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• there had been no changes in the nature of the transmissions in recent years.
The Harold E. Holt station has been in operation since 1967. VLF transmitters are
also located in other countries including the USA, UK, China, France, India, Japan
and Russia.
Estimated field strengths as a result of transmissions from the station, at the three
locations where the ADIRU-related events occurred, are provided in Table 6. Those
field strength values were significantly below the levels at which the ADIRU design
was tested during certification.
- 38 -
Table 6: Estimated field strengths as a result of transmissions from the
Harold E. Hold Naval Communication Station
Event Aircraft Approximate
distance to station
(km)
Approximate
electromagnetic
field strength
(V/m)
12 Sep 2006 VH-QPA 950 0.011
7 Oct 2008 VH-QPA 170 0.059
27 Dec 2008 VH-QPG 700 0.014
As noted in ADIRU testing, the test plan for the ADIRU 1 from QPA included
testing the unit at the frequency used by the VLF transmitter. No problems were
found with the performance of ADIRU 1. This testing only involved conducted
susceptibility.34
High Frequency (HF) radio communications site
A high frequency (HF) radio communications site is also located on North West
Cape near Learmonth. The site can transmit signals in the HF frequency band (3 to
30 MHz) at a signal power of 10 kilowatts or less. Records indicate that the site was
transmitting at the time of the 12 September 2006 and 27 December 2008 events. It
was not transmitting at the time of the 7 October 2008 event.
Cabin safety
Passenger seating disposition
There were 297 passenger seats on the aircraft: 30 located in business class (rows 1
to 5, between doors 1 and 2), 148 in the centre of the aircraft (rows 23 to 41,
between doors 2 and 3), and 119 in the rear of the aircraft (rows 45 to 60, between
doors 3 and 4).
The 303 passengers included three infants who were seated with a parent for the
takeoff. Three of the passengers on staff travel arrangements were located on nonpassenger
seats for takeoff; one in the fourth occupant seat on the flight deck and
two in the cabin crew rest area (four seats located in rows 40 and 41). During the
flight, the two passengers seated in the cabin crew rest area moved to the cabin
crew jump seats located at the front of the aircraft.
At the time of the in-flight upsets, the meal service had been completed and the
service carts were secured in the galleys. Many passengers reported that they had
recently returned from or were in the process of going to the toilets.
34 Low-frequency radiated susceptibility tests require very specialised transmitting equipment that
was not available to the investigation. In practice, low frequency susceptibility is usually
evaluated using conducted emissions as that method is both practical and representative of actual
low-frequency electromagnetic coupling.
- 39 -
Passenger questionnaire
A passenger questionnaire was developed to obtain information from passengers
about their experience and observations during the upset events. It also included
questions on safety information, use of seatbelts, injuries and use of personal
electronic devices. The questionnaire could be completed electronically or on a hard
copy form, or by interview if requested by the passenger.
Distribution of the questionnaire commenced on 28 October 2008. It was not able to
be sent to all passengers as contact details were incomplete.
As of 26 January 2009, 95 surveys had been received. Those surveys also included
details for six children. In addition to the surveys, the ATSB obtained some
information by interview or email from 29 other adult passengers, which included
information for 11 other children. In the survey responses and additional
information received, some passengers provided information about other
passengers.
Multiple attempts were made to contact passengers who were seriously injured or
attended hospital and did not respond to the survey. However, information from a
small number of those passengers was not able to be obtained.
Passenger description of first in-flight upset
Passengers reported that they noticed nothing unusual about the flight prior to the
upset. Some passengers and cabin crew reported that, a few minutes prior to the
upset, they noticed a reduction in thrust, whereas some other passengers described a
slight change in the aircraft’s flight similar to the commencement of the descent for
landing. Both of those observations were consistent with the aircraft’s descent from
37,200 ft back down to 37,000 ft following the autopilot disconnect.
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