Summary
We propose to adopt a new airworthiness directive (AD) for all Gulfstream Aerospace Corporation Model GVI airplanes. This proposed AD was prompted by reports of corrosion on in-service air non-return valves. This proposed AD would require a revision to the Emergency Procedures section of the airplane flight manual (AFM). This proposed AD would also require a revision to the maintenance or inspection program, as applicable, to incorporate airworthiness limitations for the high pressure (HP) Stage 5 air non-return valves. We are proposing this AD to ensure the flightcrew is provided with procedures to mitigate the risks associated with failure of the HP Stage 5 air non- return valve. Failure of the HP Stage 5 air non-return valve in the open position could result in engine instability and uncommanded in- flight shutdown.