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Old 24th Dec 2012, 01:03   #1 (permalink)
 
Join Date: Sep 2007
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A320 SRS Info Needed

Hi,

I have a question regarding the SRS mode and acceleration altitude.

What reference of barometer causes the SRS mode to switch to CLB mode if STD was set below the acceleration altitude?

1/For example, if i were to input an acceleration altitude of 4000ft into the PERF TAKEOFF page, and climbing out i set STD on the barometer. Will the aircraft accelerate on the PFD Altitude at 4000ft with the STD reference or 4000ft on a QNH reference???

2/ So if i decide to takeoff with 6000ft in the FCU, trans alt 6000ft, but the departure has an early constraint at 4000ft and i suddenly set STD straight away after takeoff, will ALT CST level the aircraft at 4000ft +/- the STD deviation from QNH. Say its 1003 mb at lgw (200ft amsl), i pull STD which is 300ft difference, i reach the constraint of 4000ft with STD accidently set, will the PFD altitude display 4300ft????

Last edited by JIMBO01; 24th Dec 2012 at 01:37.
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Old 24th Dec 2012, 14:50   #2 (permalink)
 
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A320 SRS Info Needed

It uses baro ref... It's an ALT. Early selection of STD will delay/advance the acceleration..
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Old 24th Dec 2012, 23:16   #3 (permalink)
 
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It all depends on the transition altitude on MCDU take off page.

In your example , MCDU transition altitude is 6000 ft. All constraints below 6000 ft will be altitude , regardless of your baro QNH/ STD setting. Early selection of STD will not affect the constraint and AP/FD will give guidance to maintain the barometric corrected FL.

So if QNH is 1003 , with 300 ft difference from STD , the AP/FD will guide the aircraft to level off at FL37 with FL37 magenta on PFD.

Opposite in descent , if TL is 45 on MCDU PERF APR page , and on FLPLN there is a FL constraint of let's say FL 80.
The FMGS will understand it as Flight level as it's above Transition Level inserted on MCDU and it will correct the barometric difference.

So early selection of QNH, will not affect the maintained FL constraint and with 300 ft difference the AP/FD will maintain 8300 ft , PFD will display FL83 magenta. Safe side!

The governing factor is the transition altitude inserted on MCDU

It goes without saying that this is not valid for OPEN modes

Last edited by Citation2; 24th Dec 2012 at 23:19.
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Old 27th Dec 2012, 15:07   #4 (permalink)
 
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Citation2
Not sure you are right about the FMGC correcting altitudes/FL's...
As I understand it, unless you have the QFE option, it flies the a/c to make the BARO altimeter read what is in the FCU window - the BARO REF setting is irrelevant. Hence the warning not to change the BARO REF when in ALT*, because that changes the target altitude.
In descent, the displayed targets on MCDU above Trans Alt do change from FL to altitude with BARO REF being STD or QNH.....
TP

Last edited by TyroPicard; 28th Dec 2012 at 10:01.
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