1. Technical Field
The present invention generally relates to identifying destination runways for use with a Terrain Awareness Warning System for use by an aircraft for adjusting aircraft terrain clearance alert values during a landing pattern of the aircraft.
2. Background Art
An important advance in aircraft flight safety has been the development of warning systems such as a Terrain Awareness Warning System (“TAWS”). These warning systems analyze the flight parameters of the aircraft and the terrain surrounding the aircraft. Based on this analysis, these warning systems provide alerts to the flight crew concerning possible inadvertent collisions with terrain or other obstacles. Unless adjusted for various phases of flight, however, such as landing and take-off, the terrain alert settings for TAWS provide false alerts to the flight crew, often called nuisance alerts, that may cause the flight crew to ignore other alerts from the TAWS altogether.
For example, during the landing operation of the aircraft, the aircraft will follow a flight path that will eventually intersect the earth at the intended runway on which the aircraft is scheduled to land. In the landing operation, if the alert settings for TAWS are not compensated for the landing pattern, the TAWS may generate constant alerts. The constant generation of alerts during landing may be a nuisance due to the added stress and confusion the alerts may impose on the flight crew. Additionally, the nuisance alerts may overshadow other critical alerts in the cockpit. For this reason, some TAWS anticipate the landing of the aircraft and disable or desensitize alerts otherwise generated by the warning system within a predetermined range of the airport, such that the TAWS will not generate nuisance alerts during landing of the aircraft.
Although disabling or desensitizing of alerts generated by the TAWS during landing eliminates problems associated with the generation of “nuisance” alerts, determining when to disable the terrain alerts also presents several problems. Specifically, several airports are located in geographic areas that are in close proximity to either natural high elevation terrain such as mountains and/or manmade terrain such as skyscrapers. Premature disablement or desensitization of the TAWS alerts may disadvantageously eliminate terrain alerting protection from these features near the airport.
Furthermore, operating the TAWS in close proximity to the airport may also cause problems. Specifically, if the TAWS is operated conservatively and the alerts remain enabled in close proximity to the airport, the TAWS is more likely to give nuisance alerts, mistaking the aircraft trajectory intersection with the runway as requiring a terrain alert. As explained previously, in these instances the flight crew may become desensitized to the alert and associate the alert with the impending landing of the aircraft, instead of the terrain or structures surrounding the airport.
Various TAWS have been designed that attempt to detect when the aircraft is entering a landing procedure to allow the terrain alerts to be disabled or desensitized in a more timely and sophisticated manner. For example, some TAWS monitor the flaps and landing gear systems of the aircraft to determine if these systems are operating in a characteristic landing configuration. Other systems monitor the rate of descent and air speed of the aircraft to determine whether the aircraft is landing.
Although these systems are designed to determine when the aircraft is beginning a landing procedure, these systems may at times be unreliable. This is because some configurations of the flaps, landing gear, air speed, and rate of descent that may appear to be part of a landing procedure, are also configurations used in the normal flight of the aircraft. Additionally, use of flap and landing gear configurations as indications of landing may not result in the TAWS alerts disabled or desensitized in a timely fashion. Specifically, because the flight crew typically configures the flaps and landing gear, the timing of the configuration of the flaps and landing gear may be different for each landing. Thus, the terrain alerts of the TAWS may either remain enabled for too long and produce unwanted nuisance alerts during a portion of the landing procedure, or the TAWS terrain alerts may be disabled too early and not provide adequate protection from terrain near the airport.
Satellite-based navigational systems, such as GPS, which can track longitude, latitude, altitude, ground track, and ground speed, are becoming an important and reliable source of information for aircraft. A TAWS' Forward Looking Terrain Avoidance (“FLTA”) function looks ahead of the aircraft during flight along and below the aircraft's lateral and vertical flight path to provide suitable alerts if a potential threat exists of the aircraft colliding or coming too close to terrain. The computation involves searching through a terrain database for terrain cells that are within the search area and violate the Required Terrain Clearance (“RTC”). The RTC is the value set by the Federal Aviation Administration as the permitted flight “floor” for various phases of aircraft flight. The RTC indicates the clearance distance from terrain below which the aircraft should not fly. Analyzing the search area and finding the cells in violation is expensive in both processor and memory resources.
The purpose of a TAWS FLTA function is to predict whether the aircraft is heading toward terrain that will cause the terrain clearance to be less than the clearance required by federal guidelines. The Federal Aviation Administration (“FAA”) establishes minimum terrain clearance levels that must be maintained for safety. The precise minimum clearance levels required for any given situation depend upon the type of aircraft, flight pattern, and other factors. The FAA also determines minimum performance standards for TAWS equipment used by an aircraft. One example of FAA TAWS equipment standards may be found in the Technical Standard Order TSO-C151b issued in December, 2002 by the FAA.
TAWS have been developed that utilize the advantages of GPS to evaluate the proximity of the aircraft to an airport and the flight altitude of the aircraft above a landing runway to determine if the aircraft is entering a landing procedure. For example, if an aircraft approaches the runway within a predetermined distance range and within a predetermined altitude range, the TAWS will determine that the aircraft is entering a landing procedure. During the landing procedure, the TAWS creates a terrain floor or minimum alert altitude surrounding the runway. An example of a system describing and explaining the use of a terrain floor and tracking of aircraft position using a Global Positioning System (“GPS”) may be found in U.S. Pat. No. 5,839,080, entitled “Terrain Awareness System.” Use of a terrain floor for calculating and providing terrain alerts during both cruising and landing procedures is well know in the art. By adjusting the aircraft terrain clearance values during a landing procedure from the minimum clearance values required during aircraft cruising flight, nuisance alerts may be reduced.
To provide higher levels of safety during landing yet reduce nuisance alerts, accurate methods of identifying when landing procedures are initiated and accurately identifying an appropriate destination runway is desirable. U.S. Pat. No. 6,304,800, entitled “Methods, Apparatus and Computer Program Products for Automated Runway Selection” discloses a method of identifying a destination runway. Particularly because TAWS is a safety system, but for other reasons as well, processor speed and reduction in the number of calculations required to perform functions is desirable. Conventional TAWS require significant processor calculation times for identifying and confirming destination runways during landing procedures.