Aviation Investigation Report A98H0003
1.6.9 Communications Systems
- 126.96.36.199 - General
- 188.8.131.52 - Interphone Call System
- 184.108.40.206 - Aircraft Communications Addressing and Reporting System
220.127.116.11 General (STI1-10)
For external communications, Swissair MD-11 aircraft are equipped with five separate radios, plus an emergency hand-held very high-frequency (VHF) radio stored in a bracket mounted on the cockpit rear wall. The five radios comprise three VHF radios and two high-frequency (HF) radios, all of which are controlled through communication radio panels installed in the aft pedestal between the two pilots seats.
Internal voice communication between the pilots is either spoken directly or through boom microphones attached to headsets. Each flight crew oxygen mask has a built-in microphone that is activated with a push-to-talk rocker switch. One position of the rocker switch is used for internal communication, and the other position is used for transmitting over the external VHF and HF radios. Additional internal communication is provided through a flight interphone system that connects all cabin attendant stations and the cockpit, and a passenger address (PA) system that enables the pilots and cabin crew to address passengers throughout the cabin and in the lavatories.
The ambient noise in the MD-11 during high-altitude cruise flight is low enough so that pilots typically do not need to use the headsets and boom microphones for internal communications. Swissair policy requires flight crews to use this equipment for flight below 15 000 feet. There are regulatory requirements in some jurisdictions that mandate the use of this equipment below certain altitudes. For example, US FAR part 121.359 (g) mandate their use below 18 000 feet for aircraft equipped to record the uninterrupted audio signal received by a boom or mask microphone in accordance with FAR part 25.1457 (c)(5). Canadian Aviation Regulations (CARs) (CAR 625.33 II (5) refers) require their use below 10 000 feet.
18.104.22.168 Interphone Call System (STI1-11)
The aircraft was equipped with an interphone call system to facilitate aircraft crew communication. In Swissair MD-11s, handsets, call buttons, and reset switches are installed at nine stations throughout the aircraft: one in the cockpit and one at each flight attendant station. Calls can be initiated from any flight attendant station to the cockpit; from the cockpit to any, or all, flight attendant stations; and from any flight attendant station to any, or all other, flight attendant stations.
The interphone call system provides both aural and visual signals to alert crew members to a station call. A visual alert is provided by the illumination of indicating lights in the reset switches. In the passenger cabin there is an additional visual alert through the use of pink call lights. At the associated area master call display, these lights would illuminate to indicate the initiation of a "pilot-to-flight-attendant" or "flight-attendant-to-flight-attendant" call. When the call button is pushed, two electro-mechanical chimes, one above the left and one above the right attendant station emit a single-stroke chime.
All cabin interphone conversations are recorded on a single cockpit voice recorder (CVR) channel. The CVR recording does not indicate which station is being used.
The occurrence aircraft was equipped with an aircraft communications addressing and reporting system (ACARS), which is a two-way digital communications link between the aircraft and the operator's flight operations centres. Typically, when the aircraft is within VHF radio range of a ground station the ACARS uses the aircraft's VHF 3 radio to communicate through a network system. (STI1-12)
The ACARS switches automatically to communicate through a satellite communications (SATCOM) system (STI1-13) when the aircraft is out of range of VHF ground stations, VHF coverage is interrupted through saturation of the system, or the VHF 3 radio in the aircraft is switched to voice mode. When VHF coverage is available, VHF is the primary path for data exchange. The SATCOM system also provides satellite telephone service available to all aircraft occupants.
The ACARS provides a means to automatically report flight information, such as engine parameters and load data, and to track aircraft movements, such as take-off and landing times. The pilots can also use the ACARS to obtain information, such as weather reports, and to exchange free-text messages.
Swissair's main service provider for the ACARS was Société Internationale de Télécommunications Aéronautiques (SITA). All communications to and from the aircraft through SITA were routed through the SITA Swissair host in Zurich. Where SITA was not able to maintain coverage, they subcontracted to Aeronautical Radio Inc. (ARINC), which is the main service provider in the USA, and to the International Maritime Satellite Organization (INMARSAT) for satellite coverage.
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