terrykohler
Well Known Member
Fellow pilot Doug Ranz forwarded the following alert after a morning discussion on the value and limitations of XM and ADS-B weather. Everyone using this service should be aware of its limitations -
Terry
Actual Age of NEXRAD Data Can Differ Significantly From Age Indicated on Display
The problem
Weather radar ?mosaic? imagery created from Next Generation Radar (NEXRAD) data is available to pilots in the cockpit via the flight information service-broadcast (FIS-B) and private satellite weather service providers.
A mosaic image presents radar data from multiple radar ground sites on a single image on the cockpit display. When a mosaic image is updated, it may not contain new information from each ground site.
The age indicator associated with the mosaic image on the cockpit display does not show the age of the actual weather conditions as detected by the NEXRAD network. Instead, the age indicator displays the age of the mosaic image created by the service provider. Weather conditions depicted on the mosaic image will ALWAYS be older than the age indicated on the display.
Due to latencies inherent in processes used to detect and deliver the NEXRAD data from the ground site to the service provider, as well as the time intervals used for the mosaic-creation process set by the service provider, NEXRAD data can age significantly by the time the mosaic image is created.
Although such situations are not believed to be typical, in extreme latency and mosaic-creation scenarios, the actual age of the oldest NEXRAD data in the mosaic can EXCEED the age indication in the cockpit by
15 to 20 minutes.1
Even small time differences between the age indicator and actual conditions can be important for safety of flight, especially when considering fast-moving weather hazards, quickly developing weather scenarios, and/or fast-moving aircraft.
1 Actual maximum age differences can vary between service type (FIS-B versus satellite) and provider.
The National Transportation Safety Board (NTSB) has recently investigated two fatal
accidents where in-cockpit NEXRAD mosaic imagery was available to pilots
operating near quickly developing and fast-moving convective weather.
o On March 25, 2010, a Eurocopter AS350 B3, N855HW, impacted
terrain near Brownsville, Tennessee. 2 During the flight, the pilot?s
cockpit display indicated that it had received one NEXRAD image
roughly halfway through the flight that indicated it was about 1 minute
old; however, the weather conditions were actually about 5 minutes old.
The image indicated that the severe weather was about 7 miles away
from the home base where the pilot was attempting to land, but the
severe weather was actually just crossing over the home base at about
the time the display received the NEXRAD image.
o On December 19, 2011, a Piper PA-32-260, N3590T, collided with
terrain following an in-flight breakup near Bryan, Texas.3 The pilot had
been diverting to avoid weather and had likely received several
NEXRAD updates in the minutes leading up to the accident. According
to the NEXRAD data the pilot likely would have received, he was flying
clear of the precipitation along the edge of the rain. Near the end of the
flight, the pilot flew into a section of the developing rain shower. His
display should have shown that he still remained clear of the
precipitation. The last three NEXRAD updates that the pilot received
should have each said that they were 1 minute old at the time they were
received; however, the actual weather conditions at the time the images
were received in the cockpit were about 6, 7, and almost 8 minutes old,
respectively.
The general issue of latency with in-cockpit NEXRAD is discussed in pilots? guides, in
industry literature, and on service providers? websites. However, the NTSB has not
found that such guidance contains details about the potential time difference between
the age indicator and actual conditions.
2 The Cockpit Display Factual Report for this accident, NTSB case number ERA10MA188, can be found at
http://dms.ntsb.gov/public/50000-50499/50404/466399.pdf, and additional information about the
accident can be found at http://www.ntsb.gov/aviationquery/index.aspx.
3 The Weather Study for this accident, NTSB case number CEN12FA108, can be found at
http://dms.ntsb.gov/public/52500-52999/52796/492704.pdf, and additional information about the
accident can be found at http://www.ntsb.gov/aviationquery/index.aspx.
What can pilots do?
Remember that the in-cockpit NEXRAD display depicts where the weather WAS, not
where it IS. The age indicator does not show the age of the actual weather conditions
but rather the age of the mosaic image. The actual weather conditions could be up to
15 to 20 minutes OLDER than the age indicated on the display. You should consider
this potential delay when using in-cockpit NEXRAD capabilities, as the movement
and/or intensification of weather could adversely affect safety of flight.
Understand that the common perception of a ?5-minute latency? with radar data is not
always correct.
Get your preflight weather briefing! Having in-cockpit weather capabilities does not
circumvent the need for a complete weather briefing before takeoff.
Use all appropriate sources of weather information to make in-flight decisions.
Let your fellow pilots know about the limitations of in-cockpit NEXRAD.
Need more information?
While some published Federal Aviation Administration (FAA) documents, including the
Aeronautical Information Manual, Advisory Circular 00-45G (?Aviation Weather Services?),
and the Advanced Avionics Handbook (FAA-H-8083-6), contain useful information about
in-cockpit NEXRAD capabilities, the NTSB is not aware of any discussion or guidance in
these documents regarding the potential amount of time difference between actual
radar-observed conditions and the age displayed in the cockpit
Terry
Actual Age of NEXRAD Data Can Differ Significantly From Age Indicated on Display
The problem
Weather radar ?mosaic? imagery created from Next Generation Radar (NEXRAD) data is available to pilots in the cockpit via the flight information service-broadcast (FIS-B) and private satellite weather service providers.
A mosaic image presents radar data from multiple radar ground sites on a single image on the cockpit display. When a mosaic image is updated, it may not contain new information from each ground site.
The age indicator associated with the mosaic image on the cockpit display does not show the age of the actual weather conditions as detected by the NEXRAD network. Instead, the age indicator displays the age of the mosaic image created by the service provider. Weather conditions depicted on the mosaic image will ALWAYS be older than the age indicated on the display.
Due to latencies inherent in processes used to detect and deliver the NEXRAD data from the ground site to the service provider, as well as the time intervals used for the mosaic-creation process set by the service provider, NEXRAD data can age significantly by the time the mosaic image is created.
Although such situations are not believed to be typical, in extreme latency and mosaic-creation scenarios, the actual age of the oldest NEXRAD data in the mosaic can EXCEED the age indication in the cockpit by
15 to 20 minutes.1
Even small time differences between the age indicator and actual conditions can be important for safety of flight, especially when considering fast-moving weather hazards, quickly developing weather scenarios, and/or fast-moving aircraft.
1 Actual maximum age differences can vary between service type (FIS-B versus satellite) and provider.
The National Transportation Safety Board (NTSB) has recently investigated two fatal
accidents where in-cockpit NEXRAD mosaic imagery was available to pilots
operating near quickly developing and fast-moving convective weather.
o On March 25, 2010, a Eurocopter AS350 B3, N855HW, impacted
terrain near Brownsville, Tennessee. 2 During the flight, the pilot?s
cockpit display indicated that it had received one NEXRAD image
roughly halfway through the flight that indicated it was about 1 minute
old; however, the weather conditions were actually about 5 minutes old.
The image indicated that the severe weather was about 7 miles away
from the home base where the pilot was attempting to land, but the
severe weather was actually just crossing over the home base at about
the time the display received the NEXRAD image.
o On December 19, 2011, a Piper PA-32-260, N3590T, collided with
terrain following an in-flight breakup near Bryan, Texas.3 The pilot had
been diverting to avoid weather and had likely received several
NEXRAD updates in the minutes leading up to the accident. According
to the NEXRAD data the pilot likely would have received, he was flying
clear of the precipitation along the edge of the rain. Near the end of the
flight, the pilot flew into a section of the developing rain shower. His
display should have shown that he still remained clear of the
precipitation. The last three NEXRAD updates that the pilot received
should have each said that they were 1 minute old at the time they were
received; however, the actual weather conditions at the time the images
were received in the cockpit were about 6, 7, and almost 8 minutes old,
respectively.
The general issue of latency with in-cockpit NEXRAD is discussed in pilots? guides, in
industry literature, and on service providers? websites. However, the NTSB has not
found that such guidance contains details about the potential time difference between
the age indicator and actual conditions.
2 The Cockpit Display Factual Report for this accident, NTSB case number ERA10MA188, can be found at
http://dms.ntsb.gov/public/50000-50499/50404/466399.pdf, and additional information about the
accident can be found at http://www.ntsb.gov/aviationquery/index.aspx.
3 The Weather Study for this accident, NTSB case number CEN12FA108, can be found at
http://dms.ntsb.gov/public/52500-52999/52796/492704.pdf, and additional information about the
accident can be found at http://www.ntsb.gov/aviationquery/index.aspx.
What can pilots do?
Remember that the in-cockpit NEXRAD display depicts where the weather WAS, not
where it IS. The age indicator does not show the age of the actual weather conditions
but rather the age of the mosaic image. The actual weather conditions could be up to
15 to 20 minutes OLDER than the age indicated on the display. You should consider
this potential delay when using in-cockpit NEXRAD capabilities, as the movement
and/or intensification of weather could adversely affect safety of flight.
Understand that the common perception of a ?5-minute latency? with radar data is not
always correct.
Get your preflight weather briefing! Having in-cockpit weather capabilities does not
circumvent the need for a complete weather briefing before takeoff.
Use all appropriate sources of weather information to make in-flight decisions.
Let your fellow pilots know about the limitations of in-cockpit NEXRAD.
Need more information?
While some published Federal Aviation Administration (FAA) documents, including the
Aeronautical Information Manual, Advisory Circular 00-45G (?Aviation Weather Services?),
and the Advanced Avionics Handbook (FAA-H-8083-6), contain useful information about
in-cockpit NEXRAD capabilities, the NTSB is not aware of any discussion or guidance in
these documents regarding the potential amount of time difference between actual
radar-observed conditions and the age displayed in the cockpit
