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HeliTorque :: View topic - Mode C Transponders
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HeliTorque Forum Index » Flight Dynamics

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Mode C Transponders
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James T Lowe
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PostPosted: Sat Mar 29, 2008 5:38 pm    Post subject: Mode C Transponders Reply with quote

Hello,

A technical question if I may.

How does a transponder obtain altitude information prior to encoding it for an interrogation response?

Is it a separate altimeter in the transponder unit, or does it link into the altimeter? Is it the same static system in use? If it's link to the altimeter, how does changing the subscale affect it?
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veeany
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PostPosted: Sat Mar 29, 2008 5:59 pm    Post subject: Reply with quote

The old way was to use an encoding altimeter, which basically does what its called. It feeds altitude information to the Transponder and the transponder sends that when interogated. Often these are expensive to repair and an replacement altimeter with no encoder is mucn cheaper.

The subscale on the encoding altimeter makes no difference as what is encoded is atltude above 1013mb, all the time.

The new way is to use an altitude encoder which is basically just an electronic altimeter without a display and that feeds the transponder.

The encoder is usually connected to the Static System with a T-Piece, so the system can then feed both the encoder and the pressure instruments.

The display at the controllers end then calculates the actual level based upon the current QNH.

Grossly simplified but hope it helps.

Quite a few with some details on http://www.aircraftspruce.com/menus/av/encoder.html

GS
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James T Lowe
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PostPosted: Sat Mar 29, 2008 6:22 pm    Post subject: Reply with quote

Yup, that helps, thanks veeany.

CAP493 1.5.10.2.4, regarding Mode C verification, states: "However, if a controller observes a discrepancy the pilot is to be asked to confirm his altimeter setting and level."

Therefore, that means that the suspicion is that the aircraft is ACTUALLY AT the wrong level, rather than incorrect data being reported? Does that sound right?

(Out of interest, I found another useful reference too http://www.airsport-corp.com/modec.htm)
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PostPosted: Sat Mar 29, 2008 10:52 pm    Post subject: Reply with quote

When the controller asks "confirm level" it does one of two things -
The first is to check that what the pilot is seeing matches the display of the controller, within limits; or

It is a subtle way of saying "Hey, Dopey! You forgot to level off from your climb/ descent!"
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PostPosted: Sun Mar 30, 2008 6:40 am    Post subject: Reply with quote

Very Happy Cool
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