TCAS test?

[Tord Hoppe]

Hey all,
does the TCAS control mode selector have to be in any specific positions for TCAS test to be performed? When I have the selector in STBY I get a TCS Test Failed aural, which I assume is as intended, just wanted to make sure.

[United744]

Hi,

No - it should pass the test. There is a condition that can cause TCAS to fail, but I forget what. I think if the IRS are not aligned that can cause failure, but assuming they are aligned and there are no faults, it should pass.

[Hardy Heinlin]

No - it should pass the test.

What is your source?

If you set the ALT RPTG selector to OFF, or the mode selector out of TA and RA/TA, the ND will indicate TCAS OFF in amber.

If the left and right radio altimeters fail, the ND will indicate TCAS FAIL in amber.

This is based on sources in AMMs and real deck observations.

In PSX, whenever these amber TCAS messages are displayed, TCAS is inoperative -- and the test will fail accordingly.


Cheers,

|-|ardy

[United744]

but assuming they are aligned and there are no faults, it should pass.

If the RA are failed/off/not powered, that would be a "fault" for example. I was assuming everything was powered for operation. If things are switched off, without knowing what, well...

does the TCAS control mode selector

I was answering his question. By TCAS mode selector I presumed he meant the STBY/ALT/TA/TARA mode selector of the transponder control panel. AFAIK the position of this switch makes no difference to the system test being successful or not.

[Hardy Heinlin]

Yes, I mean the same selector. If you set it to STBY, it's not in TA or TA/RA mode, and TCAS OFF will be annunciated.

If you're saying the test will pass even if TCAS is off, I'm wondering if PSX is wrong or if this is aircraft specific.

[Tord Hoppe]

"By TCAS mode selector I presumed he meant the STBY/ALT/TA/TARA mode selector of the transponder control panel."

Just to confirm that yes, that is the selector I meant. I was a bit thrown by getting a "TCAS Test Failed" when the switch was in the STBY position. In my mind (not backed up by any manuals) I thought one should be able to perform the test sequence without having to have the TCAS system active in TA or TA/RA.

[United744]

As the STBY position relates to the transponder, whilst the TCAS system is off as far as mode of operation, the TEST mode is not "off", and so you should see on the ND:

* TCAS OFF replaced with TCAS TEST
* TCAS test symbology displayed on ND
* TCAS test symbology displayed on PFD
* TCAS system performs functional self-test
* If all required systems for TCAS operation are providing data (no failures) then TCAS test should pass.

Source: multiple real-world aircraft installations. I can't see why 747 should be any different. The transponder shouldn't need to be actually transmitting for it to pass, which would be the implication of setting the transponder mode to TA or TARA mode.

[Hardy Heinlin]

If somebody can confirm it's the same on the 744, then I'll change it in the next update.

As far as I remember, observations on the real 744 deck during PSX development showed some results that were not quite explainable.

Anyway, setting the ALT RPTG selector to OFF failed the test on the real deck, I think. If ALT RPTG is set to OFF, the TCAS OFF message is displayed at least when the test is not running.

[United744]

Anyway, setting the ALT RPTG selector to OFF failed the test on the real deck, I think.

That would make sense as [our] transponder would not be reporting the aircraft altitude, so could not coordinate with other aircraft.

A good way to think of it is like a config check. A failure means the pilot can go looking for mis-configuration/problem.

[Jeroen Hoppenbrouwers]

I thought test failures basically mean "call tech support"?

[John H Watson]

A good way to think of it is like a config check. A failure means the pilot can go looking for mis-configuration/problem.

I thought test failures basically mean "call tech support"?

Tricky question. There seems to be many shades of "failure" when it comes to TCAS. There is probably some rule of thumb regarding TCAS fails/passes, but it's very hard to pinpoint. There are so many inconsistencies on the real aircraft. The TCAS panel may annunciate "PASS", yet the test aural may say "FAIL". The ND may show TCAS OFF, but that is not to say that the test won't generate symbols on the ND.


With the IRU's off, you will get a TCAS FAIL on the real aircraft (i.e. the aural will say FAIL during the test). To me, this is a config or input problem, rather than a TCAS problem. Then there is the TCAS OFF message which you say will not necessarily generate a failure during a test.

Here is part of a test procedure from the AMM:

Pull the Left ATC CB with the panel selected to Left ATC...

(1) Make sure the ND and EICAS shows the TCAS OFF indication

Then it says to pull the ATC Right CB

(15) Make sure the upper EICAS shows the TCAS SYSTEM advisory.

In (1), the TCAS should not operate normally (It doesn't state that the TCAS test will fail though). If only one ATC CB is pulled, the annunciation seems to be saying the system is off, but not entirely failed (as the other ATC may be selected).. So, it's up to the crew to do something with the configuration to correct the situation (I mean by selecting the other altitude source, not resetting the CB).

My training notes do say that TCAS is rendered inoperable if an ATC is not operating in Altitude Reporting Mode, but it doesn't specify if OFF or fail is shown.

The AMM also states:

If a failure is found on any of these system failure inputs, the TCAS FAIL indication is displayed on the ND:

(1) Failure inputs from the mode S transponder, radio altimeters, and PFD/ND displays
(2) Continuity failure in the TCAS antennas or antenna cables
(3) Failure data or data leakage on the ARINC 249 data bus from the mode S Transponder
(4) Internal TCAS computer failures nad failure of internal power supplies
(5) Incorrect data from the radio altimeter
(6) Failures that occur in the TCAS computer/transponder rf loop-around test

However, (1) disagrees with the earlier test  :roll:

[John H Watson]

P.S. Then there are the variations with HDG UP and TRK UP ND displays  

[Hardy Heinlin]

With the IRU's off, you will get a TCAS FAIL on the real aircraft (i.e. the aural will say FAIL during the test).

Are you sure this wasn't different on later checks? I think I had the fail in an earlier version, and then removed it.

In any case, the IRS certainly affects the TRK UP displays: when the IRS is not aligned, the TRK UP maps are blank and the TCAS targets are removed. They are not removed on the HDG UP displays.

Now, what should I do with the STBY and XPDR selector positions? Pass or fail? :-)


Cheers,

|-|ardy

[United744]

Now, what should I do with the STBY and XPDR selector positions? Pass or fail? :-)

Assuming all other required systems are powered/on/not failed, I think it should pass.

[John H Watson]

Are you sure this wasn't different on later checks? I think I had the fail in an earlier version, and then removed it.

I often heard the TCAS FAIL aural during pilot preflights because of manuals placed on top of the ATC/TCAS panel by mistake. The IRUs were not fully aligned in these situations. Our panels didn't have an OFF selection, so that would not be the cause of the fail aural (if it does affect PASS/FAIL).

Note: There was an old forum message which said that the test targets would appear on the ND prior to IRU alignment with the ND in a heading up mode (VOR, ILS... but not sure about MAP on heading up displays). I'm wondering if the aural would sound PASS if the ND was selected to a heading up mode and the IRUs were not aligned.

Anyone doing new tests would have to consider:

1) what mode the TCAS mode selector was in
2) what mode the IRUs were in (OFF, ALIGN, fully ALIGNED or ATT).
3) what CBs were set (various combinations cause different effects)
4) what input selections were made on the ATC/TCAS panel (Alt reporting off, FL mode on/off, transponder in use, etc)
5) what mode was selected on the ND (heading up/ track up)
6) (perhaps) if TCAS was selected to display on the ND
7) Air/ground status
8 ) Instrument Source Selection

Of course, some airlines don't have any track up displays (e.g. British Airways)

Rgds
JHW

[John H Watson]

P.S. On BA aircraft, with IRUs off or aligning, with ND in MAP mode, does TCAS pass the test and the test targets appear? If it does, can we assume that the display of targets is synonymous with TCAS aural PASS?

[Hardy Heinlin]

TCAS test criteria have been modified now:

http://aerowinx.com/forum/topic.php?id=2782