BA777
Topic Author
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Why Does Tcas Order Descent?

Wed Jul 03, 2002 6:10 am

Just a quick question into why the TCAS ordered the DHL 75F to descend while the system would've seen that the TU-154 was descending, so wouldnt the DHL have ascended or turned away when the square turned red, or even when it was yellow?

BA777
 
N79969
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RE: Why Does Tcas Order Descent?

Wed Jul 03, 2002 6:12 am

TCAS RA's are sometimes faulty. A BA 744 and KE 742F nearly collided because of a bad TCAS RA.
 
EGGD
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RE: Why Does Tcas Order Descent?

Wed Jul 03, 2002 6:16 am

ummmmm...... TCAS would have told the DHL 757 to descend, and the tu154 to ascend, but in this case Zurich ATC told the Tu-154 to descend, I don't know why the pilot wasn't using his TCAS...
 
MD88Captain
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RE: Why Does Tcas Order Descent?

Wed Jul 03, 2002 6:16 am

Perhaps on the initial data that was being inputed the best escape was to descend. After all they were at altitude and there is very little energy available at such a height to climb. The TCAS system may be designed to take altitude into account. I know that TCAS logic takes altitude into account at very low altitudes. TCAS will not order a descent while close to the ground.

I've had TCAS order a descent and then based on new data order a climb. It is very possible that happened to the DHL crew. The tapes will tell all.
 
EGGD
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RE: Why Does Tcas Order Descent?

Wed Jul 03, 2002 6:25 am

Why did the ATC intervene(uhhh sp?)? I would have thought that, when 2 aircraft are at the same altitude the TCAS would tell one to descend and one to climb. I think that the TCAS wouldn't have time to change from descend to climb for the DHL, after it would expect (if an electrical instrument) the DHL to descend and the tupolev to climb.
 
MD88Captain
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RE: Why Does Tcas Order Descent?

Wed Jul 03, 2002 7:03 am

Traffic separation is ATC's job. TCAS is only a last ditch safety device.
 
backfire
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RE: Why Does Tcas Order Descent?

Wed Jul 03, 2002 7:06 am

Those aircraft should have been nowhere near each other. TCAS is not a separation device, as MD88 Captain points out, in the same way that EGPWS isn't a navigation system.
 
broke
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RE: Why Does Tcas Order Descent?

Wed Jul 03, 2002 10:22 pm

The algorithm (I had to look in the dictionary to spell it  Laugh out loud) used in a TCAS system to indicate which way the crew should fly the airplane includes the following. (This is only part of the algorithm). The ceiling of the airplane (altitude), the airplane registration number, which antenna on both airplanes is receiving the signal (top, bottom, or both), the closure rate of the planes, and whether the airplanes are decending or climbing.
If the airplanes are approaching in level flight at the same altitude, the registration number code is used to determine which airplane will receive the climb indication and which will receive the descend. If one plane is close to its ceiling, it will receive the descend indication and also signal the other airplane what it is doing.
With the TU-154 descending, it was a combination of criteria in the 757 that gave the crew a descend indication. Later TCAS software will also give a go left or right indication, I thought it would be out by now, but it may not have been installed or did not provide an indication in this instance.
A few years ago, a F-16 was trying to intercept a 727 from behind, off the coast of the US. The 727 crew flew all over the place based on its TCAS indications trying to avoid what they thought was an impending mid-air.
 
jhooper
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RE: Why Does Tcas Order Descent?

Thu Jul 04, 2002 10:46 am

Traffic separation is ATC's job. TCAS is only a last ditch safety device.


The pilot is ultimately responsible. Don't know how many SAABs or Brasilias down here have almost collided with small planes because they buzz in here like they own the place with their heads buried in the cockpits not keeping a sharp watchout. Then they gripe ATC out over the radio, and the controller just comes back with "You have TCAS, I have a pair of binoculars!" PCA (CL A Airspace) might be different, but where I am there is no approach controller, no radar, and just one guy in the tower who only has responsibility for traffic on the active runway and not much else.
Last year 1,944 New Yorkers saw something and said something.

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