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call sign of “SilkAir one eight five” was often used in these transmission.
The test results indicated that the voice print characteristics of the CVR radio
transmission matched those derived from the ATC transmission. The most notable
similarity is when the F/O said the call sign “SilkAir”.
The comparison showed that the last transmission was carried out by the F/O.
1.16.4 Trajectory Studies
As some empennage parts were found away from the main impact point, trajectory
studies were carried out by BASI [Reference 12] and by NTSB [Reference 13]. The
studies were to determine the altitude at which the parts separated from the aircraft.
Both studies, see Figure 14, were based on the theory that the trajectory of a separated
object was determined by its initial condition at the time of separation, such as mass,
speed, heading, altitude and its aerodynamic characteristics as well as the wind direction.
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The BASI study showed that the rudder balance weight (which had the highest ballistic
coefficient – heavy with small frontal area) did not separate while the aircraft was
cruising at FL350 but at a lower altitude.
The wreckage distribution and the relative positions of the individual parts to one another
supported the finding from this study that the separation of the remaining components
took place at a lower altitude near or below 12,000 feet.
1.16.5 Flutter Studies
The NTSB-Boeing Report [Reference 14] showed the following:
• During cruise at 35,000 feet at 0.74 Mach
The applied static loads and aerodynamic flutter margins are well within the aircraft
certified design requirements. The loads at this flight phase did not support the
separation of the empennage structure.
• During descent from 35,000 feet to 20,000 feet
The applied static loads and aerodynamic flutter margins developed above 20,000 feet
are less than the aircraft design requirements and do not support the separation of the
empennage structure.
• During descent from 20,000 feet to ground impact
The applied static loads are less than the design ultimate load envelope and do not
support separation of the empennage during the final descent below 20,000 feet.
The estimated descent speed exceeded the 1.2 dive speed Vd analytical flutter
clearance speed at approximately18,000 feet altitude. An onset of an empennage 22
Hz anti-symmetrical flutter mode was calculated to occur later in the descent at
approximately 5,000 feet altitude (570 equivalent airspeed in knots-KEAS). A lower
frequency 12 Hz anti-symmetrical flutter mode was calculated to occur at
approximately 3,000 feet altitude (600 KEAS), see Figure 15.
1.16.6 Flight Simulation Tests
The simulation tests were performed to explore and understand the various combination
of one or more malfunctions of flight controls, aircraft systems and power plants that
could result in an extreme descent flight trajectory as suggested by the radar data points
from the accident.
Two different simulators were used to conduct these simulation tests. The first is a
Boeing “M-Cab” simulator, which is a full motion, multi-purpose, engineering flight
simulator used to simulate various types of Boeing aircraft. In the case of B737-300, the
nonlinear mathematical software has been validated up to Mach 0.87, and extrapolated
using computational data up to Mach 0.99.
The second simulator was a Garuda Indonesia full motion airline training simulator that
replicates the B737-300 aircraft and the software is validated up to the flight operations
envelope.
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The Boeing B737-300 aircraft has been designed to have a maximum operating cruise
speed Mmo of 0.83 Mach, and maximum dive speed Mmd of 0.89 Mach.
The results of these tests are tabulated in Appendix G.
1.17 Organizational and management information
• SilkAir (Singapore) Pte Ltd is a subsidiary of Singapore Airlines Ltd (SIA). It
commenced operation in 1989 under the name Tradewinds. In April 1992 Tradewinds
was renamed SilkAir, [Reference 15].
• At the time of the accident, SilkAir operated about 100 scheduled flights to about 20
business and holiday destinations in South-East Asia. SilkAir’s routes were generally
short haul regional routes, and aircraft and crew usually do not stay overnight outside
Singapore. The company operated six Boeing B737 and two Fokker F70 aircraft. In
mid-1997, SilkAir placed order for 5 new Airbus A320 and 3 new A319 aircraft to
replace the B737s and F70s.
• SilkAir is controlled by a Board of Directors. The Board sanctions major decisions
such as choice of aircraft, annual budgets and major expenditures. The General
Manager oversees the functional operations of the four divisions within SilkAir. The
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(17)
