Aeroplane Equipment and Instruments — Page 86, Lesson 136

I want to walk you through the equipment and instruments that are required on commercial aeroplanes to prevent one of the most dangerous threats in aviation: controlled flight into terrain, or CFIT. Let's start with the Ground Proximity Warning System, which is abbreviated GPWS. Too many aircraft have been lost, and crew and passengers killed, because of inadvertent controlled flight into terrain. The ATC Authority will not accept responsibility for terrain clearance, except during radar vectoring. The responsibility rests firmly with the pilots to ensure safe clearance from the ground is maintained. Misreading of altimeters, misunderstanding procedures — like the Kuala Lumpur 747 crash — poor navigation, and misidentification by ATC radar units all conspire to make CFIT a continuing danger and hazard. A GPWS fitted to an aeroplane can give warning to the pilots that the aeroplane is getting too close to the ground. That's its basic job. But there's an enhanced version called an Enhanced GPWS, or EGPWS, which is also known as a Terrain Awareness Warning System, abbreviated TAWS. The EGPWS or TAWS includes a predictive terrain hazard warning function. That means it doesn't just react to how close you already are — it looks ahead and warns you before you get into trouble. Now, which aeroplanes must have this system fitted? The requirement applies to all turbine powered CAT aeroplanes — that's commercial air transport — with a maximum take-off mass, or MTOM, greater than 5700 kilograms, or an approved passenger seating configuration of more than 9 seats. What does the GPWS actually warn you about? It must provide automatic warnings by aural and visual indications for the following circumstances: sink rate — that's descending too fast toward the ground; ground proximity — simply getting too close to the terrain; altitude loss after take-off or go-around — so if you take off or go around and then start losing height instead of climbing; incorrect landing configuration — for example, if you try to land with the gear or flaps not set properly; and downward glide slope deviation — that means you're going below the proper approach path. The terrain awareness and warning system, the TAWS, must automatically provide the flight crew, by means of visual and aural signals and a terrain awareness display, with sufficient alerting time to prevent CFIT events. It must also provide a forward-looking capability and a terrain clearance floor. The terrain clearance floor is a minimum altitude that the system uses as a hard lower boundary — if you're about to go below it, you get a warning. Let's move on to ACAS. That stands for Airborne Collision Avoidance System. All turbine powered aeroplanes with an MTOM greater than 5700 kilograms or having more than 19 passenger seats must be equipped with ACAS II. Pilots are to ensure that ACAS Resolution Advisories, or RAs, are smoothly and immediately responded to, even if this conflicts with ATC instructions. That's a critical point: if ACAS tells you to climb or descend to avoid another aircraft, you follow the RA immediately, even if ATC is telling you something different. Now, communications equipment. An aeroplane used for CAT must be fitted with radio communication equipment capable of conducting two-way communication with ATC for aerodrome control purposes and receiving meteorological information at any time during flight. Provision must be made for unserviceabilities — meaning if one radio fails, you still have a backup — so at least two transceivers must be carried. Additionally, the equipment must be capable of conducting two-way communications on the aeronautical emergency frequency, which is 121.5 megahertz. Practically, this means you need two VHF transceivers for domestic airspace, and at least one VHF and one HF for oceanic flight. HF, or high frequency, is needed over the ocean because VHF doesn't have the range. Finally, internal communications. Aeroplanes with more than 19 passenger seats must be fitted with a Public Address system, or PA, and for aircraft registered after 1965, a crew interphone. The requirement for an interphone also applies to aircraft with 19 passenger seats or less, if its MTOM is greater than 15,000 kilograms. The PA is used to talk to passengers. The interphone is for the flight deck to communicate with cabin crew. In addition, if there are more than one flight crew members, they must have a flight crew interphone, utilizing headsets and boom microphones. That ensures clear, hands-free communication between the pilots without ambient cockpit noise interfering.