Abnormal Descent Rate Notifications
This feature monitors and alerts for situations where aircraft experience Abnormal Descent Rates. This feature monitors and provides alerts for rates of descent >3000 feet/minute but excludes monitoring within a protected circle of 100NM radius around the Destination airport to avoid nuisance warnings. The system can also provide Notifications when Abnormal Descent Rates recover to less than 3000 feet/minute.
Abnormal Descent Rate 2 Minute Sampling
Data points are checked by taking a sample over a 2-minute time period and checking for a rate of descent that is >3000 feet/minute within that period but using the first and last tracked data points to make the calculation. In the diagram below, which shows some green tracking dots in a plan view (looking from above), the top row shows a 2-minute sample of tracking points within the blue rectangle.
The system begins a new 2-minute sample at 30 second intervals as shown in rows 2 to 4 in the diagram. These subsequent 2-minute samples are taken continuously from take-off until within the 100NM radius protected circle around the Destination.
Abnormal Descent Rate Calculation
The 2-minute sample is checked by comparing the altitude and time of the first and last tracking data points within the sample. The overall rate of descent is calculated by using the difference between the first and last altitude, divided by the time in minutes of the same 2 points, to give a rate of descent in feet per minute. If the average rate of descent is >3000 feet/minute, then an alert is created.
Example of Abnormal Descent Rate Calculation trigger
It is the first and last tracking data point that are important and it is these that are compared for an altitude difference. The example below begins at 30000ft and descends to 24000ft in 1.5 minutes (time between first and last tracked point), giving an overall change of 6000ft and an average rate of descent of 4000 feet/minute which triggers the alert.
The following situation demonstrates a situation where an alert is triggered because the rate of descent has exceeded >3000 feet/minute.
Example of Abnormal Descent Rate is not Triggered
The following situation demonstrates a situation where no alert is triggered because the rate of descent has not exceeded >3000 feet/minute.
The user should note that it is possible to have a transient rate of descent of >3000feet/minute and no alert is created because of the overall average between the time of first and last tracking points.
Special conditions and Limitations of the Abnormal Descent Rate Notification
The following lists some special conditions and limitations to be aware of regarding the Abnormal Descent Rate Notification:
- As the system uses samples of 2 minutes, it is possible that an Abnormal Descent Rate might not be detected if it is of short duration, if it is a rapid climb/descent or decent/climb, because it will be averaged over the time period of the first and last tracking within that 2 minute time sample. For example a 4000ft descent over one minute would not trigger the alert if there was 1.5 minutes between the first and last tracking point (4000 / 1.5 = 2667 feet/minute).
- The trigger for notifications is set to 3000 feet/minute, this cannot be changed by the user, but future functionality is planned to allow the user to define this value.
- The calculation is made over a 2 minute sample, therefore Abnormal Descent Rate alerts are historical and are provided slightly behind real-time (approx. 2 minutes after).
- After a flight enters the 100NM radius protected Destination circle no more Abnormal Descent Rate alerts are produced, even if the flight subsequently leaves the protected circle, such as when diverting to an Alternate airport. To re-start the Abnormal Descent Rate monitoring for a particular Tail Number, it would have to be detected as part of a new flight and flight plan i.e. a completely different flight.