Amendment 3 implemented the additional inspections of ASB 212-87-48, which was issued as a result of fatigue cracks being found in service. It required that visual inspections of the main rotor yoke be performed daily and every 1200 hours/24 months. These are additional to the more detailed 2400 hourly inspection specified in FAA AD 79-20-05.
Amendment 4 specifies Bell Alert SB 212-90-60, which supersedes ASB 212-87-48, and adds a yoke spindle inspection to Part III, required at 1200 hours/24 month intervals. Inspections done to earlier amendments may be considered for the purposes of determining when amendment 4 inspections are due.