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between Cape Verde and Brazil, indicated by Air France and confirmed by the
BEA at 11 h 07.
At 12 h 14, the Dassault Atlantique 2 took off bound for Cape Verde(16).
Around 13 h 00, the crew of the Dassault Atlantique 2, en route to position at
Cape Verde, received the instruction to proceed towards TASIL descending
UN 873 airway.
The Dassault Atlantique 2 arrived over the search area at 15 h 28. It landed
back in Dakar at 22 h 20 without having detected any debris.
The low altitude searches were coordinated by the Recife MRCC, the airplane
having disappeared in its zone of SAR responsibility.
1.16 Tests and Research
1.16.1 Sea Searches
1.16.1.1 Context of the searches
The estimated area of the accident is over the Atlantic Dorsal Chain. The
search is proceeding there in an unfavourable environment due to the depth
and to the topography of the seabed. This seabed is little known and presents,
over short distances, depths going from 900 metres to approximately
4,600 metres. The distance from dry land implies a lack of radar coverage and
radio communication difficulties.
The search area was initially defined based on the airplane’s route and the last
position contained in the ACARS messages. This makes an area with a radius of
40 NM, extending over more than 17,000 km2 and located more than 500 NM
from the coasts.
The searches on the surface made it possible to locate bodies and airplane
parts from 6 June onwards. The position of the floating elements allowed a
search zone to be defined based on the work on the currents and the winds.
The following figure shows the underwater search area.
(16)Take-off was
delayed for
approximately
thirty minutes
due to technical
problems.
F-GZCP - 1st June 2009
46
Size of the search area
1.16.1.2 Principle of the underwater searches
As the aircraft’s recorders were each equipped with an underwater locator
beacon, it was best to prioritise an acoustic search initially, nevertheless taking
into account the limited range of the beacons, which is about two kilometres at
most. The propagation of acoustic waves in a liquid medium, which depends on
many interdependent parameters such as the salinity and the temperature of the
water, must also be taken into account. When an acoustic wave is propagated
in the sea, it is subjected to refractions and this generates multiple trajectories.
The acoustic waves may also be deflected in such a way that there is a «shadow»
region which is never reached by these waves.
Acoustic searches using beacons which transmit at 37.5 KHz (± 1 KHz) are in
general more effective than searches using sonar, magnetometers and video
cameras. Nevertheless, the duration of the beacon transmission is limited, being
certified for a minimum transmission duration of thirty days from immersion.
Taking into account the range of the beacons, the hydrophones must be
brought closer to the source of transmission, by towing specialized equipment
near the seabed.
The underwater search devices that are used after localisation of the wreckage
must also take into account the depth and uncertainty of the area. In the
case of the accident to flight AF447, provision has been made for specialized
devices able to descend to a depth of six thousand metres.
Last known position
F-GZCP - 1st June 2009
47
1.16.1.3 Resources deployed by France
By the Ministry of Defence
The French Navy deployed the frigate Ventôse and the Mistral BPC (projection
and command ship), which have been taking part in the search and recovery
operations for the bodies and floating debris. They have been assisted by their
on-board helicopters and Naval Aviation and Air Force planes.
The Emeraude (hunter killer nuclear submarine) was sent to the area to
complete the acoustic search system.
Ventôse frigate Submarine Emeraude Mistral
By the BEA
In relation to towed acoustic devices, the BEA approached the US Navy.
The latter has two towed pinger locator (TPL) hydrophones and uses them
regularly to search for civil or military aircraft crashed at sea.
The US Navy TPLs can operate at up to a depth of six thousand metres. They
operate on a waveband between 5 and 60 KHz which includes the frequency
transmitted by the underwater locator beacons. The average detection range
of the TPLs is estimated at two kilometres at least.
To optimize the use of this equipment, the BEA chartered two available ships
from the Dutch subsidiary of Louis-Dreyfus Armateurs. These two tugs were
the "Fairmount Expedition" and the "Fairmount Glacier".
The BEA also chartered the oceanographic ship «Pourquoi Pas ?» from IFREMER
　
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