60-07-01 ALLISON: Amdt. 124 Part 507 Federal Register March 29, 1960. Applies to Models 501-D13D and 501-D13E Engines.
Compliance required within the next 25 hours of operation.
A few cases of third stage turbine blade failures have occurred due to a resonance condition at low speed ground idle. All of these failures to date have resulted in visible damage to fourth stage blades as well as fourth stage vanes. In one case continued operation of an engine with a failed blade resulted in failure of the turbine inlet case-vane case split line bolts.
(a) Aircraft not having operating engine vibration detection equipment must observe the following engine operating restriction and inspection.
(1) Low speed ground idle operation from time all engines are started to stopping all engines at end of flight not to exceed 4 minutes total time.
(2) Conduct inspection of fourth stage turbine blades at intervals not to exceed 25 hours of operation for indications of damage using adequate light and optical aid.
(b) Aircraft having operating engine vibration detection equipment shall use this equipment to detect any indications above normal and if found, the above inspection of fourth stage turbine blades shall be conducted upon arrival at the next maintenance base. If any damage is discovered as a result of (a) or (b) it is cause for more detailed inspection and/or engine removal.
(c) This restriction will not apply to engines modified in accordance with Allison Commercial Engine Bulletin No. 72-77 by installation of third stage turbine blades P/N 6794773 identified by a stripe of heat and corrosion resistant aluminum polytherm paint 1/2-inch wide and 4 inches long around contour of the inlet casing clockwise starting at the 1:00 position forward of the terminal block mounting flange.
(Allison Commercial Engine Bulletin No. 72-77 covers the same subject.)