Type Rating Knowledge — Cessna 172S NAV III

Airframe and flight controls

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An all-metal, strut-braced, four-seat airframe

The 172S is an all-metal, four-place, high wing, single-engine aeroplane on tricycle landing gear, built for general utility and training. The fuselage is semimonocoque: formed sheet metal bulkheads, stringers and skin, where the skin itself carries load rather than merely covering a frame. The pieces of structure worth knowing by name are the front and rear carry-through spars the wings attach to, a bulkhead and forgings at the base of the rear door posts that carry the main landing gear, and fittings at the base of the forward door posts that take the lower end of the wing struts. Four engine mount stringers run forward from those same forward door posts to the firewall — which is why the door posts matter more than their name suggests.

C172S structure and load pathsLEFT WINGRIGHT WINGWING STRUTSCARRY-THROUGH SPARSFORWARD DOOR POSTSREAR DOOR POSTSFIREWALL / ENGINEMAIN GEAR
  • LEFT WING — Externally braced left wing — front and rear spars, formed ribs and stringers under aluminium skin, containing the integral fuel tank.
  • RIGHT WING — Externally braced right wing, built the same way and carrying the other integral tank.
  • WING STRUTS — Wing struts — the external bracing. Their lower ends attach to fittings at the base of the FORWARD door posts, which is also where you push when moving the aeroplane by hand.
  • CARRY-THROUGH SPARS — Front and rear carry-through spars — the fuselage structure the wings attach to. The rear spars are partial span and attach to the fuselage only.
  • FORWARD DOOR POSTS — Forward door posts — they take the lower strut attachments AND the four engine mount stringers running to the firewall. Two major load paths meet here.
  • REAR DOOR POSTS — Rear door posts — a bulkhead and forgings at their base carry the main landing gear attachment.
  • FIREWALL / ENGINE — Firewall and engine mount. Four engine mount stringers run aft from the firewall to the forward door posts, so engine loads end up in the same structure as the struts.
  • MAIN GEAR — Tubular spring steel main landing gear struts — landing loads pass into the bulkhead and forgings at the base of the rear door posts.
Redrawn from POH 172SPHBUS §7, Airframe — structure restated, not traced. C172S NAV III.