System questions from another forum

[Screaming_Emu]

http://www.jetphotos.net/forums/showthread.php?t=27959

anybody able to answer #2? As I understand it, ATC said the transponder was reporting them at 9,700ft just before they crashed into the water. I guess my question is does the transponder have its own static source, or does it go off of of one (or more) of the ports that were blocked? If it was blocked, why woudl it be reporting 9,700ft? If it was a seperate one that wasn't blocked...again...why might it not be indicating correctly?

Kinda a hard question, but I'm hoping somebody with some 757 systems (or other transport cat aircraft) knowledge might be able to shed some light on. I looked at the section on flight instruments on www.smartcockpit.com for the 767 but it didn't mention the transponder at all. Thanks in advance.

 

[Alchemy]

The transponder(s) does recieve its altitude data from the same source as the aircraft's altimeter(s). For a transport category aircraft, that's usually going to be one of the Air Data Computers. I can only speak for the aircraft I fly, but the standby instrument systems uses a completely independent pitot/static sensor from the primary instruments, and the standby altimeter isn't linked to the transponder. The most common scenario where the standby instruments would be used is an electrical emergency (loss of all generators). Since the transponders will drop off line in an electrical emergency/essential power situation to conserve battery power anyway, the designers probably thought it would be pointless to include a third transponder link for the standby system.

If the aircraft encountered icing (let's assume that the static heat is malfunctiong) and the static ports froze over at 9,700 ft., the altimeters would now read 9,700 regardless of the aircraft's true altitude, until the static ports unfroze. The altimeters/ADCs measure air pressure and convert that to an altitude readout. When the ports are blocked the air at the altitude where blockage occured is "trapped" in the static system, causing a constant indicated altitude readout despite changes in true altitude. Since the xponder gets its info from the same source as the altimeters, it's also going to be indicating the altitude at which blockage occured, until the ports become unblocked.

I'm sure you already know most of this, but I figure I'd cover my bases here.

It is curious that the aircraft's transponder was reporting 9700 ft though. It seems like the altitude should be stuck on the elevation the aircraft took off from if there was tape over the static ports prior to departure. Do you know the altitude of the departure airport?

Hope this helps.

 

[Screaming_Emu]

It is curious that the aircraft's transponder was reporting 9700 ft though. It seems like the altitude should be stuck on the elevation the aircraft took off from if there was tape over the static ports prior to departure. Do you know the altitude of the departure airport?

Hope this helps.

Thanks for clearin that up about the transponder using the same static source. I thought about all the other stuff you wrote, but the part about the transponder being stuck at 9700 was the part that confused me too. I'm not sure of the takeoff airport elevation, but I'll try to look that up tomorrow when I can keep my eyes open. Thanks again.