TCAS Altitude Range, question ...

[Peter Dowson]

Is there anyone here who can answer a real-world TCAS display question,
please?

I've searched all my books, and used Google, to try to ascertain the
normal/set/real vertical limits for TCAS objects displayed on a 737NG ND screen. To
no avail so far.

The reason I ask is that with the AI traffic I've now got installed I get
some areas where the AI blips on the ND are so dense it is impossible to make
them out: they need thinning!

I noticed that many were many thousands of feet above or below me, +/-150's,
210's and so on. Surely I don't really need to see those?

I found the "TCASAltRange" parameter in the PFD.INI file and changed it from
its default of 0 (presumably for the check being switched off) to 6000.
This now only shows traffic within 6000 feet above or below me, which helps a
lot.

But what, if any, is the actual limit? Does any one here know?

Thanks,

Pete

[Tom Stian Bjerk]

I think the TCAS altitude range should act as the PMDG tcas does..

The range should be max 10000ft..
And you have to select witch way you want the max range ( above or below the
aircraft)
If u select the below. Then the TCAS range is 10000ft below and 2500ft above
the aircraft.. and visaverca...

[Peter Dowson]

On 2/28/2006 6:49:00 AM, Tom Stian Bjerk wrote:
>I think the TCAS altitude
>range should act as the PMDG
>tcas does..
>
>The range should be max
>10000ft..

Okay, that almost agrees with the real world 737NG data I received by email
in response to my question -- 18700 feet is the vertical surveillance zone,
and with the aircraft centred -- thus 9350 above and below.

>And you have to select witch
>way you want the max range (
>above or below the aircraft)
>If u select the below. Then
>the TCAS range is 10000ft
>below and 2500ft above the
>aircraft.. and visaverca...

Well, that is interesting, as none of the other (real world) data I have been
supplied suggests anything like that. 2500 feet in one direction sounds
rather dangerous -- surely if you are in cruise you need equal coverage above
and below. Only in climb or descent would such an uneven spread make a
little sense.

Can you tell me exactly which switch makes this selection, or it is some
obscure CDU setting in the PMDG implementation?

Thanks & Regards

Pete

[Tom Stian Bjerk]

http://www.precisionmanuals.com/images/products/737TNG/screenshots/radio_throttle.jpg

Here you can see the switch .. You can select above or below.. When the
switch is in center position i think the tcas altitude is 2500ft above and
below..

Best REgards
Tom Stian Bjerk

[Peter Dowson]

On 2/28/2006 9:02:32 AM, Tom Stian Bjerk wrote:
>Here you can see the switch ..

Well, yes, just about! But this image of the transponder panel doesn't match
many of the pictures I can find for a REAL 737NG. I found that switch ONLY
(oddly) in my Boeing CBT package ("Computer Based Training") for the NG --
it is there, but in a different place.

In the CBT the switch is markes "ABV", "N" (for Normal) and "BLW" or similar.
The narrative doesn't give any actual values, but it does say that the
range is dependent upon "closure rates". I assume that means it catches
aircraft from further above or below if their predicted "closest approach" is
within some value, or getting closer, but I don't really know. Nothing I've
read actually puts any values on the altitude range for display -- except
that source which gives 18700 feet in total for "surveillance".

However, I've now discovered that "surveillance" is probably NOT the same as
"display". The range (vertical or otherwise) at which blips are shown on
the ND is not involved in determining Traffic or Resolution Advisories. The
latter is purely based on the Transponder setting, and the TFC display on
the ND can be switched off.

Mind you, I'm don't think PM operates that way, at least at present. After
all, it doesn't have the appropriate transponder switch inputs at the
moment.

>When the switch is in
>center position i think the
>tcas altitude is 2500ft above
>and below..

Hmmm. 2500 is rather close considering that I suspect PM doesn't operate its
TAs and RAs like the real aircraft.

All in all, I'm now thinking that my original "guess" of 6000 for the PM
TCASAltRange parameter was reasonable, so I may go back to that instead of the
9350 I'd decided on from information received!

Thanks!

Pete

[Tom Stian Bjerk]

As I said I dont know how it really works.. But on saturday I was actually
listning to a real 737NG pilot. And he said that they "set the switch in
below position" when the was descending and above when they was climbing.
But he dident say anything the altitude for the position. But I was a little

bit late, so I dident heard all that he was saying.. But im pretty sure
there are an switch for it .. :)

But 6000ft above and below seems like an reasonable value for flightsim and

PM since we dosent have that damn switch :)

Best Regards
Tom Stian Bjerk

"Peter Dowson" skrev i melding
news:396168.79369@wb.onvix.com...
> On 2/28/2006 9:02:32 AM, Tom Stian Bjerk wrote:
>>Here you can see the switch ..
>
> Well, yes, just about! But this image of the transponder panel doesn't
> match
> many of the pictures I can find for a REAL 737NG. I found that switch
ONLY
> (oddly) in my Boeing CBT package ("Computer Based Training") for the NG --

> it is there, but in a different place.
>
> In the CBT the switch is markes "ABV", "N" (for Normal) and "BLW" or
> similar.
> The narrative doesn't give any actual values, but it does say that the
> range is dependent upon "closure rates". I assume that means it catches
> aircraft from further above or below if their predicted "closest approach"

> is
> within some value, or getting closer, but I don't really know. Nothing
> I've
> read actually puts any values on the altitude range for display -- except
> that source which gives 18700 feet in total for "surveillance".
>
> However, I've now discovered that "surveillance" is probably NOT the same

> as
> "display". The range (vertical or otherwise) at which blips are shown on
> the ND is not involved in determining Traffic or Resolution Advisories.
> The
> latter is purely based on the Transponder setting, and the TFC display on
> the ND can be switched off.
>
> Mind you, I'm don't think PM operates that way, at least at present.
After
> all, it doesn't have the appropriate transponder switch inputs at the
> moment.
>
>>When the switch is in
>>center position i think the
>>tcas altitude is 2500ft above
>>and below..
>
> Hmmm. 2500 is rather close considering that I suspect PM doesn't operate
> its
> TAs and RAs like the real aircraft.
>
> All in all, I'm now thinking that my original "guess" of 6000 for the PM
> TCASAltRange parameter was reasonable, so I may go back to that instead of

> the
> 9350 I'd decided on from information received!
>
> Thanks!
>
> Pete
>

[Michel Vandaele]

Hi Peter,
Modern TCAS is a lot more complex than just a altitude difference. The logic

is also looking for exemple into a possible collision course. So an aircraft

in RVSM conditions which is passing a at 1.000 Ft will not give you a real
traffic alert, but one which is 6000 ft lower but in a collision course
will.
The tau area is a specific "protective area" around the own aircraft. When
an intruder aircraft enters the tau area, TCAS triggers an alarm. The
threshold of the tau area is defined by time. "Tau" means time. In other
words, tau is the time-to-go to CPA, to the "Closest Point of Approach". The

time-to-go is distance divided by closure rate -- both combined vertically
and horizontally.
Example: Say, a tau value of 30 seconds is used for an alarm threshold. As
soon as the intruder's time-to-go to the CPA is dropped to 30 seconds, TCAS

triggers an alarm.
A fast approaching intruder may be, for example, 6 miles away from the CPA
when crossing its tau threshold, while another intruder half as fast may be

only 3 miles away respectively.
In additon to this logic, TCAS uses different tau values at different
altitudes. The higher the own altitude, the larger the tau area -- the
greater the sensitivity level. The exact numbers are listed on the table
below.

There are cases when a closure rate is so slow that the tau area will never

be entered, while the physical separation, however, may be just a fraction
of a mile. In such a critical scenario, the calculated closure rate is no
longer useful as a sudden increase in the closure rate would leave no room
for an advance warning.

This problem has been eliminated by an additional "Distance Modification" --

short "DMOD". Time or closure rate are not a factor here. DMOD affects only

the physical separation between the own aircraft and the target.

Above info is coming from the B744 TCAS manual, but I suppose it is also
valid for other aircraft types. As you can see a lot more than just altitude

differences.

Ciao
Michel

--
Michel VANDAELE
Member of the board FSCB
EBOS2002 Designteam
My B744 simulator project
http://users.pandora.be/michel.vandaele/sim1.htm



"Tom Stian Bjerk" wrote in message
news:396190.79369@wb.onvix.com...
> As I said I dont know how it really works.. But on saturday I was
actually
> listning to a real 737NG pilot. And he said that they "set the switch in
> below position" when the was descending and above when they was climbing.
> But he dident say anything the altitude for the position. But I was a
> little
>
> bit late, so I dident heard all that he was saying.. But im pretty sure
> there are an switch for it .. :)
>
> But 6000ft above and below seems like an reasonable value for flightsim
> and
>
> PM since we dosent have that damn switch :)
>
> Best Regards
> Tom Stian Bjerk
>
> "Peter Dowson" skrev i melding
> news:396168.79369@wb.onvix.com...
>> On 2/28/2006 9:02:32 AM, Tom Stian Bjerk wrote:
>>>Here you can see the switch ..
>>
>> Well, yes, just about! But this image of the transponder panel doesn't
>> match
>> many of the pictures I can find for a REAL 737NG. I found that switch
> ONLY
>> (oddly) in my Boeing CBT package ("Computer Based Training") for the
>> NG --
>
>> it is there, but in a different place.
>>
>> In the CBT the switch is markes "ABV", "N" (for Normal) and "BLW" or
>> similar.
>> The narrative doesn't give any actual values, but it does say that the
>> range is dependent upon "closure rates". I assume that means it catches
>> aircraft from further above or below if their predicted "closest
>> approach"
>
>> is
>> within some value, or getting closer, but I don't really know. Nothing
>> I've
>> read actually puts any values on the altitude range for display --
except
>> that source which gives 18700 feet in total for "surveillance".
>>
>> However, I've now discovered that "surveillance" is probably NOT the
same
>
>> as
>> "display". The range (vertical or otherwise) at which blips are shown on
>> the ND is not involved in determining Traffic or Resolution Advisories.
>> The
>> latter is purely based on the Transponder setting, and the TFC display
on
>> the ND can be switched off.
>>
>> Mind you, I'm don't think PM operates that way, at least at present.
> After
>> all, it doesn't have the appropriate transponder switch inputs at the
>> moment.
>>
>>>When the switch is in
>>>center position i think the
>>>tcas altitude is 2500ft above
>>>and below..
>>
>> Hmmm. 2500 is rather close considering that I suspect PM doesn't operate
>> its
>> TAs and RAs like the real aircraft.
>>
>> All in all, I'm now thinking that my original "guess" of 6000 for the PM
>> TCASAltRange parameter was reasonable, so I may go back to that instead
>> of
>
>> the
>> 9350 I'd decided on from information received!
>>
>> Thanks!
>>
>> Pete
>>
>

[Tom Stian Bjerk]

I haved asked an real 737NG pilot and I got this answer..

I had to translate it, so I have shorten it down a bit..

--

When the companies buy an Boeing Aircraft, they buying a empty Boieng. The
companies have to desied what they will equip
there aircraft with. And they can choose diffrent TCAS solutions..

RyanAir is equiped with an TCAS that only see aircrafts -+ 2700ft vertical.

Its fixed, so they cannot change that. But if there is an aircraft on
colission course above or below that 2700ft limit it will be displayed in
the ND. The vertical range the TCAS is scanning is about +- 8700ft.

Other companies can choose diffrent solution. There exist a solution like a

said in the earlier posts. Some aircrafts can be equiped with a switch that

they can choose ABOVE, NORMAL and BELOW. But aircrafts that are in collision

course will always be displayed in the ND. Here is the values for the
diffrent possition.


Above +8700/-2700 used when Climbing

Normal +2700/-2700 used in Cruise

Below -8700/+2700 used when Desending



sorry my poor english.. hope you understand the translation :)

Best regards

Tom Stian Bjerk




"Peter Dowson" skrev i melding
news:396168.79369@wb.onvix.com...
> On 2/28/2006 9:02:32 AM, Tom Stian Bjerk wrote:
>>Here you can see the switch ..
>
> Well, yes, just about! But this image of the transponder panel doesn't
> match
> many of the pictures I can find for a REAL 737NG. I found that switch
ONLY
> (oddly) in my Boeing CBT package ("Computer Based Training") for the NG --

> it is there, but in a different place.
>
> In the CBT the switch is markes "ABV", "N" (for Normal) and "BLW" or
> similar.
> The narrative doesn't give any actual values, but it does say that the
> range is dependent upon "closure rates". I assume that means it catches
> aircraft from further above or below if their predicted "closest approach"

> is
> within some value, or getting closer, but I don't really know. Nothing
> I've
> read actually puts any values on the altitude range for display -- except
> that source which gives 18700 feet in total for "surveillance".
>
> However, I've now discovered that "surveillance" is probably NOT the same

> as
> "display". The range (vertical or otherwise) at which blips are shown on
> the ND is not involved in determining Traffic or Resolution Advisories.
> The
> latter is purely based on the Transponder setting, and the TFC display on
> the ND can be switched off.
>
> Mind you, I'm don't think PM operates that way, at least at present.
After
> all, it doesn't have the appropriate transponder switch inputs at the
> moment.
>
>>When the switch is in
>>center position i think the
>>tcas altitude is 2500ft above
>>and below..
>
> Hmmm. 2500 is rather close considering that I suspect PM doesn't operate
> its
> TAs and RAs like the real aircraft.
>
> All in all, I'm now thinking that my original "guess" of 6000 for the PM
> TCASAltRange parameter was reasonable, so I may go back to that instead of

> the
> 9350 I'd decided on from information received!
>
> Thanks!
>
> Pete
>

[Peter Dowson]

On 2/28/2006 5:34:39 PM, Tom Stian Bjerk wrote:
>I haved asked an real 737NG
>pilot and I got this answer..
>...
>sorry my poor english.. hope
>you understand the translation
>:)

Yes, it is very good English, don't worry. And thank you very much.

Unfortunately (but I don't know) I suspect that the PM TCAS only gives TA's
and RA's for traffic within the altitude range I specify, so it looks like I
cannot set a "realistic" altitude such as the fixed value you mention (not
having any Above/Below switch in PM).

I think, now, we do know enough to ask Enrico, when he is back again, how his
TCAS really works and whether this aspect might be made more realistic in
future. For now I think I will have to live with the compromise I first
thought of, i.e. +/-6000 ft.

Thank you very much for your input,

Regards,

Pete