Ground proximity warning system in Aircraft

Abstract

Inventing an aircraft by The Wright Brothers at the beginning of the 20th Century had a huge impact on the world we live in now. It is generally accepted that their “Flyer” was the first powered and controlled airplane in the world. A decade later the first commercial flight took place marking the beginning of commercial air travel. However, from the very beginning the most important aim was to make air travel as safe as possible.

1. Introduction

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Since the invention of the aircraft, one of the major causes of accidents was when a fully functional aircraft was unintentionally flown into the ground or water. The number of this kind of accidents prompted the engineers at Boeing to introduce a new term in the late 1970s called Controlled Flight into terrain or CFIT.

According to Boeing, CFIT “is a leading cause of airplane accidents involving the loss of life. There have been over 9,000 deaths in different parts of the world due to this since the beginning of the commercial jet age."

There have been fewer accidents thanks to the Terrain Awareness and Warning Systems (TAWS). The TAWS which is capable of alerting the crew if they fly dangerously close to the Earth surface has substantially and effectively reduced the number of accidents caused by CFIT. It has to be stressed that the invention of the Ground.

Proximity Warning System (GPWS) in late 1960 is widely credited to a Canadian engineer Charles Donald Bateman.

Currently there are two types of TAWS used on the aircrafts. The first type is called Ground Proximity Warning System (GPWS). Admittedly, GPWS is an older system but in 1996 a second type which is an improved version on GPWS called Enhanced Ground Proximity Warning System or EGPWS was introduced. GPWS and EGPWS are one of the most vital systems on-board modern aircrafts. They are designed to alert the crew by means of aural and visual warning of immediate danger of colliding with the ground, water or an obstacle such as a mountain.

2. Principles behind how the GPWS work

The Ground Proximity Warning System automatically and continuously monitors the clearance of the aircraft above the surface. The principle of work is based on Continues Wave Radar. Continues waves are transmitted towards the ground and once reflected they are picked by the receiver. The time taken for the waves to return is measured and it indicates the distance between the plane and the Earth surface.

The heart of the system is a computer which collects data from the radio altimeter and predicts the future terrain features. As the radio altimeter only provides the distance between the aircraft and the surface directly below it, the computer has to keep track of the terrain features and predict terrain that is directly ahead of the aircraft.

The above can only be achieved and done correctly if the surface does not have significant changes in heights, but in a situation when there is a dramatic change in terrain, then invariably GPWS fails. It is also referred to as a “blind spot”.

However, the blind spot has been eliminated with the introduction of the Enhanced Ground Proximity Warning System.

3. The modes of GPWS

The Ground Proximity Warning System operates in different modes. Depending on the situation, the GPWS responds in a particular manner and provides the crew with a specific aural alert or warning and visually by illumination of warning lights. By monitoring the aircraft’s flying path and its altitude, the computer determines the risk of collision with the Earth surface and based on that it provides crew with either of the two warnings: a Soft Warning or Hard Warning.

The GPWS Alert or Soft Warning informs the crew that a potentially dangerous situation has been detected and it requires attention. It indicates that the dangerous situation may develop and it should not be ignored, but the aircraft is not in immediate danger.

The GPWS Warning or Hard Warning informs the crew that the aircraft is threatened by grave danger and that immediate action is required.

3.1. Mode 1 – Excessive descent rate

Readings from barometric altimeter and radio altimeter are combined together and the crew is informed of excessive rate of descent. When the rate of descent exceeds the limit an aural alert “SINK RATE” is issued and the amber warning light comes up. In a situation when the descent rate becomes even more severe the system generates an aural warning “PULL UP” and the red warning light illuminates in the cockpit.

3.2 Mode 2 – Excessive terrain closure

Notifications are based on the readings taken from the radio altimeter. The System monitors the clearance between the aircraft and the ground. The crew is informed of the change of the terrain below and when the aircraft is flying into the slope of a mountain or across steep features. If the aircraft’s height above the ground decreases, an amber light illuminates and an aural warning “TERRAIN” is generated 90 seconds before the aircraft flying path and the earth surface crosses. However, if no action is taken and the clearance between the aircraft and terrain decreases even more then 30 seconds from the predicted collision an aural warning “PULL UP” is issued and a red light comes up.

3.3 Mode 3 – Excessive altitude loss after take-off or go-around

Data collected from barometric altimeter, radar altimeter and indicated flight speed are combined together by the GPWS computer in order to detect an altitude loss after take-off or a missed approach. In such circumstances an aural alert “DON’T SINK” is generated and the amber light starts illuminating. During the decent this mode of the GPWS is not armed until the aircraft is 60 meters above the surface.

3.4 Mode 4 – Unsafe terrain clearance while not in landing configuration

The computer generates alerts and warnings indicating that the clearance above the ground is insufficient for the current flight path while landing gear and flaps are not in the landing configuration. Consequently, there are two alerts issued by the system – “TOO LOW GEAR” and “TOO LOW FLAPS” depending on what is causing the problem.

An amber warning light will also come up accordingly. However, if the aircraft altitude continues decreasing and the landing configuration is not corrected, the crew is issued with a voice message “TOO LOW TERRAIN” and the warning light will come up.

3.5. Mode 5 – Excessive descent rate below the instrument glide path.

The GPWS monitors the flying path of the aircraft in a situation when crew uses an Instrument Landing System (ILS) for approach and landing. The system generates an aural alert in the event of the flying path exceeding 1.3 dot below the ILS glideslope and an amber warning light illuminates in the cockpit. As the deviation increases, the voice message “GLIDESLOPE” becomes more frequent and louder.

4. Enhanced Ground Proximity Warning System

As technology advanced and so did the aviation industry. And with developed technology the basic Ground Proximity Warning System was proven to have a “blind spot” making it not 100% reliable. Therefore, inventing a new and better system capable of tracking and predicting terrain ahead of the aircraft, even with dramatic changes in the steepness of the ground became a necessity. The problem was solved with the

introduction of the Enhanced Ground Proximity Warning System. The new system was mainly based on the GPWS but in order to avoid shortcoming of the original system EGPWS was also equipped with Forward Looking Terrain Avoidance system, allowing the computer to “see” what is ahead of the aircraft.

The EGPWS computer has a worldwide digital terrain database containing natural terrain features as well as man-made obstacles and uses Global Positioning System to determine the position of the aircraft. The position of the aircraft, its altitude and ground speed is sent to the computer which combines all the information together and compares it with the map of the Earth terrain stored in the database. The information is then presented to the crew on the Terrain Awareness Display. There are different colours on the digital map each representing a different height of the terrain features relative to the aircraft current altitude (Figure 1). It has to be noted that the system is being continuously improved and some of the modern most sophisticated flight management computers are capable of analysing the whole route and inform the pilots of all potentially hazardous obstacles along the flying path.

5. The modes of EGPWS

Since the Enhanced Ground Proximity Warning System is based on the basic GPWS, it has all the five modes same as its predecessor. However as EGPWS is more advanced than GPWS, in addition to five modes described earlier on it also introduces two new modes.

5.1 Mode 6 – Excessively steep bank angle

The purpose of this mode is to inform pilots of an excessive roll attitude for the flight conditions. The system generates an audible alert “BANK ANGLE”. If the situation worsens and the roll attitude is being increased to 40% above the original callout, the voice message “BANK ANGLE” repeats itself continuously.

5.2 Mode 7 – Excessive windshear protection

This mode is designed to inform the crew of excessive wind shear during take-off and final approach. The warnings are provided between the altitudes 10 – 1500 feet above the ground level. If the wind shear exceeds the limits, the EGPWS generates an aural alert “WINDSHEAR, WINDSHEAR, WINDSHEAR” and the amber light comes up in the cockpit.

6. Conclusions

It goes without saying that the introduction of Terrain Awareness and Warning Systems has significantly reduced the number of accident caused by Controlled Flight Into Terrain. Modern Systems installed on-board civil and military aircraft are able to predict terrain ahead of the aircraft without “blind spot” which was present in the first Ground Proximity Warning Systems. Unfortunately despite advancement in technology and modern equipment, the accidents caused by CFIT still happen. They are many reasons behind that and common among them would be bad weather, navigation system failure or more often pilot error.

Most commonly these types of accidents occur during approach close to the airports. For instance in 2010 alone two major crashes took lives of many people. On 10th of April 2010, Polish Air Force Tu-154 Flight crashed during final approach in Smolensk airport in Russia killing 96 people including the president of Republic of Poland, his wife and many government and military leaders present on board that fatal flight. Three months later on 28th July 2010 another tragic crash happened in Pakistan. Airblue Flight 202 carried 146 passengers and 6 crew members. There were no survivors.

Developing new systems and improving the existing ones may one day lead to complete remove of CFIT type of crashes. As for now however caution still should be executed during pilot training to ensure that every pilot flying the aircraft equipped with EGWPS is fully trained to use the system, understands the warnings and is able to properly react to the situation.

7. References

1. http://en.wikipedia.org/wiki/Controlled_flight_into_terrain#cite_ref-boeing_0-1

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