In a good number of aircraft in the fleet today, belts drive accessories — these accessories do various things like keep the radios on… Whether for your alternator or air conditioner, the proper adjustment of accessory belts is critical to the performance of a lot things that keep you ‘comfortable’ in the cockpit.
THE SET UP: You note that your alternator belt seems loose, so you call it to the attention of your IA. She takes note of your request, and adjusts the belt back into the proper spec, which for the particular model of aircraft requires 1/2′ of free play in the belt. After the adjustment, the aircraft is returned to service, and you go flying.
THE PROBLEM: As you reach your cruising altitude, you notice oil starting to make its way up your windshield. You look down in horror and see your oil pressure headed for the bottom of the scale, and along with it, your prospects for a safe and uneventful flight. After bringing back power and turning back to the airport, you are able to squeeze enough power out of the plane to make it back to a shaky — but safe — landing.
What Happened? Every belt works at a 90-degree angle to a shaft, and every shaft has a bearing to accept that force. In this case, the bearing was near failure due to time in service, and failed as the adjustment was made. The A&P who adjusted the belt checked, and everything was fine, but the shaft to the starter drive had broken it’s bearing, and started working on the engine casing. But wait, it gets better (worse actually), as the starter drive shaft ground into the engine casing, it made a good quantity of metal fragments — and the oil system happily distributed them throughout the engine. Result: Damage was so severe that the engine had to be overhauled prior to returning it to service.
Caution: In our example above, the engine side of the equation failed, but the same thing can happen to the driven component — such as the alternator — or to both at the same time, depending on the condition of the bearings. THE BIGGEST RISK is to airplanes that are towards the end of their TBO. Substantial wear may have already taken place, so caution needs to be used when tensioning the belts on such planes to assure bearing failure does not occur.
BOTTOM LINE: While belts need to be checked and tensioned on occasion, making sure that a belt is properly tensioned is only part of the equation. Belts stretch as they age, but sudden ‘loose-ness’ in the belt system may be symptomatic of another problem. Fixing the problem means knowing the cause and correcting it — and that can mean the difference between an off field landing, and an uneventful day in the air. The more you know about the systems that run your airplane, the more likely you are to avoid problems while flying it.
