The Federal Aviation Administration today ordered new inspections of mechanisms in the tails of Boeing 737s that can cause significant issues if they’re working improperly.

The FAA order goes into effect on Sept. 9. The order stems from two incidents on 737s in March and April. In those incidents, severe in-flight vibrations developed stemming from problems with the aft attachment lugs on the elevator tab control mechanisms.

Those vibrations in extreme cases could cause loss of aircraft control or structural integrity.

In both cases the flights involved landed without incident.

Boeing issued a service bulletin suggesting more frequent inspections of those mechanisms. The bulletin advised airlines to replace any worn lug attachment points with new Boeing-built attachments.

The FAA order expands the inspection frequencies required. Boeing is developing new tab control mechanisms to address the issues involved.