CHAPTER 12. Safety Systems

Transcription

1 ELEC4504 Avionics Systems 147 CHAPTER 12. Safety Systems General Safety systems to be discussed are: a) TCAS (Traffic alert and Collision Avoidance System) b) GPWS (Ground Proximity Warning System) c) EGPWS (Extended GPWS) d) CVR (Cockpit Voice Recorder) e) FDR (Flight Data Recorder) f) ELT (Emergency Locator Transmitter) TCAS Many years ago there was an midair collision between two aircraft (DC6 and a Lockheed Constellation) over the Grand Canyon. There was a public outcry demanding that some means of preventing midair collisions should be developed. Unfortunately, the technology required for such a system has not been available until quite recently. The main problem with most of the systems proposed was not that they did not detect other aircraft in the vicinity but that they could not distinguish between real threats and normal, harmless traffic. TCAS works in conjunction with the Air Traffic Control transponder (secondary radar transponder) and is, in fact a miniature radar. The TCAS antenna is directional and sweeps 360 around the aircraft. The system emits pulses at the 1030MHz frequency and, if there are any transponder-equipped aircraft in the area they reply to the interrogation at 1090MHz. The TCAS then can determine the range, range-rate and relative bearing of the target and can determine if it is a threat. The altitude of the target is determined from the altimeter encoding from the transponder. TCAS performance is divided into two categories: I and II. i) TCAS I systems provide warnings only and alert the pilot to a potential risk. The pilot then has to resort to see and be seen procedures to avoid a collision. ii) TCAS II system provide a warning and also a conflict resolution i.e. the system will tell the pilot to climb or descend to avoid collision. TCAS I is required for all aircraft with between 10 and 30 seats operating in the United States. TCAS II is required for all aircraft with more than 30 passenger seats operating in the United States Although the number of false alarms has been decreased there is some concern about the operation of the system in congested areas especially since the transponders could become overloaded. Air traffic controllers are concerned that evasive maneuvers may introduce new threats.

2 148 ELEC4504 Avionics Systems Aside from these problems there is always the drawback that the system will not detect an aircraft that does not have a functioning transponder on board GPWS (Ground Proximity Warning System) GPWS was developed to reduce the number of CFIT (Controlled Flight Into Terrain) accidents. GPWS works with the radar altimeter to warn the pilot that the height of the aircraft above terrain is less than a preset value (about 500 ft.) If the aircraft s ground clearance is less than the preset value, and the aircraft is not in landing configuration, an aural warning too low, terrain is given together with a warning siren. If the aircraft terrain clearance decreases further the GPWS announces too low, pull up! The main problem with GPWS is that it depends on the current terrain clearance only and, as has happened on several occasions, can provide a warning too late for the crew to be able to take effective evasive action EGPWS (Allied Signal, Honeywell) Extended GPWS seeks to address the prediction problem. It uses a global terrain elevation model to determine the elevation of terrain within the predicted one minute track of the aircraft. If this terrain is less than 800 ft. below the aircraft an aural warning caution terrain is sounded. If the terrain is les is less than 400 ft. below the aircraft an aural warning terrain ahead, pull up! is sounded. Because of the size of the data base high resolution (30 arc seconds of lat and long or about 3000 ft.) blocks are used only in the immediate vicinity of airfields (16 mile radius). Between 16 and 30 miles 1 minute blocks are used and between 30 and 60 miles 2 minute blocks are used. Elsewhere the block size is 4 minutes. In the vicinity of airports in particularly rugged terrain the blocks are 15 seconds square. In addition to providing warnings, the system can also use the radar display to show dangerously high terrain in the immediate area that is not on the predicted flight path. Price for this system is about US$60,000

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4 150 ELEC4504 Avionics Systems CVR (Cockpit Voice Recorder) The CVR is primarily an aid to accident investigation. It records the cockpit sounds (mainly conversations) at several places in the cockpit. A continuous medium (tape or solid state) is used and thus only the last 30 minutes or so of sound is recorded, since the older data is constantly being overwritten. Because of the function of CVRs, much effort is made in making the recording medium shock and fire proof. These units also include an underwater sound generator to assist in retrieval in the event that the accident occurs in water. Figure 63: Flight Data Recorder FDR (Flight Data Recorder) The flight data recorder is similar to the CVR except that it records data from many sensors throughout the aircraft. Data recorded are shown below: Time Vertical acceleration Altitude Pitch attitude Airspeed Roll attitude Heading Time for each radio transmission Thrust (each engine) Trailing edge flap position

5 ELEC4504 Avionics Systems 151 Leading edge slat position Thrust reverser position Speedbrake position Marker beacon passage (on approach slope) Autopilot engagement Longitudinal acceleration Control column position Lateral acceleration Control wheel position Pitch trim position Glideslope deviation (above/below Autoflight control system mode approach) Localizer deviation (left/right of Radio altitude approach) Master warning Main gear squat switch status Angle of attack Outside air temperature Hydraulic system low pressure Groundspeed This data is recorded over a 25 hour period ELT (Emergency Locator Transmitter), The emergency locator transmitter is a small beacon usually mounted in tail of the aircraft. An acceleration - sensitive switch turns the transmitter on in the event of a crash. The transmission frequency is MHz and/or MHz

6 152 ELEC4504 Avionics Systems Early ELTs (TSO C91) had a low activation rate (25%) in actual crashes and a high false alarm rate (97%) Current ELTs (TSO C91a) have an improved rate (73%) The latest ELTs (TSO C126) operates on 406 MHz and have the capability of trasnmitting digital data such as the identity of the aircraft. These are more the twice as expensive as the C91 versions and thus owners (especially private aircraft owners) are reluctant to change over Cospas/SARSAT A Satellite system called Cospas/ SARSAT, developed by Russia, Canada, the USA and France,is designed to receive emergency transmissions and to locate the transmitter using doppler shift techniques. The satellites then trasnsmit the position of the crash to Search and Rescue facilities nearest to the accident. The system uses 4 geosynchronous (GEOSAR) and 7 low earth orbit (LEOSAR) satellites. As of 1 February of 2009, Cospas/SARSAT ceased to monitor 121.5/243 MHz signals primarily due to the many false alarms. It now monitors only 406MHz which might be an incentive for operators to install this ELT if they plan to fly in remote regions NOTE: The aeronautical communications system (and most aircraft) continue to monitor the distress frequencies but do not have the direction-finding capability to pinpoint the accident site.