The Rolls-Royce1 Gnome-engined variants of the Westland Wessex helicopter family (i.e., the twin-engined versions)2 draw air for both engines from a common intake (having been designed prior to the advent of requirements for physical separation of helicopter engine intakes), meaning that a destructive failure of one engine can potentially throw debris into the intake for the opposite engine, potentially disabling it as well; this was considered as one possible contributing factor (among many other possibilities) to the total loss of power that culminated in a fatal 1981 crash of a Gnome-engined Wessex 60, and had definitely occurred on four instances before the 1981 crash (although, in each of these four prior cases, the second engine, although damaged, continued to operate).

Early Gnome-engined Wessexes had a dividing wall separating the left and right engine intakes; however, this was removed after some time, due to difficulties it apparently caused with anti-icing the intakes:

However, in the event of compressor damage it is possible for the Gnome engine to eject debris forward out of the compressor which, in the Wessex 60, may be ingested by the other engine since the intakes are not separated. Rolls Royce (RRL) were aware of four cases in which both engines in a Wessex had been damaged in this way, although in none of the cases was the power remaining available insufficient for continued flight. Whilst the original design incorporated a dividing wall which achieved separation of the compressor intakes difficulties in achieving a satisfactory anti-icing standard led to its removal in a modification approved by the CAA but not by RRL. The possibility therefore exists that a failure in one engine could, by cross ingestion, lead to a complete loss of power. Since the Wessex 60 was certificated BCARs have been revised, and now require engine intakes to be independent of each other. [AIB3 Aircraft Accident Report 4/83, pages 21-22 (pages 25-26 of the PDF file of the report); my emphasis.]

Why did the original dividing wall cause anti-icing difficulties? Was it too thin to accommodate the wires or bleed ducts required to anti-ice it?

1: Formerly de Havilland.

2: Very early Wessexes instead used a single Napier Gazelle engine.

3: Not a typo - the AIB didn’t become the AAIB until November 1987.



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