Term Paper: Radio Altimeter Effectiveness and Cfit

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[. . .] Ratan Khatwa, a senior flight-deck research engineer at Honeywell Aerospace Electronic Systems. (Gold, 2001) He said circumstances that contributed to high safety risk during approaches and/or landings were found in many events audited in a study aimed at reducing ALA's. The study found that over 50% of fuselage loss incident were caused by ALA's, and that over 50% of these accidents involved aircraft which did not have GPWS systems installed.

Noting human mistakes, outside influences and "undesired aircraft states" that contributed to unstable approaches, Khatwa said such "behavioral markers" were present in almost one-fifth of 4,000 non-accident flights by six operators covered in the study. The markers included:

Procedural errors and violations.

Aircraft mishandling.

Threats from weather and terrain.

ATC events.

Aircraft path or speed deviation.

Aircraft misconfiguration.

Non-precision approaches increase levels of risk "significantly," said Khatwa. The danger is even higher when pilots have a low exposure to such events. (Goold, 2001)

Assuming that accident characteristics can be observed in everyday conditions, the research went on to study correlations between ALAs and data from regular aircraft operations. In addition to the 4,000 normal flights, they considered almost 300 worldwide ALAs, as well as other incidents, using International Civil Aviation Organization statistics. Some 287 fatal ALAs from 1980 to 1996 represented an annual incidence of about 17 events. The researched projected that with increasing levels of air travel, ALA incidents were likely to rise to 23 a year. (Flight Safety Foundation, 2000) About 50% of all accidents are ALAs, and the most dominant circumstance was controlled flight into terrain (CFIT). Almost 75% of ALAs involved approaches to airports with no glide slope.

Advancements in the GPWS Systems.

First-generation GPWS technology looked straight down, using the airplane's radio altimeter to provide warning of threatening terrain. Against gently rising terrain, its warning served as effective advance warning of dangerous conditions. But this GPWS would remain silent in an airplane flying across level ground -- and straight toward a vertical cliff. Regarding the Crossair incident above, its GPWS was functional throughout the flight, but when the airplane is in the landing configuration, the GPWS will not warn of insufficient terrain clearance. During landing, the system expected the plane to be nearer to the terrain than normal flight conditions.

This is the 'Achilles heel' for GPWS," says Don Bateman, chief engineer for flight safety systems at Honeywell International Inc. Honeywell is one of the leading suppliers of terrain awareness warning systems (TAWS). (Evans, 2002)

The radio altimeter as currently designed as part of a GPWS system is only partially reliable. When a plane is in decent, the lift dynamics of the vehicle are dropping, leaving the plane a slave to the physics of gravity and inertia. Even with the warning of an impending rise in the terrain, if the warning is not received early enough, the pilot may not have enough time to respond by bringing in the flaps, revving the engines, and tilting the plane back into the air in time for avoid a collision. The first generation GPWS was limited to the effectiveness of the radio altimeter. This device looks down to the ground, and in a slight arc forward of the air craft, and via radio waved, determines the height of the aircraft in relation to the ground.

For this reason, the GPWS is being replaced by the EGPWS, an enhanced ground position warning system. Designed by Honeywell, the engineers of the first GPWS, the enhanced version is a combination of a traditional radio altimeter ground warning system, a computer-based digital terrain database, and GPS system.

The radio altimeter continues to scan the ground, and the immediate airspace ahead of the plane. The combination of these devices allows the EGPWS to look farther forward than the radio altimeter can project. The EGPWS helps keep pilots aware of the aircraft's position relative to terrain. An EGPWS works as follows: It uses data from the GPS and other navigational aids and the air data sensors to determine the aircraft position both longitudinally and vertically (lat/long). Then it adds that information to data from the terrain/runway database to form a display showing the terrain elevations around the aircraft.

Early GPWS devices simply viewed the terrain below the aircraft, providing a 30-second (at best) alert prior to possible impact. But EGPWS look-ahead algorithms provide data to predict possible incursions with terrain at up to two minutes in advance. The EGPWS provides both horizontal and vertical look-ahead. With the horizontal look-ahead, the airplane can "see" at least a quarter mile on each side of the aircraft. So, if the aircraft enters into a bank turn, the EGPWS can "anticipate" the turn and warn against possible CFIT. This is in addition to the advisory callout heard when the bank angle is too steep -- a feature tailored to either air transport aircraft or business jets.

The crewmen also receive from the EGPWS an aural alert and a visual warning from a multicolor image. The green color on the image indicates terrain safely below the aircraft. Yellow represents cautionary alert 60-seconds prior to the predicted time of impact and is accompanied by a "caution terrain" aural message. And red indicates terrain that the aircraft could impact within 30 seconds; it is accompanied by an aural "terrain, terrain, pull up." Older Ground Proximity Warning Systems (GPWS), which EGPWS units replace, provide an average of about 10-15 seconds advance warning of a terrain encounter, but sometimes provide shorter warnings or no warning.

Depending on the phase of flight, EGPWS operates in seven different modes:

Mode 1 warns the crew of excessive descent rates, for example due to a navigation error during descent to approach. Also, the system will give a warning if a sink rate of 1000 ft/min is exceeded on final.

Mode 2 works as a predictive mode based on the terrain data base as described above.

Mode 3 sets off if the aircraft goes into an inadvertent descent after take-off or missed approach. The mode is automatically deactivated once the aircraft has reached a safe altitude.

Mode 4 warns if the aircraft violates a minimum terrain clearance during climb out, cruise or descent and approach. This mode is to prevent modes 1 and 2 not being able to give a timely warning because the aircraft is already flying close to the terrain.

Mode 5 is supposed to warn from excessive deviations from glide path during an ILS approach. The mode switches on when the crew selects an ILS frequency and puts the gear down.

Mode 6 is warning of excessive bank angles. Here the sensitivity goes up with decreasing altitude. (warning at 40 degrees of bank at 150 feet altitude and already at 10 degrees of bank at 30 feet altitude).

In mode 7, EGPWS is working as a windshear alerting system. (Adapted from Hess, 1999)

Honeywell claims that more than 100 airlines operate with EGPWS and that some 5,000 aircraft have flown more than 30 million hours with the well-proven system on board. That's a lot of aircraft, but it just scratches the surface of EGPWS's potential. Honeywell is looking at a general aviation market of more than 100,000 aircraft, representing 520 different types that range from single-engine piston aircraft to twin turboprops. (Jensen, 2000)

Minimum Safe Altitude Warning System (MSWA)

Installed on the ground, in 1977 the FAA determined that MSWA software and warning systems would be installed at the control towers in order to monitor air traffic expected altitudes. The system monitored all aircraft within a specific radius of the airport. The focus of the alert system is a 'control box' which scans the final approach path of air craft coming in for final landing. In the event of a plane traveling too low, the alarms in the control tower sound which can then be transmitted to the air craft. The MSWA is based on a local terrain database, like the EGPWS.

Methods and procedure for the study

The procedure for this study will include identifying 3 target groups for data collection. The first group will be companied which aircraft with EGPWS systems have installed. The second group will be companies which have aircraft with GPWS installed. The third group will be companies which have aircraft with neither installed. The procedure will be to review company records to evaluate safety records. The recorded incidents of near misses of ALA and CFIT incidents will be recorded and compared.

The data collected will evaluate incident reports between companies which had similar weather conditions, and involved similar size aircraft. The purpose of the study will be to measure quantitatively the number of incidents and near miss incidents. The second purpose of the study will be to evaluate the qualitative nature of the incidents.

Questions to consider will include:

In what ways did the warning systems add to… [END OF PREVIEW]

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