NTSB Safety Alert SA-017 - In-Cockpit NEXRAD Mosaic Imagery

In-Cockpit NEXRAD Mosaic Imagery

Actual Age of NEXRAD Data Can Differ Significantly

From Age Indicated on Display

The problem

 Weather radar “mosaic ” imagery created from Next Generation Radar (NE XRAD )

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 g round 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 indicat ed on the displa y.

 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

signific antly 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 m inutes .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 , quick ly developing weather scenarios, and/or fast-moving aircraft.

1 Actual maximum age differences can vary between service type (FIS-B versus satellite) and provider .

SA-017 June 2012 , revised December 2015  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 M arch 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 throu gh 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 diver ting 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

precip itation. The last three NEXRAD updates that the p ilot received

should have each said that they were 1 minute old at the time they were

received; however, t he 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 contain s details about the potential time diff erence 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/pubdms/search/document.cfm?docID=348568&docketID=50404&mkey=75577 , and

additional information abou t the accident can be found at

http://www.ntsb.gov/_layouts/ntsb.aviation/brief.aspx?ev_id=20100325X93604&key=1& queryId=e361d11f -

b95d -43f9-ab54 -da1dc599718c&pgno=1&pgsize=200 .

3 The Weather Study for this accident, NTSB case number CEN12FA108 , can be found at

http:/ /dms.ntsb.gov/pubdms/search/document.cfm?docID=367965&docketID=52796&mkey=82539 , and

additional information about the accident can be found at

http://www.ntsb.gov/_layouts/ntsb.aviation/brief.aspx?ev_id=20111219X15943&key=1&queryId=e361d11f -

b95d -43f9-ab54 -da1dc599718c&pgno=1&pgsize=200 .

SA-017 June 2012 , revised December 2015 What can pilots do?

 Remember that the in-cockpit NEXRAD display depict s 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 t han 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 i n-cockpit weather capabilities do es not

circumvent the need for a complete weather briefing before takeoff .

 Use all appropriate sources of weather information to make in-flight decision s.

 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, t he 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 displa yed in the cockpit .