The International Cessna^® 170 Association

A friend purchased a '61 C-172 with an AirPlanes STC'd Lycoming O-380 (180 hp). His paperwork indicates an increase of 350# (2550#). How does swapping the engine (fixed prop) do this? I "think" the engine/prop weight is about the same as the O-300 & prop.

They got approval by proving the aircraft with the modification would meet or exceed the original certification process at the new weight.

O 380? I think he means the O360. If I remember correctly the STC for the weight increase also restricts the flaps to 30 degrees.

O-380, yea isn't that that bored out O-360 should have proofread my post. I'll have to ask if the stc put a limit of 30 deg's on the 172. Sure climbs good!

One of the limiting factors in certification is "balked-landing-climb" requirement. In plain language, this is the ability of the aircraft to be able to go-around and climb with the flaps in the landing configuration without flap retraction. (in case the flaps were stuck or in case the pilot was unable for whatever reason to reduce flap deployment.)

Many engine conversions, in an effort to avoid expense (and liability) simply adopt existing certification performance...OR... state their modification "meets or exceeds" original type cert./AFM performance.

If they went to the trouble and expense to retest the performance after the modification, they apparently discovered and documented improved performance at the higher weight.

(Cessna accomplished a similar gross wt increase in the 1981 Cessna 172-line by reducing landing flap deployment from 40 to 30 degrees. Of course, there is also a maximum stall-speed allowable and the increase in gross wt affects a higher stall speed, as does the reduced 30-degree landing-flap. Therefore there is a trade-off to consider.)

On top of all that, the door posts are where the wing stress loads are concentrated, and increased operating weights place more strain on those doorposts.
In order to maintain the necessary safety-margins either the wing attach/doorposts have to be redesigned... or the weights have to be restricted.