737 : the procedure to set the QNH when cleared to an altitude it's a standard now,and the ATC is fully aware about this,and eventual separation issues.
About the transponder preset at 1013.25 , I have some questions:
- why is there a select source switch ? (Altitude SOURCE selector :enables altitude reporting from air data computer no.1 or no2. ) . We have very new planes,so it's not an old system.
-do you remember the accident involving a 757 (I'm not 100% sure ), and unreliable speed , where the crew asked the ATc about transponder altitude information, dead wrong , and they crushed in the sea?
Is it possible that the transponder reports an altitude from a selected altimeter system (cpt's or fo's ) and the TCAS uses a built-in altimeter, just for TCAS reasons?
I know that most of the time the transponder and TCAS are thought to be the same, and they are not. TCAS is a different system, incorporated and making use of the transponder system.The controlling panel is called a TCAS/ATC transponder panel.
I'm pretty sure that the altitude info the ATC gets is from the selected altimeter source ,and maybe ,the TCAS computation of the advisory/RA is done using a built-in 1013.25 preset altimeter.This preset makes some more sense, cause it will eliminate crew error (forgetting to change QNH/ STD )
Last edited by alexban; 12th February 2007 at 17:48.