Mode C

[380]

Hi all

Where does XPDR mode C have its altitude indication from?
Is it linked to the altimeter? If yes, means that that cheating is possile, by setting an other QNH?
Or has it its own altimeter with a unchangable setting on 1013?

Thanks for answers

380

[fireflybob]

Mode C data from aircraft always based on 1013 mb (29.92" Hg) - does not matter what you set on altimeter!

Why would anyone want to cheat?

[ragspanner]

The source of height information will vary with type. Generally on more modern aircraft pressure altitude information is derived from the ADC (Air Data Computer).
Other means of deriving the same information would be from a height encoding altimeter , basically a standard alt' which digitally encodes the altitude information.
I've also seen a discrete component called a digitiser ,which derives alt'information directly from the aircrafts static system.
I think they work on a standard 1013.2 ,so as far as the 'cheat' scope goes ,not sure !.
Hopefully some bright spark will have that info' !. Is this of any assistance ?

[This message has been edited by ragspanner (edited 22 May 2001).]

[Bear Cub]

As a slight aside from what you asked - did you hear of the student pilot in the USA that was asked by ATC to "squawk altitude".

As he was flying at an altitude of 7,500 feet he entered that figure into the transponder.......seven five zero zero.

ATC called him back and asked "is that squawk intentional?", not knowing about the 'codes' his answer was "yes!!".

ATC then asked "is there a gun on board the aircraft?".

The student was an off duty security officer - so his reply was "yes".

The student was subsequently amazed at the level of service offered to him by ATC - until his landing in Las Vegas.

[HugMonster]

Serves them darn well right for failing to use standard phraseology. If you diverge from standard, there will be misunderstandings

[Checkboard]

He should have used his gun to shoot the ATC officer for asking such a leading question in such circumstances!

(BTW - this means that I think the story is bogus)

[Iz]

Mode C altitude reporting works on the standard pressure plane (1013.25 hPa or 29.92 in Hg). Some of the small box-type Collins transponder sets (found in many turboprops and light jets) have a test button which will show you what altitude it's reporting. (eg. FL75 would be indicated as 7.5 on the box)

[U R NumberOne]

The way Mode C works on radar is this...

The transponder does return the height based on 1013 - when you're above the Transition Altitude that's not a problem, the Mode C readout is stated as a three figure number,.i.e. 087 for FL87 etc. For aircraft operating below the TA controllers input the current airfield QNH into the Radar Processor which will then convert any Mode C returns below the TA to an altitude and display that as two figures (given nowhere in the UK has a TA of 10000+).