC170A secy., headquarters@cessna170.org will send you one for the cost of Xerox copy.)
In any case, the original C-170 data only applies to the original airplane/engine.
Now comes your airplane. It has been modified in accordance with specifications and data contained in the STC, or field approval under which the new/replacement engine was installed. Cessna had nothing to do with that, and did not bless that, and subsequently did not publish data to support that modification. Therefore, the producer of that modification had the onus of publishing (or not) any new performance data. Most did not, they merely claimed that their mod "met or exceeded" original data. The owner of that mod (now you) is left to live with that circumstance, and there is little reason for anyone to assume the liability of publishing official data, or performance claims which do not benefit themselves monetarily. (Not to mention the expense of producing such data and getting it approved.)
Such is the case that one finds oneself when purchasing highly modified aircraft. I'm sorry to say, that I have no reliable data at my disposal to send you.
Very best regards,
George Horn
Parts/Mx TIC170A