51-21-01: Rudder Rib Flanges
FAA order 51-21-01 is an airworthiness directive. The FAA's subject line: Rudder Rib Flanges. It names Cessna 120, Cessna 140: about 2,499 aircraft on the U.S. register today across them, though an order usually covers particular versions or serial numbers rather than every one. System: flight controls.
What it applies to as the FAA lists it
- Kind of product
- Aircraft
- Makes
- Cessna Aircraft Company
- Models
- 120, 140, 140A
- On this site
- Cessna 120 660 registered
Cessna 140 1,839 registered
Dates and paperwork
- Docket
- Unknown
- Status
- Current
- The FAA's document
- Read the order on the FAA's site
The order itself the regulatory text, as published
Show the full text (2,075 characters)
51-21-01 CESSNA: Applies to Models 120 and 140 Aircraft, Serial Numbers 8001 to 10650, Inclusive, and/or Any Other Serial Numbers Not Having Steel Reinforcing Channel, Cessna P/N 0433131 or Equivalent Installed.
Compliance required prior to December 1, 1951.
Inspect bottom rib of rudder for kinks or cracks in the rib flanges just aft of the rudder horn fitting. If there is any damage, the bottom rib assembly should be replaced with Cessna P/N 0433106, since the damage may progress until the rib breaks in two with the loss of rudder control. On installing P/N 0433106, which is an assembly of the rib, the control horn and a steel reinforcing channel (P/N 0433131), AD-4 protruding head type rivets should be used wherever rivets or spotwelds were used on the original installation; six additional rivets for attachment of aft part of steel channel to bottom rib and four AD-4 protruding head type or Cherry 163-4-4 rivets added to attach the skin to each flange of the steel reinforcing channel. If no damage to the bottom rib assembly is detected, installation of the steel reinforcing channel only is necessary. This may be accomplished by drilling out existing rivets for attachment of control horn and installing the steel reinforcing channel (P/N 0433131), above the bottom rib with flanges up, using existing rivet holes. The completed installation should be the same as for the installation of the complete lower rib outlined above. The reinforcing channel, P/N 0433131, is 4 17/32 long with 5/8 flanges, planform to fit inside lower rib installed as near horn flange as possible. Flanges removed on forward inch of channel. Material 0.036 1025 steel or 0.051 24ST, ALCLAD or equivalent. It is recommended that the length of the chains to the steerable tail wheel be so adjusted that under static conditions the coil springs are not extended more than 1/8 inch, since excessive tautness of the chains contributes to the rib failures.
(Cessna Service Letter No.46, dated July 31, 1947, covers this same subject.)
This supersedes AD 47-43-07.
Related safety bulletins advice on the same designs, not orders
2015-04-15CE-15-13 · FUSELAGE Seat Belt Mounting Bracket
2010-07-30CE-10-40R1 · Aircraft Fuel System; water contamination of fuel tank systems on Cessna single engine airplanes
2009-07-24CE-09-42 · Lights - Landing Light, Beacon Light and Taxi Light Switch Applications
2006-05-24CE-06-46 ·
2005-08-09CE-05-80 ·
Source: FAA Dynamic Regulatory System, airworthiness directives. The FAA's document is the authority; the register counts are this site's, from the FAA register of November 6, 2026. Connections as data: /graph/directive/51-21-01.json · aircraft systems.