Enhanced Ground Proximity Warning System (EGWPS)
This paper will covers how an aircraft technology known as Controlled Flight Into Terrain (CFIT) relies on Terrain Awareness and Warning System (TAWS)/ Ground Proximity Warning System (GPWS) equipment to provide pilot information about the terrain. Furthermore, the Enhanced Ground Proximity Warning System provides additional information such as terrain information around the aircraft. With such equipment, there has been a significant reduction of CFIT accident worldwide. Lastly, this paper will also review the regulatory standards implemented by the relevant authority and investigate how it affects the present EGPWS system.
Keywords: Enhanced Ground Proximity Warning System, Terrain Awareness and Warning System, Ground Proximity Warning System.
There are three systems to be discussed, namely the Ground Proximity Warning System (GPWS), Terrain Avoidance and Warning System (TAWS), and Enhanced Ground Proximity Warning System (EGPWS). These systems prevent and reduce Controlled Flight into Terrain (CFIT) whereby flight crew are unable to sense the maneuvers on the aircraft towards potential hazard in a terrain.
CFIT accidents usually result in catastrophic accidents, causing large number of fatalities to passengers and flight crew. Common CFIT accidents include collision with terrain, water, or obstacle without indication of loss of control of the aircraft (IATA, 2014). In such catastrophic event, there are usually multiple contributing factors, comprising of both human errors and non-compliance with the Standard Operating Procedure (SOP). According to ICAO, after the implementing of the GPWS in 1974, there were significant reduction in CFIT accident rate (Gurney, n.d.).
Terrain Awareness and Warning System
In the early 1970’s before the GPWS was implemented, there were a number of serious CFIT-related accidents. The authority has done some studies on the pervious CFIT accidents and concluded that such accident can be avoided if a GPWS device was implemented (FAA, 2000). Specifically, it is evident that the GPWS device can reduce accident rate and avoid collision.
TWAS is a system meant to forecast potential danger in the aircraft’s path and terrain. There are warnings systems and alerts to provide caution to the flight crew of potential danger, thereby allowing them to have sufficient time to make the necessary changes to the flight path to avoid collision (UniversalCorporations, 2018).
System Safety Concepts
With exponential improvement made to aircraft technologies over the past decades. The safety features on the aircraft have to be at least on par with the likes of human mind in order to ensure that all accidents can be mitigated. In the early stages, attempts to mitigate future accidents are through post-accident investigation whereby the cause of accident is being investigated and a countering technology will be installed to avoid similar accidents from happening in future
After years of accidents, mankind has slowly progressed to implement safety system by considering all aspects of aircraft design, technologies, flight crew and terrain environment. In such system, it provides warnings and identifies potential risk so that precautionary measures can be made to avoid an accident. Terrain awareness and warning system has the GPWS implemented in the flight which gives information of potential risk when flight crew faces difficulties in having a clear visual of the terrain. Early warning is normally communicated by either warning lights or through audio. Leading factors that contribute to CFIT accidents are disorientation of the aircraft, poor visual of the environment, failure of system, close proximity to a danger area and communication breakdown. GPWS provides information to operators such as pilot or flight crew about potential hazards with warnings so that they can execute the necessary actions.
The TAWS is meant to improve the current GPWS systems by adding features so that the pilots have sufficient time to react to any impeding danger. It provides visual warning of terrain, possesses predicting capability and is normally operation during landing configuration (FAA, 2002).
Class A TAWS Equipment is under TSO-C151a, where there will be three principal alerting functions that requires a display and applies for all operators under Part 135 of turbine-powered United States registered aircraft with 10 or more passenger seats. The system has to at least provide alerts for the following (FAA, 2000):
1. Reduced required terrain clearance,
2. Imminent terrain impact,
3. Premature descent,
4. Excessive rates of decent,
5. Excessive closure rate to terrain,
6. Negative climb rate or altitude loss after take-off,
7. Flight into terrain when not in landing configuration,
8. Excessive downward deviation from an ILS glideslope,
9. Descent of the airplane to 500 feet above the terrain or nearest runway elevation (voice callout:” Five Hundred”) during a non-precision approach.
Class B TAWS equipment is under TSO-C151a, which will provide three principal alerting functions, does not require a display, and requires an independent approved GPS or an interface for horizontal position information of the aircraft. The system has to at least provide alerts as a minimum for the following (FAA, 2000):
1. Reduced required terrain clearance,
2. Imminent terrain impact
3. Premature descent
4. Excessive rates of descent
5. Negative climb rate or altitude loss after take-off
6. Descent of the airplane to 500 feet above the terrain or nearest runway elevation (voice callout:” Five Hundred”) during a non-precision approach.
Class C TAWS equipment is under TSO-C151b, which was introduced in 2002 and meant for small GA airplanes that does not required Class B TAWS equipment to be installed. In Class C TAWS equipment, it follows all Class B requirements with the small aircraft modifications (FAA, 2002). There is minimum performance standard for such modifications which include with Altitude Accuracy, Reduced Required Terrain Clearance (RTC), Voice Callouts, Cruise Descent Requirements, Cruise Level Flight Requirement, Final Approach Descent Requirements, and Landing Flight Requirement (FAA, 2002).
Ground Proximity Warning System
A Ground Proximity Warning System functionality is to provide alerts to the flight crew if the aircraft is in danger of colliding to the ground or an obstacle.
GPWS Alert Modes
In the GPWS alert mode, there will be five modes with an additional operating mode calls out. This alert mode is defined as follows (Benningfield, 2013):
Mode 1. When an aircraft is an excessive descent rate either during a landing or within close proximity to the ground, a warning will be repeatedly sound off “Sink rate”. If the issue is not rectified by the pilot, there will be a second repeated alert “Pull up.”
Mode 2. When an aircraft is flying towards a mountain with a gradient, flying across a steep gradient, or if the altitude is rapidly decreasing. This mode will sound off “Terrain” repeated followed by “Pull up.”.
Mode 3. After an aircraft takeoff where it requires to maintain a positive climb or if there is a descent in this climb after the aircraft reaches 1,000 feet, it will repeatedly sound off “Don’t sink.”
Mode 4. If there is unsafe terrain clearance during landing or when approaching the runway, warning will sound off if the landing gear or flaps are not properly configured for landing. If the clearance is inadequate in descending or if the aircraft penetrate the minimum terrain clearance floor during the flight, the red GPWS warning lamp will be light up and sound off “Too Low, Terrain”. Once the aircraft is safely back above the protective floor, the warning lamp will turn off and the voice alert will cease. As the airspeed and aircraft configuration decreases, the minimum terrain clearance floor will be automatically adjusted. If landing gear is not down at 500 feet above ground, the red GPWS lamp will be light up and sound off with “Too Low, Gear.” During the approach of landing, if the flaps are not set to landing configuration by 170 to 250 feet the GPWS lamp will be light up and sound off with “Too Low, Flaps” (FCG, 2013).
Mode 5. Flight crew making an instrument landing system (ILS), protection descent below the ILS glide slope while an ILS approach. This will be activated when landing gear is down and altitude is less than 1,000 feet. When the aircraft descent is below the glide slope of 1.3 dot, it will light up the Below G/S lamp and sound off “Glideslope”, the glideslope voice messages are initially softer and less frequent than other GPWS warning but will get louder and more frequent over time (FCG, 2013).
Mode 6. Altitude callout. It will provide information of the altitude when it is triggered at a radio altitude or at an excessive roll angle. The usual callouts are 10ft, 20ft, 30ft, 40ft, 50ft, 100ft, 200ft, 500ft, 1000ft (Honeywell, 2004).
However, there are some limitation on the GPWS systems. Flight crews cannot totally rely on the system as system may malfunction, resulting in late warning information. This cause the pilot to have insufficient time to react and rectify the flight to the right path. Therefore, flight crews are still required to apply their own experience and situation awareness to determine the situation (Johnson, 2018).
Enhanced Ground Proximity Warning System
The Enhanced Ground Proximity Warning System (EGPWS) is the improved system that overcome the limitations of GPWS, which was developed by Don Bateman at Honeywell in 1996 (HavKar, n.d.). The EGPWS provides advanced warning of steeply rising ground with more accurate information of the terrain and covers obstacles area with the runway incursions and excursions (Honeywell, 2018).
The terrain database contains higher resolution grids in airport area and lower resolution outside of the airport area. With the use of the accurate Global Positioning System (GPS), it provides information such as present position of the aircraft, track and the groundspeed. The combination of all these information, EGPWS are able to present a graphical plan view of the aircraft in the terrain which can advise the flight crew of any potential obstacles or hazards surrounding the aircraft. If there is any danger between the system and the terrain, it will automatically sound off certain alarms or warning both through audio and visual means to notify the flight crew. (Honeywell, 2014). The aircraft positioning and flightpath from the GPS and database of the EGPWS system. This can offer flight crew of a visual display in the terrain obstacle which improves the awareness of the pilots flying in and around mountain or congested areas with more ample time where it can reduce the CFIT.
The EGPWS is a much-improved system for GPWS but there is still some limitation since the database uses topographical map in the system which they might not have updated or accurate data especially for the less developed countries. During a Honeywell safety system, Bateman, pilots and his engineering group has discovered that the database has incorrect information when they found a mountain which is 1,000 feet higher in real life compared to the EGPWS system (Pope, 2008). Due to this testing, they have been constantly updating with satellite data, and Honeywell provide free download to EGPWS owners to constantly update.
As shown Figure 1. statistic table below illustrates information regarding CFIT risk for aircraft during 1965 to 2002 and shows the implementation of GPWS and EGPWS has resulted a drastic drop in accident rate.
Figure 1. CFIT risk for commercial jet aircraft – North America during 1965-2002 (Honeywell, n.d.)
According to the Accident Investigation Board Norway report, On 19 December 2009, 2127 Hrs, Aeroflot Russian Airlines Flight AFL211, A320-214, had an incident when approaching Oslo Gardermoen Airport (ENGM). Applying the Preliminary Hazard Analysis (PHA) on this incident as to identify the possible hazards or contributing factors.
|1||Human Factors||Misunderstanding between ATC and pilots||Flight crew took the wrong runway and might cause collision to other aircrafts.||1C||Unacceptable risk due to injuries or even fatalities to flight crew and passengers on board.||1. Flight crew should clarify with ATC to ensure the right information.|
|2||Environment||Low visibility due to fog at the airport||Increase of risk during landing, which might land out of the runway.||2C||Undesirable risk which can cause collision on runway.||1. A system that can guide the pilot on the right track.
2. Ensure lighting of the runway are visible.
|3||Human Factors||Flight crew in high stress||Their focus of flying changes to adjusting the landing configuration but not flying the aircraft.||2C||Undesirable risk due to flight crew might key or give the wrong information.||1. The flight crew must be trained to handle in different situations with the amount of stress level.|
|4||Human Factors||Pilot disorientated||Loss of situational awareness which causing the pilot to disorientated by delaying corrective actions.||1C||Unacceptable risk as pilot might crash the aircraft from maneuvering in the wrong direction.||1. Flight crew must always refer to cockpit display to prevent any disorientated.|
|5||Human Factors||Mental stress with overload information.||Pilot unsure of which runway that they are landing.||2D||Undesirable risk due to giving wrong information to other flight crew.||1. The flight crew must be trained to handle in different situations with the amount of stress level.
2. Develop a method that remove stress and allow the flight crew to stay calm.
The PHA is the effort in hazard analysis for this case study of Aeroflot Russian Airlines Flight AFL211 occurs during a flight operation.
Hazard Risk Item
Hazard Risk Item 1: Misunderstanding between ATC and pilots.
There is a poor communication between the ATC and the pilots where changing of runway 19R to 19L which during the approach of landing is changed to 19R again (The Accident Investigation Board Norway, 2013)..
Risk Assessment for Item 1: 1C
It described as “Occasional” and “Catastrophic” if this event occurred. The hazard risk assessment matrix indicates that a risk classification of 1C is unacceptable. Hence, to resolve such problem, flight crew should clarify with ATC to ensure the right information.
Hazard Risk Item 2: Low visibility due to fog at the airport.
With low visibility can reduce and limit flight crew view when approach of the runway, which might cause runway excursion.
Risk Assessment for Item 2: 2C
It described as “Occasional” and “Critical” if this event occurred. The hazard risk assessment matrix indicates that a risk classification of 2C is unacceptable. Hence, to resolve such problem, a system that can guide the pilot on the right track and ensure lighting of the runway are visible.
Hazard Risk Item 3: Flight crew in high stress
During the incident, the flight crew was so stressed up at a point that they focus on changing the aircraft landing configuration rather than focusing on flying the aircraft.
Risk Assessment for Item 3: 2C
It described as “Occasional” and “Critical” if this event occurred. The hazard risk assessment matrix indicates that a risk classification of 2C is unacceptable. Hence, to resolve such problem, the flight crew must be trained to handle in different situations with the amount of stress level.
Hazard Risk Item 4: Pilot disorientated
When the pilot is in loss of situational awareness when the autopilot intensively banked to the left and the pilot was putting inputs to the FMGS control panel and making the aircraft aggressively pitching up instead of bank. When a pilot is disorientated it might cause a delay to execute the corrective actions (The Accident Investigation Board Norway, 2013).
Risk Assessment for Item 4: 1C
It described as “Occasional” and “Catastrophic” if this event occurred. The hazard risk assessment matrix indicates that a risk classification of 1C is unacceptable. Hence, to resolve such problem, flight crew must always refer to cockpit display to prevent any disorientated.
Hazard Risk Item 5: Mental stress with overload information.
During the incident, the commander was experienced with flights to Gardermoen and had full control of the situation, whereas his first officer was inexperienced and became significantly stressed as a result of the late runway change (The Accident Investigation Board Norway, 2013). Pilot unsure of which runway that they are landing.
Risk Assessment for Item 5: 2D
It described as “Remote” and “Critical” if this event occurred. The hazard risk assessment matrix indicates that a risk classification of 2D is undesirable. Hence, to resolve such problem, the flight crew must be trained to handle in different situations with the amount of stress level and develop a method that remove stress and allow the flight crew to stay calm.
Aircraft Design, Development & Retrofit
Honeywell contains a total of nine products of EGPWS, Runway awareness and advisory system, and Smart Runway and Smart Landing. The range of product versions includes MARK V, MARK V-A, MARK VI, MARK VII, MARK VIII, MARK XXI, MARK XXII EGPWS (Honeywell, 2018). Out of the 9 models, MARK XXI and MARK XXII are for helicopters where the former is for light weight helicopter that falls under the Class B TAWS, while the latter is suitable for larger transport and corporate rotorcraft which falls under the Class A performance (Jensen, 2000)
Other than major aviation organizational, Honeywell also caters to anybody who owns an aircraft. The most basic EGPWS (MARK V) costs at least $68,000 USD, and Honeywell aims to have EGPWS in every single aircraft worldwide (Jensen, 2000).
One of the latest development with Honeywell is EGPWS compatibility with weather radar, allowing the flight crew to see the terrain profile together with the weather above it (Jensen, 2000).
Although if the first implementation of TWAS, flight crews cannot rely totally on GPWS as there are limitations. After past accidents and constantly improving technology over the years, it mitigates CFIT by improving the awareness of terrain. The EGPWS were the most advances technology that overcomes all GPWS limitations by the use of accurate GPS and the FMS information it enhances the system. As we can see from the statistics, there is a drastic fall on the accident rate with such systems such been implemented on the aircraft.
Therefore, the FAA must ensure software developing with Honeywell and constantly updating of EGPWS of data or any form of errors found within the system, so that to prevent any accidents which causes by insufficient data or incorrect data. The FAA should ensure all operators using EGPWS are trained with the latest version.
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