Chapter 7 - Section 1: Introduction

Question 1

How are messages downlinked from the aircraft?

Answer 1

• VHF
• HF
• SatCom radio
• Land lines
• Central ARINC control stations

Question 2

What does ACARS system contain and what OOOI information can it report?

Answer 2

GMT clock

It can report:-

• Out-of-gate
• Off the ground
• On the ground
• In the gate

Question 3

What other information can ACARS provide?

Answer 3

• Flight and air identification
• Delays
• Fuel
• Weather
• Engine performance data

Question 4

What are the ‘other’ benefits of ACARS?

Answer 4

• Ground monitoring capability of aircraft engines and other parameters
• Efficient exchange of information concerning arrival and connecting flights
• Reduction of multiple frequency changes
• More reliable aircraft selective calling system

Question 5

What are the usual communications systems utilised for ACARS operation?

Answer 5

VHF 3 and HF 2

Question 6

What is the maximum message block length?

Answer 6

220 characters of text

Question 7

What is the eighth bit added to each character?

Answer 7

A parity bit

Question 8

What two modes does the ACARS system operate in?

Answer 8

• Polled mode
• Demand mode

Question 9

What does demand mode allow?

Answer 9

Airborne station or the ground processor to initiate communications

Question 10

How does the MU ensure garbled messages are sent without becoming garbled again?

Answer 10

Subsequent retransmissions are transmitted at random intervals, preventing synchronous retransmission

Question 11

What are the steps after a ground station receives a transmission from the airborne station?

Answer 11

• Processor performs a block check sequence (BCS) error check
• If free from errors, message is routed to its proper destination
• Acknowledgement signal (ACK) is transmitted to airborne station to clear message and return to normal operations
• If no ACK is received, airborne station will retransmit original message
• After six failed attempts, the aircrew are alerted

Question 12

How will uplink and downlink messages differ with regards to the error checking process?

Answer 12

Negative acknowledgement (NAK) is sent for error in uplink messages

Question 13

When will an airborne system in polled mode automatically enter the demand mode?

Answer 13

When the ON event occurs (aircraft on ground)

Question 14

Where is SatCom not generally used?

Answer 14

Polar regions due to lack of satellite signal