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B773, en-route, east northeast of Anchorage AK USA, 2015

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Summary
On 30 December 2015, a Boeing 777-300 making an eastbound Pacific crossing en-route to Toronto encountered forecast moderate to severe clear air turbulence associated with a jet stream over mountainous terrain. Some passengers remained unsecured and were injured, one seriously and the flight diverted to Calgary. The Investigation found that crew action had mitigated the injury risk but that more could have been achieved. It was also found that the pilots had not been in possession of all relevant information and that failure of part of the air conditioning system during the turbulence was due to an improperly installed clamp.
Event Details
When December 2015
Actual or Potential
Event Type
Airworthiness, Category:Cabin Safety, Weather
Day/Night Day
Flight Conditions VMC
Flight Details
Aircraft BOEING 777-300
Operator Air Canada
Domicile Canada
Type of Flight Public Transport (Passenger)
Origin Shanghai Pudong International
Intended Destination Toronto/Lester B. Pearson International Airport
Take off Commenced Yes
Flight Airborne Yes
Flight Completed Yes
Flight Phase Cruise
ENR
Location En-Route
Origin Shanghai Pudong International
Destination Toronto/Lester B. Pearson International Airport
General
Tag(s) Extra flight crew (no training),
CVR overwritten
WX
Tag(s) CAT encounter,
Mountain Wave/Rotor Conditions
CS
Tag(s) Pax Turbulence Injury - Seat Belt Signs on
AW
System(s) Air Conditioning and Pressurisation
Contributor(s) Component Fault in service
Outcome
Damage or injury Yes
Aircraft damage Minor
Injuries Few occupants
Causal Factor Group(s)
Group(s) Aircraft Operation,
Aircraft Technical
Safety Recommendation(s)
Group(s) None Made
Investigation Type
Type Independent

Description

On 30 December 2015, a Boeing 777-300ER (C-FRAM) being operated by Air Canada on a scheduled international passenger flight from Shanghai Pudong to Toronto as ACA 088 with an augmented crew encountered a period of moderate to severe turbulence in day VMC as it crossed southern Alaska at FL 330 eight hours after take-off from Shanghai. One of the two air conditioning systems also failed. One of the 332 passengers was seriously injured and 20 others sustained minor injuries. There was some damage to cabin fittings.

The track flown and event location (a hollow circle) plotted the forecast sig weather chart. [Reproduced from the Official Report]

Investigation

An Investigation was carried out by the Canadian Transportation Safety Board. Relevant data was successfully recovered from the DFDR but since the aircraft did not land until 2 hours 45 minutes after the investigated event and the CVR continued to run, none of its data, in particular use of the cabin PA system, were of any use. It was noted that the DFDR did not record the position of the flight deck switch which is used to switch the seat belt sign on and off.

It was noted that the Captain had accumulated 22,529 total flying hours which included 5,061 hours on type and the First Officer 13,906 total flying hours including 5,814 hours on type. The two augmenting crew members had 19,738 and 5,221 total flying hours of which respectively 1,459 and 1,433 hours were on type. The turbulence occurred with the augmenting crew in control and the First Officer acting as PF.

It was established that three Air Canada fights were taking a similar eastbound track across Alaska with AC 088 being the third. Another 777 was 90 minutes ahead and a 787 was 30 minutes ahead. Whilst still over the Pacific and almost three hours ahead of the encounter, a SIGMET advising of occasionally severe turbulence over southern Alaska between FL 260 and FL 400 was issued. About an hour later, the augmenting crew routinely took over and were briefed accordingly. ATC were contacted to see if any reports of more than moderate turbulence had been received for their cruise level of FL 330 and were told that none had. Soon after this, the 777 ahead encountered moderate to severe turbulence at FL 350 within the area covered by the SIGMET and had initially descended to FL 330 where the turbulence was less but then returned to FL 350 to stay clear of cloud. A further short period of similar turbulence followed and in a direct ACARS communication with the 787 which was at FL390, the lead aircraft crew "strongly advised" a diversion off track to the south which was subsequently made and the turbulence experienced in the same vicinity was no more than moderate.

Approximately 35 minutes before the area of forecast turbulence was expected to be reached, the cabin crew were instructed to secure the cabin and appropriate announcements were made in English, French and Mandarin with checks following. About half an hour later, ATC advised of reports of moderate to severe turbulence in the notified area at both FL 280 and FL 350. The crew decided to stay on their flight-planned track at FL 330 based on information they had obtained from Air Canada dispatch and on "visual cues suggesting that FL 330 would have the least turbulence". At about the same time, with all cabin crew secured in their seats, a passenger in row 1 got up to use the toilet and ignored a request to return to his seat. As he returned from the toilet, turbulence subsequently described by the crew as moderate to severe began and he was thrown to the ceiling and then onto the floor. This turbulence lasted 2 minutes and after it ceased, the crew turned 30° to the right and noted an indication that the right air conditioning system had failed. For the next 8 minutes, light to moderate turbulence prevailed before a second 7½ minute period of moderate to severe turbulence occurred. It was reported that it was during the second period of significant turbulence that most passenger injuries were sustained.

The aircraft track showing the turbulence encountered and the boundary of the SIGMET-notified area of 'moderate to severe' turbulence. [Reproduced from the Official Report]

Once the turbulence had subsided, the operating crew returned and took over with the augmenting crew remaining on the flight to assist. Given that the aircraft now had only one working air conditioning system, a diversion to Calgary to enable medical attention to be given to the injured passengers was preferred to the nearer Vancouver for terrain clearance reasons and this was accomplished without further event. It was found that "most of the passengers who were physically injured were aware that they were required to wear their seat belts, but chose not to, with two others having been asleep without seat belts on and not heard the cabin crew announcement".

An analysis of the meteorological situation which had prevailed noted that the SIGMET forecast of severe turbulence covered an area where it was likely that mountain wave activity could be created by the jet stream. It was concluded that when the crew had decided to stay at FL330 to cross the notified turbulence area, the aircraft had happened to be in an area "between prominent mountain ranges where there was less mountain wave activity created by the jet stream". It was noted that the 787 which had been at FL 390 half an hour ahead on the same track had not encountered severe turbulence because it had been able to fly significantly higher than the tropopause, a material factor in turbulence avoidance.

It was found that training of Air Canada pilots and dispatchers in respect of CAT weather and jet streams made no specific reference to the conditions which were likely to be produced by jet streams which crossed over mountainous terrain although the likelihood of increased intensity of windshear and CAT in these circumstances is known.

The Investigation undertook a detailed review of the issues surrounding non use of seat belts by some of the passengers, noting that most of the passengers were Chinese nationals. It was found that use of car seat belts by drivers in two cities near to Shanghai was less than 50%, their use by front seat car passengers less than 10% and for back seat passengers even lower. These figures compared with a 95% rate for all Canadian car occupants and an 87% daytime rate for all front seat occupants in the United States. The alternative ways of influencing personal behaviour in seat belt use - conformity, compliance, obedience, persuasion and enforcement were reviewed and it was considered that a combination of these was the only way to increase use. It was noted that the FAA, Transport Canada and IATA had all issued recommendations for seat belt use in recent years.

Finally, maintenance personnel found that a clamp had failed and the air supply duct it was securing had been detached from the flow control valve thus signalling an air conditioning system failure. It was noted that on a properly fitted clamp, only about 0.5 inch of thread should be visible on the securing bolt whereas on the failed clamp, about twice this was exposed. It was found that the failed clamp had been removed and replaced during a change of the associated flow control valve 4 weeks earlier. It was also noted that as the result of a series of similar clamp failures in service, an improved clamp design had been introduced on new build aircraft in 2010 and a SB for retrofit made available in March 2011 but not taken up by Air Canada.

The formally-stated Findings as to Causes and Contributing Factors were as follows:

  1. Air Canada flight 088 entered an area of moderate to severe turbulence that was caused by the jet stream traversing the southern coastal mountains of Alaska.
  2. The acceleration forces encountered resulted in passengers who were not wearing seat belts contacting various furnishings and surfaces in the cabin causing a variety of injuries, including one serious injury.

The formally stated Findings as to Risk were as follows:

  1. If training material does not contain complete information pertaining to all of the factors that contribute to turbulence, then there is a risk that the best course of action will not be taken.
  2. If flight crews and dispatchers do not receive all pertinent information relating to flight conditions, optimal decisions on a course of action may not be made, increasing the risk of exposure to adverse conditions.
  3. If systems such as an aircraft communications addressing and report system are not utilized to their full capacity, then there is a risk that more detailed information about flight conditions will not be available.
  4. If cabin lighting is not sufficiently bright, then there is a risk that cabin crew members will miss unfastened seat belts due to the lack of contrast of the seat belt material and concealment by blankets.
  5. If there is no visual or audible indicator associated with not wearing a seat belt, then there is an increased risk of non-conformity with respect to seat belt use.
  6. If seat belt announcements do not contain sufficient detailed information on anticipated turbulence, then there is a risk that passengers will not immediately comply and maintain compliance with an instruction to fasten seat belts.
  7. If safety announcements made by cabin crew do not use language that conveys the expectation of compliance, there is a risk that passengers will perceive these announcements to be less authoritative, which may result in non-compliance.
  8. If passenger safety briefings lack information on the effects turbulence can have on individual passengers, their possessions, and on others, then there is a risk that it will reduce the probability of seat belt use.
  9. If passengers do not expect consequences and enforcement for non-seat belt use, passengers may not perceive the use of seat belts as mandatory, when so directed.
  10. If the approach to improve seat belt use does not include an understanding of the cultural and social norms of passengers, education to increase awareness, improved attitudes and an associated enforcement program to ensure corresponding changes in behaviour, then there is a risk that passengers will not wear their seat belts.
  11. If V-clamps are not installed using the proper procedure and torque, then there is a risk of V-clamp failure and subsequent partial loss of an air conditioning system and cabin pressurization.

One Other Finding was also identified:

  • The Air Canada flight 088 flight crew’s decision to secure the cabin and reduce to turbulence penetration speed contributed to preventing significant numbers of injuries in the cabin and potential damage to the aircraft.

Safety Action taken as a result of the occurrence by Air Canada was noted as having included the provision of improved guidance on CAT for pilots, amplified procedures in respect of forecasts of moderate or severe turbulence and the commencement of a programme to change all air conditioning supply duct couplings with the improved one already available but not previously implemented.

The Final Report of the Investigation was authorised for release on 4 January 2017 and it was officially released on 20 February 2017. No Safety Recommendations were made.

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