Description

On 23 August 2017, a Boeing 767-400ER (N68061) being operated by United Airlines on a scheduled international passenger flight from Zurich to Washington Dulles by an augmented crew was approaching FL 100 in unrecorded day flying conditions when the cabin altitude was seen to be increasing at an excessive rate and the climb was stopped at that level until it was believed that it had been possible to address the problem after which the climb was resumed. However as the aircraft approached FL 200, the same problem recurred and it was decided to don oxygen masks, declare a MAYDAY and commence an immediate descent to FL100 following which a return to Zurich was made without further event.

Investigation

An Investigation was carried out by the Swiss Transportation Safety Investigation Board (STSB). The experience of the members of the flight crew was not recorded but it was noted that the Captain had been designated as PM and the augmenting First Officer was occupying the supernumerary seat behind the operating crew throughout the two hour flight. No reference was made to the investigation of an event which occurred in French airspace being undertaken by the STSB but return of the aircraft to Zurich meant that the Investigation could proceed more easily.