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.
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.
The departure of the flight from Zurich was normal but climbing through approximately FL 080, the Captain reported having experienced ‘ear popping’ to a degree which he had considered unusual and upon checking the cabin altitude saw that it was increasing at an abnormally high rate. As the aircraft approached FL 100 climbing as cleared to FL 120, a request to stop climb at FL 100 was made and approved. By this time, an EICAS indication had appeared in respect of the forward equipment valve along with a warning light on the equipment cooling panel which indicated that one of the valves associated with this system was not in the normal position. The augmenting First Officer ran the corresponding checklist which included turning the respective selector switch from AUTO to STBY. After this, the cabin pressure began to “decrease and stabilise” but the panel warning light which had come on remained. The checklist stated that in this circumstance, “pressurisation could not be ensured”.
The Captain stated that as the pressurisation system had now been working normally, “he had wanted to climb further as he had been feeling unsafe at FL 100 due to possible VFR traffic at this altitude and because of the mountainous terrain around Zurich” and that he wanted to deal with any further pressurisation problem at a higher altitude. In respect of the Captain’s expressed concern about terrain proximity on track, the Investigation noted that the MSA in the area was 4,500 feet.
ATC were advised that the intention was to continue en-route rather than remain near Zurich and the flight was cleared to climb to FL 230. However, on passing FL 195, the Captain reported having experienced “severe ear popping” and observing that the cabin altitude was again increasing at an abnormally high rate. It was decided to request an immediate descent and a MAYDAY was declared. All three pilots donned their oxygen masks and ATC quickly approved the descent and provided radar vectors. The descent was made with the AP disconnected and with idle thrust and speed brakes extended. As the cabin altitude exceeded 10,000 feet, the corresponding audio and visual warnings were annunciated but the increasing cabin altitude did not reach the 13,500 feet trigger for an automatic deployment of the cabin oxygen masks.
Once at FL 100 a holding pattern was commenced approximately 27 nm east-north-east of Dijon to allow the Captain to discuss the problem with the airline’s maintenance contractor in Zurich but no solution was available and it was therefore decided to return to Zurich. At the lower altitude, there was no immediate urgency to achieve a landing and so fuel was reduced to MLW by a combination of dumping and early extension of the landing gear and flaps and the aircraft eventually landed back at Zurich after a two hour flight.
Examination of the pressurisation system
On an initial examination, it was found that the equipment cooling overboard exhaust valve was fully open and that its electrical connector was un-plugged. The air duct connected to this valve had sustained severe damage (see the illustration below). No material from the damaged duct was found and the equipment cooling panel overhead panel rotary switch was in the STBY position as selected earlier by the crew.
It was then established that prior to the departure of the flight, the crew had called for assistance from the airline’s contracted engineering support organisation at Zurich, SR Technics, because of an EICAS status message ‘FWD GND EXH VAL’. Two suitably qualified technicians attended and concluded that one of the air conditioning exhaust valves - the ‘AVS/IFE inboard/outboard exhaust valve’ - was defective. It was noted that this valve functioned as part of the Alternative Ventilation System (AVS) when airborne and as part of the Inflight Entertainment System (IFE) cooling system when on the ground. After consulting United Airlines Maintenance Control Centre, it was decided that the aircraft could be dispatched in accordance with the MEL provided that the faulty valve was deactivated. It was established that one of the two technicians who had originally attended the aircraft had checked what the agreed deactivation required and noted that in addition to pulling the appropriate CB, the electrical supply to the disabled valve had to be disconnected and secured and the valve then opened manually. A corresponding information sticker was then required to be placed on the flight deck equipment cooling panel. The technician who had reviewed the task then returned to the aircraft without taking any documentation with him and, working from memory, proceeded to de-activate the equipment overboard exhaust valve instead of the valve which had been identified as defective. He then passed the flight deck information sticker to the flight crew to attach themselves. It was found that the text description of the faulty AVS/IFE valve differed slightly between the AMM and the United Airlines MEL - in some places it was referred to as an inboard / outboard exhaust valve and in others as an inboard/overboard exhaust valve.
The damaged air duct connected to the equipment overboard exhaust valve. [Reproduced from the Official Report]
It was noted that the equipment cooling overboard exhaust valve automatically should close when the rotary switch on the flight deck equipment cooling panel is selected to AUTO but because the electrical supply to this valve had been mistakenly disconnected, the valve would have remained fully open when the aircraft was in the air and remained so when the switch was selected to SBY by the crew in accordance with the specified checklist actions. The air duct to which the valve is connected was not designed to withstand the increased differential which builds up in flight and to which it was exposed because the valve was open and had burst. It was considered that the reason why the cabin altitude had begun to slightly decrease after the crew had initially responded to the pressurisation problem by switching the equipment cooling panel rotary selector switch from AUTO to STBY could probably be attributed to the fact that one effect of this change was that cabin air was no longer being actively moved by a circulation fan. Subsequently, when the climb was resumed, the pressurisation system was again no longer able to compensate for the loss of pressure occurring due to the open overboard exhaust valve and so the unwanted increase in cabin altitude recurred.
- The air duct to which the equipment cooling overboard exhaust valve was attached was unable to withstand the increased strain resulting from the developing differential pressure to which it was subjected as the aircraft climbed and it eventually burst.
- The flight crew’s decision to continue the climb along the intended flight path to a flying altitude of over 10,000 feet amsl even though pressurisation of the cabin was not ensured according to the actioned checklist, was not in accordance with the applicable operating procedures and was risky.
The Cause of the investigated Serious Incident was “the mix-up by the technician, who deactivated the wrong valve and left it in the open position”.
A Contributory Factor was also identified as the technician’s approach of working from memory, without taking a written form of the operating procedures described in the MEL with him.
A Risk Factor was identified as the inconsistent use of valve designations in the AMM and MEL.
Safety Action taken by SR Technics included the introduction of a two-man principle when carrying out maintenance work in accordance with the MEL.
The Final Report of the Investigation was completed on 11 April 2018 and subsequently published on 25 May 2018. No Safety Recommendations were made.