For all the problems that could result from a drive belt failure in flight, stopping them before they can actually happen is fairly simple — if you know how. All you need to be an effective belt checker is a good set of eyes, hands, and maybe a flashlight…
Just like the belts in your car, the belts in your airplane should always be in good condition — or they should be replaced. Check the belts for:
- Cracks — in the surface of the belt and *especially* in the “V” section of the belt. If you can see cracks in the surface or “V” section, belt replacement is warranted.
- Checking — that is, any signs of small, square or irregularly sided divots in the surface of the belt. If you can see reinforcing fibers in the belt, it needs to be replaced. Some belts are toothed, that is, instead of having a smooth “V” section; it is notched at regular intervals. All the notches need to be in place in order for the belt to be considered airworthy.
- Abnormal Wear — To complete our visual inspection, look at the pulleys that the belts ride on, and check them for obvious cracks. If scoring is found on the side of a “V” type belt, look for a damaged pulley — it is very likely that there is one is causing the scoring.
Use your hands to determine if the belt is properly tensioned. Reach into the cowling, and, with a single finger, apply pressure at the center of the belt’s span. You should be able to deflect most belts around 1/2 of an inch without *too much* effort (see below.)
Too TIGHT! If the belt is tight like a bowstring and won’t deflect, it is likely too tight. Danger: Running with a belt that is too tight places excessive load on the associated bearings, which can result in failure of those bearings, the loss of the belt, and loss of the use of accessories while in flight. Have your mechanic check the service manual and set the belt up properly.
Too LOOSE! If the belt can be deflected more than 1/2 an inch, the belt is probably too loose. Danger: The consequences of running with a belt that is too loose can be the same as the loss of the belt — anything from a weak battery to a complete loss of electrical power in flight! Again, refer the adjustment to your mechanic and have the belt set up properly.
THE BOTTOM LINE: Unless you like to experience a loss of electrical power in flight, and the exciting, extra training in the loss of an electrical system that comes with it, keeping an eye on your belts should be a critical part of the preflight for those planes that use them. By taking this approach, you will be able to better manage the systems driven by the belts, and by doing so, avoid any “extra training drills” while you are flying.