Impulse coupling -500hr inspection

Why you really can't neglect magneto maintenance!. Some Failure Modes and Effects Analysis involved here. During a 500hr inspection the impulse coupling pawl plates and attaching rivets are checked for excessive wear. If excessive wear of these rivets is allowed to go undetected (e.g. by ignoring or delaying a 500hr inspection), the heel end of the pawl plate may start contacting the stop pin which in turn will cause the latching end to move radially outwards into contact with the stop pin. This will result in a catastrophic failure of the impulse coupling. Given that the impulse coupling actually resides inside the engine accessory drive housing, the debris from the coupling will cause further damage to the drivetrain. This Failure Mode is by far the most serious as it will result in both magnetos and the engine being put out of action (and if it is a single engine aeroplane) an emergency landing will be unavoidable. Broken coupling springs, missing teeth on the plastic distributor and rotor gears etc will result in incorrect timing and erratic engine noise and vibration, but switching to the other magneto will usually eliminate the problem and give the pilot time to make a normal landing at the earliest opportunity. Therefore those Failure Modes are less serious. Bottom line is don't put off doing a 500 hour inspection on your magnetos! "Ní hé lá na gaoithe lá na scolb" - get the 500 hr inspection done when it's due.