Descent - climb climb now


Time Position Groundspeed Altitude
18:21 3655N/7532W 454 35700
18:22 3654N/7541W 459 34600
18:23 3654N/7550W 459 33600
18:24 3654N/7600W 459 32400
18:25 3654N/7610W 465 31500
18:26 3652N/7619W 465 30400
18:27 3648N/7628W 460 29300
18:28 3645N/7636W 455 28200
18:29 3641N/7644W 455 27100
18:30 3638N/7652W 455 30000
18:31 3635N/7701W 455 30000
18:32 3632N/7709W 450 30000
18:33 3629N/7717W 445 30000
18:34 3625N/7726W 445 30000
18:35 3622N/7734W 445 25700
18:36 3619N/7742W 445 24700
18:37 3616N/7750W 445 23600
18:38 3614N/7759W 433 22500
18:39 3612N/7807W 427 22000

This starts to be a perfect 1000fpm descent until FL270 !??
Thereafter again from FL300 (after 5 minutes) until FL220.
Are we talking about TCAS or something else ?


Check and see what alt the flight filed for. I think if the postion report does not have and alt (perhaps the secondary radar return was not received by ATC) they (I assume ATC) subs the filed alt in the data. Notice very often the first one or two postion reports after departure are for the files alt, then “descent” to an actual. It also occurs very often during approach descent, possibly because during approach, the aircraft is transiting between high alt long distance radar coverage to local systems.


Well, that flight was deep within US.
Started obviously an enroute descent into CLT.


Below are the first several reports for COM153, 4/12/06 JAX-BOS. Filed for alt FL330. Notice the 7:25 reports Alt 33000 from 5500. Pretty good performance for a CRJ.

07:22AM 3030N/8140W 116 1000
07:23AM 3033N/8140W 210 2700 climbing
07:24AM 3037N/8141W 232 5500 climbing
07:25AM 3039N/8141W 232 33000 climbing
07:26AM 3044N/8141W 271 8100 descending
07:27AM 3048N/8141W 271 10200 climbing
07:28AM 3053N/8140W 278 12300 climbing
07:29AM 3058N/8138W 289 14500 climbing
07:30AM 3103N/8136W 306 16600 climbing
07:31AM 3108N/8134W 311 18100 climbing

The secondary radar data is based on an interogation method. When the primary radar signal hits the aircraft, the transponder “squawks”- it transmits back a quick stream of data which includes the squawk code (so ATC systems can positivily ID the target and alt as read from the aircraft’s navagation systems. This alt is much more reliable than can be gathered from the primary radar return (radio echo). The primary return is good at getting range and bearing, giving a position. Lots of factors could result in momentary loss of the secondary signal. In addition to switching radar facilities, a large number of secondary returns (high traffic) can cause some of the returns to be lost.

Editing to add that this current topic is discussing this same issue.


Please see this similar thread that has an explanation.


I work the sector in the Washington ARTCC that does the CLT arrivals over LYH for the MAJIC9 arrival. One of the radar site for our mosaic is over ROA, during a decent, it is not uncommon for the computer to miss the MODE C readouts.