When fabric is applied to a wooden or metal wing structure, the covering itself must be firmly attached to every rib beneath it. Without this attachment, aerodynamic pressure — particularly the suction forces on the upper surface — would simply peel the fabric away from the structure during flight. Rib lacing is the time-honored technique that solves this problem: a continuous length of specially rated cord is stitched through pre-punched holes (or through the fabric itself) and around the rib cap strip, creating a series of closely spaced loops that lock the fabric in place along every rib's entire chord length. Understanding rib lacing is fundamental for any Aviation Maintenance Technician working under FAA-approved methods, and it is a core subject area in the AMT Airframe curriculum.
The principles and acceptable practices for rib lacing are described in Advisory Circular 43.13-1B, which the FAA accepts as an approved data source for aircraft fabric covering repairs and recovering projects. The governing standards for maintenance practices fall under 14 CFR Part 43, while specific fabric strength requirements are addressed in Part 23 standards and the applicable Supplemental Type Certificate (STC) or Type Certificate Data Sheet for each aircraft.
Why Rib Lacing Is Necessary
Fabric-covered wings experience both positive pressure on the lower surface and negative pressure (suction) on the upper surface. The net aerodynamic load acts perpendicular to the wing chord and must be transferred from the fabric into the ribs, then into the spars and ultimately the fuselage. Without rib lacing, only the fabric cement and the trailing- and leading-edge tapes would hold the covering in place — clearly insufficient for flight loads. The lacing stitches create a direct mechanical connection between fabric and rib, distributing the load over many individual stitch loops rather than concentrating it at a few tack points. Each stitch loop, when tied correctly, is essentially a small load-bearing element; the cumulative strength of dozens of properly tied stitches along a single rib provides the margin of safety required by FAA standards.
Approved Lacing Cord
The FAA requires that lacing cord meet specific strength standards. The most commonly used material is waxed linen cord, although braided nylon cord of equivalent strength is also acceptable on many recovering projects when approved data permits it. The cord must have a minimum breaking strength appropriate to the design load of the aircraft. AC 43.13-1B specifies that the cord used for rib lacing on certificated aircraft should have a minimum breaking strength of at least 80 pounds for most general aviation applications, though some higher-performance or heavier aircraft may require cord rated higher. Always verify the required cord strength against the aircraft's approved data — the Type Certificate Data Sheet, the aircraft maintenance manual, or an applicable STC.
Waxed linen cord is preferred for traditional recovering projects because the wax coating provides some weather resistance, reduces abrasion against the rib cap strip, and helps the cord stay in position during the lacing process. Synthetic cords must be proven compatible with the dope or coating system being used, since some solvents in nitrate or butyrate dope can degrade certain synthetic fibers. The technician must confirm compatibility before beginning the job.
The Standard Rib Lacing Stitch
The most widely used stitch pattern is the modified seine knot (also called the standard rib lacing knot or seine knot). Each completed knot consists of two half-hitches oriented so that the locking portion of the knot sits on top of the rib cap strip rather than on the fabric surface. This positioning is critical: a knot that rides on the fabric can chafe through the covering during vibration. The seine knot is preferred because it is self-locking — once pulled tight, it resists loosening under cyclic aerodynamic loading — yet it can be formed quickly and consistently by a skilled technician.
The lacing process begins at the leading edge and works toward the trailing edge, or follows the sequence specified in the approved data. The cord is threaded through a large-eyed rib-lacing needle. A starting knot anchors the cord to the first rib hole or around the rib flange, and then the technician alternates stitches between the upper and lower fabric surfaces with each stitch locked by a seine knot on the rib cap. At the trailing edge, the cord is terminated with a finishing knot — typically two half-hitches backed up by a surgeon's knot — to prevent any possibility of unraveling.
Reinforcing Tape
Before lacing begins, reinforcing tape (also called anti-tear tape or rib lace tape) is applied over the fabric directly above each rib. This tape distributes the load of each stitch loop over a wider area of fabric, preventing the cord from cutting through the weave under load. The tape runs the full chord length of the rib on both the upper and lower surfaces. Tape width and material must conform to approved data; AC 43.13-1B provides guidance on minimum widths. The tape is typically made from the same fabric grade as the covering, or from a heavier grade if called for, and is attached with aircraft dope before lacing begins.
Stitch Spacing Requirements
Stitch spacing — the distance between successive knots along the rib — is one of the most safety-critical dimensions in the entire fabric covering process. Closer stitch spacing means more knots per unit length, more load-sharing points, and greater resistance to fabric separation. The FAA provides specific spacing requirements based on aircraft never-exceed speed (VNE) in AC 43.13-1B:
- VNE up to 150 mph: Maximum stitch spacing is 3 inches in the slipstream area (propeller wash region) and 4 inches on the non-slipstream areas of the wing.
- VNE 151–200 mph: Maximum stitch spacing is 2 inches in the slipstream area and 3 inches outside the slipstream area.
- VNE 201–250 mph: Maximum stitch spacing is 2 inches in the slipstream area and 3 inches outside the slipstream area.
- VNE above 250 mph: Maximum stitch spacing is 2 inches throughout, or as specified by approved data for that aircraft.
The slipstream area is generally defined as the region of the wing and fuselage directly behind the propeller disc, where aerodynamic loads and turbulence are greatest. Identifying the slipstream boundary on a specific aircraft requires reference to the aircraft's maintenance manual or the applicable STC data — do not estimate this boundary on your own. Always use the tightest applicable spacing when in doubt, because closer spacing never reduces airworthiness.
These spacing values assume standard grades of aircraft fabric meeting the strength requirements of AC 43.13-1B. If a lighter-weight experimental or ultralight fabric is being used under a different approval, the approved data for that material may specify different — potentially closer — spacing requirements.
Knot Inspection and Quality Checks
After lacing is complete and before any additional coats of dope are applied, the technician must inspect every knot along every rib. Each knot should sit squarely on the rib cap strip, be drawn uniformly tight without cutting the fabric, and show no signs of slipping. The spacing between knots must be measured and verified against the approved data. Any knot that is loose, incorrectly positioned, or formed from a single half-hitch rather than a proper seine knot must be re-laced. Once the first coat of dope (the pinning coat) is applied, the knots are locked in place and correction becomes extremely difficult — thorough inspection before doping is therefore mandatory.
Common Test Traps
- Confusing slipstream and non-slipstream spacing: The FAA knowledge test frequently asks about spacing in each zone separately. Remember that slipstream areas always require equal or closer spacing than non-slipstream areas — never the other way around.
- Assuming one spacing rule covers all speeds: The tightest maximum spacing applies only to aircraft with the lowest VNE range. Faster aircraft require progressively tighter spacing; applying the loosest spacing to a faster aircraft would be an airworthiness violation.
- Skipping reinforcing tape: Some candidates think lacing cord alone is sufficient. The reinforcing tape is a required element, not optional, because without it the cord can tear through the fabric weave.
- Knot placement on fabric surface: Knots must land on top of the rib cap strip, not on the fabric. A knot sitting on the fabric surface will chafe through the covering — a common error that inspectors look for.
- Cord strength requirements: The test may present scenarios where a technician substitutes a lower-rated cord to save cost. Using cord below the minimum breaking strength required by approved data is never acceptable, regardless of the aircraft's age or condition.