What Causes the Vertical Stabilizer Fittings To Fail

What causes the Vertical Stabilizer Fittings to Fail on Cessna 150s and 152s?

Written by: Dave McFarlane, Founder

On the swept tail Cessna 150s and 152s, cracks start to show on the aft vertical stabilizer attach fittings around 5,500 hours or more. The cracks begin at the aft end of the fitting and progress forward. At first, the crack looks like a small scratch in the paint. Fortunately, the location of these cracks is easy to spot – generally on the outboard leg of the fittings. Cessna SE78-62 addresses this issue by mandating periodic inspections.

So, what is causing the fittings to fail? After speaking to several salvage yards over the years, they never sell a fitting for the old straight tail airplanes, but they have sold all their fittings for the swept tail airplanes. Cessna has attempted to change the swept tail fittings designs and fasteners over the years – but all seem to fail. So, why do swept tail fittings fail when the straight fittings never fail? And what is causing the cracks? Why do the fittings crack on the outboard leg, and not the inboard leg?

In-flight, the vertical stabilizer has considerable high-speed air flowing past it on both sides. This airflow fives the vertical stabilizer resistance to horizontal motion. Every time an engine cylinder fires, a reaction occurs that wants to move the engine opposite to the piston motion. This engine motion is transferred to the airframe, and then mostly stopped by the stability of the vertical stabilizer.

The rotation of the propeller creates a swirling motion of the slip stream air, which then creates pulses of unequal force on both sides of the vertical stabilizer. To give perspective, combine the number of cylinder firings (2,400 per minute times 2) with the number of prop wash pulses (another 2 per revolution), which equals 9,600 vibration cycles per minute (576,000 per hour). At 5,000 hours, that equals 2,880,000,000 cycles – and that does not include the effect of turbulence.

The height of the vertical stabilizer has a large lever effect on the mounting fittings, giving the fittings a mechanical disadvantage. Have you listened to the vertical and horizontal stabilizer when the engine starts but has not yet reached speed? The stabilizers make noticeable noise from minor buckling of the sheet metal. There is tremendous energy transfer from the engine motion and inertia of the stabilizers wanting to stay in a stable state.

So, again, why do swept tail fittings fail when straight tail fittings do not? One reason is the geometry of the vertical stabilizer. With the swept tail airplanes, the center of gravity and the center of aerodynamic loading are further aft. Compared to the straight tail airplanes, the swept tail airplane aft fittings carry more of the stabilizer loads than the front fittings. The larger issue is the geometry of the fitting and the angle that the bending loads are applied (see illustration).

The Cessna fitting features an angular geometry that applies bending stress to the intersection of the aft end of the fitting and the bend axis (a straight line between the forward and aft fittings). This concentrated angular stress can start a crack.

The fitting geometry and its perpendicular placement to the bend axis will withstand much more fatigue bending cycles than fittings that intersect the bend axis at an angle. This was proven through our testing. Using an endurance stimulation, we compared the McFarlane fitting with the Cessna fitting, bending them at the same time with the same amount of forces. The Cessna fitting failed first and the McFarlane fitting failed after many more abusive cycles.

The McFarlane fittings are much more fatigue-resistant and crack-resistant because the intersection of the back edge of the fitting and the bending axis is perpendicular to each other (see illustration. McFarlane also uses a more crack-resistant aluminum alloy material.

Cracks almost always appear on the outer legs of the fittings because more motion occurs in the metal as distance increases from the center of the vertical stabilizer. The outer fitting legs are subjected to more bending strain.

When one fitting cracks, the other fitting that has experienced the same number and intensity of fatigue cycles has likely also failed. McFarlane recommends replacing your fittings in pairs.