Swiss investigators have found that cabin crew on board an Airbus A330-300 experienced considerable difficulty with using protective breathing equipment during an incident in cruise over the English Channel.

The Swiss-operated jet (HB-JHL) had been flying at 34,000ft en route from Zurich to Newark on 11 July 2023.

Both pilots donned oxygen masks in response to an unusual smell while several flight attendants chose to put on the protective equipment after experiencing headaches, eye irritation and burning sensations.

The crew opted to return to Zurich.

While Swiss investigation authority SUST could not determine the source of the smell – or whether passengers perceived it – the inquiry says several flight attendants encountered “significant difficulties” with using the protective equipment.

These problems ranged from unpacking and unfolding the equipment from its vacuum-sealed packaging, to putting it on, activating and using it.

“Several [protective equipment] units exhibited technical defects and were therefore not, or only partially, functional,” it adds. “This represents a significant safety risk.”

Technical design of the equipment was such that trained cabin crew were unable to activate it within a “reasonable amount of time”, it says. The time required, up to 3min, was “several times longer” than the interval of 15s specified in certification criteria.

Seven cabin crew members, each of whom used a different protective equipment unit, experienced a number of problems. Two used scissors or a knife to open the packaging, one found a retaining strap broke after fitting, others experienced pressure on the face and high temperatures.

Communication was also “severely impaired” and, in cases, “almost impossible” when the equipment was in use, the inquiry says.

SUST found that the crew had been trained exclusively with “dummy” equipment, provided for the purpose, but this differed “considerably” from the actual emergency equipment.

Dummy equipment, it points out, does not simulate the “strong heat development” after activation or the difficult breathing after donning it.

“As trainers do not usually gain experience with real [equipment] at any point in their professional careers, they cannot draw on their own experiences with any difficulties,” the inquiry states.

“Accordingly, they are unable to raise the trainees’ awareness of difficulties in a targeted manner.”

SUST is recommending that training equipment and conditions of use be as realistic as possible, and adds that trainees should be prepared for the ‘startle’ effect of an emergency and forewarned over what to expect when using real equipment.

The inquiry says recurring maintenance measures on the protective equipment were “inadequate” with defects remaining undetected.

SUST says European and US regulators should review certification and maintenance criteria for such equipment and, if necessary, adapt them.

It adds that there is “no reliable systematic inspection” of protective equipment in use that would provide information on failure rates. SUST suggests testing functionality at the end of service life before disposal.