The Federal Aviation Administration has ordered airlines to address a problem affecting Rolls-Royce-powered Boeing 757s that can cause hot engine air to strike and weaken engine struts.
The issue involves potential concurrent failures of two components on RB211-535-powered 757s, the regulator says in a proposed airworthiness directive released on 31 March.
Those components include thermal switches within an engine strut overheat detection system, and pre-coolers, which are used to cool hot engine air.
The FAA is aware of seven instances involving pre-coolers on RB211-powered 757s that failed due to a “wear-out condition”. The cores within the pre-coolers had “cracked or ruptured”, allowing hot engine air to leak and strike an engine strut.
Additionally, the FAA found that the strut overheat detection systems include “thermal switches with a latent failure mode”.
“The combination of a failed pre-cooler and latently failed overheat detection thermal switches may result in prolonged high temperature heat exposure on the strut, which could lead to separation of the engine strut-to-wing-box connection,” the proposed rule says.
Boeing addressed the issue in January when it issued an Alert Requirements Bulletin to operators.
That bulletin “specifies procedures for a general visual inspection for heat damage on the left and right engine strut structure, repetitive thermal switch temperature tests and continuity tests of the ground wire”, says the FAA.
The bulletin also calls for airlines to replace pre-coolers at intervals not to exceed 45,000 flight hours.
If approved, the FAA’s proposal, would mandate that airlines complete actions specified in the bulletin.
